Microbial engineering for the production of alpha-substituted 3-hydroxy acids

Abstract

Disclosed herein are biological pathways and recombinant microorganisms useful, in some aspects, for the production of a-substituted 3-hydroxy acids and precursors thereof, as well as methods of their use.

Description

[0001]MICROBIAL ENGINEERING FOR THE PRODUCTION OF ALPHA-SUBSTITUTED 3-HYDROXY ACIDS RELATED APPLICATIONS This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No.63 / 658,134 filed June 10, 2024, and entitled “MICROBIAL ENGINEERING FOR THE PRODUCTION OF ALPHA-SUBSTITUTED 3 HYDROXY ACIDS,” the content of which is hereby incorporated by reference herein in its entirety for all purposes. REFERENCE TO AN ELECTRONIC SEQUENCE LISTING The contents of the electronic sequence listing (M065670534WO00-SEQ-KGC.xml; Size: 190,937 bytes; and Date of Creation: June 5, 2025) is herein incorporated by reference in its entirety. FIELD Disclosed herein are cells, compositions, and methods for the production of α-substituted 3-hydroxy acids from carbon sources. BACKGROUND Polyhydroxyalkanoates (PHAs) represent a rapidly growing sector of global plastics production. Natural PHAs are renewably derived, biodegradable polyesters produced by several genera of bacteria. They represent a sustainable alternative to conventional plastics and have found utility primarily in packaging and textiles. According to European Bioplastics, PHA production capacity is expected to grow by more than 128% over the next five years. Poly(3-hydroxybutyrate) (P3HB) is one of the most well studied PHAs. However, thermal instability limits the utility of P3HB. SUMMARY Described herein are compositions and methods useful relating to stereospecific biosynthetic production of α-3HAs and / or branched acid α-3HA precursors from suitable carbon sources. In some aspects, biosynthetic pathways to produce α-3HA precursors are provided. In some aspects, biosynthetic pathways to produce α-3HAs are provided. Some aspects of the present disclosure relate to recombinant microbial cells, comprising a nucleic acid molecule encoding an acetohydroxyacid synthase, a nucleic acid molecule encoding an acetohydroxy acid isomeroreductase, a nucleic acid molecule encoding a dihydroxy acid dehydratase, a nucleic acid molecule encoding an alpha-ketoisovalerate decarboxylase, a nucleic acid molecule encoding an aldehyde dehydrogenase, and a nucleic acid molecule encoding a threonine deaminase; wherein the microbial cell overexpresses an aspartokinase, a homoserine kinase, and a threonine synthase relative to a wildtype microbial cell. In some embodiments, the nucleic acid molecule encoding an acetohydroxyacid synthase encodes IlvGM from Klebsiella aerogenes. In some embodiments, the nucleic acid molecule encoding an acetohydroxyacid synthase encodes AlsS from Bacillus subtilis. In some embodiments, the nucleic acid molecule encoding acetohydroxy acid isomeroreductase encodes IlvC from Escherichia coli. In some embodiments, the nucleic acid molecule encoding dihydroxy acid dehydratase encodes IlvD from Escherichia coli. In some embodiments, the nucleic acid molecule encoding alpha-ketoisovalerate decarboxylase encodes KivD from Lactococcus lactis. In some embodiments, the nucleic acid molecule encoding aldehyde dehydrogenase encodes Fjoh_2967 from Flavobacterium johnsonaie or AldH from Escherichia coli. In some embodiments, the nucleic acid molecule encoding threonine deaminase encodes IlvA from Corynebacterium glutamicum. In some embodiments, the recombinant microbial cell further comprises a nucleic acid molecule encoding an aspartokinase, a nucleic acid molecule encoding a homoserine kinase, and a nucleic acid molecule encoding a threonine synthase. In some embodiments, the nucleic acid molecule encoding an aspartokinase encodes ThrA from Escherichia coli. In some embodiments, the ThrA is ThrAG433Rfrom Escherichia coli. In some embodiments, the nucleic acid molecule encoding a homoserine kinase encodes ThrB from Escherichia coli. In some embodiments, the nucleic acid molecule encoding a threonine synthase encodes ThrC from Escherichia coli. In some embodiments, the recombinant microbial cell further comprises an inducible promoter or constitutive promoter operably linked to: the nucleic acid molecule encoding acetohydroxyacid synthase II, the nucleic acid molecule encoding acetohydroxy acid isomeroreductase, the nucleic acid molecule encoding dihydroxy acid dehydratase , the nucleic acid molecule encoding alpha-ketoisovalerate decarboxylase, the nucleic acid molecule encoding aldehyde dehydrogenase, and / or the nucleic acid molecule encoding threonine deaminase. Some aspects of the present disclosure relate to methods of producing short-chain fatty acids by culturing a recombinant microbial cell described herein with a carbon source. In some embodiments, the short-chain fatty acids are 2-methylbutyric acid (2MB) and / or isobutyric acid (IBA). In some embodiments, the short-chain fatty acids are precursors of α-substituted 3-hydroxy acids (α-3HAs). In some embodiments, the culture of the recombinant microbial cell contains at least 0.2 g L-12MB. In some embodiments, the carbon source is glucose. In some embodiments, the carbon source is glycerol. In some embodiments, the carbon source is a pentose. Some aspects of the present disclosure relate to a recombinant microbial cell comprising a nucleic acid molecule encoding an activating enzyme; a nucleic acid molecule encoding acyl-CoA oxidase (ACOX) or acyl-CoA dehydrogenase (ACDH); a nucleic acid molecule encoding a (R)-specific enoyl-CoA hydratase; and one or more thioesterases. In some embodiments, the nucleic acid molecule encoding an activating enzyme encodes propionyl-CoA transferase (Pct). In some embodiments, the Pct is EC.2.8.3.1 from Megasphaera elsdenii. In some embodiments, the nucleic acid molecule encoding an activating enzyme encodes an irreversible activating enzyme. In some embodiments, the irreversible activating enzyme is IbuA from Rhodopseudomonas palustris. In some embodiments, the irreversible activating enzyme is 2-methylbutyryl-CoA synthetase (MACS) from Methanosarcina acetivorans. In some embodiments, the nucleic acid molecule encoding (R)-specific enoyl-CoA hydratase encodes PhaJ from Aeromonas caviae. In some embodiments, the nucleic acid molecule encoding ACOX encodes ACX4 from Arabidopsis thaliana. In some embodiments, a recombinant cell comprising a nucleic acid molecule encoding ACOX overexpresses KatE relative to a wildtype microbial cell. In some embodiments, the nucleic acid molecule encoding ACDH encodes FadE from Escherichia coli. In some embodiments, the nucleic acid molecule encoding ACDH encodes Acd from Pseudomonas putida KT2440. In some embodiments, the recombinant cell further comprises a nucleic acid molecule encoding an ETFBAD from Pseudomonas putida KT2440. In some embodiments, the recombinant microbial cell further comprises an inducible promoter or constitutive promoter operably linked to the nucleic acid molecule encoding an activating enzyme, the nucleic acid molecule encoding acyl-CoA oxidase (ACOX) or acyl- CoA dehydrogenase (ACDH), and / or the nucleic acid molecule encoding a (R)-specific enoyl-CoA hydratase. Some aspects of the present disclosure relate to methods of producing α-substituted 3- hydroxy acids (α-3HAs). In some embodiments, the method comprises culturing a recombinant microbial cell described herein with isobutyric acid (IBA) or 2-methylbutyric acid (2MB). In some embodiments, the (α-3HAs) are (2S)-3-hydroxyisobutyric acid (3HIB) or (2S, 3R)-3-hydroxy-2-methylbutyric acid (3H2MB). In some embodiments, the culture of the recombinant microbial cell contains at least 0.02 g L-13HIB. In some embodiments, the culture of the recombinant microbial cell contains at least 0.05 g L-13H2MB. Some aspects relate to a recombinant microbial cell, comprising: a nucleic acid molecule encoding acetohydroxyacid synthase; a nucleic acid molecule encoding acetohydroxy acid isomeroreductase; a nucleic acid molecule encoding dihydroxy acid dehydratase; a nucleic acid molecule encoding alpha-ketoisovalerate decarboxylase; a nucleic acid molecule encoding aldehyde dehydrogenase; a nucleic acid molecule encoding threonine deaminase; a nucleic acid molecule encoding an activating enzyme; a nucleic acid molecule encoding acyl-CoA oxidase (ACOX) or acyl-CoA dehydrogenase (ACDH); a nucleic acid molecule encoding a (R)-specific enoyl-CoA hydratase; a nucleic acid molecule encoding thioesterases; and overexpressing an aspartokinase, a homoserine kinase, and a threonine synthase relative to a wildtype microbial cell. In some embodiments, the recombinant cell further comprises a nucleic acid molecule encoding an isobutyryl-CoA mutase (IcmF). In some embodiments, the IcmF is IcmF from Cupriavidus metallidurans. In some embodiments, the recombinant microbial cell further comprises a nucleic acid molecule encoding a 3-hydroxyacyl-CoA dehydrogenase. In some embodiments, the 3- hydroxyacyl-CoA dehydrogenase is FadB from Escherichia coli. In some embodiments, the recombinant microbial cell further comprises a nucleic acid molecule encoding a thiolase. In some embodiments, the thiolase is AtoB from Escherichia coli. In some embodiments, the recombinant microbial cell further comprises a nucleic acid molecule encoding an aspartokinase, a nucleic acid molecule encoding a homoserine kinase, and a nucleic acid molecule encoding a threonine synthase. In some embodiments, the nucleic acid molecule encoding an aspartokinase encodes ThrA from Escherichia coli. In some embodiments, the ThrA is ThrAG433Rfrom Escherichia coli. In some embodiments, the nucleic acid molecule encoding a homoserine kinase encodes ThrB from Escherichia coli. In some embodiments, the nucleic acid molecule encoding a threonine synthase encodes ThrC from Escherichia coli. In some embodiments, the recombinant microbial cell further comprises an inducible promoter or constitutive promoter operably linked to: the nucleic acid molecule encoding acetohydroxyacid synthase; the nucleic acid molecule encoding acetohydroxy acid isomeroreductase; the nucleic acid molecule encoding dihydroxy acid dehydratase; the nucleic acid molecule encoding alpha-ketoisovalerate decarboxylase; the nucleic acid molecule encoding aldehyde dehydrogenase; the nucleic acid molecule encoding threonine deaminase; the nucleic acid molecule encoding the activating enzyme; the nucleic acid molecule encoding acyl-CoA oxidase (ACOX) or acyl-CoA dehydrogenase (ACDH); and / or the nucleic acid molecule encoding a (R)-specific enoyl-CoA hydratase. Some aspects relate to methods of producing α-substituted 3-hydroxy acids in a recombinant microbial cell, the method comprising culturing a recombinant cell described herein with a carbon source. In some embodiments, the α-substituted 3-hydroxy acids are (2S)-3-hydroxyisobutyric acid (3HIB) or (2S, 3R)-3-hydroxy-2-methylbutyric acid (3H2MB). In some embodiments, the carbon source is glucose. In some embodiments, the culture of the recombinant microbial cell contains at least 0.14 g L-13H2MB. In some embodiments, the culture of the recombinant microbial cell contains at least 0.01 g L-13HIB. In some embodiments, culturing the recombinant cell with a carbon source further produces isobutyric acid (IBA) or 2-methylbutyric acid (2MB). In some embodiments, the recombinant microbial cell contains at least 0.25 g L-1IBA. In some embodiments, the recombinant microbial cell contains at least 0.05 g L-12MB. In some embodiments, a recombinant microbial cell described herein is a bacterial, fungal, protist, or algal cell. In some embodiments, the bacterial cell is an Escherichia coli cell. Some aspects relate to a cell culture comprising a recombinant microbial cell described herein. Some aspects relate to a supernatant of a cell culture comprising a recombinant microbial cell described herein. Some aspects relate to a recombinant microbial cell, comprising: (i) a nucleic acid molecule encoding an acetohydroxyacid synthase; (ii) a nucleic acid molecule encoding an acetohydroxy acid isomeroreductase; (iii) a nucleic acid molecule encoding a dihydroxy acid dehydratase; (iv) a nucleic acid molecule encoding an alpha-ketoisovalerate decarboxylase; (v) a nucleic acid molecule encoding an aldehyde dehydrogenase; and (vi) a nucleic acid molecule encoding a threonine deaminase; wherein the microbial cell overexpresses an aspartokinase, a homoserine kinase, and a threonine synthase relative to a wildtype microbial cell. In some embodiments, (i) the nucleic acid molecule encoding an acetohydroxyacid synthase encodes IlvGM from Klebsiella aerogenes or AlsS from Bacillus subtilis; (ii) the nucleic acid molecule encoding acetohydroxy acid isomeroreductase encodes IlvC from Escherichia coli; (iii) the nucleic acid molecule encoding dihydroxy acid dehydratase encodes IlvD from Escherichia coli; (iv) the nucleic acid molecule encoding alpha- ketoisovalerate decarboxylase encodes KivD from Lactococcus lactis; (v) the nucleic acid molecule encoding aldehyde dehydrogenase encodes Fjoh_2967 from Flavobacterium johnsonaie or AldH from Escherichia coli; and (vi) the nucleic acid molecule encoding threonine deaminase encodes IlvA from Corynebacterium glutamicum. In some embodiments, the recombinant microbial cell further comprises: (vii) a nucleic acid molecule encoding an aspartokinase; (viii) a nucleic acid molecule encoding a homoserine kinase; and (ix) a nucleic acid molecule encoding a threonine synthase. In some embodiments, (vii) the nucleic acid molecule encoding an aspartokinase encodes ThrA from Escherichia coli, optionally wherein the ThrA is ThrAG433Rfrom Escherichia coli; (viii) the nucleic acid molecule encoding a homoserine kinase encodes ThrB from Escherichia coli; and (ix) the nucleic acid molecule encoding a threonine synthase encodes ThrC from Escherichia coli. In some embodiments, the recombinant microbial cell further comprises an inducible promoter or constitutive promoter operably linked to the nucleic acid molecule of (i), the nucleic acid molecule of (ii), the nucleic acid molecule of (iii), the nucleic acid molecule of (iv), the nucleic acid molecule of (v), and / or the nucleic acid molecule of (vi). Some aspects relate to a method of producing short-chain fatty acids in a recombinant microbial cell described herein, the method comprising culturing the recombinant microbial cell with a carbon source, wherein the carbon source optionally comprises glucose, glycerol, pentose, or a combination thereof. In some embodiments, the short-chain fatty acids are 2- methylbutyric acid (2MB) and / or isobutyric acid (IBA), wherein the short-chain fatty acids are optionally precursors of α-substituted 3-hydroxy acids (α-3HAs). In some embodiments, the culture of the recombinant microbial cell contains at least 0.2 g L-12MB. Some aspects relate to a recombinant microbial cell comprising: (i) a nucleic acid molecule encoding an activating enzyme; (ii) a nucleic acid molecule encoding acyl-CoA oxidase (ACOX) or acyl-CoA dehydrogenase (ACDH); (iii) a nucleic acid molecule encoding a (R)-specific enoyl-CoA hydratase; and (iv) one or more thioesterases. In some embodiments, (i) the nucleic acid molecule encoding an activating enzyme encodes (a) propionyl-CoA transferase (Pct), wherein the Pct is optionally EC.2.8.3.1 from Megasphaera elsdenii; or (b) an irreversible activating enzyme, wherein the irreversible activating enzyme is optionally IbuA from Rhodopseudomonas palustris or 2-methylbutyryl-CoA synthetase (MACS) from Methanosarcina acetivorans; (ii) the nucleic acid molecule encoding ACOX or ACDH encodes: (a) ACX4 from Arabidopsis thaliana, wherein KatE is optionally overexpressed relative to a wildtype microbial cell; or (b) FadE from Escherichia coli or Acd from Pseudomonas putida KT2440, wherein the recombinant microbial cell optionally further comprises a nucleic acid molecule encoding an ETFBAD from Pseudomonas putida KT2440; and (iii) the nucleic acid molecule encoding (R)-specific enoyl-CoA hydratase encodes PhaJ from Aeromonas caviae. In some embodiments, the recombinant microbial cell further comprises an inducible promoter or constitutive promoter operably linked to the nucleic acid molecule of (i), the nucleic acid of (ii), and / or the nucleic acid molecule of (iii). Some aspects relate to a method of producing α-substituted 3-hydroxy acids (α- 3HAs), the method comprising culturing a recombinant microbial cell described herein with isobutyric acid (IBA) or 2-methylbutyric acid (2MB), optionally wherein the α-3HAs are (2S)-3-hydroxyisobutyric acid (3HIB) or (2S, 3R)-3-hydroxy-2-methylbutyric acid (3H2MB). In some embodiments, the culture of the recombinant microbial cell contains at least 0.02 g L-13HIB. Some aspects relate to a recombinant microbial cell, comprising: a first module comprising: (i) a nucleic acid molecule encoding an acetohydroxyacid synthase; (ii) a nucleic acid molecule encoding an acetohydroxy acid isomeroreductase; (iii) a nucleic acid molecule encoding a dihydroxy acid dehydratase; (iv) a nucleic acid molecule encoding an alpha- ketoisovalerate decarboxylase; (v) a nucleic acid molecule encoding an aldehyde dehydrogenase; and (vi) a nucleic acid molecule encoding a threonine deaminase; and a second module comprising: (i) a nucleic acid molecule encoding an activating enzyme; (ii) a nucleic acid molecule encoding acyl-CoA oxidase (ACOX) or acyl-CoA dehydrogenase (ACDH); (iii) a nucleic acid molecule encoding a (R)-specific enoyl-CoA hydratase; and (iv) one or more thioesterases; wherein the microbial cell overexpresses an aspartokinase, a homoserine kinase, and a threonine synthase relative to a wildtype microbial cell. In some embodiments, the first module comprises: (i) the nucleic acid molecule encoding an acetohydroxyacid synthase encodes IlvGM from Klebsiella aerogenes or AlsS from Bacillus subtilis; (ii) the nucleic acid molecule encoding acetohydroxy acid isomeroreductase encodes IlvC from Escherichia coli; (iii) the nucleic acid molecule encoding dihydroxy acid dehydratase encodes IlvD from Escherichia coli; (iv) the nucleic acid molecule encoding alpha-ketoisovalerate decarboxylase encodes KivD from Lactococcus lactis; (v) the nucleic acid molecule encoding aldehyde dehydrogenase encodes Fjoh_2967 from Flavobacterium johnsonaie or AldH from Escherichia coli; and (vi) the nucleic acid molecule encoding threonine deaminase encodes IlvA from Corynebacterium glutamicum; and / or the second module comprises: (i) the nucleic acid molecule encoding an activating enzyme encodes (a) propionyl-CoA transferase (Pct), wherein the Pct is optionally EC.2.8.3.1 from Megasphaera elsdenii; or (b) an irreversible activating enzyme, wherein the irreversible activating enzyme is optionally IbuA from Rhodopseudomonas palustris or 2- methylbutyryl-CoA synthetase (MACS) from Methanosarcina acetivorans; (ii) the nucleic acid molecule encoding ACOX or ACDH encodes: (a) ACX4 from Arabidopsis thaliana, wherein KatE is optionally overexpressed relative to a wildtype microbial cell; or (b) FadE from Escherichia coli or Acd from Pseudomonas putida KT2440, wherein the recombinant microbial cell optionally further comprises a nucleic acid molecule encoding an ETFBAD from Pseudomonas putida KT2440; and (iii) the nucleic acid molecule encoding (R)-specific enoyl-CoA hydratase encodes PhaJ from Aeromonas caviae. In some embodiments, the first module further comprises an inducible promoter or constitutive promoter operably linked to the nucleic acid molecule of (i), the nucleic acid of (ii), the nucleic acid molecule of (iii), the nucleic acid molecule of (iv), the nucleic acid molecule of (v), and / or the nucleic acid molecule of (vi); and the second module further comprises an inducible promoter or constitutive promoter operably linked to the nucleic acid molecule of (i), the nucleic acid of (ii), and / or the nucleic acid molecule of (iii). In some embodiments, the recombinant microbial cell further comprises: a nucleic acid molecule encoding an isobutyryl-CoA mutase (IcmF), a nucleic acid molecule encoding a 3-hydroxyacyl-CoA dehydrogenase, and / or a thiolase. In some embodiments, the recombinant microbial cell further comprises a nucleic acid molecule encoding an aspartokinase, wherein the aspartokinase is optionally ThrA from Escherichia coli, further optionally wherein the ThrA is ThrAG433Rfrom Escherichia coli;a nucleic acid molecule encoding a homoserine kinase, wherein the homoserine kinase is optionally ThrB from Escherichia coli; and a nucleic acid molecule encoding a threonine synthase, wherein the nucleic acid molecule encoding a threonine synthase optionally encodes ThrC from Escherichia coli. Some aspects relate to a method of producing α-substituted 3-hydroxy acids (α- 3HAs), the method comprising culturing a recombinant microbial cell described herein with a carbon source, wherein the α-3HAs are optionally (2S)-3-hydroxyisobutyric acid (3HIB) or (2S, 3R)-3-hydroxy-2-methylbutyric acid (3H2MB), further optionally wherein the culture of the recombinant microbial cell contains at least 0.14 g L-13H2MB and / or at least 0.01 g L-13HIB. In some embodiments, the method further comprises producing isobutyric acid (IBA) or 2-methylbutyric acid (2MB), wherein the culture of the recombinant microbial cell optionally contains at least 0.25 g L-1IBA and / or the culture of the recombinant microbial cell contains at least 0.05 g L-12MB. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings: FIG.1 shows a biological pathway for the production of (2S)-3-hydroxyisobutyric acid (3HIB) and (2S,3R)-3-hydroxy-2-methylbutyric acid (3H2MB) from glucose in recombinant E. coli via Module 1 and / or Module 2. Threonine hyperproduction (left) provides a pathway from phosphoenolpyruvate (metabolized from glucose) into threonine. Module 1 (top) provides pathways from pyruvate, metabolized from phosphoenolpyruvate, or 2-ketobutyrate, metabolized from threonine to isobutyric acid (IBA) and / or 2-methylbutyric acid (2MB), respectively. Module 2 (bottom) provides a pathway from IBA and / or 2MB to 3HIB and / or 3H2MB, respectively. FIG.2 shows production of IBA and 2MB from glucose in recombinant E. coli harboring the Module 1 pathway. Strains of recombinant E. coli were engineered to express a combination of acetolactate synthase (ALS; AlsS from B. subtilis or IlvGM from K. aerogenes), ketol-acid reductoisomerase (IlvC from E.coli ), dihydroxyacid dehydratase (IlvD from E. coli), and aldehyde dehydrogenase (ALDH; Fjoh_2967 from F. johnsonaie or AldH from E. coli), resulting in seven different strains. In some cases, genes for threonine hyperproduction are expressed or over-expressed, for example, genes encoding feedback resistant, fused aspartate kinase / homoserine dehydrogenase ( ThrAG433Rfrom E. coli), homoserine kinase (ThrB from E. coli), threonine synthase (ThrC from E. coli), and feedback resistant threonine deaminase (IlvA from Corynebacterium glutamicum). Filled bars correspond to metabolite (IBA or 2MB) titer; vertical error bars correspond to standard deviation. Abbreviations used: F = Fjoh_2967, A = AldH. FIG.3 shows an IBA-recycling Module 1 pathway. The Module 1 pathway was modified to favor 2MB production via IBA recycling. Recombinant E. coli were engineered to express Module 1 and further express vitamin B12-dependent isobutyryl-CoA mutase (IcmF) from C. metallidurans. Isobutyryl-CoA is produced per the “promiscuous” Module 1 pathway (Module 1 only); in bacteria harboring IcmF, isobutyryl-CoA is converted into butyryl-CoA by IcmF; butyryl-CoA is then degraded into acetyl-CoA by the E. coli native short-chain fatty acid degradation pathway. Acetyl-CoA is then converted into threonine, then re-enters Module 1 to preferentially produce 2MB. FIG.4 shows production of IBA and 2MB from glucose in recombinant E. coli harboring the IBA-recycling Module 1 pathway. Strains of recombinant E. coli were engineered to express: IcmF only; Module 1(A) (including IlvGM from K. aerogenes and aldehyde dehydrogenase AldH from E. coli [A]) only; Module 1(A) and IcmF; Module 1(F) (including IlvGM from K. aerogenes and Fjoh_2967 from F. johnsonaie [F]) only; Module 1(F) and IcmF. Strains were optionally fed IBA. Filled bars correspond to metabolite (IBA or 2MB) and butyric acid (BA) titer; vertical error bars correspond to standard deviation. FIG.5 shows production of 3HIB and 3H2MB from IBA and 2MB in recombinant E. coli harboring the Module 2 pathway. Strains of recombinant E. coli were engineered to express EC.2.8.3.1 from M. elsdenii, PhaJ from A. cavie, thioesterases, and one of the following: Acx4 from A. thaliana; Acx4 from A. thaliana and KatE from E. coli; Acd from P. putida KT2440 and EtfBAD (genome locus tags: PP_0312, PP_0313, and PP_4203) from P. putida KT2440; FadE from E. coli; ACOX3 from Y. lipolytica; and Acd from P. putida KT2440 and EtfBAD (genome locus tags: PP_4202, PP_4201, and PP_4203) from P. putida KT2440; AcdH from Streptomyces avermitilis and EtfBAD (genome locus tags: PP_4202, PP_4201, and PP_4203) from P. putida KT2440, and resulting in seven different strains. Filled bars correspond to α-3HA (3HIB or 3H2MB) titer; vertical error bars correspond to standard deviation. BLOQ = below limit of quantification; ND = not detected. FIG.6 shows production of 3H2MB from glucose in recombinant E. coli harboring Module 1 and Module 2 pathways. Strains differed in expression of activation enzyme (propionyl-CoA transferase (Pct) from M. elsdenii, isobutyryl-CoA synthase (IbuA) from R. palustris, or 2- methylbutyryl-CoA synthetase (MACS) from M. acetivorans) and optionally IcmF (IcmF from C. metallidurans). Filled bars correspond to titers of 3H2MB. FIG.7 shows production of IBA and 3HIB from glucose in recombinant E. coli harboring Module 1 and Module 2 pathways. Strains differed in expression of AcdH / ACOX (ACX4 from A. thaliana or FadE from E. coli), and AldH (AldH from E. coli or Fjoh_2967 from F. johnsonai). Filled bars correspond to titers of IBA and 3HIB. FIGs.8A-8B are schematics depicting plasmids comprising a Module 1 pathway. FIG.8A is a schematic depicting one embodiment of a plasmid encoding enzymes characteristic of a Module 1 pathway for IBA production, comprising transgenes encoding AlsS from B. subtilis, IlvC from E.coli, and IlvD from E. coli. FIG.8B is a schematic depicting one embodiment of a plasmid encoding enzymes characteristic of a Module 1 pathway for IBA and / or 2MB production, comprising transgenes encoding KivD from L. lactis and Fjoh_2967 from F. johnsonaie. FIG.9 is a schematic depicting one embodiment of a plasmid comprising a Module 1 pathway for IBA and / or 2MB production, comprising transgenes encoding KivD from L. lactis and AldH from E. coli. FIGs.10A-10D are schematics depicting plasmids comprising a 2MB-favoring Module 1 and / or 3H2MB-favoring Module 1+2 pathway. FIG.10A is a schematic depicting one embodiment of a plasmid encoding enzymes characteristic of a Module 1 pathway for 2MB production, comprising transgenes encoding IlvGM from K. aerogenes, IlvC from E.coli, and IlvD from E. coli FIG.10B is a schematic depicting one embodiment of a plasmid comprising a threonine hyperproduction pathway, comprising transgenes encoding ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, and IlvA from Corynebacterium glutamicum. FIG.10C is a schematic depicting one embodiment of a plasmid encoding enzymes characteristic of a Module 1+23H2MB-favoring pathway, the plasmid comprising: KivD from L. lactis, AldH from E. coli, and IcmF from C. metallidurans. FIG.10D is a schematic depicting one embodiment of a plasmid encoding enzymes characteristic of a Module 1+23H2MB-favoring pathway, the plasmid comprising transgenes encoding: EC.2.8.3.1 from M. elsdenii, PhaJ from A. caviae, and Acx4 from A. thaliana. DETAILED DESCRIPTION Polyhydroxyalkanoates (PHAs) are renewably derived, biodegradable polyesters which represent a sustainable alternative to conventional plastics. Unlike synthetic plastics, which are formed from nonrenewable hydrocarbons (e.g., fossil fuels), PHAs are produced by microorganisms under nutrient limited and carbon excessive conditions. A promising alternative to traditional plastics, PHA has found ample utility in packaging and textiles, and demand has risen significantly. However, thermal instability of widespread and commercially available PHAs, like poly(3-hydroxybutyrate) (P3HB), limits their utility. Though PHA copolymers like poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and poly(3-hydroxybutyrate- co-3-hydroxyhexanoate) exhibit some improvement in thermal properties (e.g., see: Choi, S. R. (2020). Metabolic engineering for the synthesis of polyesters: A 100-year journey from polyhydroxyalkanoates to non-natural microbial polyesters. Metabolic engineering, 58, 47- 81; and Koller, M., & Mukherjee, A. (2022). A new wave of industrialization of PHA biopolyesters. Bioengineering, 9(2), 74.), the fabrication of PHAs containing α-substituted 3- hydroxy acids (α-3HAs) stands to greatly improve PHA thermal properties beyond those of industry standards. PHAs with an α-substitution have been shown to avoid thermal degradation by mechanisms shown to occur in P3HB and its co-polymers (e.g., see: Furutate, S., et al. (2021). Superior thermal stability and fast crystallization of a novel biodegradable α- methylated bacterial polyester. NPG Asia Materials, 13(1), 31.). Indeed, one report showed a 34% increase in degradation temperature in PHAs with an α-substitution compared to P3HB (e.g., see: Zhou, L., et al. (2023). Chemically circular, mechanically tough, melt-processable polyhydroxyalkanoates. Science, 380(6640), 64-69.). However, attempts thus far to produce suitable α-3HAs for production of PHAs are lacking. For example, biological production of a branched α-3HA, (2S)-3-hydroxyisobutyric acid (3HIB) has been shown previously (e.g., see: Lang, K., et al. (2015). Multistep synthesis of (s)-3-hydroxyisobutyric acid from glucose using Pseudomonas taiwanensis VLB120 B83 T7 catalytic biofilms. Advanced Synthesis & Catalysis, 357(8), 119-1927.), but these routes failed to produce R stereochemistry at C3, an essential property for biodegradability. Other groups have also reported a reverse β-oxidation pathway to another α-3HA, (2S,3S)-3-hydroxy-2-methylbutyric acid; however, this pathway relies on the activity of a branched-chain specific thiolase to condense acetyl-CoA and exogenously supplied propionate (e.g., see: Blaisse, M. D., & Chang, M. (2017). Discovery and engineering of pathways for production of α-branched organic acids. Journal of the American Chemical Society, 139(41), 14526-14532.). There is a need for cost-effective, efficient, and sustainable alternatives to production of stable, biodegradable PHAs. Similarly, there is a need for alternative, efficient methods of producing α-3HA and α-3HA precursors from renewable sources. Described herein are compositions and methods relating to the efficient, stereospecific biosynthetic production of α-3HAs and / or branched acid α-3HA precursors from suitable carbon sources. In some aspects, biosynthetic pathways (e.g., expressed by recombinant microbes) to produce α-3HA precursors are provided. In some aspects, biosynthetic pathways (e.g., expressed by recombinant microbes) to produce α-3HAs are provided. Non-limiting aspects and embodiments of the compositions and methods are described below. Synthesis of α-3HA precursors (Module 1) In some aspects, biosynthetic pathways (i.e., “modules”) disclosed herein provide routes for bioconversion of a suitable carbon source (e.g., glucose) into α-3HA precursors. In some embodiments, α-3HA precursors are isobutyric acid (IBA) and / or, preferably, 2- methylbutyric acid (2MB). Biosynthetic pathways for the production of IBA and / or 2MB are referred to hereinafter as “Module 1.” Module 1 pathways utilize microbial enzymes to produce α-3HA precursors from a carbon source. Exemplary pathways for α-3HA precursors are shown in FIG.1 (top). In some embodiments, Module 1 pathways favor the production of IBA from a suitable carbon source, referred to hereinafter as “IBA-favoring Module 1.” A Module 1 favoring production of IBA comprises one or more enzymes which enable (1) metabolism of a carbon source (e.g. glucose) into pyruvate, (2) condensation of pyruvate into an acetohydroxy acid, (3) reduction of the acetohydroxy acid into a dihydroxy valerate, (4) dehydration of the dihydroxy valerate into 2-keto acids, (5) decarboxylation of the 2-keto acids into an aldehyde, and (6) oxidation of the aldehyde into IBA. In a preferred embodiment, Module 1 pathways favor the production of 2MB from a suitable carbon source, referred to hereinafter as “2MB-favoring Module 1.” A Module 1 favoring production of 2MB comprises one or more enzymes which enable (1) metabolism of a carbon source (e.g., glucose) into threonine (2) conversion of threonine to a 2-ketobutyrate, (3) condensation of the 2-ketobutyrate into acetohydroxy acid, (4) reduction of the acetohydroxy acid into a dihydroxy valerate, (5) dehydration of the dihydroxy valerate into a 2-keto acid, (6) decarboxylation of the 2-keto acid into an aldehyde, and (7) oxidation of the aldehyde into 2MB. Both IBA-favoring and 2MB-favoring Module 1 pathways comprise an acetohydroxyacid synthase (also known as acetolactate synthase), an acetohydroxy acid isomeroreductase, a dihydroxy dehydratase, an alpha-ketoisovalerate decarboxylase, and an aldehyde dehydrogenase. However, the enzymes of each pathway may differ in preferred substrate (e.g., pyruvate, threonine) and stereospecificity which favor production of a particular α-3HA precursor. In some embodiments, the acetohydroxyacid synthetase is an AHAS II. In some embodiments, the AHAS II is acetolactate synthase (AlsS) from Bacillus subtilis. In some embodiments, AlsS from B. subtilis comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 1. In some embodiments, AlsS from B. subtilis comprises SEQ ID NO: 1. In some embodiments, AlsS from B. subtilis is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 2. In some embodiments, AlsS from B. subtilis is encoded by a nucleic acid sequence comprising SEQ ID NO: 2. In some embodiments, the AHAS II is IlvGM from Klebsiella aerogenes. In some embodiments, IlvGM from K. aerogenes comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 3. In some embodiments, IlvGM from K. aerogenes comprises SEQ ID NO: 3. In some embodiments, IlvGM from K. aerogenes is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 4. In some embodiments, IlvGM from K. aerogenes is encoded by a nucleic acid sequence comprising SEQ ID NO: 4. In some embodiments, the acetohydroxy acid isomeroreductase is IlvC from Escherichia coli. In some embodiments, IlvC from E. coli comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 5. In some embodiments, IlvC from E. coli comprises SEQ ID NO: 5. In some embodiments, IlvC from E. coli is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 6. In some embodiments, IlvC from E. coli is encoded by a nucleic acid sequence comprising SEQ ID NO: 6. In some embodiments, the dihydroxy dehydratase is IlvD from Escherichia coli. In some embodiments, IlvD from E. coli comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 7. In some embodiments, IlvD from E. coli comprises SEQ ID NO: 7. In some embodiments, IlvD from E. coli is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 8. In some embodiments, IlvD from E. coli is encoded by a nucleic acid sequence comprising SEQ ID NO: 8. In some embodiments, the alpha-ketoisovalerate decarboxylase is KivD from Lactococcus lactis. In some embodiments, KivD from L. lactis comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 9. In some embodiments, KivD from L. lactis comprises SEQ ID NO: 9. In some embodiments, KivD from L. lactis is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 10. In some embodiments, KivD from L. lactis is encoded by a nucleic acid sequence comprising SEQ ID NO: 10. In some embodiments, the aldehyde dehydrogenase is an aldehyde dehydrogenase capable of oxidizing isobutyraldehyde. In some embodiments, the aldehyde dehydrogenase is Fjoh_2967 from Flavobacterium johnsonaie. In some embodiments, Fjoh_2967 from F. johnsonaie comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 11. In some embodiments, Fjoh_2967 from F. johnsonaie comprises SEQ ID NO: 11. In some embodiments, Fjoh_2967 from F. johnsonaie is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 12. In some embodiments, Fjoh_2967 from F. johnsonaie is encoded by a nucleic acid sequence comprising SEQ ID NO: 12. In some embodiments, the aldehyde dehydrogenase is AldH from Escherichia coli. In some embodiments, the aldehyde dehydrogenase is AldH from E. coli. In some embodiments, AldH from E. coli comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 13. In some embodiments, AldH from E. coli comprises SEQ ID NO: 13. In some embodiments, AldH from E. coli is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 14. In some embodiments, AldH from E. coli is encoded by a nucleic acid sequence comprising SEQ ID NO: 14. Non-limiting examples of modifications to Module 1 pathways which favor a particular α-3HA precursor include: specificity of the acetohydroxyacid synthetase for a given reaction product, and end products of carbon source (e.g., glucose) metabolism which are processed by the pathway. Acetohydroxyacid synthetases perform a variety of condensation reactions. In some embodiments, an acetohydroxyacid synthetase performs a condensation reaction which preferentially leads to IBA production. In some embodiments, an acetohydroxyacid synthetase that performs a condensation reaction which preferentially leads to IBA production is AlsS from Bacillus subtilis. In some embodiments, an acetohydroxyacid synthetase performs a condensation reaction which preferentially leads to 2MB production. In some embodiments, an acetohydroxyacid synthetase that performs a condensation reaction which preferentially leads to 2MB production is IlvGM from Klebsiella aerogenes. Carbon source metabolism may provide a number of substrates which are processed by the pathway. In some embodiments, the end product of carbon source metabolism is pyruvate. In some embodiments, the end product of carbon source metabolism is threonine. Pyruvate and threonine are bioconverted preferentially into IBA and 2MB, respectively. Because metabolism of a carbon source (e.g., glucose) to pyruvate generally occurs more readily over metabolism of the same carbon source to threonine, increasing threonine is important to favor production of 2MB. In some embodiments, Module 1 pathway modifications that increase the production of threonine and favor the production of 2MB include threonine hyperproduction, and addition of a threonine deaminase. In some embodiments, a 2MB-favoring Module 1 pathway comprises enzymes enabling threonine hyperproduction. In some embodiments, enzymes enabling threonine hyperproduction are heterologous enzymes enabling threonine hyperproduction. In some embodiments, enzymes enabling threonine hyperproduction are endogenous enzymes enabling threonine hyperproduction. In some embodiments, a 2MB-favoring Module 1 pathway comprises overexpression of endogenous enzymes which enable threonine hyperproduction. Exemplary pathways for threonine hyperproduction are shown in FIG.1 (left). In some embodiments, enzymes enabling threonine hyperproduction comprise an aspartokinase, a homoserine kinase, and a threonine synthase. In some embodiments, the aspartokinase is a ThrA from E. coli. In some embodiments, the aspartokinase is a feedback resistant fused aspartate kinase / homoserine dehydrogenase. In some embodiments, the feedback resistant fused aspartate kinase / homoserine dehydrogenase is ThrAG433Rfrom E. coli. In some embodiments, ThrAG433Rfrom E. coli comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 15. In some embodiments, ThrAG433Rfrom E. coli comprises SEQ ID NO: 15. In some embodiments, ThrAG433Rfrom E. coli is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 16. In some embodiments, ThrAG433Rfrom E. coli is encoded by a nucleic acid sequence comprising SEQ ID NO: 16. In some embodiments, the homoserine kinase is ThrB from E. coli. In some embodiments, ThrB from E. coli comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 17. In some embodiments, ThrB from E. coli comprises SEQ ID NO: 17. In some embodiments, ThrB from E. coli is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 18. In some embodiments, ThrB from E. coli is encoded by a nucleic acid sequence comprising SEQ ID NO: 18. In some embodiments, the threonine synthase is ThrC from E. coli. In some embodiments, ThrC from E. coli comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 19. In some embodiments, ThrC from E. coli comprises SEQ ID NO: 19. In some embodiments, ThrC from E. coli is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 20. In some embodiments, ThrC from E. coli is encoded by a nucleic acid sequence comprising SEQ ID NO: 20. In some embodiments, a 2MB-favoring Module 1 pathway comprises a threonine deaminase. In some embodiments, a 2MB-favoring Module 1 pathway comprises a heterologous threonine deaminase. In some embodiments, a 2MB-favoring Module 1 pathway comprises overexpression of endogenous threonine deaminase. In some embodiments, the threonine deaminase is a feedback resistant threonine deaminase. In some embodiments, the threonine deaminase is IlvA from Corynebacterium glutamicum. In some embodiments, IlvA from C. glutamicum comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 21. In some embodiments, IlvA from C. glutamicum comprises SEQ ID NO: 21. In some embodiments, IlvA from C. glutamicum is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 22. In some embodiments, IlvA from C. glutamicum is encoded by a nucleic acid sequence comprising SEQ ID NO: 22. In some embodiments, a 2MB-favoring Module 1 pathway comprises an acetohydroxyacid synthase II, an acetohydroxy acid isomeroreductase, a dihydroxy dehydratase, a decarboxylase, an aldehyde dehydrogenase, a fused aspartate kinase / homoserine dehydrogenase, a homoserine kinase, a threonine synthase, and a threonine deaminase. In some embodiments, a 2MB-favoring Module 1 pathway comprises an acetohydroxyacid synthase II, an acetohydroxy acid isomeroreductase, a dihydroxy dehydratase, a decarboxylase, an isobutyraldehyde-preferring aldehyde dehydrogenase, a fused aspartate kinase / homoserine dehydrogenase, a homoserine kinase, a threonine synthase, and a threonine deaminase. In some embodiments, a 2MB-favoring Module 1 pathway comprises one or more of IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, AldH from E. coli, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, and IlvA from C. glutamicum. In some embodiments, a 2MB-favoring Module 1 pathway comprises IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, Fjoh_2967 from F. johnsonaie, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, and IlvA from C. glutamicum. In some embodiments, a 2MB-favoring Module 1 pathway comprises IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, AldH from E. coli, IlvA from C. glutamicum, ThrAG433Rfrom E. coli, ThrB from E. coli, and ThrC from E. coli, and overexpression of a thrA gene, relative to a wildtype microbial cell. In some embodiments, a 2MB-favoring Module 1 pathway comprises IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, Fjoh_2967 from F. johnsonaie, and IlvA from C. glutamicum, and overexpression of ThrAG433R, ThrB, and ThrC, relative to a wildtype microbial cell. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway recombinantly expresses an acetohydroxyacid synthase II, an acetohydroxy acid isomeroreductase, a dihydroxy dehydratase, a decarboxylase, an aldehyde dehydrogenase, a fused aspartate kinase / homoserine dehydrogenase, a homoserine kinase, a threonine synthase, and a threonine deaminase. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway recombinantly expresses IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, AldH from E. coli, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, and IlvA from C. glutamicum. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway recombinantly expresses IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, Fjoh_2967 from F. johnsonaie, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, and IlvA from C. glutamicum. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway recombinantly expresses IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, AldH from E. coli, and IlvA from C. glutamicum, and overexpresses ThrAG433R, ThrB, ThrC, relative to a wildtype microbial cell. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway recombinantly expresses IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, Fjoh_2967 from F. johnsonaie, and IlvA from C. glutamicum, and overexpresses ThrAG433R, ThrB, ThrC, relative to a wildtype microbial cell. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway recombinantly expresses an acetohydroxyacid synthase II, an acetohydroxy acid isomeroreductase, a dihydroxy dehydratase, a decarboxylase, an isobutyraldehyde-preferring aldehyde dehydrogenase, and a threonine deaminase; and overexpresses a fused aspartate kinase / homoserine dehydrogenase, a homoserine kinase, and a threonine synthase relative to a wildtype microbial cell. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway recombinantly expresses IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, AldH from E. coli, and IlvA from C. glutamicum; and overexpresses ThrA, ThrB, and ThrC relative to a wildtype microbial cell. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway recombinantly expresses IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, Fjoh_2967 from F. johnsonaie, and IlvA from C. glutamicum; and overexpresses ThrA, ThrB, and ThrC relative to a wildtype microbial cell. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway comprises: a nucleic acid molecule encoding an acetohydroxyacid synthase II, a nucleic acid molecule encoding an acetohydroxy acid isomeroreductase, a nucleic acid molecule encoding a dihydroxy dehydratase, a nucleic acid molecule encoding a decarboxylase, a nucleic acid molecule encoding an aldehyde dehydrogenase, a nucleic acid molecule encoding a fused aspartate kinase / homoserine dehydrogenase, a nucleic acid molecule encoding a homoserine kinase, a nucleic acid molecule encoding a threonine synthase, and a nucleic acid molecule encoding a threonine deaminase. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway comprises: a nucleic acid molecule encoding an acetohydroxyacid synthase II, a nucleic acid molecule encoding an acetohydroxy acid isomeroreductase, a nucleic acid molecule encoding a dihydroxy dehydratase, a nucleic acid molecule encoding a decarboxylase, a nucleic acid molecule encoding an aldehyde dehydrogenase, and a nucleic acid molecule encoding a threonine deaminase; and overexpresses a fused aspartate kinase / homoserine dehydrogenase, a homoserine kinase, and a threonine synthase relative to a wildtype microbial cell. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway comprises: a nucleic acid molecule encoding IlvGM from K. aerogenes, a nucleic acid molecule encoding IlvC from E. coli, a nucleic acid molecule encoding IlvD from E. coli, a nucleic acid molecule encoding KivD from L. lactis, a nucleic acid molecule encoding AldH from E. coli, a nucleic acid molecule encoding ThrAG433Rfrom E. coli, a nucleic acid molecule encoding ThrB from E. coli, a nucleic acid molecule encoding ThrC from E. coli, and a nucleic acid molecule encoding IlvA from C. glutamicum. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway comprises: a nucleic acid molecule encoding IlvGM from K. aerogenes, a nucleic acid molecule encoding IlvC from E. coli, a nucleic acid molecule encoding IlvD from E. coli, a nucleic acid molecule encoding KivD from L. lactis, a nucleic acid molecule encoding AldH from E. coli, and a nucleic acid molecule encoding IlvA from C. glutamicum; and overexpresses ThrAG433R, ThrB, and ThrC relative to a wildtype microbial cell. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway comprises: a nucleic acid molecule encoding IlvGM from K. aerogenes, a nucleic acid molecule encoding IlvC from E. coli, a nucleic acid molecule encoding IlvD from E. coli, a nucleic acid molecule encoding KivD from L. lactis, a nucleic acid molecule encoding Fjoh_2967 from F. johnsonaie, a nucleic acid molecule encoding ThrAG433Rfrom E. coli, a nucleic acid molecule encoding ThrB from E. coli, a nucleic acid molecule encoding ThrC from E. coli, and a nucleic acid molecule encoding IlvA from C. glutamicum. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway comprises: a nucleic acid molecule encoding IlvGM from K. aerogenes, a nucleic acid molecule encoding IlvC from E. coli, a nucleic acid molecule encoding IlvD from E. coli, a nucleic acid molecule encoding KivD from L. lactis, a nucleic acid molecule encoding Fjoh_2967 from F. johnsonaie, and a nucleic acid molecule encoding IlvA from C. glutamicum; and overexpresses ThrAG433R, ThrB, and ThrC relative to a wildtype microbial cell. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway comprises: a nucleic acid molecule encoding IlvGM from K. aerogenes, a nucleic acid molecule encoding IlvC from E. coli, a nucleic acid molecule encoding IlvD from E. coli, a nucleic acid molecule encoding KivD from L. lactis, a nucleic acid molecule encoding Fjoh_2967 from F. johnsonaie, and a nucleic acid molecule encoding IlvA from C. glutamicum; and overexpresses ThrAG433R, ThrB, ThrC relative to a wildtype microbial cell. In some embodiments, a recombinant microbial cell harboring a 2MB-favoring Module 1 pathway comprises: a nucleic acid molecule encoding IlvGM from K. aerogenes, a nucleic acid molecule encoding IlvC from E. coli, a nucleic acid molecule encoding IlvD from E. coli, a nucleic acid molecule encoding KivD from L. lactis, a nucleic acid molecule encoding Fjoh_2967 from F. johnsonaie, and a nucleic acid molecule encoding IlvA from C. glutamicum; and overexpresses ThrAG433R, ThrB, and ThrC relative to a wildtype microbial cell. Recombinant microbial cells harboring Module 1 pathways, as described herein, bioconvert carbon sources (i.e., feedstocks) into α-3HA precursors (e.g., IBA, 2MB). A suitable carbon source for the Module 1 pathways described herein includes any carbon source that can be natively metabolized by a host cell (e.g., E. coli) to pyruvate or 2- ketobutyrate. Non-limiting examples of suitable carbon sources include glucose, glycerol, and pentose (e.g., arabinose, xylose). In some embodiments, a carbon source comprises glucose, glycerol, pentose, or a combination thereof. In some embodiments, a carbon source is selected from glucose, glycerol, and pentose. In some embodiments, recombinant microbial cells harboring a Module 1 pathway are provided with one carbon source. In some embodiments, recombinant microbial cells harboring a Module 1 pathway are provided with two or more carbon sources. In some embodiments, recombinant microbial cells harboring a Module 1 pathway are cultured with a carbon source to produce isobutyric acid (IBA). In a preferred embodiment, recombinant microbial cells are cultured with a carbon source to produce 2-methylbutyric acid (2MB). In some embodiments, recombinant microbial cells harboring a Module 1 pathway are cultured with a carbon source to produce IBA and 2MB. Total molar yield of α- 3HA precursors (e.g., 2MB, IBA) may vary with carbon source concentration. In some embodiments, the concentration of carbon source(s) (weight of carbon source / volume culture medium) is about 0.2% wt / vol, about 0.3% wt / vol, about 0.4% wt / vol, about 0.5% wt / vol, about 0.6% wt / vol, about 0.7% wt / vol, about 0.8% wt / vol, about 0.9% wt / vol, about 1.0% wt / vol, about 1.1% wt / vol, about 1.2% wt / vol, about 1.3% wt / vol, about 1.4% wt / vol, about 1.5% wt / vol, about 1.6% wt / vol, about 1.7% wt / vol, about 1.8% wt / vol, about 1.9% wt / vol, about 2.0% wt / vol, about 2.1% wt / vol, about 2.2% wt / vol, about 2.3% wt / vol, about 2.4% wt / vol, about 2.5% wt / vol, about 2.6% wt / vol, about 2.7% wt / vol, about 2.8% wt / vol, about 2.9% wt / vol, about 3.0% wt / vol, about 3.1% wt / vol, about 3.2% wt / vol, about 3.3% wt / vol, about 3.4% wt / vol, about 3.5% wt / vol, about 3.6% wt / vol, about 3.7% wt / vol, about 3.8% wt / vol, about 3.9% wt / vol or more than about 3.9% wt / vol. In some embodiments, the concentration of carbon sources is about 1% wt / vol. In some embodiments, recombinant microbial cells harboring a Module 1 pathway produce a 2MB titer of at least 0.13 g L-1. For example the titer may be at least about 1 mg L-1, about 2 mg L-1, about 3 mg L-1, about 4 mg L-1, about 5 mg L-1, about 6 mg L-1, about 7 mg L-1, about 8 mg L-1, about 9 mg L-1, about 10 mg L-1, about 15 mg L-1, about 20 mg L-1, about 25 mg L-1, about 30 mg L-1, about 35 mg L-1, about 40 mg L-1, about 45 mg L-1, about 50 mg L-1, about 51 mg L-1, about 52 mg L-1, about 53 mg L-1, about 54 mg L-1, about 55 mg L-1, about 56 mg L-1, about 57 mg L-1, about 58 mg L-1, about 59 mg L-1, about 60 mg L-1, about 61 mg L-1, about 62 mg L-1, about 63 mg L-1, about 64 mg L-1, about 65 mg L-1, about 66 mg L-1, about 67 mg L-1, about 68 mg L-1, about 69 mg L-1, about 70 mg L-1, about 71 mg L-1, about 72 mg L-1, about 73 mg L-1, about 74 mg L-1, about 75 mg L-1, about 76 mg L-1, about 77 mg L-1, about 78 mg L-1, about 79 mg L-1, about 80 mg L-1, about 81 mg L-1, about 82 mg L-1, about 83 mg L-1, about 84 mg L-1, about 85 mg L-1, about 86 mg L-1, about 87 mg L-1, about 88 mg L-1, about 89 mg L-1, about 90 mg L-1, about 91 mg L-1, about 92 mg L-1, about 93 mg L-1, about 94 mg L-1, about 95 mg L-1, about 96 mg L-1, about 97 mg L-1, about 98 mg L-1, about 99 mg L-1, about 100 mg L-1, about 105 mg L-1, about 110 mg L-1, about 115 mg L-1, about 120 mg L-1, about 130 mg L-1, about 140 mg L-1, about 150 mg L-1, about 160 mg L-1, about 170 mg L-1, about 180 mg L-1, about 190 mg L-1, about 200 mg L-1, about 210 mg L-1, about 220 mg L-1, about 230 mg L-1, about 240 mg L-1, about 250 mg L-1, about 260 mg L-1, about 270 mg L-1, about 280 mg L-1, about 290 mg L-1, about 300 mg L-1, about 350 mg L-1, about 400 mg L-1, about 450 mg L-1, about 500 mg L-1, about 550 mg L-1, about 600 mg L-1, about 650 mg L-1, about 700 mg L-1, about 750 mg L-1, about 800 mg L-1, about 850 mg L-1, about 900 mg L-1, about 950 mg L-1, about 1 g L-1, about 1.5 g L-1, about 2 g L-1, about 2.5 g L-1, about 3 g L-1, about 3.5 g L-1, about 4 g L-1, about 4.5 g L-1, about 5 g L-1, or more than 5 g L-1. In some embodiments, recombinant microbial cells harboring a Module 1 pathway produces an IBA titer of at least 0.13 g L-1. For example the titer may be at least about 1 mg L-1, about 2 mg L-1, about 3 mg L-1, about 4 mg L-1, about 5 mg L-1, about 6 mg L-1, about 7 mg L-1, about 8 mg L-1, about 9 mg L-1, about 10 mg L-1, about 15 mg L-1, about 20 mg L-1, about 25 mg L-1, about 30 mg L-1, about 35 mg L-1, about 40 mg L-1, about 45 mg L-1, about 50 mg L-1, about 51 mg L-1, about 52 mg L-1, about 53 mg L-1, about 54 mg L-1, about 55 mg L-1, about 56 mg L-1, about 57 mg L-1, about 58 mg L-1, about 59 mg L-1, about 60 mg L-1, about 61 mg L-1, about 62 mg L-1, about 63 mg L-1, about 64 mg L-1, about 65 mg L-1, about 66 mg L-1, about 67 mg L-1, about 68 mg L-1, about 69 mg L-1, about 70 mg L-1, about 71 mg L-1, about 72 mg L-1, about 73 mg L-1, about 74 mg L-1, about 75 mg L-1, about 76 mg L-1, about 77 mg L-1, about 78 mg L-1, about 79 mg L-1, about 80 mg L-1, about 81 mg L-1, about 82 mg L-1, about 83 mg L-1, about 84 mg L-1, about 85 mg L-1, about 86 mg L-1, about 87 mg L-1, about 88 mg L-1, about 89 mg L-1, about 90 mg L-1, about 91 mg L-1, about 92 mg L-1, about 93 mg L-1, about 94 mg L-1, about 95 mg L-1, about 96 mg L-1, about 97 mg L-1, about 98 mg L-1, about 99 mg L-1, about 100 mg L-1, about 105 mg L-1, about 110 mg L-1, about 115 mg L-1, about 120 mg L-1, about 130 mg L-1, about 140 mg L-1, about 150 mg L-1, about 160 mg L-1, about 170 mg L-1, about 180 mg L-1, about 190 mg L-1, about 200 mg L-1, about 210 mg L-1, about 220 mg L-1, about 230 mg L-1, about 240 mg L-1, about 250 mg L-1, about 260 mg L-1, about 270 mg L-1, about 280 mg L-1, about 290 mg L-1, about 300 mg L-1, about 350 mg L-1, about 400 mg L-1, about 450 mg L-1, about 500 mg L-1, about 550 mg L-1, about 600 mg L-1, about 650 mg L-1, about 700 mg L-1, about 750 mg L-1, about 800 mg L-1, about 850 mg L-1, about 900 mg L-1, about 950 mg L-1, or about 1 g L-1, about 1.1 g L-1, about 1.2 g L-1, about 1.3 g L-1, about 1.4 g L-1, about 1.5 g L-1, about 1.6 g L-1, about 1.7 g L-1, about 1.8 g L-1, about 1.9 g L-1, about 2 g L-1, about 2.1 g L-1, about 2.2 g L-1, about 2.3 g L-1, about 2.4 g L-1, about 2.5 g L-1, about 2.6 g L-1, about 2.7 g L-1, about 2.8 g L-1, about 2.9 g L-1, about 3 g L-1, about 3.5 g L-1, about 4 g L-1, about 4.5 g L-1, about 5 g L-1, or more than 5 g L-1. In some embodiments, recombinant microbial cells harboring a Module 1 pathway produce an IBA titer of less than about 0.4 g L-1and a 2MB titer of at least 0.6 g L-1. For example the IBA titer may be about 0 mg L-1, about 1 mg L-1, about 2 mg L-1, about 3 mg L-1, about 4 mg L-1, about 5 mg L-1, about 6 mg L-1, about 7 mg L-1, about 8 mg L-1, about 9 mg L-1, about 10 mg L-1, about 15 mg L-1, about 20 mg L-1, about 25 mg L-1, about 30 mg L-1, about 35 mg L-1, about 40 mg L-1, about 45 mg L-1, about 50 mg L-1, about 51 mg L-1, about 52 mg L-1, about 53 mg L-1, about 54 mg L-1, about 55 mg L-1, about 56 mg L-1, about 57 mg L-1, about 58 mg L-1, about 59 mg L-1, about 60 mg L-1, about 61 mg L-1, about 62 mg L-1, about 63 mg L-1, about 64 mg L-1, about 65 mg L-1, about 66 mg L-1, about 67 mg L-1, about 68 mg L-1, about 69 mg L-1, about 70 mg L-1, about 71 mg L-1, about 72 mg L-1, about 73 mg L-1, about 74 mg L-1, about 75 mg L-1, about 76 mg L-1, about 77 mg L-1, about 78 mg L-1, about 79 mg L-1, about 80 mg L-1, about 81 mg L-1, about 82 mg L-1, about 83 mg L-1, about 84 mg L-1, about 85 mg L-1, about 86 mg L-1, about 87 mg L-1, about 88 mg L-1, about 89 mg L-1, about 90 mg L-1, about 91 mg L-1, about 92 mg L-1, about 93 mg L-1, about 94 mg L-1, about 95 mg L-1, about 96 mg L-1, about 97 mg L-1, about 98 mg L-1, about 99 mg L-1, about 100 mg L-1, about 105 mg L-1, about 110 mg L-1, about 115 mg L-1, about 120 mg L-1, about 130 mg L-1, about 140 mg L-1, about 150 mg L-1, about 160 mg L-1, about 170 mg L-1, about 180 mg L-1, about 190 mg L-1, about 200 mg L-1, about 210 mg L-1, about 220 mg L-1, about 230 mg L-1, about 240 mg L-1, about 250 mg L-1, about 260 mg L-1, about 270 mg L-1, about 280 mg L-1, about 290 mg L-1, about 300 mg L-1, about 350 mg L-1, or about 400 mg L-1, and the 2MB titer may be at least about 600 mg L-1, about 650 mg L-1, about 700 mg L-1, about 750 mg L-1, about 800 mg L-1, about 850 mg L-1, about 900 mg L-1, about 950 mg L-1, about 1 g L-1, about 1.5 g L-1, about 2 g L-1, about 2.5 g L-1, about 3 g L-1, about 3.5 g L-1, about 4 g L-1, about 4.5 g L-1, about 5 g L-1, or more than 5 g L-1. Synthesis of α-3HAs (Module 2) In some aspects, biosynthetic pathways disclosed herein provide routes for bioconversion of α-3HA precursors into α-3HAs. In some embodiments, α-3HAs are (2S)-3- hydroxyisobutyric acid (3HIB) and / or, preferably, (2S,3R)-3-hydroxy-2-methylbutyric acid (3H2MB). Biosynthetic pathways for the production of 3HIB and / or 3H2MB are referred to hereinafter as “Module 2”. Module 2 pathways utilize microbial enzymes to produce α-3HAs from IBA and / or 2MB. Like the IBA-favoring and 2MB-favoring Module 1 pathways, 3HIB-favoring and 3H2MB-favoring Module 2 share a basic pathway differing primarily in the substrate which is bioconverted to 3HIB or 3H2MB. A Module 2 pathway comprises one or more enzymes enabling (1) activation of an α-3HA precursor into an acyl-CoA, (2) dehydrogenation of the acyl-CoA into an enoyl-CoA, (3) stereospecific hydration of the enoyl-CoA to a (3R)-3- hydroxy acid, and (4) hydrolysis of the (3R)-3-hydroxy acid into an α-3HA. Both 3HIB- favoring and 3H2MB-favoring Module 2 pathways comprise an activating enzyme, an acyl- CoA dehydrogenase (ACDH) or acyl-CoA oxidase (ACOX), an (R)-specific enoyl-CoA hydratase, and one or more thioesterases. Exemplary pathways for α-3HAs are shown in FIG. 1 (bottom). Generally, a Module 2 pathway (e.g., a 3HIB-favoring or 3H2MB-favoring Module 2 pathway) comprises an activating enzyme, an ACDH, an (R)-specific enoyl-CoA hydratase, and one or more thioesterases. As used herein, the term “activating enzyme” refers to an enzyme which can activate carboxylic acids (e.g., IBA, 2MB) into acyl-CoA. In some embodiments, the activating enzyme is a propionyl-CoA transferase (Pct). In some embodiments, the Pct is EC.2.8.3.1 from Megasphaera elsdenii. In some embodiments, EC.2.8.3.1 from M. elsdenii comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 23. In some embodiments, EC.2.8.3.1 from M. elsdenii comprises SEQ ID NO: 23. In some embodiments, EC.2.8.3.1 from M. elsdenii is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 24. In some embodiments, EC.2.8.3.1 from M. elsdenii is encoded by a nucleic acid sequence comprising SEQ ID NO: 24. In some embodiments, the activating enzyme is an irreversible activating enzyme. In some embodiments, the irreversible activating enzyme is an isobutyryl-CoA synthase. In some embodiments, the isobutyryl-CoA synthase is IbuA from Rhodopseudomonas palustris. In some embodiments, IbuA from R. palustris comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 49. In some embodiments, IbuA from R. palustris comprises SEQ ID NO: 49. In some embodiments, IbuA from R. palustris is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 50. In some embodiments, IbuA from R. palustris is encoded by a nucleic acid sequence comprising SEQ ID NO: 50. In some embodiments, the irreversible activating enzyme is 2-methylbutyryl-CoA synthetase (MACS). In some embodiments, the MACS is MACS from Methanosarcina acetivorans. In some embodiments, is MACS from M. acetivorans comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 47. In some embodiments, MACS from M. acetivorans comprises SEQ ID NO: 47. In some embodiments, MACS from M. acetivorans is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 48. In some embodiments, MACS from M. acetivorans is encoded by a nucleic acid sequence comprising SEQ ID NO: 48. In some embodiments, the ACDH is FadE from E. coli. In some embodiments, the ACDH is ACD from Pseudomonas putida KT2440. In some embodiments, is ACD from P. putida KT2440 comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 27. In some embodiments, ACD from P. putida KT2440 comprises SEQ ID NO: 27. In some embodiments, ACD from P. putida KT2440 is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 28. In some embodiments, ACD from P. putida KT2440 is encoded by a nucleic acid sequence comprising SEQ ID NO: 28. In some embodiments, a Module 2 pathway comprising ACD from P. putida KT2440 further comprises electron transfer flavoproteins (ETFs). In some embodiments, the ETF is EtfBAD (gene locus tags: PP_4201-PP_4203) from P. putida KT2440. In some embodiments, the EtfBAD comprises EtfA(PP_4201) from P. putida KT2440. In some embodiments, EtfA(PP_4201) from P. putida KT2440 comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 31. In some embodiments, EtfA(PP_4201) from P. putida KT2440 comprises SEQ ID NO: 31. In some embodiments, EtfA(PP_4201) from P. putida KT2440 is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 32. In some embodiments, EtfA(PP_4201) from P. putida KT2440 is encoded by a nucleic acid sequence comprising SEQ ID NO: 32. In some embodiments, the EtfBAD comprises EtfB(PP_4202) from P. putida KT2440. In some embodiments, EtfB(PP_4202) from P. putida KT2440, comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 29. In some embodiments, EtfB(PP_4202) from P. putida KT2440 comprises SEQ ID NO: 29. In some embodiments, EtfB(PP_4202) from P. putida KT2440 is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 30. In some embodiments, EtfB(PP_4202) from P. putida KT2440 is encoded by a nucleic acid sequence comprising SEQ ID NO: 30. In some embodiments, EtfBAD comprises EtfD(PP_4203) from P. putida KT2440. In some embodiments, EtfD(PP_4203) from P. putida KT2440 comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 33. In some embodiments, EtfD(PP_4203) from P. putida KT2440 comprises SEQ ID NO: 33. In some embodiments, EtfD(PP_4203) from P. putida KT2440 is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 34. In some embodiments, EtfD(PP_4203) from P. putida KT2440 is encoded by a nucleic acid sequence comprising SEQ ID NO: 34. In some embodiments, EtfBAD comprises EtfB(PP_4202), EtfA(PP_4201), and EtfD(PP_4203) from P. putida KT2440. In some embodiments, the EtfBAD comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 39. In some embodiments, the EtfBAD comprises SEQ ID NO: 39. In some embodiments, the EtfBAD is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 40. In some embodiments, the EtfBAD is encoded by a nucleic acid sequence comprising SEQ ID NO: 40. In some embodiments, the ETF is EtfABD (gene locus tags: PP_0312, PP_0313, and PP_4203). In some embodiments, the EtfABD comprises EtfA(PP_0313) from P. putida KT2440. In some embodiments, EtfA(PP_0313) from P. putida KT2440 comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 35. In some embodiments, EtfA(PP_0313) from P. putida KT2440 comprises SEQ ID NO: 35. In some embodiments, EtfA(PP_0313) from P. putida KT2440 is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 36. In some embodiments, EtfA(PP_0313) from P. putida KT2440 is encoded by a nucleic acid sequence comprising SEQ ID NO: 36. In some embodiments, the EtfABD comprises EtfA(PP_0312) from P. putida KT2440. In some embodiments, EtfA(PP_0312) from P. putida KT2440 comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 37. In some embodiments, EtfA(PP_0312) from P. putida KT2440 comprises SEQ ID NO: 37. In some embodiments, EtfA(PP_0312) from P. putida KT2440 is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 38. In some embodiments, EtfA(PP_0312) from P. putida KT2440 is encoded by a nucleic acid sequence comprising SEQ ID NO: 38. In some embodiments, the EtfABD comprises EtfD(PP_4203) from P. putida KT2440. In some embodiments, EtfD(PP_4203) from P. putida KT2440 comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 33. In some embodiments, EtfD(PP_4203) from P. putida KT2440 comprises SEQ ID NO: 33. In some embodiments, EtfD(PP_4203) from P. putida KT2440 is encoded by a nucleic acid sequence comprising a nucleic acid sequence having 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 34. In some embodiments, EtfD(PP_4203) from P. putida KT2440 is encoded by a nucleic acid sequence comprising SEQ ID NO: 34. In some embodiments, the ACOX is ACX4 from Arabidopsis thaliana. In some embodiments, ACX4 from A. thaliana comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 41. In some embodiments, ACX4 from A. thaliana comprises SEQ ID NO: 41. In some embodiments, ACX4 from A. thaliana is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 42. In some embodiments, ACX4 from A. thaliana is encoded by a nucleic acid sequence comprising SEQ ID NO: 42. In some embodiments, the ACOX is ACOX3 from Yarrowia lipolytica. In some embodiments, ACOX3 from Y. lipolytica comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 74. In some embodiments, ACOX3 from Y. lipolytica comprises SEQ ID NO: 74. In some embodiments, ACOX3 from Y. lipolytica is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 75. In some embodiments, ACOX3 from Y. lipolytica is encoded by a nucleic acid sequence comprising SEQ ID NO: 75. Unlike ACDH, ACOX performs a dehydrogenation reaction in which oxygen is a final electron receptor, resulting in production of H2O2as a byproduct. As H2O2accumulation can be toxic some instances, decreasing production of H2O2byproducts may be desired. Thus, in some embodiments, a Module 2 pathway comprising ACX4 from A. thaliana or ACOX3 from Y. lipolytica further comprises an H2O2 scavenger, for example, a catalase. In some embodiments, the catalase is a KatE catalase. In some embodiments, the KatE catalase is KatE from E. coli. In some embodiments, KatE from E. coli comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 43. In some embodiments, KatE from E. coli comprises SEQ ID NO: 43. In some embodiments, KatE from E. coli is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 44. In some embodiments, KatE from E. coli is encoded by a nucleic acid sequence comprising SEQ ID NO: 44. In some embodiments, the (R)-specific enoyl-CoA hydratase is PhaJ from Aeromonas caviae. In some embodiments, PhaJ from A. caviae comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 45. In some embodiments, PhaJ from A. caviae comprises SEQ ID NO: 45. In some embodiments, PhaJ from A. caviae is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 46. In some embodiments, PhaJ from A. caviae is encoded by a nucleic acid sequence comprising SEQ ID NO: 46. In some embodiments, the one or more thioesterases are one or more thioesterases from E. coli. In some embodiments, the thioesterase is acyl-CoA thioesterase II, TesB, from E. coli. In some embodiments, TesB from E. coli comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 59. In some embodiments, TesB from E. coli comprises SEQ ID NO: 59. In some embodiments, TesB from E. coli is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 60. In some embodiments, TesB from E. coli is encoded by a nucleic acid sequence comprising SEQ ID NO: 60. In some embodiments, the thioesterase is phenylacetyl-CoA thioesterase, PaaI from E. coli. In some embodiments, PaaI from E. coli comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 51. In some embodiments, PaaI from E. coli comprises SEQ ID NO: 51. In some embodiments, PaaI from E. coli is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 52. In some embodiments, PaaI from E. coli is encoded by a nucleic acid sequence comprising SEQ ID NO: 52. In some embodiments, the thioesterase is 1,4-dihydroxy-2-naphthoyl-CoA hydrolase, YdiI from E. coli. In some embodiments, YdiI from E. coli comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 53. In some embodiments, YdiI from E. coli comprises SEQ ID NO: 53. In some embodiments, YdiI from E. coli is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 54. In some embodiments, YdiI from E. coli is encoded by a nucleic acid sequence comprising SEQ ID NO: 54. In some embodiments, the thioesterase is long chain acyl-CoA thioesterase, FadM from E. coli. In some embodiments, FadM from E. coli comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 55. In some embodiments, FadM from E. coli comprises SEQ ID NO: 55. In some embodiments, FadM from E. coli is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 56. In some embodiments, FadM from E. coli is encoded by a nucleic acid sequence comprising SEQ ID NO: 56. In some embodiments, the thioesterase is acyl-CoA thioesterase, YigI from E. coli. In some embodiments, YigI from E. coli comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 57. In some embodiments, YigI from E. coli comprises SEQ ID NO: 57. In some embodiments, YigI from E. coli is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 58. In some embodiments, YigI from E. coli is encoded by a nucleic acid sequence comprising SEQ ID NO: 58. In some embodiments, a Module 2 pathway (e.g., a 3HIB-favoring or 3H2MB- favoring Module 2 pathway) comprises an activating enzyme, an ACDH, an (R)-specific enoyl-CoA hydratase, and one or more thioesterases. In some embodiments, a Module 2 pathway comprises EC.2.8.3.1 from M. elsdenii, FadE from E. coli, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway comprises IbuA from R. palustris, FadE from E. coli, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway comprises MACS from M. acetivorans, FadE from E. coli, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses an activating enzyme, an ACDH, an (R)-specific enoyl-CoA hydratase, and one or more thioesterases. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses EC.2.8.3.1 from M. elsdenii, FadE from E. coli, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses IbuA from R. palustris, FadE from E. coli, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses MACS from M. acetivorans, FadE from E. coli, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding an activating enzyme, a nucleic acid molecule encoding an ACDH, a nucleic acid molecule encoding an (R)-specific enoyl-CoA hydratase, and a nucleic acid molecule encoding one or more thioesterases. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding EC.2.8.3.1 from M. elsdenii, a nucleic acid molecule encoding FadE from E. coli, a nucleic acid molecule encoding PhaJ from A. caviae, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding IbuA from R. palustris, a nucleic acid molecule encoding FadE from E. coli, a nucleic acid molecule encoding PhaJ from A. caviae, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding MACS from M. acetivorans, a nucleic acid molecule encoding FadE from E. coli, a nucleic acid molecule encoding PhaJ from A. caviae, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway comprises an activating enzyme, an ACDH, an ETF, an (R)-specific enoyl-CoA hydratase, and one or more thioesterases. In some embodiments, a Module 2 pathway comprises EC.2.8.3.1 from M. elsdenii, ACDH from P. putida KT2440, ETFBAD from P. putida KT2440, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway comprises IbuA from R. palustris, ACDH from P. putida KT2440, ETFBAD from P. putida KT2440, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway comprises MACS from M. acetivorans, ACDH from P. putida KT2440, ETFBAD from P. putida KT2440, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses an activating enzyme, an ACDH, an ETF, an (R)-specific enoyl-CoA hydratase, and one or more thioesterases. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses EC.2.8.3.1 from M. elsdenii, ACDH from P. putida KT2440, ETFBAD from P. putida KT2440, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses IbuA from R. palustris, ACDH from P. putida KT2440, ETFBAD from P. putida KT2440, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses MACS from M. acetivorans, ACDH from P. putida KT2440, ETFBAD from P. putida KT2440, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding an activating enzyme, a nucleic acid molecule encoding an ACDH, a nucleic acid molecule encoding an ETF, an (R)-specific enoyl-CoA hydratase, and a nucleic acid molecule encoding one or more thioesterases. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding EC.2.8.3.1 from M. elsdenii, a nucleic acid molecule encoding ACDH from P. putida KT2440, a nucleic acid molecule encoding ETFBAD from P. putida KT2440, a nucleic acid molecule encoding PhaJ from A. caviae, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding IbuA from R. palustris, a nucleic acid molecule encoding ACDH from P. putida KT2440, a nucleic acid molecule encoding ETFBAD from P. putida KT2440, a nucleic acid molecule encoding PhaJ from A. caviae, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding MACS from M. acetivorans, a nucleic acid molecule encoding ACDH from P. putida KT2440, a nucleic acid molecule encoding ETFBAD from P. putida KT2440, a nucleic acid molecule encoding PhaJ from A. caviae, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway (e.g., a 3HIB-favoring or 3H2MB- favoring Module 2 pathway) comprises an activating enzyme, an ACOX, an (R)-specific enoyl-CoA hydratase, and one or more thioesterases. In some embodiments, a Module 2 pathway comprises EC.2.8.3.1 from M. elsdenii, ACX4 from A. thaliana, PhaJ from A. caviae and one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway comprises IbuA from R. palustris, ACX4 from A. thaliana, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway comprises MACS from M. acetivorans, ACX4 from A. thaliana, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway comprises EC.2.8.3.1 from M. elsdenii, ACX4 from A. thaliana, PhaJ from A. caviae and one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway comprises IbuA from R. palustris, ACOX3 from Y. lipolytica, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway comprises MACS from M. acetivorans, ACOX3 from Y. lipolytica, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses an activating enzyme, an ACOX, an (R)-specific enoyl-CoA hydratase, and one or more thioesterases. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses EC.2.8.3.1 from M. elsdenii, ACX4 from A. thaliana, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses IbuA from R. palustris, ACX4 from A. thaliana, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses MACS from M. acetivorans, ACX4 from A. thaliana, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses IbuA from R. palustris, ACOX3 from Y. lipolytica, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses MACS from M. acetivorans, ACOX3 from Y. lipolytica, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding an activating enzyme, a nucleic acid molecule encoding an ACOX, a nucleic acid molecule encoding an (R)-specific enoyl-CoA hydratase, and a nucleic acid molecule encoding one or more thioesterases. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding EC.2.8.3.1 from M. elsdenii, a nucleic acid molecule encoding ACX4 from A. thaliana, PhaJ from A. caviae, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding IbuA from R. palustris, a nucleic acid molecule encoding ACX4 from A. thaliana, PhaJ from A. caviae, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding a nucleic acid molecule encoding MACS from M. acetivorans, a nucleic acid molecule encoding ACX4 from A. thaliana, PhaJ from A. caviae, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding EC.2.8.3.1 from M. elsdenii, a nucleic acid molecule encoding ACOX3 from Y. lipolytica, PhaJ from A. caviae, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding IbuA from R. palustris, a nucleic acid molecule encoding ACOX3 from Y. lipolytica, PhaJ from A. caviae, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding a nucleic acid molecule encoding MACS from M. acetivorans, a nucleic acid molecule encoding ACOX3 from Y. lipolytica, PhaJ from A. caviae, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway (e.g., a 3HIB-favoring or 3H2MB- favoring Module 2 pathway) comprises an activating enzyme, an ACOX, a catalase, an (R)- specific enoyl-CoA hydratase, and one or more thioesterases. In some embodiments, a Module 2 pathway comprises EC.2.8.3.1 from M. elsdenii, ACX4 from A. thaliana, KatE from E. coli, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway comprises IbuA from R. palustris, ACX4 from A. thaliana, KatE from E. coli, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway comprises MACS from M. acetivorans, ACX4 from A. thaliana, KatE from E. coli, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway comprises EC.2.8.3.1 from M. elsdenii, ACOX3 from Y. lipolytica, KatE from E. coli, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway comprises IbuA from R. palustris, ACOX3 from Y. lipolytica, KatE from E. coli, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a Module 2 pathway comprises MACS from M. acetivorans, ACOX3 from Y. lipolytica, KatE from E. coli, PhaJ from A. caviae, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses an activating enzyme, an ACOX, a catalase, an (R)-specific enoyl- CoA hydratase, and one or more thioesterases. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses EC.2.8.3.1 from M. elsdenii, ACX4 from A. thaliana, KatE from E. coli, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses IbuA from R. palustris, ACX4 from A. thaliana, KatE from E. coli, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses MACS from M. acetivorans, ACX4 from A. thaliana, KatE from E. coli, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses EC.2.8.3.1 from M. elsdenii, ACOX3 from Y. lipolytica, KatE from E. coli, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses IbuA from R. palustris, ACOX3 from Y. lipolytica, KatE from E. coli, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway recombinantly expresses MACS from M. acetivorans, ACOX3 from Y. lipolytica, KatE from E. coli, and one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding an activating enzyme, a nucleic acid molecule encoding an ACOX, a nucleic acid molecule encoding a catalase, a nucleic acid molecule encoding an (R)-specific enoyl-CoA hydratase, and a nucleic acid molecule encoding one or more thioesterases. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding EC.2.8.3.1 from M. elsdenii, a nucleic acid molecule encoding ACX4 from A. thaliana, a nucleic acid molecule encoding KatE from E. coli, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding IbuA from R. palustris, a nucleic acid molecule encoding ACX4 from A. thaliana, a nucleic acid molecule encoding KatE from E. coli, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding MACS from M. acetivorans, a nucleic acid molecule encoding ACX4 from A. thaliana, a nucleic acid molecule encoding KatE from E. coli, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding EC.2.8.3.1 from M. elsdenii, a nucleic acid molecule encoding ACOX3 from Y. lipolytica, a nucleic acid molecule encoding KatE from E. coli, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding IbuA from R. palustris, a nucleic acid molecule encoding ACOX3 from Y. lipolytica, a nucleic acid molecule encoding KatE from E. coli, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 2 pathway comprises: a nucleic acid molecule encoding MACS from M. acetivorans, a nucleic acid molecule encoding ACOX3 from Y. lipolytica, a nucleic acid molecule encoding KatE from E. coli, and a nucleic acid molecule encoding one or more thioesterases from E. coli. In some aspects, Module 2 pathway can be used to produce high titers of α-3HAs from α-3HA precursors. Total molar yield of α-3HAs may vary with α-3HA precursor concentrations. In some embodiments, recombinant microbial cells harboring a Module 2 pathway are provided α-3HA precursors exogenously, for example, as a supplement to a culture of recombinant microbial cells harboring a Module 2 pathway. In some embodiments, recombinant microbial cells harboring a Module 2 pathway are provided with 2MB to produce high titers of 3H2MB. In some embodiments, recombinant microbial cells harboring a Module 2 pathway are provided with IBA to produce high titers of 3HIB. In some embodiments, about 1g L-1to 2g L-1of α-3HA precursors are provided exogenously to recombinant microbial cells harboring a Module 2 pathway. In some embodiments, about 0.7 g L-1, about 0.8 g L-1, about 0.9 g L-1, about 1g L-1, about 1.1 g L-1, about 1.2 g L-1, about 1.3 g L-1, about 1.4 g L-1, about 1.5 g L-1, about 1.6 g L-1, about 1.7 g L-1, about 1.8 g L-1, about 1.9 g L-1, about 2.0 g L-1, about 2.1 g L-1, about 2.2 g L-1, about 2.3 g L-1, about 2.4 g L-1, about 2.5 g L-1of α-3HA precursors are provided exogenously to recombinant microbial cells harboring a Module 2 pathway. In some embodiments, recombinant microbial cells harboring a Module 2 pathway can produce a 3HIB titer of at least 20 mg L-1. For example the titer may be at least about 20 mg L-1, about 25 mg L-1, about 30 mg L-1, about 35 mg L-1, about 40 mg L-1, about 45 mg L-1, about 50 mg L-1, about 51 mg L-1, about 52 mg L-1, about 53 mg L-1, about 54 mg L-1, about 55 mg L-1, about 56 mg L-1, about 57 mg L-1, about 58 mg L-1, about 59 mg L-1, about 60 mg L-1, about 61 mg L-1, about 62 mg L-1, about 63 mg L-1, about 64 mg L-1, about 65 mg L-1, about 66 mg L-1, about 67 mg L-1, about 68 mg L-1, about 69 mg L-1, about 70 mg L-1, about 71 mg L-1, about 72 mg L-1, about 73 mg L-1, about 74 mg L-1, about 75 mg L-1, about 76 mg L-1, about 77 mg L-1, about 78 mg L-1, about 79 mg L-1, about 80 mg L-1, about 81 mg L-1, about 82 mg L-1, about 83 mg L-1, about 84 mg L-1, about 85 mg L-1, about 86 mg L-1, about 87 mg L-1, about 88 mg L-1, about 89 mg L-1, about 90 mg L-1, about 91 mg L-1, about 92 mg L-1, about 93 mg L-1, about 94 mg L-1, about 95 mg L-1, about 96 mg L-1, about 97 mg L-1, about 98 mg L-1, about 99 mg L-1, about 100 mg L-1, about 105 mg L-1, about 110 mg L-1, about 115 mg L-1, about 120 mg L-1, about 130 mg L-1, about 140 mg L-1, about 150 mg L-1, about 160 mg L-1, about 170 mg L-1, about 180 mg L-1, about 190 mg L-1, about 200 mg L-1, about 210 mg L-1, about 220 mg L-1, about 230 mg L-1, about 240 mg L-1, about 250 mg L-1, about 260 mg L-1, about 270 mg L-1, about 280 mg L-1, about 290 mg L-1, about 300 mg L-1, about 350 mg L-1, about 400 mg L-1, about 450 mg L-1, about 500 mg L-1, about 550 mg L-1, about 600 mg L-1, about 650 mg L-1, about 700 mg L-1, about 750 mg L-1, about 800 mg L-1, about 850 mg L-1, about 900 mg L-1, about 950 mg L-1or more. In some embodiments, recombinant microbial cells harboring a Module 2 pathway can produce a 3H2MB titer of at least 70 mg L-1. For example the titer may be at least about 70 mg L-1, about 71 mg L-1, about 72 mg L-1, about 73 mg L-1, about 74 mg L-1, about 75 mg L-1, about 76 mg L-1, about 77 mg L-1, about 78 mg L-1, about 79 mg L-1, about 80 mg L-1, about 81 mg L-1, about 82 mg L-1, about 83 mg L-1, about 84 mg L-1, about 85 mg L-1, about 86 mg L-1, about 87 mg L-1, about 88 mg L-1, about 89 mg L-1, about 90 mg L-1, about 91 mg L-1, about 92 mg L-1, about 93 mg L-1, about 94 mg L-1, about 95 mg L-1, about 96 mg L-1, about 97 mg L-1, about 98 mg L-1, about 99 mg L-1, about 100 mg L-1, about 105 mg L-1, about 110 mg L-1, about 115 mg L-1, about 120 mg L-1, about 130 mg L-1, about 140 mg L-1, about 150 mg L-1, about 160 mg L-1, about 170 mg L-1, about 180 mg L-1, about 190 mg L-1, about 200 mg L-1, about 210 mg L-1, about 220 mg L-1, about 230 mg L-1, about 240 mg L-1, about 250 mg L-1, about 260 mg L-1, about 270 mg L-1, about 280 mg L-1, about 290 mg L-1, about 300 mg L-1, about 350 mg L-1, about 400 mg L-1, about 450 mg L-1, about 500 mg L-1, about 550 mg L-1, about 600 mg L-1, about 650 mg L-1, about 700 mg L-1, about 750 mg L-1, about 800 mg L-1, about 850 mg L-1, about 900 mg L-1, about 950 mg L-1, about 1 g L-1, about 1.5 g L-1, about 2 g L-1, about 2.5 g L-1, about 3 g L-1, about 3.5 g L-1, about 4 g L-1, about 4.5 g L-1, about 5 g L-1, or more than 5 g L-1. In some aspects, biosynthetic pathways disclosed herein are practiced separately, such that bioconversion of a suitable carbon source into an α-3HA occurs within separate environments, for example, a first recombinant microbial cell harboring a Module 1 pathway and a second recombinant microbial cell harboring a Module 2 pathway. However, in some aspects, biosynthetic pathways disclosed herein are combined, such that bioconversion of a suitable carbon source into an α-3HA occurs within the same environment, for example, within a recombinant microbial cell harboring both a Module 1 pathway and a Module 2 pathway. Any combination of a Module 1 pathway and a Module 2 pathway are hereinafter referred to as “Module 1+2 pathway.” Recombinant microbial cells harboring a Module 1 pathway and a Module 2 pathway are hereinafter referred to as “Module 1+2 microbial cell.” Module 1+2 microbial cells may preferentially produce one α-3HA over another. For example, a recombinant microbial cell harboring a 2MB-favoring Module 1 and a Module 2 pathway and provided a suitable carbon source (e.g., glucose) can preferentially bioconvert the carbon source into 2MB and 3H2MB, such that IBA and 3HIB titers produced by the Module 1+2 microbial cell are reduced relative to 2MB and 3H2MB titers produced, respectively. “Module 1+2” pathways may comprise any combination of any Module 1 pathway and Module 2 pathway described herein. In some embodiments, a Module 1+2 pathway favors production of 3HIB. A 3HIB- favoring Module 1+2 comprises one or more enzymes which enable (1) metabolism of a carbon source (e.g. glucose) into pyruvate, (2) condensation of pyruvate into an acetohydroxy acid, (3) reduction of the acetohydroxy acid into a dihydroxy valerate, (4) dehydration of the dihydroxy valerate into 2-keto acids, (5) decarboxylation of the 2-keto acid into an aldehyde, (6) oxidation of the aldehyde into IBA, (7) activation of the IBA into an acyl-CoA, (8) dehydrogenation of the acyl-CoA into an enoyl-CoA, (9) stereospecific hydration of the enoyl Co-A to a (3R)-3-hydroxy acid, and (10) hydrolysis of the (3R)-3-hydroxy acid into 3HIB. In some embodiments, a Module 1+2 pathway favors production of 3H2MB. A 3H2MB-favoring Module 1+2 pathway comprises one or more enzymes which enable (1) metabolism of a carbon source (e.g., glucose) into threonine (2) conversion of threonine to a 2-ketobutyrate, (3) condensation of the 2-ketobutyrate into acetohydroxy acid, (4) reduction of the acetohydroxy acid into a dihydroxy valerate, (5) dehydration of the dihydroxy valerate into a 2-keto acid, (6) decarboxylation of the 2-keto acid into an aldehyde, (7) oxidation of the aldehyde into 2MB, (8) activation of 2MB into an acyl-CoA, (9) dehydrogenation of the acyl-CoA into a enoyl-CoA, (10) stereospecific hydration of the enoyl Co-A to a (3R)-3- hydroxy acid, and (11) hydrolysis of the (3R)-3-hydroxy acid into 3H2MB. In some embodiments, a 3H2MB-favoring Module 1+2 pathway further comprises one or more enzymes enabling threonine hyperproduction. IBA Recycle Though 3H2MB-preferring Module 1+2 pathways typically comprise a 2MB-favoring Module 1 pathway, further reduction of co-synthesis of IBA and 3HIB may be desirable. Also provided herein is an “IBA recycle” which, surprisingly, converts intermediates of the IBA- to-3HIB route of Module 1+2 into intermediates favored by the 2MB-to-3H2MB pathway, via central carbon metabolism. An IBA recycle comprises one or more enzymes which enable (1) isomerization of isobutyryl-CoA to butyryl-CoA, (2) dehydrogenation of butyryl-CoA to produce crotonyl- CoA, (3) dehydrogenation of crotonyl-CoA to acetoactyl-CoA, and (4) cleavage of acetoactyl-CoA to acetyl-CoA. An exemplary IBA recycle pathway is shown in FIG.3. In an exemplary embodiment, an IBA recycle comprises an isobutyryl-CoA mutase (IcmF), an ACDH, a 3-hydroxyacyl-CoA dehydrogenase, and a thiolase. In this exemplary IBA recycle, IcmF isomerizes isobutyryl-CoA, a byproduct of the IBA-to-3HIB route of Module 2 pathways, to butyryl-CoA. The ACDH dehydrogenates butyryl-CoA to produce crotonyl-CoA, which is converted by the 3-hydroxyacyl-CoA dehydrogenase to produce acetoactyl-CoA. The resulting acetoactyl-CoA is cleaved by the thiolase to produce two molecules of acetyl-CoA. Endogenous enzymes of the tricarboxylic acid cycle in a microbial cell convert acetyl-CoA into threonine, providing additional substrate for the 2MB-to- 3H2MB route of Module 2 pathways. In some embodiments, the IcmF is a vitamin B12-dependent IcmF. In some embodiments, the IcmF is IcmF from Cupriavidus metallidurans. In some embodiments, IcmF from C. metallidurans comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 61. In some embodiments, IcmF from C. metallidurans comprises SEQ ID NO: 61. In some embodiments, IcmF from C. metallidurans is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 62. In some embodiments, IcmF from C. metallidurans is encoded by a nucleic acid sequence comprising SEQ ID NO: 62. In some embodiments, the ACDH is FadE from E. coli. In some embodiments, the 3-hydroxyacyl-CoA dehydrogenase is FadB from E. coli. In some embodiments, FadB from E. coli comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 63. In some embodiments, FadB from E. coli comprises SEQ ID NO: 63. In some embodiments, FadB from E. coli is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 64. In some embodiments, FadB from E. coli is encoded by a nucleic acid sequence comprising SEQ ID NO: 64. In some embodiments, the thiolase is AtoB from E. coli. In some embodiments, AtoB from E. coli comprises an amino acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 65. In some embodiments, AtoB from E. coli comprises SEQ ID NO: 65. In some embodiments, AtoB from E. coli is encoded by a nucleic acid sequence comprising a nucleic acid sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more identity to SEQ ID NO: 66. In some embodiments, AtoB from E. coli is encoded by a nucleic acid sequence comprising SEQ ID NO: 66. Thus, in some embodiments, Module 1+2 pathways comprise an IBA recycle. In some embodiments, an IBA recycle comprises an IcmF, an ACDH, a 3- hydroxyacyl-CoA dehydrogenase, and a thiolase. In some embodiments, an IBA recycle comprises a vitamin B12-dependent IcmF, an ACDH, a 3-hydroxyacyl-CoA dehydrogenase, and a thiolase. In some embodiments, an IBA recycle comprises IcmF from Cupriavidus metallidurans, FadE from E. coli, FadB from E. coli, and AtoB from E. coli. In some embodiments, a recombinant microbial cell harboring an IBA recycle recombinantly expresses an IcmF, an ACDH, a 3-hydroxyacyl-CoA dehydrogenase, and a thiolase. In some embodiments, a recombinant microbial cell harboring an IBA recycle recombinantly expresses a vitamin B12-dependent IcmF, an ACDH, a 3-hydroxyacyl-CoA dehydrogenase, and a thiolase. In some embodiments, a recombinant microbial cell harboring an IBA recycle recombinantly expresses IcmF from Cupriavidus metallidurans, FadE from E. coli, FadB from E. coli, and AtoB from E. coli. In some embodiments, a recombinant microbial cell harboring an IBA recycle comprises: a nucleic acid molecule encoding an IcmF, a nucleic acid molecule encoding an ACDH, a nucleic acid molecule encoding a 3-hydroxyacyl-CoA dehydrogenase, and a nucleic acid molecule encoding a thiolase. In some embodiments, a recombinant microbial cell harboring an IBA recycle comprises: a nucleic acid molecule encoding a vitamin B12- dependent IcmF, a nucleic acid molecule encoding an ACDH, a nucleic acid molecule encoding a 3-hydroxyacyl-CoA dehydrogenase, and a nucleic acid molecule encoding a thiolase. In some embodiments, a recombinant microbial cell harboring an IBA recycle comprises: a nucleic acid molecule encoding IcmF from Cupriavidus metallidurans, a nucleic acid molecule encoding FadE from E. coli, a nucleic acid molecule encoding FadB from E. coli, and a nucleic acid molecule encoding AtoB from E. coli. Preferred Module 1 + 2 Pathways In a preferred embodiment, a 3H2MB-favoring Module 1+2 pathway comprises an IBA recycle. A preferred embodiment of a 3H2MB-favoring Module 1+2 pathway comprises, for example, one or more enzymes which enable (1) metabolism of a carbon source (e.g., glucose) into threonine (2) conversion of threonine to a 2-ketobutyrate, (3) condensation of the 2-ketobutyrate into acetohydroxy acid, (4) reduction of the acetohydroxy acid into a dihydroxy valerate, (5) dehydration of the dihydroxy valerate into a 2-keto acid, (6) decarboxylation of the 2-keto acid into an aldehyde, (7) oxidation of the aldehyde into 2MB, (8) activation of 2MB into an acyl-CoA, (9) dehydrogenation of the acyl-CoA into a enoyl- CoA, (10) stereospecific hydration of the enoyl Co-A to a (3R)-3-hydroxy acid, and (11) hydrolysis of the (3R)-3-hydroxy acid into 3H2MB; and further comprises an IBA recycle comprising one or more enzymes which enable: (12) isomerization of isobutyryl-CoA to butyryl-CoA, (13) dehydrogenation of butyryl-CoA to produce crotonyl-CoA, (14) dehydrogenation of crotonyl-CoA to acetoactyl-CoA, and (15) cleavage of acetoactyl-CoA to acetyl-Co; and an IBA recycle. In some embodiments, the 3H2MB-favoring Module 1+2 pathway comprising an IBA recycle further comprises one or more enzymes enabling threonine hyperproduction. An exemplary 3H2MB-favoring Module 1+2 pathway comprises IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, AldH from E. coli, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, IlvA from C. glutamicum, EC.2.8.3.1 from M. elsdenii, Acx4 from A. thaliana, one or more thioesterases from E. coli, IcmF from C. metallidurans, FadE from E. coli, FadB from E. coli, and AtoB from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 1+2 pathway recombinantly expresses IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, AldH from E. coli, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, IlvA from C. glutamicum, EC.2.8.3.1 from M. elsdenii, Acx4 from A. thaliana, one or more thioesterases from E. coli, IcmF from C. metallidurans, FadE from E. coli, FadB from E. coli, and AtoB from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 1+2 pathway comprises: a nucleic acid molecule encoding IlvGM from K. aerogenes, a nucleic acid molecule encoding IlvC from E. coli, a nucleic acid molecule encoding IlvD from E. coli, a nucleic acid molecule encoding KivD from L. lactis, a nucleic acid molecule encoding AldH from E. coli, a nucleic acid molecule encoding ThrAG433Rfrom E. coli, a nucleic acid molecule encoding ThrB from E. coli, a nucleic acid molecule encoding ThrC from E. coli, a nucleic acid molecule encoding IlvA from C. glutamicum, a nucleic acid molecule encoding EC.2.8.3.1 from M. elsdenii, a nucleic acid molecule encoding Acx4 from A. thaliana, a nucleic acid molecule encoding one or more thioesterases from E. coli, a nucleic acid molecule encoding IcmF from C. metallidurans, a nucleic acid molecule encoding FadE from E. coli, a nucleic acid molecule encoding FadB from E. coli, and a nucleic acid molecule encoding AtoB from E. coli. Another exemplary 3H2MB-favoring Module 1+2 pathway comprises IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, AldH from E. coli, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, IlvA from C. glutamicum, EC.2.8.3.1 from M. elsdenii, Acx4 from A. thaliana, KatE from E. coli, one or more thioesterases from E. coli, IcmF from C. metallidurans, FadE from E. coli, FadB from E. coli, and AtoB from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 1+2 pathway recombinantly expresses IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, AldH from E. coli, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, IlvA from C. glutamicum, EC.2.8.3.1 from M. elsdenii, Acx4 from A. thaliana, KatE from E. coli, one or more thioesterases from E. coli, IcmF from C. metallidurans, FadE from E. coli, FadB from E. coli, and AtoB from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 1+2 pathway comprises: a nucleic acid molecule encoding IlvGM from K. aerogenes, a nucleic acid molecule encoding IlvC from E. coli, a nucleic acid molecule encoding IlvD from E. coli, a nucleic acid molecule encoding KivD from L. lactis, a nucleic acid molecule encoding AldH from E. coli, a nucleic acid molecule encoding ThrAG433Rfrom E. coli, a nucleic acid molecule encoding ThrB from E. coli, a nucleic acid molecule encoding ThrC from E. coli, a nucleic acid molecule encoding IlvA from C. glutamicum, a nucleic acid molecule encoding EC.2.8.3.1 from M. elsdenii, a nucleic acid molecule encoding Acx4 from A. thaliana, a nucleic acid molecule encoding KatE from E. coli, a nucleic acid molecule encoding one or more thioesterases from E. coli, a nucleic acid molecule encoding IcmF from C. metallidurans, a nucleic acid molecule encoding FadE from E. coli, a nucleic acid molecule encoding FadB from E. coli, and a nucleic acid molecule encoding AtoB from E. coli. Another exemplary 3H2MB-favoring Module 1+2 pathway comprises IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, AldH from E. coli, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, IlvA from C. glutamicum, EC.2.8.3.1 from M. elsdenii, Acd from P. putida KT2440 and ETFBAD from P. putida KT2440, one or more thioesterases from E. coli, IcmF from C. metallidurans, FadE from E. coli, FadB from E. coli, and AtoB from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 1+2 pathway recombinantly expresses IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, AldH from E. coli, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, IlvA from C. glutamicum, EC.2.8.3.1 from M. elsdenii, Acd from P. putida KT2440 and ETFBAD from P. putida KT2440, one or more thioesterases from E. coli, IcmF from C. metallidurans, FadE from E. coli, FadB from E. coli, and AtoB from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 1+2 pathway comprises: a nucleic acid molecule encoding IlvGM from K. aerogenes, a nucleic acid molecule encoding IlvC from E. coli, a nucleic acid molecule encoding IlvD from E. coli, a nucleic acid molecule encoding KivD from L. lactis, a nucleic acid molecule encoding AldH from E. coli, a nucleic acid molecule encoding ThrAG433Rfrom E. coli, a nucleic acid molecule encoding ThrB from E. coli, a nucleic acid molecule encoding ThrC from E. coli, a nucleic acid molecule encoding IlvA from C. glutamicum, a nucleic acid molecule encoding EC.2.8.3.1 from M. elsdenii, a nucleic acid molecule encoding Acd from P. putida KT2440, a nucleic acid molecule encoding ETFBAD from P. putida KT2440, a nucleic acid molecule encoding one or more thioesterases from E. coli, a nucleic acid molecule encoding IcmF from C. metallidurans, a nucleic acid molecule encoding FadE from E. coli, a nucleic acid molecule encoding FadB from E. coli, and a nucleic acid molecule encoding AtoB from E. coli. Another exemplary 3H2MB-favoring Module 1+2 pathway comprises IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, Fjoh_2967 from F. johnsonaie, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, IlvA from C. glutamicum, EC.2.8.3.1 from M. elsdenii, Acx4 from A. thaliana, one or more thioesterases from E. coli, IcmF from C. metallidurans, FadE from E. coli, FadB from E. coli, and AtoB from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 1+2 pathway recombinantly expresses IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, Fjoh_2967 from F. johnsonaie, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, IlvA from C. glutamicum, EC.2.8.3.1 from M. elsdenii, Acx4 from A. thaliana, one or more thioesterases from E. coli, IcmF from C. metallidurans, FadE from E. coli, FadB from E. coli, and AtoB from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 1+2 pathway comprises: a nucleic acid molecule encoding IlvGM from K. aerogenes, a nucleic acid molecule encoding IlvC from E. coli, a nucleic acid molecule encoding IlvD from E. coli, a nucleic acid molecule encoding KivD from L. lactis, a nucleic acid molecule encoding Fjoh_2967 from F. johnsonaie, a nucleic acid molecule encoding ThrAG433Rfrom E. coli, a nucleic acid molecule encoding ThrB from E. coli, a nucleic acid molecule encoding ThrC from E. coli, a nucleic acid molecule encoding IlvA from C. glutamicum, a nucleic acid molecule encoding EC.2.8.3.1 from M. elsdenii, a nucleic acid molecule encoding Acx4 from A. thaliana, a nucleic acid molecule encoding one or more thioesterases from E. coli, a nucleic acid molecule encoding IcmF from C. metallidurans, a nucleic acid molecule encoding FadE from E. coli, a nucleic acid molecule encoding FadB from E. coli, and a nucleic acid molecule encoding AtoB from E. coli. Another exemplary 3H2MB-favoring Module 1+2 pathway comprises IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, Fjoh_2967 from F. johnsonaie, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, IlvA from C. glutamicum, EC.2.8.3.1 from M. elsdenii, Acx4 from A. thaliana, KatE from E. coli, one or more thioesterases from E. coli, IcmF from C. metallidurans, FadE from E. coli, FadB from E. coli, and AtoB from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 1+2 pathway recombinantly expresses IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, Fjoh_2967 from F. johnsonaie, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, IlvA from C. glutamicum, EC.2.8.3.1 from M. elsdenii, Acx4 from A. thaliana, KatE from E. coli, one or more thioesterases from E. coli, IcmF from C. metallidurans, FadE from E. coli, FadB from E. coli, and AtoB from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 1+2 pathway comprises: a nucleic acid molecule encoding IlvGM from K. aerogenes, a nucleic acid molecule encoding IlvC from E. coli, a nucleic acid molecule encoding IlvD from E. coli, a nucleic acid molecule encoding KivD from L. lactis, a nucleic acid molecule encoding Fjoh_2967 from F. johnsonaie, a nucleic acid molecule encoding ThrAG433Rfrom E. coli, a nucleic acid molecule encoding ThrB from E. coli, a nucleic acid molecule encoding ThrC from E. coli, a nucleic acid molecule encoding IlvA from C. glutamicum, a nucleic acid molecule encoding EC.2.8.3.1 from M. elsdenii, a nucleic acid molecule encoding Acx4 from A. thaliana, KatE from E. coli, a nucleic acid molecule encoding one or more thioesterases from E. coli, a nucleic acid molecule encoding IcmF from C. metallidurans, a nucleic acid molecule encoding FadE from E. coli, a nucleic acid molecule encoding FadB from E. coli, and a nucleic acid molecule encoding AtoB from E. coli. Another exemplary 3H2MB-favoring Module 1+2 pathway comprises IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, Fjoh_2967 from F. johnsonaie, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, IlvA from C. glutamicum, EC.2.8.3.1 from M. elsdenii, Acd from P. putida KT2440, ETFBAD from P. putida KT2440, one or more thioesterases from E. coli, IcmF from C. metallidurans, FadE from E. coli, FadB from E. coli, and AtoB from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 1+2 pathway recombinantly expresses IlvGM from K. aerogenes, IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, Fjoh_2967 from F. johnsonaie, ThrAG433Rfrom E. coli, ThrB from E. coli, ThrC from E. coli, IlvA from C. glutamicum, EC.2.8.3.1 from M. elsdenii, Acd from P. putida KT2440, ETFBAD from P. putida KT2440one or more thioesterases from E. coli, IcmF from C. metallidurans, FadE from E. coli, FadB from E. coli, and AtoB from E. coli. In some embodiments, a recombinant microbial cell harboring a Module 1+2 pathway comprises: a nucleic acid molecule encoding IlvGM from K. aerogenes, a nucleic acid molecule encoding IlvC from E. coli, a nucleic acid molecule encoding IlvD from E. coli, a nucleic acid molecule encoding KivD from L. lactis, a nucleic acid molecule encoding Fjoh_2967 from F. johnsonaie, a nucleic acid molecule encoding ThrAG433Rfrom E. coli, a nucleic acid molecule encoding ThrB from E. coli, a nucleic acid molecule encoding ThrC from E. coli, a nucleic acid molecule encoding IlvA from C. glutamicum, a nucleic acid molecule encoding EC.2.8.3.1 from M. elsdenii, a nucleic acid molecule encoding Acd from P. putida KT2440, a nucleic acid molecule encoding ETFBAD from P. putida KT2440, a nucleic acid molecule encoding one or more thioesterases from E. coli, a nucleic acid molecule encoding IcmF from C. metallidurans, a nucleic acid molecule encoding FadE from E. coli, a nucleic acid molecule encoding FadB from E. coli, and a nucleic acid molecule encoding AtoB from E. coli In some embodiments, a Module 1+2 pathway produces an IBA titer of less than 0.4 g L-1. For example the titer is less than or about 1 mg L-1, about 2 mg L-1, about 3 mg L-1, about 4 mg L-1, about 5 mg L-1, about 6 mg L-1, about 7 mg L-1, about 8 mg L-1, about 9 mg L-1, about 10 mg L-1, about 15 mg L-1, about 20 mg L-1, about 25 mg L-1, about 30 mg L-1, about 35 mg L-1, about 40 mg L-1, about 45 mg L-1, about 50 mg L-1, about 51 mg L-1, about 52 mg L-1, about 53 mg L-1, about 54 mg L-1, about 55 mg L-1, about 56 mg L-1, about 57 mg L-1, about 58 mg L-1, about 59 mg L-1, about 60 mg L-1, about 61 mg L-1, about 62 mg L-1, about 63 mg L-1, about 64 mg L-1, about 65 mg L-1, about 66 mg L-1, about 67 mg L-1, about 68 mg L-1, about 69 mg L-1, about 70 mg L-1, about 71 mg L-1, about 72 mg L-1, about 73 mg L-1, about 74 mg L-1, about 75 mg L-1, about 76 mg L-1, about 77 mg L-1, about 78 mg L-1, about 79 mg L-1, about 80 mg L-1, about 81 mg L-1, about 82 mg L-1, about 83 mg L-1, about 84 mg L-1, about 85 mg L-1, about 86 mg L-1, about 87 mg L-1, about 88 mg L-1, about 89 mg L-1, about 90 mg L-1, about 91 mg L-1, about 92 mg L-1, about 93 mg L-1, about 94 mg L-1, about 95 mg L-1, about 96 mg L-1, about 97 mg L-1, about 98 mg L-1, about 99 mg L-1, about 100 mg L-1, about 105 mg L-1, about 110 mg L-1, about 115 mg L-1, about 120 mg L-1, about 130 mg L-1, about 140 mg L-1, about 150 mg L-1, about 160 mg L-1, about 170 mg L-1, about 180 mg L-1, about 190 mg L-1, about 200 mg L-1, about 210 mg L-1, about 220 mg L-1, about 230 mg L-1, about 240 mg L-1, about 250 mg L-1, about 260 mg L-1, about 270 mg L-1, about 280 mg L-1, about 290 mg L-1, about 300 mg L-1, about 350 mg L-1, about 400 mg L-1, about 450 mg L-1, or about 500 mg L-1. In some embodiments, a Module 1+2 pathway produces a 3HIB titer less than 0.1 g L-1. For example the titer may be at least about 1 mg L-1, about 2 mg L-1, about 3 mg L-1, about 4 mg L-1, about 5 mg L-1, about 6 mg L-1, about 7 mg L-1, about 8 mg L-1, about 9 mg L-1, about 10 mg L-1, about 15 mg L-1, about 20 mg L-1, about 25 mg L-1, about 30 mg L-1, about 35 mg L-1, about 40 mg L-1, about 45 mg L-1, about 50 mg L-1, about 51 mg L-1, about 52 mg L-1, about 53 mg L-1, about 54 mg L-1, about 55 mg L-1, about 56 mg L-1, about 57 mg L-1, about 58 mg L-1, about 59 mg L-1, about 60 mg L-1, about 61 mg L-1, about 62 mg L-1, about 63 mg L-1, about 64 mg L-1, about 65 mg L-1, about 66 mg L-1, about 67 mg L-1, about 68 mg L-1, about 69 mg L-1, about 70 mg L-1, about 71 mg L-1, about 72 mg L-1, about 73 mg L-1, about 74 mg L-1, about 75 mg L-1, about 76 mg L-1, about 77 mg L-1, about 78 mg L-1, about 79 mg L-1, about 80 mg L-1, about 81 mg L-1, about 82 mg L-1, about 83 mg L-1, about 84 mg L-1, about 85 mg L-1, about 86 mg L-1, about 87 mg L-1, about 88 mg L-1, about 89 mg L-1, about 90 mg L-1, about 91 mg L-1, about 92 mg L-1, about 93 mg L-1, about 94 mg L-1, about 95 mg L-1, about 96 mg L-1, about 97 mg L-1, about 98 mg L-1, about 99 mg L-1, about 100 mg L-1, about 105 mg L-1, about 110 mg L-1, about 115 mg L-1, about 120 mg L-1, about 130 mg L-1, about 140 mg L-1, about 150 mg L-1, about 160 mg L-1, about 170 mg L-1, about 180 mg L-1, about 190 mg L-1, or about 200 mg L-1. In some embodiments, a 3H2MB-favoring Module 1+2 pathway produces a 2MB titer of at least 0.2 g L-1. For example, the titer may be at least about 200 mg L-1, , about 250 mg L-1, about 300 mg L-1, about 350 mg L-1, about 400 mg L-1, about 450 mg L-1, about 500 mg L-1, about 550 mg L-1, about 600 mg L-1, about 650 mg L-1, about 700 mg L-1, about 750 mg L-1, about 800 mg L-1, about 850 mg L-1, about 900 mg L-1, about 950 mg L-1, about 1 g L-1, about 1.5 g L-1, about 2 g L-1, about 2.5 g L-1, about 3 g L-1, about 3.5 g L-1, about 4 g L-1, about 4.5 g L-1, about 5 g L-1, or more than 5 g L-1. In some embodiments, a 3H2MB-favoring Module 1+23H2MB titer of at least 0.15 g L-1. For example the titer may be at least about 100 mg L-1, about 105 mg L-1, about 110 mg L-1, about 115 mg L-1, about 120 mg L-1, about 125 mg L-1, about 130 mg L-1, about 135 mg L-1, about 140 mg L-1, about 145 mg L-1, about 150 mg L-1, about 155 mg L-1, about 160 mg L-1, about 165 mg L-1, about 170 mg L-1, about 175 mg L-1, about 180 mg L-1, about 185 mg L-1, about 190 mg L-1, about 195 mg L-1, about 200 mg L-1, about 205 mg L-1, about 210 mg L-1, about 215 mg L-1, about 220 mg L-1, about 225 mg L-1, about 230 mg L-1, about 235 mg L-1, about 240 mg L-1, about 245 mg L-1, about 250 mg L-1, about 255 mg L-1, about 260 mg L-1, about 265 mg L-1, about 270 mg L-1, about 275 mg L-1, about 280 mg L-1, about 285 mg L-1, about 290 mg L-1, about 295 mg L-1, about 300 mg L-1, about 310 mg L-1, about 320 mg L-1, about 330 mg L-1, about 340 mg L-1, about 350 mg L-1, about 360 mg L-1, about 370 mg L-1, about 380 mg L-1, about 390 mg L-1, about 400 mg L-1, about 450 mg L-1, about 500 mg L-1, about 550 mg L-1, about 600 mg L-1, about 650 mg L-1, about 700 mg L-1, about 750 mg L-1, about 800 mg L-1, about 850 mg L-1, about 900 mg L-1, about 1 g L-1, about 1.5 g L-1, about 2 g L-1, about 2.5 g L-1, about 3 g L-1, about 3.5 g L-1, about 4 g L-1, about 4.5 g L-1, about 5 g L-1, or more than 5 g L-1. In preferred embodiments, a 3H2MB-favoring Module 1+2 pathway produces an IBA titer of less than 0.5 g L-1. For example, the titer may be less than about 550 mg L-1, about 500 mg L-1, about 400 mg L-1, about 300 mg L-1, about 200 mg L-1, about 100 mg L-1, about 90 mg L-1, about 80 mg L-1, about 70 mg L-1, about 60 mg L-1, about 50 mg L-1, about 40 mg L-1, about 30 mg L-1, about 20 mg L-1, about 10 mg L-1, about 9 mg L-1, about 8 mg L-1, about 7 mg L-1, about 6 mg L-1, about 5 mg L-1, about 4 mg L-1, about 3 mg L-1, about 2 mg L-1, about 1 mg L-1, about 0.9 mg L-1, about 0.8 mg L-1, about 0.7 mg L-1, about 0.5 mg L-1, about 0.4 mg L-1, about 0.3 mg L-1, about 0.2 mg L-1, about 0.1 mg L-1, about 0.05 mg L-1, about 0.025 mg L-1, about 0.0125 mg L-1or less. Microbial Cells Aspects of the disclosure relate to biological pathways harbored by a recombinant microbial cell. The compositions and methods described herein may be practiced using any suitable microbial host cell. Host cells suitable for the compositions and methods described herein are microbial cells which can be engineered, using techniques known to persons of ordinary skill in the art, to recombinantly express or be modified by at least one heterologous nucleic acid or gene product. A host cell may be a wild type (i.e., naturally occurring) cell or an existing recombinant cell (i.e., already engineered to express or be modified by at least one heterologous nucleic acid). A host cell may be a prokaryotic or eukaryotic cell. In some embodiments, a host cell is a bacterial cell. A bacterial cell may be Gram- negative, such as a species of Escherichia, or Gram-positive, such as a species of Bacillus. Exemplary bacterial cells include Acinetobacter spp., Actinomycetes spp., Aeromonas spp., Alcaligenes spp., Aphanocapsa spp., Aquaspirillum spp., Asospirillum spp., Aspergillus spp., Asticcaulus spp., Azatobacter spp., Azohydromonas spp., Aizomonas spp., Bacillus spp., Bdellovibrio spp., Botryococcus spp., Burkholderia spp., Caulobacter spp., Chlorofrexeus spp., Chlorogloea spp., Chromobacterium spp., Comamonas spp., Cupriavidus spp., Dechloromonas spp., Delftia spp., Ectothiorhodospira spp., Erwinia spp., Escherichia spp., Ferrobacillus spp., Gloeothece spp., Haemophilus spp., Halobacterium spp., Haloferax spp., Halomonas spp., Hydrogenophaga spp., Hyphomicrobium spp., Lamprocystis spp., Lampropedia spp., Leptothrix spp., Methanomonas spp., Methylobacterium spp., Methylocystis spp., Methylomicrobium spp., Methylosinus spp., Nitrobacter spp., Nitrococcus spp., Oceanospirillum spp., Paracoccus spp., Pseudomonas spp., Ralstonia spp., Rhizobium spp., Rhodobacter spp., Rhodococcus spp., Rhodopseudomonas spp., Rhodospirillum spp., Saccharophagus spp., Salmonella spp., Staphylococcus spp., Streptomyces spp., Thermus spp., Wautersia spp., and Zobellella spp. (e.g., see: Anjum, A. Z. (2016). Microbial production of polyhydroxyalkanoates (PHAs) and its copolymers: A review of recent advancements. International Journal of Biological Macromolecules, 89, 161-174; and Muneer, F. R. (2020). Microbial polyhydroxyalkanoates (PHAs): efficient replacement of synthetic polymers. Journal of Polymers and the Environment, 28, 201-2323.). In some embodiments, a host cell is E. coli. In some embodiments, a host cell is a fungal cell (e.g. Acremonium spp., Allomyces spp., Arxula spp., Aspergillus spp., Botrytis spp., Fusarium spp., Magnaporthe spp., Neurospora spp., Paffia spp., Penicillium spp., Pichia spp., Rhizopus spp., Saccharomyces spp., Sordaria spp., Trichoderma spp., Ustilago spp., Wickerhamomyces spp., Yarrowia spp. (e.g., see: Muneer, 2020). In some embodiments, a host cell is an algal cell (e.g. Aspergillus spp., Aulosira spp., Botryococcus spp., Chlorella spp., Nostoc spp., Sacharomyces spp., Spirulina spp., Synechococcus spp., Synechocystis spp. (e.g., see: Muneer, 2020). In some embodiments, a host cell is a protist. A microbial cell can be considered to “harbor” a certain pathway if it contains within it: (i) one or more enzymes which enable the bioconversion of a carbon source to an α-3HA precursor according to the reaction steps disclosed herein; (ii) one or more enzymes which enable the bioconversion of an α-3HA precursor or an α-3HA according to the reaction steps disclosed herein; (iii) one or more nucleic acids encoding the enzyme(s) of (i) or (ii); or (iv) any combination of (i)-(iii). Recombinant Cells and Heterologous Nucleic Acids Aspects of the disclosure relate to recombinant microbial cells. As used herein, a “recombinant microbial cell” is a microbial cell (e.g., host cell) which has been engineered to express or be modified by at least one heterologous nucleic acid (e.g., nucleic acid molecule). As used herein, a “heterologous” nucleic acid is (a) foreign to the host cell (i.e. derived from another species; derived from the same species but substantially modified; synthetic); (b) native to the host cell (i.e. naturally occurring in wild type cells of the same species), but with mutations which alter function or expression relative to a wild type cell of the same species; or (c) native to the host cell, but in unnatural amounts (i.e. significantly increased or decreased), relative to wild type cell of the same species. In recombinant cells, desired heterologous nucleic acids are placed under operable control of regulatory sequences to permit the expression of the heterologous nucleic acids in the cell. Heterologous expression of genes is demonstrated in the Examples using E. coli. The combinations of genes and gene products also be recombinantly expressed in other bacterial cells, fungi (e.g., yeast), algae, protists, etc. according to standard techniques well known in the art. Heterologous nucleic acids may contain portions of nucleic acids that are naturally occurring, but as a whole, heterologous nucleic acids do not occur naturally in a host cell, and their presence in a host cell requires human intervention. Nucleic acids naturally occurring in a host cell may be referred to as “endogenous nucleic acids.” In some embodiments, a recombinant microbial cell comprises 1 or more unique heterologous nucleic acids. In some embodiments, a recombinant microbial cell comprises 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or more than 20 unique heterologous nucleic acids. A heterologous nucleic acid may be DNA or RNA. A heterologous nucleic acid can comprise one or more genes. As used herein, a gene is a nucleotide sequence which encodes a gene product. A gene product may be a peptide (e.g., protein, polypeptide, or fragment thereof) or non-coding nucleic acid (e.g., functional RNA). In some embodiments, a heterologous nucleic acid comprises a gene encoding an enzyme. In some embodiments, heterologous nucleic acids comprise one or more genes encoding a gene product foreign to a host cell (i.e., a gene product not naturally occurring in wild type cells of the same species). In some embodiments, heterologous nucleic acids comprise one or more genes encoding gene products which are endogenous to a host cell, but which comprise mutations which alter function or expression relative to a wild type cell of the same species. Genes in a recombinant microbial cell harboring a certain pathway may be modified or mutated; for example, a gene may be mutated by substitution, repetition, or deletion of one or more nucleotides. In some embodiments, expression of a mutated gene results in one or more substitutions or deletions of amino acids in a peptide. In some embodiments, a mutated gene results in altered function or expression of the gene product in the host cell. In some embodiments, a gene has been modified for optimal expression (e.g., codon optimized) in a host cell. Codon usages for a variety of organisms can be accessed in the Codon Usage Database (kazusa.or.jp / codon / ). Codon optimization, including identification of optimal codons for a variety of organisms and methods for achieving codon optimization, is known to persons of skill int the art. In some embodiments, one or more mutations may be combined in the same gene, regulatory sequence, nucleic acid, or cell. A heterologous nucleic acid comprising a gene may further comprise a regulatory sequence. A regulatory sequence is a non-coding sequence which regulates transcription of a downstream gene, such that transcription is initiated, facilitated, inhibited, or silenced. Regulatory sequences vary between species and cell types but generally include 5’ non- transcribed and 5’ non-translated sequences involved in the regulation of transcription and translation respectively, such as a TATA box, capping sequence, CAAT sequence, and the like. Such 5’ non-transcribed regulatory sequences include promoter regions, which includes a promoter sequence for transcriptional control of the operably joined gene. Regulatory sequences may also include enhancer sequences, upstream activator sequences, and silencing sequences, as desired. Regulatory sequences may be endogenous to (e.g., naturally occurring in) or foreign to the host cell. In some embodiments, a regulatory sequence is constitutive (i.e., unregulated by external inducing factors). In some embodiments, a regulatory sequence is inducible. An inducible regulatory sequence (e.g., promoter) is one which is controlled by the presence or absence of an inducing factor (e.g., chemical agent). In some embodiments, a regulatory sequence is modified to alter its regulation of transcription. In some embodiments, a modified regulatory sequence (e.g., promoter) exhibits increased or decreased transcriptional activity as compared to its unmodified counterpart. In some embodiments, the nucleotide sequence of a regulatory element has been altered by one or more deletion, insertion, substitution, or other mutation, or combination thereof. In some embodiments, all or part of the endogenous promoters of an endogenous enzyme are replaced with an inducible promoter or a constitutively active promoter. Methods for targeted modification of a regulatory sequence are well known to those of skill in the art. As used herein, a gene is “operably” linked to a regulatory sequence when they are covalently linked in such a manner that transcription of the gene is under influence or control of the regulatory sequence. In some embodiments, a heterologous nucleic acid comprises a gene operably linked to an inducible regulatory sequence. In some embodiments, a heterologous nucleic acid comprises a gene operably linked to an inducible promoter, such that introduction of an inducing factor effects transcription of the gene. Genes may be overexpressed in a recombinant microbial cell, relative to expression of an unaltered version of the same gene in a wild type cell of the same species. Overexpression of a gene may be achieved using any method known in the art. In some embodiments, overexpressing a gene comprises altering the genomic DNA of a microbial cell, non-limiting examples of which include: introducing one or more copies of the gene (e.g., amplifying at the chromosomal position by tandem repeat), introducing a promoter sequence, altering a regulatory sequence, altering one or more different genes involved in the regulation of expression the gene, or deleting a repressor gene in the genomic DNA of the cell. In some embodiments, overexpressing a gene comprises introducing copies of RNA transcripts of the gene into the microbial cell. In some embodiments, a recombinant microbial cell overexpresses one or more genes relative to expression of the same genes in a wildtype microbial cell. In some embodiments, a recombinant microbial cell overexpresses 2 genes, 3 genes, 4 genes, 5 genes, 6 genes, 7, genes, 8 genes, 9 genes, 10 genes, 11 genes, 12 genes, 13 genes, 14 genes, 15 genes, 16 genes, 17 genes, 18 genes, 19 genes, 20 genes, or more than 20 genes relative to expression of the same genes in a wildtype microbial cell. A recombinant microbial cell may comprise one or more heterologous nucleic acids encoding one or more gene products from a single species. In some embodiments, a heterologous nucleic acid comprises a gene obtained from a different species (i.e., a transgene). In some embodiments, a recombinant microbial cell comprises one or more heterologous nucleic acids encoding one or more gene products from one or more unique species. In some embodiments, a recombinant microbial cell comprises one or more heterologous nucleic acids encoding one or more gene products from 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or more than 15 unique species. In some embodiments, a recombinant microbial cell comprises one or more heterologous nucleic acids encoding one or more gene products from Bacillus subtilis, Klebsiella aerogenes, Escherichia coli, Lactococcus lactis, Flavobacterium johnsonaie, Corynebacterium glutamicum, Megasphaera elsdenii, Aeromonas caviae, Arabidopsis thaliana, Pseudomonas putida, Cupriavidus metallidurans, Rhodopseudomonas palustris, and / or any combination thereof. Heterologous transgenes introduced to a host cell (e.g., by transformation, transfection, transduction) may be integrated into the genome of the host cell. Heterologous transgenes introduced to a host cell (e.g., by transformation, transfection, transduction) may be transiently expressed by a host cell. In some embodiments, a transgene is recombinantly expressed by a host cell (e.g., by transcription, translation) such that that the host cell expresses one or more foreign gene products (e.g., heterologous enzymes). In some embodiments, expression and activity of one or more endogenous or enzymes involved in the Module 1 or Module 2 pathways disclosed herein is altered for achieving increased production of IBA, 2MB, 3HIB, and / or 3H2MB. Increased or decreased (e.g., reduced or eliminated) expression and / or activity of endogenous enzymes can be achieved by manipulation of one or more of the genes encoding the endogenous enzymes, their promoters, and / or their ribosome binding sites. In some embodiments, heterologous nucleic acids described herein comprise one or more recombinant copies of endogenously expressed enzymes, either on one or more vectors (e.g., plasmids) or through integration into the chromosome. Cell-free Environments Aspects of the disclosure relate to methods of producing α-3HAs and / or branched acid α-3HA precursors in a cell-free environment. Several methods for cell-free metabolic engineering are known in the art (See: Lim HJ and Kim D-M. Methods and Protocols. (2019); 2(2):33. doi.org / 10.3390 / mps2020033; Ullah, M. W., et al. Biochemical Engineering Journal. (2016);105:391-405.). In some embodiments, the methods of producing α-3HAs and / or branched acid α- 3HA precursors in a cell-free environment includes lysing (e.g., thermal, mechanical, chemical, or enzymatic lysis) cultured cells to produce at least one (e.g., at least two, at least three, or at least four) cell lysate. “Lysing” refers to the process by which cells are broken down, for example, by thermal, mechanical, chemical, or enzymatic mechanisms. A “cell lysate” refers to a fluid containing the contents of lysed cells, including, for example, proteins (e.g., enzymes), nucleic acids, organelles, and membrane lipids. Cell lysates of the present disclosure may be produced by lysing any population of recombinant microbial, as provided herein. It should be understood that multiple cell lysates (and thus multiple cell populations, e.g., from the same organism (e.g., bacteria) or from different organisms (e.g., bacteria, yeast and or plant cells)) may be used in an enzymatic pathway as provided herein. For example, one cell population may be engineered to express one or more enzymes(s) of the Module 1 pathway, while another cell population (or several other cell populations) may be engineered to express another (at least one other) enzyme of the Module 1 pathway. Following lysis of the cells, the cell lysates are combined such that the enzymes are present in a single cell lysate / reaction mixture. In some embodiments, at least one (e.g., at least two or at least three) purified enzyme is added to the cell lysate / reaction mixture. Thus, a reaction mixture, in some embodiments, includes a combination of enzymes present in the cell lysate (expressed by the engineered host cell(s)) and at least one purified enzyme. In some embodiments, cell-free methods comprise: (a) culturing at least two cell populations, wherein cells of each population are engineered to express at least one enzyme selected from a Module 1, Module 2, Module 1+2, IBA recycle, and / or thioesterases hyperproduction pathway described herein to produce at least two cultured populations of cells expressing different enzymes, (b) lysing cells of the at least two cultured populations to produce at least two cell lysates, (c) combining the at least two cell lysates to produce a cell lysate mixture that comprises two enzymes selected from a Module 1, Module 2, Module 1+2, IBA recycle, and / or thioesterases hyperproduction pathway described herein, and (d) incubating the reaction mixture in the presence of a carbon source (e.g., glucose) to produce the α-3HAs and / or branched acid α-3HA precursors. Vectors The heterologous nucleic acid molecules described herein can be introduced into a host cell using methods and techniques that are standard in the art. Heterologous nucleic acid molecules can be introduced by standard protocols such as transformation including, without limitation, transformation, transfection (chemical (e.g. calcium phosphate, cationic polymers, or liposomes) or non-chemical (e.g. electroporation, sonoporation, impalefection, optical transfection, hydrodynamic transfection)), transduction (e.g. viral transduction), particle bombardment, etc. In some embodiments, the heterologous nucleic acids described herein are inserted into a host cell using a vector. As used herein, a “vector” is a vehicle for the transfer of one or more nucleic acids between different genetic environments (e.g., for expression in a host cell). A vector may be composed of DNA or RNA. Suitable vectors include plasmids, fosmids, phagemids, viral genomes, and artificial chromosomes. In some embodiments, a vector is a cloning vector. A “cloning vector” is one which is able to replicate autonomously or be integrated in the genome in a host cell, and which is further characterized by one or more endonuclease restriction sites at which the vector may be cut in a determinable fashion. Desired nucleotide sequences may be ligated into endonuclease restriction such that the new recombinant cloning vector retains its ability to replicate in the host cell. Cloning vectors include plasmids and phages. In the case of plasmids, replication of a desired nucleotide sequence may occur many times as the plasmid increases in copy number within the host cell or only a single time per host before the host reproduces by mitosis. In the case of phages, replication may occur actively during a lytic phase or passively during a lysogenic phase. In some embodiments, a vector is an expression vector. An “expression vector” is a vector into which a desired nucleotide sequence may be inserted by restriction and ligation such that the desired sequence is operably joined to regulatory sequences and may be expressed as an RNA transcript. Vectors may further contain one or more marker sequences (e.g., fluorescent proteins, antibiotic resistance genes) suitable for use in the identification of cells which have or have not been transformed or transfected with the vector. Non-limiting examples of suitable markers include genes encoding proteins which increase or decrease either resistance or sensitivity to antibiotics or other compounds, genes which encode enzymes whose activities are detectable by standard assays known in the art (e.g., β- galactosidase, luciferase, or alkaline phosphatase), and genes which visibly affect the phenotype of transformed or transfected cells, hosts, colonies or plaques (e.g., green fluorescent protein). Preferred vectors are those capable of autonomous replication and expression of the structural gene products present in the nucleotide segments to which they are operably joined. The vectors disclosed herein may optionally include 5' leader or signal sequences. The choice and design of an appropriate vector is within the ability and discretion of one of ordinary skill in the art. Expression vectors containing all the necessary elements for expression are commercially available and known to those skilled in the art. See, e.g., Sambrook et al., Molecular Cloning: A Laboratory Manual, Second Edition, Cold Spring Harbor Laboratory Press, 1989. Schematics of exemplary plasmids encoding enzymes characteristic of Module 1 and / or Module 2 pathways described herein are shown in FIGs. 8A, 8B, 9, and 10A-10D. In some embodiments, genes encoding one or more enzymes are expressed together on an inducible plasmid. In some embodiments, an inducible plasmid can be induced using IPTG, tetracycline, or anhydro-tetracycline. As one of ordinary skill would appreciate, many different types of inducible plasmids are compatible with the heterologous nucleic acids described herein. In some embodiments, a plasmid encodes enzymes characteristic of a Module 1 pathway. In some embodiments, a plasmid encoding enzymes characteristic of a Module 1 pathway comprises the nucleic acid sequence of any one of SEQ ID NOs: 67-71 or 73. In some embodiments, enzymes characteristic of an IBA-favoring Module 1 pathway are encoded by a plasmid comprising a nucleic acid sequence set forth in SEQ ID NO: 67. In some embodiments, enzymes characteristic of an IBA-favoring Module 1 pathway are encoded by a plasmid comprising a nucleic acid sequence set forth in SEQ ID NO: 68. In some embodiments, enzymes characteristic of a 2MB-favoring Module 1 pathway are encoded by a plasmid comprising a nucleic acid sequence set forth in SEQ ID NO: 70. In some embodiments, enzymes characteristic of a 2MB-favoring Module 1 pathway are encoded by a plasmid comprising a nucleic acid sequence set forth in SEQ ID NO: 71. In some embodiments, enzymes characteristic of a 2MB-favoring Module 1 pathway are encoded by a plasmid comprising a nucleic acid sequence set forth in SEQ ID NO: 73. In some embodiments, a plasmid encodes enzymes characteristic of a Module 2 pathway. In some embodiments, a plasmid encoding enzymes characteristic of a Module 2 pathway comprises the nucleic acid sequence of SEQ ID NO: 72. In some embodiments, enzymes characteristic of a Module 2 pathway are encoded by a plasmid comprising a nucleic acid sequence set forth in SEQ ID NO: 72. In some embodiments, enzymes characteristic of a 3H2MB-favoring Module 1+2 pathway are encoded by SEQ ID NO: 69-73. In some embodiments, enzymes characteristic of a 3HIB-favoring Module 1+2 pathway are encoded by SEQ ID NOs: 67-68 and 72. Cell Culturing Typically, the recombinant microbial cells described herein are cultured. As used herein, “culturing” is the process by which cells (e.g., recombinant microbial cells) are grown under controlled conditions. Cells may be grown as cell suspensions in liquid nutrient broth (e.g., culture media). The recombinant microbial cells described herein can be cultured in media of any type (rich or minimal) and any composition. In some embodiments, recombinant microbial cells described herein are cultured in minimal media. Various media supplements can be added to the culture media, for example, carbon sources, α-3HA precursors, yeast extract, and isopropyl β-d-1-thiogalactopyranoside (IPTG) for gene induction, ATCC Trace Mineral Supplement, and antibiotics. In some embodiments, cell culture factors and conditions such as the concentration and amount of any media supplements can be adjusted as needed. In some embodiments, the frequency and duration of cell culture factors (e.g., media supplements) before harvesting α- 3HA precursors and α-3HAs are adjusted. Other cell culture factors and conditions (e.g., pH, temperature, culture time) may also be adjusted. Cell culture factors and conditions, such as media, media supplements, temperature, and pH can influence the overall titer and / or yield of α-3HA precursors and / or α-3HAs. In some embodiments, cell culture factors and conditions can be adjusted to obtain a titer of α- 3HA precursors and / or α-3HAs in the milligrams per liter (mg L-1) scale or greater. In some embodiments the concentration of yeast extract is adjusted. In certain embodiments, the concentration of yeast extract (weight of yeast extract / volume culture medium) is about 0.01%, about 0.02%, about 0.03%, about 0.04%, about 0.05%, about 0.06%, about 0.07%, about 0.08%, about 0.09%, about 0.1%, about 0.15%, about 0.2%, about 0.25%, about 0.3%, about 0.35%, about 0.4%, about 0.45%, about 0.5%, about 0.55%, about 0.6%, about 0.65%, about 0.7%, about 0.75%, about 0.8%, about 0.85%, about 0.9%, about 0.95%, about 1.0%, about 1.1%, about 1.2%, about 1.3%, about 1.4%, about 1.5%, about 1.6%, about 1.7%, about 1.8%, about 1.9%, or more than about 2.0%. In some embodiments the concentration of an inducer molecule is optimized. In certain embodiments, the inducer is IPTG and the concentration of IPTG is about 10 µM, about 20 µM, about 30 µM, about 40 µM, about 50 µM, about 60 µM, about 70 µM, about 80 µM, about 90 µM, about 100 µM, about 110 µM, about 120 µM, about 130 µM, about 140 µM, about 150 µM, about 160 µM, about 170 µM, about 180 µM, about 190 µM, about 200 µM, about 210 µM, about 220 µM, about 230 µM, about 240 µM, about 250 µM, about 260 µM, about 270 µM, about 280 µM, about 290 µM, about 300 µM, about 310 µM, about 320 µM, about 330 µM, about 340 µM, about 350 µM, about 360 µM, about 370 µM, about 380 µM, about 390 µM, about 400 µM, about 410 µM, about 420 µM, about 430 µM, about 440 µM, about 450 µM, about 460 µM, about 470 µM, about 480 µM, about 490 µM, about 500 µM, about 525 µM, about 550 µM, about 575 µM, about 600 µM, about 625 µM, about 650 µM, about 675 µM, about 700 µM, about 725 µM, about 750 µM, about 775 µM, about 800 µM, about 825 µM, about 850 µM, about 875 µM, about 900 µM, about 925 µM, about 950 µM, about 975 µM, about 1000 µM, about 1025 µM, about 1050 µM, about 1100 µM or is more than about 1100 µM. In some embodiments, the inducer is tetracycline or anhydro-tetracycline (ATC) and the concentration of tetracycline or anhydro-tetracycline (ATC) is about 10 ng / ml, about 20 ng / ml, about 30 ng / ml, about 40 ng / ml, about 50 ng / ml, about 60 ng / ml, about 70 ng / ml, about 80 ng / ml, about 90 ng / ml, about 100 ng / ml, about 110 ng / ml, about 120 ng / ml, about 130 ng / ml, about 140 ng / ml, about 150 ng / ml, about 160 ng / ml, about 170 ng / ml, about 180 ng / ml, about 190 ng / ml, about 200 ng / ml, about 210 ng / ml, about 220 ng / ml, about 230 ng / ml, about 240 ng / ml, about 250 ng / ml, about 260 ng / ml, about 270 ng / ml, about 280 ng / ml, about 290 ng / ml, about 300 ng / ml, about 310 ng / ml, about 320 ng / ml, about 330 ng / ml, about 340 ng / ml, about 350 ng / ml, about 360 ng / ml, about 370 ng / ml, about 380 ng / ml, about 390 ng / ml, about 400 ng / ml, about 410 ng / ml, about 420 ng / ml, about 430 ng / ml, about 440 ng / ml, about 450 ng / ml, about 460 ng / ml, about 470 ng / ml, about 480 ng / ml, about 490 ng / ml, about 500 ng / ml, about 510 ng / ml, about 520 ng / ml, about 530 ng / ml, about 540 ng / ml, about 550 ng / ml, about 560 ng / ml, about 570 ng / ml, about 580 ng / ml, about 590 ng / ml, about 600 ng / ml, about 610 ng / ml, about 620 ng / ml, about 630 ng / ml, about 640 ng / ml, about 650 ng / ml, about 660 ng / ml, about 670 ng / ml, about 680 ng / ml, about 690 ng / ml, about 700 ng / ml, about 725 ng / ml, about 750 ng / ml, about 775 ng / ml, about 800 ng / ml, about 825 ng / ml, about 850 ng / ml, about 875 ng / ml, about 900 ng / ml, about 925 ng / ml, about 950 ng / ml, about 975 ng / ml, about 1000 ng / ml or more than about 1000 ng / ml. It should be appreciated that other inducer molecules are also compatible with methods described here and that optimization of the concentrations of such inducer molecules can be achieved through routine experimentation. In some embodiments, recombinant microbial cells described herein may be cultured in a temperature between about 30 and about 37ºC. For example, recombinant microbial cells may be cultured at a temperature of 30, 31, 32, 33, 34, 35, 36, 37º, or any value in between. In certain embodiments, recombinant microbial cells are cultured at a temperature between about 30 and about 32ºC, including 30, 31, and 32ºC and any value in between. As would be understood by one of ordinary skill in the art, the optimal temperature in which to culture a cell for production of α-3HA precursors and / or α-3HAs may be influenced by many factors, including cell type, culture media, and culture conditions. ADDITIONAL EMBODIMENTS Embodiment 1. A recombinant microbial cell, comprising: a nucleic acid molecule encoding an acetohydroxyacid synthase, a nucleic acid molecule encoding an acetohydroxy acid isomeroreductase, a nucleic acid molecule encoding a dihydroxy acid dehydratase, a nucleic acid molecule encoding an alpha-ketoisovalerate decarboxylase, a nucleic acid molecule encoding an aldehyde dehydrogenase, and a nucleic acid molecule encoding a threonine deaminase; and wherein the microbial cell overexpresses an aspartokinase, a homoserine kinase, and a threonine synthase relative to a wildtype microbial cell. Embodiment 2. The recombinant microbial cell of embodiment 1, wherein the nucleic acid molecule encoding an acetohydroxyacid synthase encodes IlvGM from Klebsiella aerogenes. Embodiment 3. The recombinant microbial cell of embodiment 1, wherein the nucleic acid molecule encoding an acetohydroxyacid synthase encodes AlsS from Bacillus subtilis. Embodiment 4. The recombinant microbial cell of embodiment 1 or embodiment 2, wherein the nucleic acid molecule encoding acetohydroxy acid isomeroreductase encodes IlvC from Escherichia coli. Embodiment 5. The recombinant microbial cell of any one of embodiments 1 to 4, wherein the nucleic acid molecule encoding dihydroxy acid dehydratase encodes IlvD from Escherichia coli . Embodiment 6. The recombinant microbial cell of any one of embodiments 1 to 5, wherein the nucleic acid molecule encoding alpha-ketoisovalerate decarboxylase encodes KivD from Lactococcus lactis. Embodiment 7. The recombinant microbial cell of any one of embodiments 1 to 6, wherein the nucleic acid molecule encoding aldehyde dehydrogenase encodes Fjoh_2967 from Flavobacterium johnsonaie or AldH from Escherichia coli. Embodiment 8. The recombinant microbial cell of any one of embodiments 1 to 7, wherein the nucleic acid molecule encoding threonine deaminase encodes IlvA from Corynebacterium glutamicum. Embodiment 9. The recombinant microbial cell of any one of embodiments 1 to 8, further comprising a nucleic acid molecule encoding an aspartokinase, a nucleic acid molecule encoding a homoserine kinase, and a nucleic acid molecule encoding a threonine synthase. Embodiment 10. The recombinant microbial cell of embodiment 9, wherein the nucleic acid molecule encoding an aspartokinase encodes ThrA from Escherichia coli. Embodiment 11. The recombinant microbial cell of embodiment 10, wherein the ThrA is ThrAG433Rfrom Escherichia coli. Embodiment 12. The recombinant microbial cell of any one of embodiments 9 to 11, wherein the nucleic acid molecule encoding a homoserine kinase encodes ThrB from Escherichia coli. Embodiment 13. The recombinant microbial cell of any one of embodiments 9 to 12, wherein the nucleic acid molecule encoding a threonine synthase encodes ThrC from Escherichia coli. Embodiment 14. The recombinant microbial cell of any one of embodiments 1 to 13, further comprising an inducible promoter or constitutive promoter operably linked to the nucleic acid molecule encoding acetohydroxyacid synthase II, the nucleic acid molecule encoding acetohydroxy acid isomeroreductase, the nucleic acid molecule encoding dihydroxy acid dehydratase , the nucleic acid molecule encoding alpha-ketoisovalerate decarboxylase, the nucleic acid molecule encoding aldehyde dehydrogenase, and / or the nucleic acid molecule encoding threonine deaminase. Embodiment 15. A method of producing short-chain fatty acids in a recombinant microbial cell, the method comprising culturing the recombinant microbial cell of any one of embodiments 1 to 9 with a carbon source. Embodiment 16. The method of embodiment 15, wherein the short-chain fatty acids are 2-methylbutyric acid (2MB) and / or isobutyric acid (IBA). Embodiment 17. The method of embodiment 15 or 16, wherein the short-chain fatty acids are precursors of α-substituted 3-hydroxy acids (α-3HAs). Embodiment 18. The method of any one of embodiments 15 to 17, wherein the culture of the recombinant microbial cell contains at least 0.2 g L-12MB. Embodiment 19. The method of any one of embodiments 15 to 18, wherein the carbon source is glucose. Embodiment 20. The method of any one of embodiments 15 to 19, wherein the carbon source is glycerol. Embodiment 21. The method of any one of embodiments 15 to 19, wherein the carbon source is a pentose. Embodiment 22. A recombinant microbial cell comprising a nucleic acid molecule encoding an activating enzyme; a nucleic acid molecule encoding acyl-CoA oxidase (ACOX) or acyl-CoA dehydrogenase (ACDH); a nucleic acid molecule encoding a (R)-specific enoyl- CoA hydratase; and one or more thioesterases. Embodiment 23. The recombinant microbial cell of embodiment 22, wherein the nucleic acid molecule encoding an activating enzyme encodes propionyl-CoA transferase (Pct). Embodiment 24. The recombinant microbial cell of embodiment 22, wherein the Pct is EC.2.8.3.1 from Megasphaera elsdenii. Embodiment 25. The recombinant microbial cell of embodiment 22, wherein the nucleic acid molecule encoding an activating enzyme encodes an irreversible activating enzyme. Embodiment 26. The recombinant microbial cell of embodiment 25, wherein the irreversible activating enzyme is IbuA from Rhodopseudomonas palustris. Embodiment 27. The recombinant microbial cell of embodiment 25, wherein the irreversible activating enzyme is 2-methylbutyryl-CoA synthetase (MACS) from Methanosarcina acetivorans. Embodiment 28. The recombinant microbial cell of any one of embodiments 22 to 27, wherein the nucleic acid molecule encoding (R)-specific enoyl-CoA hydratase encodes PhaJ from Aeromonas caviae. Embodiment 29. The recombinant microbial cell of any one of embodiments 22 to 28, wherein the nucleic acid molecule encoding ACOX encodes ACX4 from Arabidopsis thaliana. Embodiment 30. The recombinant microbial cell of embodiment 29, wherein KatE is overexpressed relative to a wildtype microbial cell. Embodiment 31. The recombinant microbial cell of any one of embodiments 22 to 30, wherein the nucleic acid molecule encoding ACDH encodes FadE from Escherichia coli. Embodiment 32. The recombinant microbial cell of any one of embodiments 22 to 30, wherein the nucleic acid molecule encoding ACDH encodes Acd from Pseudomonas putida KT2440. Embodiment 33. The recombinant microbial cell of embodiment 31 or 32, further comprising a nucleic acid molecule encoding an ETFBAD from Pseudomonas putida KT2440. Embodiment 34. The recombinant microbial cell of any one of embodiments 22 to 33, further comprising an inducible promoter or constitutive promoter operably linked to the nucleic acid molecule encoding an activating enzyme, the nucleic acid molecule encoding acyl-CoA oxidase (ACOX) or acyl-CoA dehydrogenase (ACDH), and / or the nucleic acid molecule encoding a (R)-specific enoyl-CoA hydratase. Embodiment 35. A method of producing α-substituted 3-hydroxy acids (α-3HAs), the method comprising culturing the recombinant microbial cell of any one of embodiments 22 to 34 with isobutyric acid (IBA) or 2-methylbutyric acid (2MB). Embodiment 36. The method of embodiment 35, wherein the (α-3HAs) are (2S)-3- hydroxyisobutyric acid (3HIB) or (2S, 3R)-3-hydroxy-2-methylbutyric acid (3H2MB). Embodiment 37. The method of embodiment 36, wherein the culture of the recombinant microbial cell contains at least 0.02 g L-13HIB. Embodiment 38. The method of embodiment 36, wherein the culture of the recombinant microbial cell contains at least 0.05 g L-13H2MB. Embodiment 39. A recombinant microbial cell, comprising: a nucleic acid molecule encoding acetohydroxyacid synthase; a nucleic acid molecule encoding acetohydroxy acid isomeroreductase; a nucleic acid molecule encoding dihydroxy acid dehydratase; a nucleic acid molecule encoding alpha-ketoisovalerate decarboxylase; a nucleic acid molecule encoding aldehyde dehydrogenase; a nucleic acid molecule encoding threonine deaminase; a nucleic acid molecule encoding an activating enzyme; a nucleic acid molecule encoding acyl- CoA oxidase (ACOX) or acyl-CoA dehydrogenase (ACDH); a nucleic acid molecule encoding a (R)-specific enoyl-CoA hydratase; a nucleic acid molecule encoding thioesterases; and wherein the recombinant microbial cell overexpresses an aspartokinase, a homoserine kinase, and a threonine synthase relative to a wildtype microbial cell. Embodiment 40. The recombinant microbial cell of embodiment 39, wherein the nucleic acid molecule encoding acetohydroxyacid synthase encodes IlvGM from Klebsiella aerogenes. Embodiment 41. The recombinant microbial cell of embodiment 39 or 40, wherein the nucleic acid molecule encoding acetohydroxy acid isomeroreductase encodes IlvC from Escherichia coli. Embodiment 42. The recombinant microbial cell of any one of embodiments 39 to 41, wherein the nucleic acid molecule encoding dihydroxy acid dehydratase encodes IlvD from Escherichia coli. Embodiment 43. The recombinant microbial cell of any one of embodiments 39 to 42, wherein the nucleic acid molecule encoding alpha-ketoisovalerate decarboxylase encodes KivD from Lactococcus lactis. Embodiment 44. The recombinant microbial cell of any one of embodiments 39 to 43, wherein the nucleic acid molecule encoding aldehyde dehydrogenase encodes Fjoh_2967 from Flavobacterium johnsonaie or AldH from Escherichia coli. Embodiment 45. The recombinant microbial cell of any one of embodiments 39 to 44, wherein the nucleic acid molecule encoding threonine deaminase encodes IlvA from Corynebacterium glutamicum. Embodiment 46. The recombinant microbial cell of any one of embodiments 39 to 45, wherein the nucleic acid molecule encoding an activating enzyme encodes propionyl-CoA transferase (Pct). Embodiment 47. The recombinant microbial cell of embodiment 46, wherein the Pct is EC. 2.8.3.1 from Megasphaera elsdenii. Embodiment 48. The recombinant microbial cell of any one of embodiments 39 to 45, wherein the nucleic acid molecule encoding an activating enzyme encodes an irreversible activating enzyme. Embodiment 49. The recombinant microbial cell of embodiment 48, wherein the irreversible activating enzyme is IbuA from Rhodopseudomonas palustris. Embodiment 50. The recombinant microbial cell of embodiment 48, wherein the irreversible activating enzyme is 2-methylbutyryl-CoA synthetase (MACS) from Methanosarcina acetivorans. Embodiment 51. The recombinant microbial cell of any one of embodiments 39 to 50, wherein the nucleic acid molecule encoding (R)-specific enoyl-CoA hydratase encodes PhaJ from Aeromonas caviae. Embodiment 52. The recombinant microbial cell of any one of embodiments 39 to 51, wherein the nucleic acid molecule encoding ACOX encodes ACX4 from Arabidopsis thaliana. Embodiment 53. The recombinant microbial cell of embodiment 52, wherein KatE is overexpressed relative to a wildtype microbial cell. Embodiment 54. The recombinant microbial cell of any one of embodiments 39 to 51, wherein the nucleic acid molecule encoding ACDH encodes FadE from Escherichia coli. Embodiment 55. The recombinant microbial cell of any one of embodiments 39 to 51, wherein the nucleic acid molecule encoding ACDH encodes Acd from Pseudomonas putida KT2440. Embodiment 56. The recombinant microbial cell of embodiment 55, further comprising a nucleic acid molecule encoding ETFBAD from Pseudomonas putida KT2440. Embodiment 57. The recombinant microbial cell of any one of embodiments 39 to 56, further comprising a nucleic acid molecule encoding an isobutyryl-CoA mutase (IcmF). Embodiment 58. The recombinant microbial cell of embodiment 57, wherein the IcmF is IcmF from Cupriavidus metallidurans. Embodiment 59. The recombinant microbial cell of embodiment 57 or 58, further comprising a nucleic acid molecule encoding a 3-hydroxyacyl-CoA dehydrogenase. Embodiment 60. The recombinant microbial cell of embodiment 59, wherein the 3- hydroxyacyl-CoA dehydrogenase is FadB from Escherichia coli. Embodiment 61. The recombinant microbial cell of any one of embodiments 57 to 60, further comprising a nucleic acid molecule encoding a thiolase. Embodiment 62. The recombinant microbial cell of embodiment 61, wherein the thiolase is AtoB from Escherichia coli. Embodiment 63. The recombinant microbial cell of any one of embodiments 39 to 62, further comprising a nucleic acid molecule encoding an aspartokinase, a nucleic acid molecule encoding a homoserine kinase, and a nucleic acid molecule encoding a threonine synthase. Embodiment 64. The recombinant microbial cell of embodiment 63, wherein the nucleic acid molecule encoding an aspartokinase encodes ThrA from Escherichia coli. Embodiment 65. The recombinant microbial cell of embodiment 64, wherein the ThrA is ThrAG433Rfrom Escherichia coli. Embodiment 66. The recombinant microbial cell of any one of embodiments 63 to 65, wherein the nucleic acid molecule encoding a homoserine kinase encodes ThrB from Escherichia coli. Embodiment 67. The recombinant microbial cell of any one of embodiments 63 to 66, wherein the nucleic acid molecule encoding a threonine synthase encodes ThrC from Escherichia coli. Embodiment 68. The recombinant microbial cell of any one of embodiments 39 to 67, further comprising an inducible promoter or constitutive promoter operably linked to the nucleic acid molecule encoding acetohydroxyacid synthase; the nucleic acid molecule encoding acetohydroxy acid isomeroreductase; the nucleic acid molecule encoding dihydroxy acid dehydratase; the nucleic acid molecule encoding alpha-ketoisovalerate decarboxylase; the nucleic acid molecule encoding aldehyde dehydrogenase; the nucleic acid molecule encoding threonine deaminase; the nucleic acid molecule encoding the activating enzyme; the nucleic acid molecule encoding acyl-CoA oxidase (ACOX) or acyl-CoA dehydrogenase (ACDH); and / or the nucleic acid molecule encoding a (R)-specific enoyl-CoA hydratase. Embodiment 69. A method of producing α-substituted 3-hydroxy acids, the method comprising culturing the recombinant microbial cell of any of embodiments 39 to 68 with a carbon source. Embodiment 70. The method of embodiment 69, wherein the α-substituted 3-hydroxy acids are (2S)-3-hydroxyisobutyric acid (3HIB) or (2S, 3R)-3-hydroxy-2-methylbutyric acid (3H2MB). Embodiment 71. The method of embodiment 69 or 70, wherein the carbon source is glucose. Embodiment 72. The method of any one of embodiments 69 to 71, wherein the culture of the recombinant microbial cell contains at least 0.14 g L-13H2MB. Embodiment 73. The method of any one of embodiments 69 to 72, wherein the culture of the recombinant microbial cell contains at least 0.01 g L-13HIB. Embodiment 74. The method of any one of embodiments 69 to 73, further comprising producing isobutyric acid (IBA) or 2-methylbutyric acid (2MB). Embodiment 75. The method of any one of embodiments 69 to 74, wherein the culture of the recombinant microbial cell contains at least 0.25 g L-1IBA. Embodiment 76. The method of any one of embodiments 69 to 75, wherein the culture of the recombinant microbial cell contains at least 0.05 g L-12MB. Embodiment 77. The recombinant microbial cell of any one of embodiments 1 to 14, 22 to 34, or 39 to 68, wherein the recombinant microbial cell is a bacterial, fungal, protist, or algal cell. Embodiment 78. The recombinant microbial cell of embodiment 77, wherein the bacterial cell is an Escherichia coli cell. Embodiment 79. A cell culture comprising the recombinant microbial cell of any one of embodiments 1 to 14, 22 to 34, or 39 to 68. Embodiment 80. A supernatant of a cell culture comprising the recombinant microbial cell of any one of embodiments 1 to 14, 22 to 34, or 39 to 68. Embodiment 81. The supernatant of embodiment 80, wherein the recombinant microbial cell is a bacterial, fungal, protist, or algal cell. Embodiment 82. The supernatant of embodiment 81, wherein the bacterial cell is an Escherichia coli cell. EXAMPLES Example 1: Module 1-mediated production of 2MB from glucose Production of 2MB in Module 1 pathways was evaluated in strains of recombinant E. coli expressing different Module 1 pathways, some of which included enzymes for threonine hyperproduction. E. coli were transfected with IPTG inducible, T7 promoter-controlled plasmids for expression of Module 1 pathway consisting of transgenes for the expression of: AlsS from B. subtilis or IlvGM from K. aerogenes, and optionally AldH from E. coli or Fjoh_2967 from F. johnsonaie, IlvC from E. coli, IlvD from E. coli, and KivD from L. lactis. Plasmids including transgenes for expression of IlvGM also included transgenes for expression of IlvA from C. glutamicum and over-expression of ThrAG433Rfrom E. coli, ThrB from E. coli, and ThrC from E. coli. The recombinant E. coli were grown in M9 medium supplemented with 1% glucose and 0.25% yeast extract. Strains expressing IlvGM produced 3x more 2MB and 30% less IBA than strains expressing AlsS (FIG.2). Example 2: Increased production of 2MB with Module 1 and IBA recycle Production of 2MB by Module 1 pathways with IBA recycles were evaluated. IBA recycle was first validated in a control strain of E. coli heterologously expressing pct (a gene encoding a propionyl-CoA transferase from Megasphaera elsdenii) and IcmF from C. metallidurans. The control strain was fed IBA, and was found to produce low levels of 2MB, as well as exhibit butyric acid (BA) accumulation (FIG.4, left). Test strains of E. coli were transfected with IPTG inducible, T7 promoter-controlled plasmids for expression of a 2MB- favoring Module 1 pathway and, optionally, IcmF from C. metallidurans. Plasmids for expression of a 2MB-favoring Module 1 pathway included transgenes for expression of: IlvGM from K. aerogenes, AldH from E. coli (A) or Fjoh_2967 from F. johnsonaie (F), IlvC from E. coli, IlvD from E. coli, KivD from L. lactis, IlvA from C. glutamicum, and over- expression of ThrAG433Rfrom E. coli, ThrB from E. coli, and ThrC from E. coli. Strains expressing Icmf produced 80% less IBA than those without it (FIG.4), illustrating that the IcmF-mediated IBA recycle is an effective tool to reduce byproduct formation from module 1. Example 3: Module 2-mediated production of 3HIB and 3H2MB from IBA and 2MB Production of 3HIB and 3H2MB by Module 2 pathways were evaluated in recombinant E. coli. Strains of recombinant E. coli were engineered to express Module 2 pathways by transforming a background strain with the relevant plasmids encoding: EC.2.8.3.1 from M. elsdenii, PhaJ from A. cavie, thioesterases, and one of the following: FadE from E. coli, acyl-CoA dehydrogenase (Acd) from P. putida KT2440 and EtfBAD from P. putida KT2440, ACX4 from A. thaliana, ACX4 from A. thaliana and KatE overexpression, or ACX4 from A. thaliana, resulting in six different strains.. Separate overnight cultures corresponding to each biological triplicate were started from freezer stocks. These overnights were grown in LB broth at 37 °C, 250 rpm, with supplementation of the appropriate antibiotic(s). After ~ 18 hr of growth, cultures were spun down at 4500 rpm for 5 min and resuspended in 1X M9 salts. These concentrated cells were used to inoculate M9 media + 10 g / L glucose + 2.5 g / L yeast extract to an optical density at 600 nm (OD600) = 0.1. Cultures were grown at 30 °C and 250 rpm according to: (1) condition A - 15 mL culture in 55 mL glass tube or (2) condition B - 50 mL culture in 250 mL baffled shake flasks. Cultures were grown to early exponential phase (OD600 = 0.6 - 0.9). Subsequently, the cells were induced with 0.25 mM isopropyl ^-d-1-thiogalactopyranoside (IPTG) and allowed to growth for 72 hr. Strains were fed IBA or 2MB. PhaJ from A. caviae in combination with Acx4 (ACOX from A. thaliana), Acd / EtfBAD (ACDH and ETFs from P. putida KT2440), or FadE (ACDH from E. coli) were shown to produce α-3HAs from the α-3HA precursors (FIG.5). Interestingly, FadE is able to catalyze the dehydrogenation reaction without overexpressing accessory ETFs; however, the low α-3HA titers exhibited by FadE versions of module 2 are attributed to substrate specificity since FadE is known to act primarily on straight chain acyl- CoAs from fatty acid degradation. Compared to FadE, strains possessing Acx4 or Acd / EtfBAD produced more α-3HAs, with 3H2MB titer reaching as high as 60% of the theoretical maximum in Acx4 strains. Further, 3H2MB production by versions of module 2 containing Acd / EtfBAD represents one of the first times that a complete ACDH system has been expressed heterologously and shows that EtfBAD is a sufficient electron transfer system for this ACDH.3HIB titers were lower than 3H2MB titers for all versions of the pathway indicating that dehydrogenation enzymes specific for shorter substrates could be used to increase titer here. Finally, there was no significant difference between 3HIB or 3H2MB titers when katE – a gene encoding for KatE catalase - was overexpressed to scavenge toxic H2O2in versions of Module 2 with Acx4. This suggests that H2O2generated by Acx4-mediated versions of Module 2 was low or that another endogenous H2O2 scavenging system was being used. Example 4: Module 1+2-mediated production of 3HIB and 3H2MB from glucose Versions of IBA-favoring Module 1 or 2MB-favoring Module 1 pathways were combined with Module 2 pathways to yield recombinant E. coli capable of producing 3HIB or 3H2MB from glucose. α-3HA production from glucose was measured. Strains were prepared as in Examples 1-3. Each strain harbored a Module 2 pathway including Acx4 from A. thaliana or ACDH from P. putida KT2440 and FadE / EtfBAD from P. putida KT2440, and AldH from E. coli or Fjoh_2967 from F. johnsonaie. Module 1+2 strains with a 2MB- favoring pathway also included a variable activation enzyme (Pct from M. elsdenii, IbuA from R. palustris, or MACS from M. acetivorans). Module 1+2 strains with a 2MB-favoring pathway and Pct as the activation enzyme, produced 192 mg / L 3H2MB. Addition of IcmF which encodes the only heterologous step in the IBA recycle improved 3H2MB titer to 261 mg / L. (FIG.6). Notably, replacing Pct with the irreversible activating enzymes IbuA or MACS did not improve 6).3H2MB titer. Module 1+2 strains with Acx4 produced increased 3HIB titers compared to those with FadE / EtfBAD, while AldH did not seem to affect 3HIB titer (FIG.7). Notably, 3HIB titers from strains possessing Module 2 only and Module 1+2 were comparable, suggesting that the limiting factor to produce a particular α-3HA depends on enzyme specificity for the substrates of the pathway. EQUIVALENTS While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and / or structures for performing the functions and / or obtaining the results and / or one or more of the advantages described herein, and each of such variations and / or modifications is deemed to be within the scope of the present invention. For example, the skilled artisan will understand that methods of administering compositions described herein also contemplate compositions for use in said methods and for the manufacture of medicaments. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and / or configurations will depend upon the specific application or applications for which the teachings of the present invention is / are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, and / or method described herein. In addition, any combination of two or more such features, systems, articles, materials, and / or methods, if such features, systems, articles, materials, and / or methods are not mutually inconsistent, is included within the scope of the present invention. The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” The phrase “and / or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Other elements may optionally be present other than the elements specifically identified by the “and / or” clause, whether related or unrelated to those elements specifically identified unless clearly indicated to the contrary. Thus, as a non-limiting example, a reference to “A and / or B,” when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A without B (optionally including elements other than B); in another embodiment, to B without A (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc. As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and / or” as defined above. For example, when separating items in a list, “or” or “and / or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e., “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law. As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and / or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. As used herein in the specification and in the claims, the phrase “about” means approximately. In the context of numerical values, the term “about” refers to ±5% of the numerical values cited. In the context of numerical ranges, the term “about” refers to ±5% of each numerical value cited (inclusive). For example, “about 1g L-1to about 1.1g L-1” should be understood to include any value between 0.95g L-1and 1.155g L-1(inclusive). Some embodiments may be embodied as a method, of which various examples have been described. The acts performed as part of the methods may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include different (e.g., more or less) acts than those that are described, and / or that may involve performing some acts simultaneously, even though the acts are shown as being performed sequentially in the embodiments specifically described above. Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.

Claims

CLAIMS What is claimed is:

1. A recombinant microbial cell, comprising: (i) a nucleic acid molecule encoding an acetohydroxyacid synthase; (ii) a nucleic acid molecule encoding an acetohydroxy acid isomeroreductase; (iii) a nucleic acid molecule encoding a dihydroxy acid dehydratase; (iv) a nucleic acid molecule encoding an alpha-ketoisovalerate decarboxylase; (v) a nucleic acid molecule encoding an aldehyde dehydrogenase; and (vi) a nucleic acid molecule encoding a threonine deaminase; wherein the microbial cell overexpresses an aspartokinase, a homoserine kinase, and a threonine synthase relative to a wildtype microbial cell.

2. The recombinant microbial cell of claim 1, wherein: (i) the nucleic acid molecule encoding an acetohydroxyacid synthase encodes IlvGM from Klebsiella aerogenes or AlsS from Bacillus subtilis; (ii) the nucleic acid molecule encoding acetohydroxy acid isomeroreductase encodes IlvC from Escherichia coli; (iii) the nucleic acid molecule encoding dihydroxy acid dehydratase encodes IlvD from Escherichia coli; (iv) the nucleic acid molecule encoding alpha-ketoisovalerate decarboxylase encodes KivD from Lactococcus lactis; (v) the nucleic acid molecule encoding aldehyde dehydrogenase encodes Fjoh_2967 from Flavobacterium johnsonaie or AldH from Escherichia coli; and (vi) the nucleic acid molecule encoding threonine deaminase encodes IlvA from Corynebacterium glutamicum.

3. The recombinant microbial cell of claim 1 or 2, further comprising: (vii) a nucleic acid molecule encoding an aspartokinase; (viii) a nucleic acid molecule encoding a homoserine kinase; and (ix) a nucleic acid molecule encoding a threonine synthase.

4. The recombinant microbial cell of claim 3, wherein:(vii) the nucleic acid molecule encoding an aspartokinase encodes ThrA from Escherichia coli, optionally wherein the ThrA is ThrAG433Rfrom Escherichia coli; (viii) the nucleic acid molecule encoding a homoserine kinase encodes ThrB from Escherichia coli; and (ix) the nucleic acid molecule encoding a threonine synthase encodes ThrC from Escherichia coli.

5. The recombinant microbial cell of any one of claims 1 to 4, further comprising an inducible promoter or constitutive promoter operably linked to the nucleic acid molecule of (i), the nucleic acid molecule of (ii), the nucleic acid molecule of (iii), the nucleic acid molecule of (iv), the nucleic acid molecule of (v), and / or the nucleic acid molecule of (vi).

6. A method of producing short-chain fatty acids in a recombinant microbial cell, the method comprising culturing the recombinant microbial cell of any one of claims 1 to 5 with a carbon source, optionally wherein the carbon source comprises glucose, glycerol, pentose, or a combination thereof.

7. The method of claim 6, wherein the short-chain fatty acids are 2-methylbutyric acid (2MB) and / or isobutyric acid (IBA), optionally wherein the short-chain fatty acids are precursors of α-substituted 3- hydroxy acids (α-3HAs).

8. The method of claim 6 or 7, wherein the culture of the recombinant microbial cell contains at least 0.2 g L-12MB.

9. A recombinant microbial cell comprising: (i) a nucleic acid molecule encoding an activating enzyme; (ii) a nucleic acid molecule encoding acyl-CoA oxidase (ACOX) or acyl-CoA dehydrogenase (ACDH); (iii) a nucleic acid molecule encoding a (R)-specific enoyl-CoA hydratase; and (iv) one or more thioesterases.

10. The recombinant microbial cell of claim 9, wherein:(i) the nucleic acid molecule encoding an activating enzyme encodes (a) propionyl-CoA transferase (Pct), optionally wherein the Pct is EC.2.8.3.1 from Megasphaera elsdenii; or (b) an irreversible activating enzyme, optionally wherein the irreversible activating enzyme is IbuA from Rhodopseudomonas palustris or 2-methylbutyryl-CoA synthetase (MACS) from Methanosarcina acetivorans; (ii) the nucleic acid molecule encoding ACOX or ACDH encodes: (a) ACX4 from Arabidopsis thaliana, optionally wherein KatE is overexpressed relative to a wildtype microbial cell; or (b) FadE from Escherichia coli or Acd from Pseudomonas putida KT2440, optionally wherein the recombinant microbial cell further comprises a nucleic acid molecule encoding an ETFBAD from Pseudomonas putida KT2440; and (iii) the nucleic acid molecule encoding (R)-specific enoyl-CoA hydratase encodes PhaJ from Aeromonas caviae.

11. The recombinant microbial cell of claim 9 or 10, further comprising an inducible promoter or constitutive promoter operably linked to the nucleic acid molecule of (i), the nucleic acid of (ii), and / or the nucleic acid molecule of (iii).

12. A method of producing α-substituted 3-hydroxy acids (α-3HAs), the method comprising culturing the recombinant microbial cell of any one of claims 9 to 11 with isobutyric acid (IBA) or 2-methylbutyric acid (2MB), optionally wherein the α-3HAs are (2S)-3-hydroxyisobutyric acid (3HIB) or (2S, 3R)- 3-hydroxy-2-methylbutyric acid (3H2MB).

13. The method of claim 12, wherein the culture of the recombinant microbial cell contains at least 0.02 g L-13HIB.

14. A recombinant microbial cell, comprising: a first module comprising:(i) a nucleic acid molecule encoding an acetohydroxyacid synthase; (ii) a nucleic acid molecule encoding an acetohydroxy acid isomeroreductase; (iii) a nucleic acid molecule encoding a dihydroxy acid dehydratase; (iv) a nucleic acid molecule encoding an alpha-ketoisovalerate decarboxylase; (v) a nucleic acid molecule encoding an aldehyde dehydrogenase; and (vi) a nucleic acid molecule encoding a threonine deaminase; and a second module comprising: (i) a nucleic acid molecule encoding an activating enzyme; (ii) a nucleic acid molecule encoding acyl-CoA oxidase (ACOX) or acyl-CoA dehydrogenase (ACDH); (iii) a nucleic acid molecule encoding a (R)-specific enoyl-CoA hydratase; and (iv) one or more thioesterases; wherein the microbial cell overexpresses an aspartokinase, a homoserine kinase, and a threonine synthase relative to a wildtype microbial cell.

15. The recombinant microbial cell of claim 14, wherein: the first module comprises: (i) the nucleic acid molecule encoding an acetohydroxyacid synthase encodes IlvGM from Klebsiella aerogenes or AlsS from Bacillus subtilis. (ii) the nucleic acid molecule encoding acetohydroxy acid isomeroreductase encodes IlvC from Escherichia coli; (iii) the nucleic acid molecule encoding dihydroxy acid dehydratase encodes IlvD from Escherichia coli; (iv) the nucleic acid molecule encoding alpha-ketoisovalerate decarboxylase encodes KivD from Lactococcus lactis; (v) the nucleic acid molecule encoding aldehyde dehydrogenase encodes Fjoh_2967 from Flavobacterium johnsonaie or AldH from Escherichia coli; and (vi) the nucleic acid molecule encoding threonine deaminase encodes IlvA from Corynebacterium glutamicum; and / or the second module comprises: (i) the nucleic acid molecule encoding an activating enzyme encodes (a) propionyl-CoA transferase (Pct), optionally wherein the Pct is EC.2.8.3.1 from Megasphaera elsdenii; or(b) an irreversible activating enzyme, optionally wherein the irreversible activating enzyme is IbuA from Rhodopseudomonas palustris or 2-methylbutyryl-CoA synthetase (MACS) from Methanosarcina acetivorans; (ii) the nucleic acid molecule encoding ACOX or ACDH encodes: (a) ACX4 from Arabidopsis thaliana, optionally wherein KatE is overexpressed relative to a wildtype microbial cell; or (b) FadE from Escherichia coli or Acd from Pseudomonas putida KT2440, optionally wherein the recombinant microbial cell further comprises a nucleic acid molecule encoding an ETFBAD from Pseudomonas putida KT2440; and (iii) the nucleic acid molecule encoding (R)-specific enoyl-CoA hydratase encodes PhaJ from Aeromonas caviae.

16. The recombinant microbial cell of claim 14 or 15, wherein: the first module further comprises an inducible promoter or constitutive promoter operably linked to the nucleic acid molecule of (i), the nucleic acid of (ii), the nucleic acid molecule of (iii), the nucleic acid molecule of (iv), the nucleic acid molecule of (v), and / or the nucleic acid molecule of (vi); and the second module further comprises an inducible promoter or constitutive promoter operably linked to the nucleic acid molecule of (i), the nucleic acid of (ii), and / or the nucleic acid molecule of (iii).

17. The recombinant microbial cell of any one of claims 14 to 16, further comprising: a nucleic acid molecule encoding an isobutyryl-CoA mutase (IcmF), a nucleic acid molecule encoding a 3-hydroxyacyl-CoA dehydrogenase, and / or a thiolase.

18. The recombinant microbial cell of any one of claims 14 to 17, further comprising: a nucleic acid molecule encoding an aspartokinase, optionally wherein the aspartokinase is ThrA from Escherichia coli, further optionally wherein the ThrA is ThrAG433Rfrom Escherichia coli;a nucleic acid molecule encoding a homoserine kinase, optionally wherein the homoserine kinase is ThrB from Escherichia coli; and a nucleic acid molecule encoding a threonine synthase, optionally wherein the nucleic acid molecule encoding a threonine synthase encodes ThrC from Escherichia coli.

19. A method of producing α-substituted 3-hydroxy acids (α-3HAs), the method comprising culturing the recombinant microbial cell of any one of claims 14 to 18 with a carbon source, optionally wherein the α-3HAs are (2S)-3-hydroxyisobutyric acid (3HIB) or (2S, 3R)- 3-hydroxy-2-methylbutyric acid (3H2MB), further optionally wherein the culture of the recombinant microbial cell contains at least 0.14g L-13H2MB and / or at least 0.01g L-13HIB.

20. The method of claim 19, further comprising producing isobutyric acid (IBA) or 2- methylbutyric acid (2MB), optionally wherein the culture of the recombinant microbial cell contains at least 0.25g L-1IBA and / or the culture of the recombinant microbial cell contains at least 0.05g L-12MB.

Images