The present invention provides conjugates between a substrate, e.g., peptide, glycopeptide, lipid, etc., and a modified saccharyl fragment bearing a modifying group such as a water-soluble polymer, therapeutic moiety or a biomolecule. The conjugates are linked via the enzymatic conversion of the activated modified saccharyl fragment into a glycosyl linking group that is interposed between and covalently attached to the substrate and the modifying group. The conjugates are formed from substrates by the action of a sugar transferring enzyme, e.g., a glycosyltransferase. For example, when the substrate is a peptide, the enzyme conjugates a modified saccharyl fragment moiety onto either an amino acid or glycosyl residue of the peptide. Also provided are pharmaceutical formulations that include the conjugates. Methods for preparing the conjugates are also within the scope of the invention.