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Flexibility in the donor substrate specificity of beta 1,4-galactosyltransferase: application in the synthesis of complex carbohydrates.

Biosynthetically, bovine N-acetylglucosamine beta 1,4-galactosyltransferase (GalT) catalyses the transfer of galactosyl residues from UDP-Gal to the 4-position of GlcNAc units, resulting in the production of N-acetyllactosamine sequences. UDP-Glc and UDP-GalNAc were also found to act as donors for this enzyme, allowing the preparation of beta Glc(1----4)-beta GlcNAc and beta GalNAc(1----4) beta GlcNAc terminating structures on the milligram scale. GalT could thus be used to add beta GalNAc to beta GlcNAc(1----2) alpha Man terminating structures, converting them to the beta GalNAc(1----4) beta GlcNAc(1----2) alpha Man sequences found on glycoprotein hormones. GalT did not transfer GlcNAc residues from UDP-GlcNAc, but it could utilize UDP-GlcNH2 as a donor. Synthesis of beta GlcNAc(1----4) beta GlcNAc sequences could therefore be accomplished by transfer of GlcNH2 from its UDP derivative, followed by N-acetylation of the product amino-disaccharide using acetic anhydride in methanol. The products of the enzymatic reactions were characterized by 1H-NMR-spectroscopy and fast-atom bombardment mass spectrometry. This work expands the scope of the combined chemical-enzymatic synthesis of complex carbohydrates, using glycosyltransferases, to the production of oligosaccharides different from those for which these enzymes were designed. These unnatural reactions should find application in glycoprotein and glycolipid remodelling.