Enzymes involved in the biosynthesis of glycoconjugates. A UDP-2-acetamido-2-deoxy-D-glucose: beta-D-galactopyranosyl-(1 leads to 4)-saccharide (1 leads to 3)-2-acetamido-2-deoxy-beta-D- glucopyranosyltransferase in human serum.

A 2-acetamido-2-deoxy-beta-D-glucopyranosyltransferase (N-acetylglucosaminyltransferase) that catalyses the transfer of 2-acetamido-2-deoxy-D-glucose from UDP-2-acetamido-2-deoxy-D-glucose to terminal nonreducing beta-D-galactosyl residues in disaccharides, oligosaccharides, glycoproteins, and glycolipids has been detected in human serum. The preferred acceptors are those with beta-D-galactosyl residues linked (1 leads to 4) to the subterminal sugar residue. Activity is greatest when the second sugar residue is 2-acetamido-2-deoxy-D-glucose but beta-D-Galp-(1 leads to 4)-D-Glc and beta-D-Galp-(1 leads to 4)-D-Man are also good acceptors. Compounds with beta-D-galactosyl groups linked (1 leads to 3) to 2-acetamido-2-deoxy-D-glucose are relatively poor acceptors and beta-D-Galp-(1 leads to 6)-D-GlcNAc is inactive. Oligosaccharides substituted with an L-fucose unit on the beta-D-galactosyl unit or on the adjacent sugar residue failed to accept 2-acetamido-2-deoxy-D-glucose. Similarly, glycoproteins with L-fucose or sialic acid substituents are less effective acceptors before removal of these sugars to expose more beta-D-galactosyl end-groups. The transferred 2-acetamido-2-deoxy-beta-D-glucopyranosyl residue is cleaved from the enzymic reaction products by the N-acetyl-beta-D-glucosaminidase from Jack beans. Methylation analysis of the products of 2-acetamido-2-deoxy-D-glucosyl transfer to N-acetyllactosamine [beta-D-Galp-(1 leads to 4)-D-GlcNAc] and lactose [beta-D-Galp-(1 leads to 4)-D-Glc] revealed that the terminal, nonreducing D-galactosyl group in both these acceptors had been 3-O-substituted with 2-acetamido-2-deoxy-D-glucose. Hence, the enzyme in human serum catalyses, in the presence of Mn2+, the reaction UDP-GlcNAc + beta-D-Galp-(1 leads to 4)-R leads to beta-D-GlcpNAc(1 leads to 3)-beta-D-Galp-(1 leads to 4)-R + UDP and is a UDP-2-acetamido-2-deoxy-D-glucose: beta-D-galactopyranosyl-(1 leads to to 3)-2-acetamido-2-deoxy-beta-D-glucopyranosyltransferase.