Recognition of the acceptor beta-D-GlcpNAc-(1-->2)-alpha-D-Manp- (1-->6)-beta-D-Glcp-OR by N-acetyl-glucosaminyltransferase-V: none of the hydroxyl groups on the Glc-residue are important

The enzyme, N-acetylglucosaminyltransferase-V (GlcNAcT-V, E.C. 2.4.1.155), transfer a beta-D-GlcpNAc residue, from UDP-GlcNAc, to the OH-6 group of the Man residue in the synthetic acceptor beta-D-GlcpNAc-(1-->2)-alpha-D-Manp-(1-->6)-beta-D-Glcp-O(CH2)7 CH3 (3). Trisaccharide 3 is an excellent substrate for the enzyme from hamster kidney with a Km value of 26 microM. In this paper we examine the contribution of the Glc residue in 3 to acceptor recognition by this enzyme. beta-D-GlcpNAc-(1-->2)-alpha-D-Manp-O(CH2)7CH3 where the Glc residue in 3 has been deleted, was synthesized and found to be a very poor substrate with a Km value elevated to almost 2 mM. Two other analogues of 3, where the Glc residue was O-trimethylated (6) or O-tribenzylated (7), respectively, possessed Km values very near to those of 3. The Glc residue in 3 is thereby shown to present an important recognition element for GlcNAcT-V, but none of the free hydroxyl groups are required. This observation should facilitate the design of more hydrophobic and membrane-permeable analogues of 3 that are expected to function as specific glycosylation inhibitors.