ACCEPTOR SUBSTRATE RECOGNITION BY N-ACETYLGLUCOSAMINYLTRANSFERASE-V - CRITICAL ROLE OF THE 4''-HYDROXYL GROUP IN BETA-D-GLCPNAC-(1-)2)-ALPHA-D-MANP(1-)6)-BETA-D-GLCP-OR

The enzyme N-acetylglucosaminyltransferase-V (GlcNAcT-V) transfers GlcNAc from UDP-GlcNAc to the OH-6' group of oligosaccharides terminating in the sequence beta-D-GlcpNAc-(1 --> 2)-alpha-D-Man p-(1 --> 6)-beta-D-Glc p (or Man p)-OR (5, R = (CH2)7CH3) to yield the sequence beta-D-GlcpNAc-(1 --> 2)-[beta-D-Glc p NAc-(1 --> 6)]-alpha-D-Man p-(1 --> 6)-beta-D-Glc p (or Man p)-OR. Biosynthetically, if beta-(1 --> 4)-galactosyltransferase acts first on 5, the product beta-D-Gal p-(1 --> 4)-beta-D-Glc p NAc-(1 --> 2)-alpha-D-Man p-(1 --> 6)-beta-D-Glc p (or Man p)-OR (7) is no longer a substrate for GlcNAcT-V even though it retains the active OH-6' group. The reason for this loss in activity is examined in this paper. Six analogues of the acceptor trisaccharide 5, all with the reducing-end D-gluco configuration, were chemically synthesized. A key feature of the synthetic scheme is the use of 1,2-diaminoethane for the efficient removal of N-phthalimdo protecting groups. In these analogues OH-4 of the terminal sugar unit, the site of galactosylation by GalT in the normal GlcNAc-terminating trisaccharide 5, was systematically replaced by OMe, F, NH2, NHAc, and H, as well as inverted to the galacto configuration. The interactions of the resulting trisaccharide analogues with GlcNAcT-V from hamster kidney were then evaluated kinetically. All six compounds were found to be essentially inactive either as acceptors or as inhibitors of GlcNAcT-V. The conclusion is reached that galactosylation of natural acceptors for GlcNAcT-V destroys acceptor activity, not by introduction of the steric bulk of an added sugar residue, but by destroying an important hydrogen-bonding interaction of terminal OH-4 of the GlcNAc residues with the enzyme. This OH-4 group is therefore designated as a key polar group for GlcNAcT-V.