SYNTHESIS AND ENZYMATIC EVALUATION OF 2 CONFORMATIONALLY RESTRICTED TRISACCHARIDE ANALOGS AS SUBSTRATES FOR N-ACETYLGLUCOSAMINYLTRANSFERASE-V

The synthetic trisaccharide beta-GlcNAc(1 --> 2)alpha-Man(1 --> 6)beta-GLc-O(CH2)7CH3 (5) is an acceptor substrate for the enzyme N-acetylglucosaminyltransferase V (GlcNAcT-V) which adds an additional beta-GlCNAc residue to the 6-position of the central alpha-Man unit. Two trisaccharide analogues of 5 have been chemically synthesized where the possibility for rotation about the C5-C6 bond of the beta-Glc residue has been eliminated by linking 04 and C6 with an ethylene bridge. The resulting fused-ring trisaccharides 8 and 9 represent conformationally restricted models for the "gt" and "gg" conformations, respectively, of asparagine-linked oligosaccharides. Both 5 and its conformationally restricted gg analogue 9 showed similar acceptor properties toward the enzyme from hamster kidney, while the gt trisaccharide 8 was an extremely poor substrate. These results demonstrate that GlcNAcT-V preferentially recognizes the carbohydrate chains of asparagine-linked glycoproteins in their gg conformations.