The influence of an intramolecular hydrogen bond in differential recognition of inhibitory acceptor analogs by human ABO(H) blood group A and B glycosyltransferases

Human ABO( H) blood group glycosyltransferases GTA and GTB catalyze the final monosaccharide addition in the biosynthesis of the human A and B blood group antigens. GTA and GTB utilize a common acceptor, the H antigen disaccharide alpha-L-Fucp-(1-->2)-beta-D-Galp-OR, but different donors, where GTA transfers GalNAc from UDP-GalNAc and GTB transfers Gal from UDP-Gal. GTA and GTB are two of the most homologous enzymes known to transfer different donors and differ in only 4 amino acid residues, but one in particular (Leu/Met-266) has been shown to dominate the selection between donor sugars. The structures of the A and B glycosyltransferases have been determined to high resolution in complex with two inhibitory acceptor analogs alpha-L-Fucp( 1-->2)-beta-D-(3-deoxy)-Galp-OR and alpha-L-Fucp-(1-->2)-beta-D-(3-amino)- Galp-OR, in which the 3-hydroxyl moiety of the Gal ring has been replaced by hydrogen or an amino group, respectively. Remarkably, although the 3-deoxy inhibitor occupies the same conformation and position observed for the native H antigen in GTA and GTB, the 3-amino analog is recognized differently by the two enzymes. The 3-amino substitution introduces a novel intramolecular hydrogen bond between O-2' on Fuc and N-3' on Gal, which alters the minimum-energy conformation of the inhibitor. In the absence of UDP, the 3-amino analog can be accommodated by either GTA or GTB with the L-Fuc residue partially occupying the vacant UDP binding site. However, in the presence of UDP, the analog is forced to abandon the intramolecular hydrogen bond, and the L-Fuc residue is shifted to a less ordered conformation. Further, the residue Leu/Met-266 that was thought important only in distinguishing between donor substrates is observed to interact differently with the 3-amino acceptor analog in GTA and GTB. These observations explain why the 3-deoxy analog acts as a competitive inhibitor of the glycosyltransferase reaction, whereas the 3-amino analog displays complex modes of inhibition.