Abstract
Glycosidase-catalyzed transglycosylation for practical synthesis of oligosaccharides has been developed by using a thermostable CLONEZYMETM glycosidase library. Convenient syntheses of N-acetyllactosamine, xylose containing oligosaccharides, and galactobiosides by the use of regioselective transglycosylation were described. Furthermore, transglycosylation could be used to modify hydroxyalkyl polysaccharides.alpha-Galactosyl epitopes are carbohydrate structures bearing a Galalpha1 → 3Gal terminus. The interaction of these epitopes on the surface of animal cells with anti alpha-Gal antibodies in human serum is believed to be the main cause in antibody-mediated hyperacute rejection in xenotransplantation. An efficient chemoenzymatic approach based on the use of recombinant alpha1 → 3 galactosyltransferase has been developed to synthesize alpha-Gal epitopes. Simultaneously, chemical synthesis of alpha-Galactosyl epitopes was accomplished using thioglycoside chemistry.Conformational analysis of an N-linked alpha-Gal trisaccharide epitope was conducted in terms of each monosaccharide residue conformation, primary hydroxymethyl group configuration and interglycosidic conformations. Selective 2D J-delta INEPT experiments have been carried out at different temperatures to evaluate the three-bond long range 13C, 1H coupling constants for the alpha1 → 3 linkage. The NMR experimental data were complemented by theoretical calculations based on energy minimization, grid search, and Metropolis Monte Carlo simulations. The results indicated that the trisaccharide had a restricted flexibility around the crucial alpha1 → 3 linkage. The determination of this conformation set the foundation for the design of conformationally restricted alpha-Gal mimetics. The hydrogen bond between HO-2' and HO-4 in Galalpha1 → 3Gal disaccharide was replaced by a methylene bridge, thus introducing a highly rigid and preorganized conformation.Glycopolymer mediators bearing Galalpha1 → 3Gal termini as multivalent xenoactive antigens and alpha-mannosyl termini as specific multivalent ligands for bacterial cells were prepared through chemoenzymatic synthesis. The resulting glycopolymers binding to bacterial cells and human natural anti-Gal antibody were demonstrated. It holds the possibility of removing bacterial cells by redirecting human natural immunity through this alpha-Gal-mannose glycoconjugates.