Synthesis of fluorinated mucin core 2 branched oligosaccharides with the potential of novel substrates and enzyme inhibitors for glycosyltransferases and sulfotransferases

Syntheses of fluorinated mucin core 2 tri- and tetrasaccharides modified at the C-3 or C-4 position of the pertinent galactose residue are reported. These compounds were used for the study of sialyltransferases and 3-O- sulfotransferases involved in the biosynthesis of O-glycans. Our acceptor substrate specificity studies on three cloned sialyltransferases (Sia-Ts) revealed that a 3- or 4-fluoro substituent in β1,4Gal resulted in poor acceptors for α2,6(N)Sia-T and α2,3(N)Sia-T, whereas 4-fluoro-Galβ1,3GalNAcα was a good acceptor for α2,3(O)Sia-T. Uniquely, 4-F-Galβ1,4GlcNAcβ1,6(Galβ1,3)GalNAcα-OBn was an inhibitor of α2,6(N)Sia-T activity but not α2,3(N)Sia-T activity. Further we found that the activities of only Gal 3-O-sulfotransferases and not sialyltransferases were adversely affected by a C-3 fluoro substituent at the other Gal terminal of mucin core 2. The strategy of building branched mucin core 2 structures by three glycosidation sequence coupling three classes of glycosyl donors with the reactivity-matching acceptors proved to be successful in syntheses of modified mucin-type core structures of O-glycan. The relative poor yields of the glycosylations using fluorinated galactosyl donors indicated that the fluorine modification dramatically decreased the donor reactivity due to electron-withdrawing effect.