Side Chain Conformation Restriction in the Catalysis of Glycosidic Bond Formation by Leloir Glycosyltransferases, Glycoside Phosphorylases, and Transglycosidases.

Carbohydrate side chain conformation is an important factor in the control of reactivity at the anomeric center, ie, in the making and breaking of glycosidic bonds, whether chemically or, for hydrolysis, by glycoside hydrolases. In nature glycosidic bond formation is catalyzed out by glycosyltransferases (GTs), glycoside phosphoryases, and transglycosidases. By analysis of 118 crystal structures of sugar nucleotide dependent (Leloir) GTs, 136 crystal structures of glycoside phosphorylases, and 54 crystal structures of transglycosidases bound to hexopyranosides or their analogs at the donor site (-1 site), we determined that most enzymes that catalyze glycoside synthesis, be they GTs, glycoside phosphorylases or transglycosidases, restrict their substrate side chains to the most reactive gauche,gauche (gg) conformation to achieve maximum stabilization of the oxocarbenium ion-like transition state for glycosyl transfer. The galactose series deviates from this trend, with α-galactosyltransferases preferentially restricting their substrates to the second-most reactive gauche,trans (gt) conformation, and β-galactosyltransferases favoring the least reactive trans,gauche (tg) conformation. This insight will help progress the design and development of improved, conformationally-restricted GT inhibitors that take advantage of these inherent side chain preferences.