Side Chain Conformation Restriction in the Catalysis of Glycosidic Bond Formation by Leloir Glycosyltransferases, Glycoside Phosphorylases, and Transglycosidases
Jonathan C.K. Quirke, David Crich
Abstract
Carbohydrate side chain conformation is an important factor in the control of reactivity at the anomeric center, i.e. in the making and breaking of glycosidic bonds, whether by chemical means or, for hydrolysis, by glycoside hydrolases. In nature glycosidic bond formation is catalyzed out by glycosyltransferases (GTs), glycoside phosphorylases, and transglycosidases. By an 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 analogues 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 secondmost reactive gauche,trans (gt) conformation and β-galactosyltransferases favoring the least reactive trans,gauche (tg) conformation. This insight will help promote the design and development of improved, conformationally restricted GT inhibitors that take advantage of these inherent side chain preferences.