Glycosyltransferases: structures, functions, and mechanisms.
Glycosyltransferases catalyze glycosidic bond formation using sugar donors containing a nucleoside phosphate or a lipid phosphate leaving group. Only two structural folds, GT A and GT B, have been identified for the nucleotide sugar dependent enzymes, but other folds are now appearing for the soluble domains of lipid phosphosugar dependent glycosyl transferases. Structural and kinetic studies have provided new insights. Inverting glycosyltransferases utilize a direct displacement S(N)2 like mechanism involving an enzymatic base catalyst. Leaving group departure in GT A fold enzymes is typically facilitated via a coordinated divalent cation, whereas GT B fold enzymes instead use positively charged side chains and/or hydroxyls and helix dipoles. The mechanism of retaining glycosyltransferases is less clear. The expected two step double displacement mechanism is rendered less likely by the lack of conserved architecture in the region where a catalytic nucleophile would be expected. A mechanism involving a short lived oxocarbenium ion intermediate now seems the most likely, with the leaving phosphate serving as the base.
Annual review of biochemistry