Glycosyltransferases: Structures, Functions, and Mechanisms

Abstract

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.

N-Linked Protein Glycosylation in the Endoplasmic Reticulum

“…Based on their primary sequence the enzymes adopt similar folds and all use lipid-bound phosphate-activated sugars as donor substrates. They are inverting enzymes, resulting in an a configuration of the transferred glycan unit (Lairson et al 2008).…”

The Biochemistry and Structural Biology of Plant Cell Wall Deconstruction

“…The data have revealed only two major folds for these enzymes, while also providing insights into the likely catalytic mechanisms displayed by inverting and retaining glycosyltransferases (for review, see Lairson et al, 2008). These studies, however, have focused, almost exclusively, on enzymes that are not membrane associated, and currently, there is no high-resolution crystal structural information on glycosyltransferases that contribute to plant cell wall synthesis.…”

A Membrane-located Glycosyltransferase Complex Required for Biosynthesis of the d-Galactan I Lipopolysaccharide O Antigen in Klebsiella pneumoniae

“…WbbN belongs to GT-2, one of the largest families within the GT-A superfamily (34,35). Unlike WbbO and WbbM, WbbN (290 residues, 33.2 kDa) is predicted to contain a single domain (Fig.…”