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

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.

Influence of Side Chain Conformation on the Activity of Glycosidase Inhibitors

“… [8] The rate of glycoside hydrolysis was found to be gg > gt > tg , as corroborated in the galacto series by our laboratory [9] . In addition, our recent findings have revealed that the influence of side chain conformation on reactivity is exploited by nature, where many GHs and GTs bind their ligands with specific side chain conformations [13, 14] . For example, >80 % of glucosidases restrict the side chain of glucose in their active sites to the gg conformation: considering the ≈50 : 50 unbound population of gg : gt conformers, these enzymes appear to have evolved to take the advantage of the substrate side chain conformation to gain additional transition state stabilization [13] .…”

Influence of Configuration at the 4- and 6-Positions on the Conformation and Anomeric Reactivity and Selectivity of 7-Deoxyheptopyranosyl Donors: Discovery of a Highly Equatorially Selective l-glycero-d-gluco-Heptopyranosyl Donor

“…In recent years, it has become increasingly apparent that the conformation of the side chain of glycosyl donors contributes significantly to their reactivity and selectivity, with the initial ground rules determined using conformationally locked bicyclic donors. − More recently, we have demonstrated by the study of a series of sialic acid and Kdo donors isomeric at the 5- and/or 7-positions that side-chain conformation also contributes to reactivity and selectivity in monocyclic donors (Figure ). − Finally, recent studies have highlighted the role of side-chain conformation in glycan recognition and as a contributing factor to catalysis by the glycosyl hydrolases, glycosyltransferases, transglycosidases, and glycoside phosphorylases during enzymatic cleavage and the formation of glycosidic bonds …”

Synthesis of C6-modified mannose 1-phosphates and evaluation of derived sugar nucleotides against GDP-mannose dehydrogenase

“…model allowed unambiguous assignment of the solid state molecular structure and absolute configuration. As shown in Figure 2, the desired axial anomeric phosphate was clearly visible alongside an interesting gg rotameric form for the C6-chloro side chain substituent (pyranose side chain conformation has recently been shown to be an important factor contributing to anomeric reactivity of canonical pyranoses [12]). One phenyl ring of the phosphate ester group occupied a position folded underneath the pyranose ring and engaged in an intramolecular C-H•••π interaction with the axial C5 proton (Figure 2, dotted line).…”