Opinion
Stereochemical Divergence of Polyprenol Phosphate Glycosyltransferases

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Trends

Despite their considerable sequence and structural homology, polyprenol phosphate glycosyltransferases can act with different reaction mechanisms and stereochemical outcomes.

In N-linked protein glycosylation, sugars added to polyprenol monophosphate carriers present either the α- or the β-configuration.

Novel roles for glycosylated phosphoprenol carriers other than contributing to N-glycosylation have been described.

In the three domains of life, lipid-linked glycans contribute to various cellular processes ranging from protein glycosylation to glycosylphosphatidylinositol anchor biosynthesis to peptidoglycan assembly. In generating many of these glycoconjugates, phosphorylated polyprenol-based lipids are charged with single sugars by polyprenol phosphate glycosyltransferases. The resultant substrates serve as glycosyltransferase donors, complementing the more common nucleoside diphosphate sugars. It had been accepted that these polyprenol phosphate glycosyltransferases acted similarly, given their considerable sequence homology. Recent findings, however, suggest that matters may not be so simple. In this Opinion we propose that the stereochemistry of sugar addition by polyprenol phosphate glycosyltransferases is not conserved across evolution, even though the GT-A fold that characterizes such enzymes is omnipresent.

Section snippets

Glycosylation of Linear, Long-Chain Polyprenol Phosphates Across Evolution

In addition to serving as key constituents of the basic building blocks of biological membranes, as currency for energy storage in the form of triacylglycerides, and as vital intracellular signaling molecules (1, 2, 3 for recent reviews), lipids also serve as platforms upon which diverse glycoconjugates are assembled 4, 5. In many cases glycoconjugate biosynthesis begins with the addition of a single carbohydrate to a phosphorylated, membrane-associated polyprenol (see Glossary)-based lipid

Different Mechanisms for Different Polyprenol Phosphate GTs?

It had been generally accepted that polyprenol phosphate GTs, assigned to the abundant CAZy (carbohydrate-active enzyme; www.cazy.org/Welcome-to-the-Carbohydrate-Active.html) GT2 glycosyltransferase family [40], may act similarly, given their considerable structure and sequence homology. Indeed, examination of an archaeal DPM synthase from P. furiosus [36] and GtrB, a bacterial DPG synthase from Synechocystis sp. PCC6803 that participates in O-antigen biosynthesis [41], the only two polyprenol

Why Does Polyprenol Phosphosugar Stereochemistry Matter?

Across evolution, the transfer of lipid-linked glycans to target asparagine residues is mediated by an oligosaccharyltransferase. Bacterial PglB and archaeal AglB, each acting alone, and eukaryal Stt3, acting alone in lower eukaryotes or as part of a heterooligomeric complex in higher organisms, are homologous proteins that catalyze the reaction 25, 45, 46. These enzymes rely on an inverting mechanism whereby an α-linked glycan in the lipid-linked substrate is processed to yield the asparagine

Concluding Remarks and Future Perspectives

If eukaryal and archaeal polyprenol phosphate GTs indeed catalyze similar reactions, albeit with different stereochemistries, then one would imagine that the two versions of the enzyme can be distinguished at the sequence and/or structural levels. Because preliminary comparisons of the sequences and membrane topologies of archaeal enzymes involved in N-glycosylation with their eukaryal counterparts fails to reveal any obvious distinction, the basis for the apparent reliance on mechanisms

Acknowledgments

J.E. was supported by grants from the Israel Science Foundation (ISF) (grant 109/16), the ISF within the ISF-University Grant Council (UGC) joint research program framework (grant 2253/15), the ISF-Natural Science Foundation of China (NSFC) joint research program (grant 2193/16) and the German-Israeli Foundation for Scientific Research and Development (grant I-1290-416.13/2015). B.I. was supported by the National Institutes of Health (grant GM-039334).

Glossary

Anomeric center
In the cyclized pyranose or furanose forms, monosaccharides feature an asymmetric center created by the formation of an intramolecular acetal (or ketal) between a sugar hydroxyl group and the aldehyde (or ketone) group. Two stereoisomers called anomers are formed because the anomeric hydroxyl group can assume two possible configurations. When the configurations are the same at the anomeric carbon and the stereogenic center furthest from the anomeric carbon, the monosaccharide is

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