MurG is an essential glycosyltransferase that forms the glycosidic linkage between N-acetyl muramyl pentapeptide and N-acetyl glucosamine in the biosynthesis of the bacterial cell wall. This enzyme is a member of a major superfamily of NDP-glycosyltransferases for which no x-ray structures containing intact substrates have been reported. Here we present the 2.5-Å crystal structure of Escherichia coli MurG in complex with its donor substrate, UDPGlcNAc. Combined with genomic analysis of other superfamily members and site-specific mutagenesis of E. coli MurG, this structure sheds light on the molecular basis for both donor and acceptor selectivity for the superfamily. This structural analysis suggests that it will be possible to evolve new glycosyltransferases from prototypical superfamily members by varying two key loops while maintaining the overall architecture of the family and preserving key residues.

Bacterial transglycosylases are enzymes that couple the disaccharide subunits of peptidoglycan to form long carbohydrate chains. These enzymes are the target of the pentasaccharide antibiotic moenomycin as well as the proposed target of certain glycopeptides that overcome vancomycin resistance. Because bacterial transglycosylases are difficult enzymes to study, it has not previously been possible to evaluate how moenomycin inhibits them or to determine whether glycopeptide analogues directly target them. We have identified transglycosylase assay conditions that enable kinetic analysis of inhibitors and have examined the inhibition of Escherichia coli penicillin-binding protein 1b (PBP1b) by moenomycin as well as by various glycopeptides. We report that chlorobiphenyl vancomycin analogues that are incapable of binding substrates nevertheless inhibit E. coli PBP1b, which shows that these compounds interact directly with the enzyme. These findings support the hypothesis that chlorobiphenyl vancomycin derivatives overcome vanA resistance by targeting bacterial transglycosylases. We have also found that moenomycin is not competitive with respect to the lipid II substrate of PBP1b, as has long been believed. With the development of suitable methods to evaluate bacterial transglycosylases, it is now possible to probe the mechanism of action of some potentially very important antibiotics.

From a functional standpoint, glycosyltransferases (GTases) comprise one the most diverse group of enzymes in existence. Every category of biopolymer (oligosaccharides, proteins, nucleic acids, and lipids) plus numerous natural products are modified by GTases, with remarkably varied effects. Given the structural and functional diversity of the products of glycosyl transfer combined with the often distant evolutionary relationships between glycosyltransferases, it is not surprising that sequence homologies between glycosyltransferases are low. What is surprising is that the majority of glycosyltransferases belong to only two structural superfamilies, implying that nature has come up with only a few solutions to the ubiquitous problem of how to catalyze glycosyl transfer. The conservation of GTase structure suggests that it will be simpler to manipulate glycosyltransferases for various applications than previously envisioned. A new age in glycoconjugate chemistry is beginning.

Ramoplanin is a lipglycodepsipeptide antibiotic that inhibits peptidoglycan biosynthesis. Its mechanism of action has been the subject of debate. It was originally proposed to inhibit the MurG step of peptidoglycan synthesis by binding Lipid I. In this paper, we report that ramoplanin inhibits bacterial transglycosylases by binding to Lipid II, the substrate for these enzymes. The inhibition curves reveal that the inhibitory species has a stoichiometry of 2:1 ramoplanin:Lipid II. A Job titration confirms that ramoplanin binds as a dimer to Lipid II. The apparent dissociation constant is in the nanomolar range, which is unusually low given the nature of the interacting species. We show that Lipid II binding is coupled to the formation of a higher order species, which may explain the tight binding. We also present a testable model for the binding-competent dimeric conformation of ramoplanin.

MurG, the last enzyme involved in the intracellular phase of peptidoglycan synthesis, is a membrane-associated glycosyltransferase that couples N-acetyl glucosamine to the C4 hydroxyl of a lipid-linked N-acetyl muramic acid derivative (lipid I) to form the beta-linked disaccharide (lipid II) that is the minimal subunit of peptidoglycan. Lipid I is anchored to the bacterial membrane by a 55 carbon undecaprenyl chain. Because this long lipid chain impedes kinetic analysis of MurG, we have been investigating alternative substrates containing shortened lipid chains. We now describe the intrinsic lipid preferences of MurG and show that the optimal substrate for MurG in the absence of membranes is not the natural substrate. Thus, while the undecaprenyl carrier lipid may be critical for certain steps in the biosynthetic pathway to peptidoglycan, it is not required-in fact, is not preferred-by MurG. Using synthetic substrate analogues and products containing different length lipid chains, as well as a synthetic dead-end acceptor analogue, we have also shown that MurG follows a compulsory ordered Bi Bi mechanism in which the donor sugar binds first. This information should facilitate obtaining crystals of MurG with substrates bound, an important goal because MurG belongs to a major superfamily of NDP-glycosyltransferases for which no structures containing intact substrates have yet been solved.