Dissecting ramoplanin: mechanistic analysis of synthetic ramoplanin analogues as a guide to the design of improved antibiotics.

Ramoplanin is a potent cyclic lipoglycodepsipeptide antibiotic that disrupts bacterial cell wall synthesis by binding to the peptidoglycan intermediate Lipid II and blocking its polymerization to form the carbohydrate chains of peptidoglycan. Although ramoplanin is a promising compound for certain indications, it has limitations that impede IV administration for systemic use. However, it may be possible to overcome these limitations with analogues. In this manuscript, we dissect the effects of structural changes to ramoplanin. The studies described here combine total synthesis with enzyme kinetics, NMR analysis, and MIC measurements to shed light on the roles of key structural features in this antibiotic in Lipid II binding, transglycosylation inhibition, and biological activity. The results should serve as a foundation for the design of synthetically accessible analogues with improved biological properties.

Identification of selective inhibitors for the glycosyltransferase MurG via high-throughput screening.

Nucleotide-glycosyltransferases (NDP-Gtfs) play key roles in a wide range of biological processes. It is difficult to probe the roles of individual glycosyltransferases or their products because, with few exceptions, selective glycosyltransferase inhibitors do not exist. Here, we investigate a high-throughput approach to identify glycosyltransferase inhibitors based on a fluorescent donor displacement assay. We have applied the screen to E. coli MurG, an enzyme that is both a potential antibiotic target and a paradigm for a large family of glycosyltransferases. We show that the compounds identified in the donor-displacement screen of MurG are selective for MurG over other enzymes that use similar or identical substrates, including structurally related enzymes. The donor displacement assay described here should be adaptable to many other NDP-Gtfs and represents a new strategy to identify selective NDP-Gtf inhibitors.

Ramoplanin inhibits bacterial transglycosylases by binding as a dimer to lipid II.

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.

Identification of active-site inhibitors of MurG using a generalizable, high-throughput glycosyltransferase screen.

MurG is a glycosyltransferase involved in the biosynthesis of bacterial peptidoglycan. It is a potentially important antibiotic target, but no inhibitors of the enzyme have been reported. In general, inhibitors of glycosyltransferases have been difficult to design. Furthermore, no glycosyltransferase inhibitors have been identified through high-throughput screening, perhaps because appropriate screens for glycosyltransferase inhibition have not been developed. In this manuscript, we describe the development of a high-throughput screen for MurG that was used to screen a 50 000 compound library for inhibitors. The screen, which can be generalized to other glycosyltransferases, led to the identification of a family of active-site directed MurG inhibitors. The family of inhibitors contains a five-membered heterocyclic core that appears to function as a diphosphate mimic with respect to the presentation of substituents. We discuss the implications of this result and the utility of the screen for identifying inhibitors of other glycosyltransferases.

Rethinking ramoplanin: the role of substrate binding in inhibition of peptidoglycan biosynthesis.

Ramoplanin is a cyclicdepsipeptide antibiotic that inhibits peptidoglycan biosynthesis. It was proposed in 1990 to block the MurG step of peptidoglycan synthesis by binding to the substrate of MurG, Lipid I. The proposed mechanism of MurG inhibition has become widely accepted even though it was never directly tested. In this paper, we disprove the accepted mechanism for how ramoplanin functions, and we present an alternative mechanism. This work has implications for the design of ramoplanin derivatives and may influence how other proposed substrate binding antibiotics are studied.