Characterization of a transglycosylase domain of Streptococcus pneumoniae PBP1b

Optimizations of lipid II synthesis: an essential glycolipid precursor in bacterial cell wall synthesis and a validated antibiotic target

Lipid II is an essential glycolipid found in bacteria. Accessing this valuable cell wall precursor is important both for studying cell wall synthesis and for studying/identifying novel antimicrobial compounds. Herein, we describe optimizations to the modular chemical synthesis of lipid II and unnatural analogues. In particular, the glycosylation step, a critical step in the formation of the central disaccharide unit (GlcNAc-MurNAc), was optimized. This was achieved by employing the use of glycosyl donors with diverse leaving groups. The key advantage of this approach lies in its adaptability, allowing for the generation of a wide array of analogues through the incorporation of alternative building blocks at different stages of synthesis.

Observation of the time-course for peptidoglycan lipid intermediate II polymerization by Staphylococcus aureus monofunctional transglycosylase

The polymerization of lipid intermediate II by the transglycosylase activity of penicillin-binding proteins (PBPs) represents an important target for antibacterial action, but limited methods are available for quantitative assay of this reaction, or screening potential inhibitors. A new labelling method for lipid II polymerization products using Sanger's reagent (fluoro-2,4-dinitrobenzene), followed by gel permeation HPLC analysis, has permitted the observation of intermediate polymerization products for Staphylococcus aureus monofunctional transglycosylase MGT. Peak formation is inhibited by 6 µM ramoplanin or enduracidin. Characterization by mass spectrometry indicates the formation of tetrasaccharide and octasaccharide intermediates, but not a hexasaccharide intermediate, suggesting a dimerization of a lipid-linked tetrasaccharide. Numerical modelling of the time-course data supports a kinetic model involving addition to lipid-linked tetrasaccharide of either lipid II or lipid-linked tetrasaccharide. Observation of free octasaccharide suggests that hydrolysis of the undecaprenyl diphosphate lipid carrier occurs at this stage in peptidoglycan transglycosylation.

Prospects for novel inhibitors of peptidoglycan transglycosylases

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We examine key aspects of transglycosylase inhibitor design.

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Low to high throughput assays suitable for transglycosylase drug discovery.

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Existing chemical start points for transglycosylase active site targeting.

The lack of novel antimicrobial drugs under development coupled with the increasing occurrence of resistance to existing antibiotics by community and hospital acquired infections is of grave concern. The targeting of biosynthesis of the peptidoglycan component of the bacterial cell wall has proven to be clinically valuable but relatively little therapeutic development has been directed towards the transglycosylase step of this process. Advances towards the isolation of new antimicrobials that target transglycosylase activity will rely on the development of the enzymological tools required to identify and characterise novel inhibitors of these enzymes. Therefore, in this article, we review the assay methods developed for transglycosylases and review recent novel chemical inhibitors discovered in relation to both the lipidic substrates and natural product inhibitors of the transglycosylase step.

Peptidoglycan biosynthesis machinery: a rich source of drug targets

The range of antibiotic therapy for the control of bacterial infections is becoming increasingly limited because of the rapid rise in multidrug resistance in clinical bacterial isolates. A few diseases, such as tuberculosis, which were once thought to be under control, have re-emerged as serious health threats. These problems have resulted in intensified research to look for new inhibitors for bacterial pathogens. Of late, the peptidoglycan (PG) layer, the most important component of the bacterial cell wall has been the subject of drug targeting because, first, it is essential for the survivability of eubacteria and secondly, it is absent in humans. The last decade has seen tremendous inputs in deciphering the 3-D structures of the PG biosynthetic enzymes. Many inhibitors against these enzymes have been developed using virtual and high throughput screening techniques. This review discusses the mechanistic and structural properties of the PG biosynthetic enzymes and inhibitors developed in the last decade.

High-throughput identification of antibacterials against methicillin-resistant Staphylococcus aureus (MRSA) and the transglycosylase

To identify new transglycosylase inhibitors with potent anti-methicillin-resistant Staphylococcus aureus (MRSA) activities, a high-throughput screening against Staphylococcus aureus was conducted to look for antibacterial cores in our 2M compound library that consists of natural products, proprietary collection, and synthetic molecules. About 3600 hits were identified from the primary screening and the subsequent confirmation resulted in a total of 252 compounds in 84 clusters which showed anti-MRSA activities with MIC values as low as 0.1 μg/ml. Subsequent screening targeting bacterial transglycosylase identified a salicylanilide-based core that inhibited the lipid II polymerization and the moenomycin-binding activities of transglycosylase. Among the collected analogues, potent inhibitors with the IC(50) values below 10 μM against transglycosylase were identified. The non-carbonhydrate scaffold reported in this study suggests a new direction for development of bacterial transglycosylase inhibitors.

Synthesis and evaluation of a new fluorescent transglycosylase substrate: lipid II-based molecule possessing a dansyl-C20 polyprenyl moiety

The preparation of a novel fluorescent lipid II-based substrate for transglycosylases (TGases) is described. This substrate has characteristic structural features including a shorter lipid chain, a fluorophore tag at the end of the lipid chain rather than on the peptide chain, and no labeling with a radioactive atom. This fluorescent substrate is readily utilized in TGase activity assays to characterize TGases and also to evaluate the activities of TGase inhibitors.

Isolated peptidoglycan glycosyltransferases from different organisms produce different glycan chain lengths

Peptidoglycan is an essential component of bacterial cell wall. The glycan strands of peptidoglycan are synthesized by enzymes called peptidoglycan glycosyltransferases (PGTs). Using a high-resolution SDS-PAGE assay, we compared the glycan strand lengths of four different PGTs from three different organisms (Escherichia coli, Enterococcus faecalis, and Staphylococcus aureus). We report that each enzyme makes a polymer having an intrinsic characteristic length that is independent of the enzyme:substrate ratio. The glycan strand lengths vary considerably, depending on the enzyme. These results indicate that each enzyme must have some mechanism, as yet unknown, for controlling product length. The observation that different PGTs produce different length glycan chains may have implications for their cellular roles and for the three-dimensional structure of bacterial peptidoglycan.