Bacterial transglycosylase inhibitors Bohdan Ostash and Suzanne Walker The spread of bacterial resistance to known antibiotics has inspired interest in previously underexploited drug targets. The transglycosylation reaction remains a ‘black box’ in the generally well-studied process of bacterial peptidoglycan biosynthesis, which is a very attractive target for chemotherapeutic intervention. Here, we summarize recent progress in the study of bacterial transglycosylases and the compounds that inhibit them. The transglycosylation reaction is readily targeted by several different classes of natural products, implying that it should be possible to develop drugs that inhibit this process once efficient high-throughput screens and appropriate compound libraries have been developed. Addresses Department of Microbiology and Molecular Genetics, Harvard Medical School, 200 Longwood Avenue, Boston, MA 02115, USA Corresponding author: Walker, Suzanne (suzanne_walker@hms.harvard.edu) Current Opinion in Chemical Biology 2005, 9:459–466 This review comes from a themed issue on Mechanisms Edited by Rowena G Mattews and Christopher T Walsh Available online 22nd August 2005 1367-5931/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. DOI 10.1016/j.cbpa.2005.08.014 Introduction Antibiotic-resistantbacterial infections pose a serious threat to human health. Resistance has emerged to every class of antibiotics in clinical use, and many common pathogens are resistant to several different antibiotics [1]. The need for new drugs to treat antibiotic-resistant infections has led to a resurgence of interest in bacterial metabolism and patho- genesis, and it is hoped that a detailed understanding of important bacterial enzymes and processes will lead even- tually to the discovery of new antibiotics. Most broad-spectrum antibiotics function by inhibiting one of a small number of highly conserved metabolic processes in bacteria [2]. The biosynthesis of peptidogly- can, the major component of the bacterial cell wall, is one of those processes. Peptidoglycan is a polymeric mesh that surrounds bacterial cells and functions as a protective exoskeleton. It comprises linear strands of repeating GlcNAc-b-(1,4)-MurNAc (NAG-NAM) disaccharide units that are linked together via crossbridges between peptide moieties attached to the MurNAc sugars (Figure 1). One of its major functions is to stabilize bacterial membranes against high internal osmotic pres- sures, and anything that disrupts the integrity of the peptidoglycan layers therefore threatens the viability of bacterial cells [3]. A large number of natural-product antibiotics, including penicillin, cephalosporin, fosfomy- cin, cycloserine and vancomycin interfere with peptido- glycan biosynthesis, attesting to the importance of this pathway as a target for therapeutic intervention [2]. The emergence of antibiotic resistance has prompted efforts to obtain detailed structural and mechanistic infor- mation on every enzyme in the biosynthetic pathway to peptidoglycan and to discover new inhibitors for each enzyme. Significant progress has been made on both fronts for most of the intracellular enzymes and for several types of extracellular penicillin binding proteins ([4– 6,7  ,8]; [9] and references therein). However, for one group of enzymes, the bacterial transglycosylases (TGs) that catalyze the polymerization of the carbohydrate chains of peptidoglycan, progress has been exceptionally slow. Here, we review recent work on these enzymes and discuss the compounds that are known to inhibit them. We consider the validity of these enzymes as antibiotic targets; and address what might be done to accelerate the discovery of useful inhibitors. Bacterial transglycosylases The carbohydrate chains of peptidoglycan consist of repeating NAG-NAM disaccharide units. The NAG- NAM subunit is synthesized inside the bacterial cell as an activated undecaprenyl diphospho-sugar, which is com- monly known as Lipid II because it is the second mem- brane-anchored intermediate in the biosynthetic pathway starting from UDP-GlcNAc (Figure 1). Once made, Lipid II is transported to the external surface of the bacterial membrane where it is proposed to react with the reducing end of the growing peptidoglycan chain in a reaction catalyzed by TGs. The peptide chains attached to the MurNAc sugars of the growing glycan chains are then crosslinked by transpeptidases (TPs). All organisms con- tain multiple TGs, which come in two known forms: as N- terminal domains of bifunctional proteins that also contain TP domains; and as separate proteins. The former are called Class A PBPs and the latter are known as mono- functional glycosyltransferases (MGTs) [10]. Recent work suggests that there are other types of TGs as well, but their genes have not yet been identified [11  ,12  ]. The first biochemical work on bacterial TGs was carried out in the 1960s [13], yet more than 40 years later there is www.sciencedirect.com Current Opinion in Chemical Biology 2005, 9:459–466