Formation of the glycan chains in the synthesis of bacterial peptidoglycan

The main structural features of bacterial peptidoglycan are linear glycan chains interlinked by short peptides. The glycan chains are composed of alternating units of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc), all linkages between sugars being beta,1-->4. On the outside of the cytoplasmic membrane, two types of activities are involved in the polymerization of the peptidoglycan monomer unit: glycosyltransferases that catalyze the formation of the linear glycan chains and transpeptidases that catalyze the formation of the peptide cross-bridges. Contrary to the transpeptidation step, for which there is an abundant literature that has been regularly reviewed, the transglycosylation step has been studied to a far lesser extent. The aim of the present review is to summarize and evaluate the molecular and cellullar data concerning the formation of the glycan chains in the synthesis of peptidoglycan. Early work concerned the use of various in vivo and in vitro systems for the study of the polymerization steps, the attachment of newly made material to preexisting peptidoglycan, and the mechanism of action of antibiotics. The synthesis of the glycan chains is catalyzed by the N-terminal glycosyltransferase module of class A high-molecular-mass penicillin-binding proteins and by nonpenicillin-binding monofunctional glycosyltransferases. The multiplicity of these activities in a given organism presumably reflects a variety of in vivo functions. The topological localization of the incorporation of nascent peptidoglycan into the cell wall has revealed that bacteria have at least two peptidoglycan-synthesizing systems: one for septation, the other one for elongation or cell wall thickening. Owing to its location on the outside of the cytoplasmic membrane and its specificity, the transglycosylation step is an interesting target for antibacterials. Glycopeptides and moenomycins are the best studied antibiotics known to interfere with this step. Their mode of action and structure-activity relationships have been extensively studied. Attempts to synthesize other specific transglycosylation inhibitors have recently been made.