Which antibiotic binds to peptidoglycan precursors, inhibiting transpeptidation?
Glycopeptide antibiotic activity depends on extracellular substrate capture rather than direct inhibition of enzyme active site, an unusual mechanism among cell wall agents. During late-stage peptidoglycan assembly, the membrane-anchored disaccharide-pentapeptide Lipid II carrying a terminal D-Ala-D-Ala sequence at the stem terminus is the natural recognition motif for penicillin-binding proteins that perform transpeptidation cross-linking. Vancomycin forms a sophisticated network of five hydrogen bonds with carbonyl oxygens and amide nitrogens of this D-Ala-D-Ala dipeptide, creating a large cup-shaped hydrophobic pocket complex that physically shields the scissile bond from enzymatic approach. Consequently both transglycosylation reaction that elongates glycan strands and transpeptidation reaction that cross-links stem peptides are blocked, as glycosyltransferases and transpeptidases cannot access their natural Lipid II substrate. The net effect is accumulation of uncross-linked precursors, futile cycling, weakening of nascent septal wall and activation of endogenous autolysins such as LytA leading to osmotic lysis. Rifampin inhibits RNA polymerase, tetracycline and gentamicin block bacterial ribosomes, so precursor sequestration uniquely describes vancomycin mechanism distinguishing it from beta-lactams that covalently modify PBPs.
Ref: NCBI Bookshelf, Molecular Biology of the Cell, Section: Vancomycin and Lipid II D-Ala-D-Ala Binding.