Practice question

Peptidoglycan assembly begins in cytoplasm with formation of UDP-MurNAc-pentapeptide containing terminal D-alanyl-D-alanine, followed by linkage to lipid carrier undecaprenyl phosphate, also termed bactoprenol, which flips precursor across inner membrane to periplasmic side. Once externalized, class A transglycosylases polymerize glycan chains extending the sacculus and DD-transpeptidases, members of penicillin-binding protein family such as PBP1a, PBP1b, PBP2 and PBP3, catalyze crosslink between D-alanine of one stem and diamino acid of neighboring stem, releasing terminal D-alanine and forming robust mesh. Penicillin and other beta-lactams are structural analogs of D-Ala-D-Ala dipeptide; they enter active site and acylate catalytic serine irreversibly, blocking transpeptidation and leaving newly inserted material poorly crosslinked. Growing cells with weakened wall cannot contain internal turgor and undergo lysis aided by dysregulated autolytic amidases and lytic transglycosylases. Streptomycin binds 30S ribosomal subunit causing misreading, ciprofloxacin inhibits DNA gyrase, rifampin inhibits beta subunit of bacterial RNA polymerase, distinguishing wall-active agents from other classes. This mechanistic insight is relevant for competitive examinations such as CSIR-NET and GATE, where understanding molecular detail rather than memorization enables accurate interpretation of experimental data and pathway interconnections.