Which enzyme is involved in the cross-linking of peptidoglycan?
After glycan polymerization by transglycosylases elongating alternating N-acetylglucosamine and N-acetylmuramic acid strands, final strength requires crosslinking of stem peptides attached to MurNAc. Stem typically comprises L-alanine, D-glutamate, meso-diaminopimelic acid or L-lysine, and terminal D-alanyl-D-alanine dipeptide. DD-transpeptidases, members of penicillin-binding protein family including PBP1a, PBP1b, PBP2a, and PBP3, perform nucleophilic attack where serine in active site forms bond with penultimate D-alanine, releasing terminal D-alanine, then transfers acyl-intermediate to amino group of adjacent diamino acid, creating 4-3 crosslink. Some bacteria also have LD-transpeptidases forming 3-3 linkages. Lysozyme hydrolyzes glycan backbone rather than crosslinks, ATP synthase generates ATP from proton motive force, DNA gyrase introduces negative supercoiling. Inhibition of transpeptidase by beta-lactams that mimic D-Ala-D-Ala substrate leaves nascent peptidoglycan poorly crosslinked, compromising mechanical strength so that turgor pressure causes lysis. This step is crucial for shape determination and antibiotic susceptibility, explaining why PBP mutations confer resistance. Recent cryo-EM structures capture PBP2 in active conformation with nascent peptidoglycan strand threaded through donor site, revealing how transpeptidase orients peptide for crosslinking, and how beta-lactams occupy same pocket mimicking acyl-D-Ala-D-Ala, explaining structure-activity relationships used to design carbapenems and cephalosporins that evade certain beta-lactamases.
Ref: Vollmer et al., FEMS Microbiol Rev 2008, Peptidoglycan Crosslinking; Lovering et al., Ann Rev Biochem 2012, PBPs.