Peptidoglycan assembly: what is the final step in bacterial cell wall synthesis that beta-lactam antibiotics are designed to block?

Introduction / Context:
Bacterial cell wall synthesis proceeds through cytoplasmic precursor formation, membrane-linked assembly, translocation, polymerization, and finally cross-linking. Understanding the terminal cross-linking step clarifies how beta-lactam antibiotics inhibit growth.

Given Data / Assumptions:

• Peptidoglycan monomers consist of NAM-NAG disaccharides with a peptide stem on NAM.
• Monomers are assembled on bactoprenol (a lipid carrier), flipped across the membrane, and polymerized into glycan chains.
• The final step is transpeptidation (cross-linking) between peptide stems of adjacent strands.

Concept / Approach:
Transpeptidation forms peptide cross-links catalyzed by penicillin-binding proteins (PBPs, transpeptidases). Beta-lactam antibiotics mimic the D-Ala-D-Ala substrate and irreversibly acylate PBPs, halting cross-link formation and weakening the cell wall.

Step-by-Step Solution:

Verification / Alternative check:
Laboratory effects of beta-lactams include cell lysis due to osmotic fragility, consistent with disrupted cross-linking.

Why Other Options Are Wrong:
A: Peptide attachment to NAM occurs earlier. C: Attachment to a membrane lipid (bactoprenol) is a precursor step. D: Describes drug action (binding to PBP) rather than a biosynthetic step and is not the physiological step of peptidoglycan assembly.

Common Pitfalls:
Confusing transglycosylation (sugar chain elongation) with transpeptidation (peptide cross-linking). Both are catalyzed by PBPs but are distinct steps.

Final Answer:
attaching two amino acids to form a cross-link