Peptidoglycan assembly — identifying the next-to-last step of biosynthesis During bacterial cell-wall synthesis, which process occurs just before the final transpeptidation (cross-linking) step?

Introduction / Context:
Bacterial peptidoglycan is built by a well-ordered sequence: cytosolic precursor synthesis, membrane linkage to a lipid carrier, flipping to the periplasmic side, polymerization of glycan chains, and cross-linking of peptide stems. Knowing the order clarifies how antibiotics (for example, glycopeptides, beta-lactams) disrupt wall assembly.

Given Data / Assumptions:

• Lipid II (MurNAc-peptide–GlcNAc) is flipped across the membrane on bactoprenol.
• Transglycosylases catalyze glycan-strand elongation.
• Transpeptidases (penicillin-binding proteins) make peptide cross-links as the final step.

Concept / Approach:
The final event is transpeptidation (peptide cross-linking). The step immediately prior is transglycosylation, in which the disaccharide-peptide unit is transferred from lipid II to the non-reducing end of the growing glycan chain, releasing bactoprenol for recycling. This sequencing explains why vancomycin (binding D-Ala-D-Ala) blocks transglycosylation and indirectly prevents subsequent cross-linking, whereas beta-lactams directly inhibit transpeptidation.

Step-by-Step Solution:
Arrange steps: precursor synthesis → lipid carrier linkage → flipping → transglycosylation → transpeptidation.Identify “next-to-last” as the step before cross-linking.Recognize transglycosylation as that penultimate step.Select the option describing addition of the disaccharide-peptide to the growing chain.

Verification / Alternative check:
Mechanistic diagrams of cell-wall synthesis show PBPs with bifunctional domains; the glycosyltransferase activity precedes the transpeptidase reaction in strand maturation.

Why Other Options Are Wrong:

• A: Cytosolic precursor synthesis occurs much earlier.
• B: “Removal” wording is imprecise; lipid II donates its unit during transglycosylation.
• D: This is the last step, not the next-to-last.
• E: Autolysin activity facilitates insertion but is not the ordered penultimate biosynthetic step.

Common Pitfalls:
Confusing transglycosylation (glycan elongation) with transpeptidation (peptide cross-linking). Remember: sugars first, peptides second.

Final Answer:
Transglycosylation: adding the disaccharide-peptide unit to the growing peptidoglycan chain