Glycopeptide mechanism — vancomycin target: Vancomycin inhibits peptidoglycan synthesis by binding to which component in the bacterial cell wall precursor?

Introduction / Context:
Classifying antibiotics by target clarifies spectrum and resistance patterns. Glycopeptides like vancomycin act at a unique step in cell-wall assembly distinct from β-lactams.

Given Data / Assumptions:

• Vancomycin is bactericidal against most Gram-positive organisms.
• Mechanism involves blocking cell-wall polymerization.
• We must identify the molecular binding target.

Concept / Approach:
Vancomycin binds the D-Ala–D-Ala dipeptide on lipid II peptidoglycan precursors, sterically hindering transglycosylation and transpeptidation. This substrate binding contrasts with β-lactams, which inhibit PBPs directly. Resistance (e.g., vanA/vanB) replaces the terminal D-Ala with D-Lac, lowering binding affinity.

Step-by-Step Solution:
Recall that vancomycin recognizes the peptide termini, not the enzyme.Exclude enzyme targets (alanine racemase, D-Ala–D-Ala ligase, PBPs) which are inhibited by other antibiotic classes.Choose the substrate target ‘‘D-alanyl – D-alanine termini’’.

Verification / Alternative check:
Phenotypic resistance with D-Ala–D-Lac substitution confirms the substrate-binding model.

Why Other Options Are Wrong:
They describe enzymes or sites targeted by other drug classes (e.g., cycloserine inhibits alanine racemase and D-Ala–D-Ala ligase; β-lactams bind PBPs).

Common Pitfalls:
Conflating glycopeptide substrate binding with β-lactam enzyme inhibition.

Final Answer:
D-alanyl – D-alanine termini of peptidoglycan precursors.