Mechanism of action of cycloserine in bacterial cell wall synthesis Cycloserine inhibits which enzyme targets involved in peptidoglycan biosynthesis?

Introduction / Context:Cycloserine is a second-line antitubercular antibiotic. Its mechanism highlights early cytoplasmic steps of peptidoglycan synthesis rather than the later transpeptidation step targeted by beta-lactams.

Given Data / Assumptions:

• Peptidoglycan synthesis requires formation of D-Ala–D-Ala dipeptide.
• Two key enzymes: alanine racemase (converts L-Ala to D-Ala) and D-Ala–D-Ala ligase (synthetase).
• Cycloserine is known to inhibit both enzymes.

Concept / Approach:By inhibiting alanine racemase and D-Ala–D-Ala synthetase, cycloserine depletes the D-Ala–D-Ala dipeptide necessary for peptidoglycan cross-link precursor formation (UDP-MurNAc–pentapeptide). This undermines cell wall assembly and leads to bactericidal activity.

Step-by-Step Solution:Map the pathway: L-Ala --(alanine racemase)--> D-Ala.Then: D-Ala + D-Ala --(D-Ala–D-Ala synthetase)--> D-Ala–D-Ala.Cycloserine inhibits both enzymes, blocking dipeptide formation.Select “Both alanine racemase and D-alanyl–D-alanine synthetase.”

Verification / Alternative check:Mechanistic studies show competitive inhibition of the pyridoxal phosphate–dependent racemase and ATP-dependent ligase by cycloserine analogs.

Why Other Options Are Wrong:Single-enzyme answers are incomplete; amidase and PBPs are not cycloserine targets.

Common Pitfalls:Confusing cycloserine with beta-lactams, which act at later periplasmic transpeptidation steps.

Final Answer:Both alanine racemase and D-alanyl–D-alanine synthetase.