Question

All of the following statements about Penicillin binding proteins (PBPs) are true, except:

• A. PBPs are localized on the outer face of the cell wall
• B. PBPs are essential for cell wall synthesis
• C. PBPs act as carboxypeptidases and transpeptidases
• D. Alteration in PBPs is the primary basis of resistance in MRSA

Hint:

Ask where these enzymes physically sit relative to the membrane and the wall.

Answer 1

The Correct Option is A

Step 1: The question asks for the FALSE statement about penicillin binding proteins (PBPs).
Step 2: PBPs are enzymes that sit on the cytoplasmic membrane, not on the outer face of the cell wall. They face the periplasmic side and build the peptidoglycan layer from inside out. So option A is the wrong (false) statement, and that is the answer.
Step 3: Option B is true. PBPs catalyse the final cross-linking steps of peptidoglycan, so they are essential for cell wall synthesis. Blocking them stops the wall from forming and the cell lyses.
Step 4: Option C is true. PBPs work as transpeptidases (they cross-link the peptide chains) and as carboxypeptidases (they trim terminal D-alanine residues). Beta-lactams mimic the D-Ala-D-Ala terminus and bind the active site.
Step 5: Option D is true. MRSA carries the mecA gene that codes for PBP2a, a low-affinity PBP. Beta-lactams cannot bind it, so altered PBP is the main resistance mechanism in MRSA.
Conclusion: The false statement is that PBPs are localized on the outer face of the cell wall. Answer: option A.