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

Penicillin binding proteins are the molecular targets of beta-lactam drugs, and this item wants the one claim that does not hold. Think about where these proteins actually live. They are membrane-bound enzymes anchored in the cytoplasmic membrane, with their active sites turned toward the growing peptidoglycan, not parked on the outside surface of the finished cell wall. That single fact makes statement A incorrect, so A is what we pick. Now check the rest. Statement B is sound because PBPs run the last assembly steps of the murein sacculus; if they stop working, the wall cannot be sealed and the organism bursts under its own turgor. Statement C is also correct, since these proteins behave as transpeptidases that stitch adjacent glycan strands together and as carboxypeptidases that clip off the terminal D-alanine, and the beta-lactam ring imitates that D-Ala-D-Ala end to jam the enzyme. Statement D is true as well, because methicillin-resistant Staphylococcus aureus expresses PBP2a from the mecA gene, a variant with poor drug affinity that lets wall synthesis continue despite the antibiotic. Everything except the location claim is accurate. $A$ is the false option. \[\boxed{\text{PBPs are localized on the outer face of cell wall}}\]