Beta-lactam antibiotics exert their effect by inhibiting which bacterial enzyme family?

• A. 70S ribosome
• B. DNA gyrase
• C. Transpeptidases (PBPs)
• D. RNA polymerase

Answer: (C)

Beta-lactam antibiotics work by blocking the enzymes that build the bacterial cell wall. They target transpeptidases, the penicillin-binding proteins, which normally catalyze the final cross-linking step in peptidoglycan synthesis. The beta-lactam ring mimics the normal substrate (the D-Ala-D-Ala portion) and binds irreversibly to the active site serine of these enzymes, preventing cross-link formation. Without proper cross-links, the cell wall weakens and the bacterium cannot maintain its integrity, leading to cell lysis and death.

Other targets listed are not how beta-lactams work: the 70S ribosome is involved in protein synthesis and is targeted by other classes of antibiotics; DNA gyrase controls DNA topology and is targeted by fluoroquinolones; RNA polymerase is involved in transcription and is targeted by rifamycins.