Quinolones inhibit which bacterial enzyme?

• A. RNA Polymerase
• B. DNA Gyrase
• C. Topoisomerase I
• D. Helicase

Answer: (B)

Quinolones disrupt bacterial DNA replication by targeting type II topoisomerases, with DNA gyrase being the main enzyme affected in many bacteria. DNA gyrase normally introduces negative supercoils into DNA to relieve the torsional stress that happens as the DNA double helix unwinds during replication. When gyrase is inhibited, the DNA cannot be properly supercoiled, replication stalls, and drug-stabilized enzyme-DNA complexes lead to breaks that kill the cell. This is why quinolones are bactericidal. The other enzymes listed aren’t the targets of quinolones: RNA polymerase is inhibited by rifamycins, not quinolones; Topoisomerase I is a different enzyme and not the primary quinolone target; helicase unwinds DNA and isn’t inhibited by quinolones. (Note: some quinolones can affect topoisomerase IV as well, but DNA gyrase is the classic target described here.)