What is the key enzyme that sulfa drugs inhibit in bacterial metabolism?

Sulfa drugs inhibit the enzyme dihydropteroate synthase, which plays a crucial role in bacterial folate synthesis. This enzyme is involved in the condensation of para-aminobenzoic acid (PABA) with pteridine to form dihydropteroate, a key intermediate in the production of folic acid, which is essential for bacterial growth and replication.

By inhibiting dihydropteroate synthase, sulfa drugs effectively disrupt the synthesis of folate, leading to a depletion of folic acid levels in bacteria. Since folate is necessary for the synthesis of nucleic acids and other critical biomolecules, the inhibition of this pathway ultimately slows down or halts bacterial growth.

In contrast, other options listed serve different functions within bacterial metabolism. Dihydrofolate reductase is involved in converting dihydrofolate to tetrahydrofolate, but it is not the target of sulfa drugs. DNA gyrase and RNA polymerase are involved in DNA replication and transcription respectively, but they are not directly affected by sulfa drugs. Thus, the focus on dihydropteroate synthase highlights the specific mechanism through which sulfa drugs exert their antibacterial effects.