Which mechanism below best describes the method by which sulfa drugs inhibit the folic acid production pathway?

• A. Sulfa drugs block the conversion of tetrahydrofolic acid into folic acid.
• B. Sulfa drugs bind to PABA, preventing it from being converted to tetrahydrofolic acid.
• C. Sulfa drugs denature the enzyme that converts dihydrofolic acid to tetrahydrofolic acid.
• D. Sulfa drugs bind to the same enzyme that converts PABA to dihydrofolic acid.

Answer: (D)

Verified step by step guidance

1. Understand the folic acid synthesis pathway in bacteria: it involves the conversion of para-aminobenzoic acid (PABA) to dihydrofolic acid by the enzyme dihydropteroate synthase, which is then further converted to tetrahydrofolic acid.
2. Recognize that sulfa drugs are structural analogs of PABA, meaning they resemble PABA closely enough to interact with the same enzyme.
3. Identify that sulfa drugs competitively inhibit the enzyme dihydropteroate synthase by binding to its active site, preventing the normal substrate PABA from binding.
4. Note that this competitive inhibition blocks the production of dihydrofolic acid, thereby halting the synthesis of tetrahydrofolic acid, which is essential for bacterial DNA synthesis and growth.
5. Conclude that the mechanism of sulfa drugs is best described as binding to the enzyme that converts PABA to dihydrofolic acid, effectively blocking the folic acid production pathway.