The step blocked by sulfonamides and PAS is which step?

The step blocked by sulfonamides and PAS is which step?

• A. The fourth step
• B. The second step
• C. The first step
• D. The third step

Answer: (C)

Sulfonamides and PAS block the initial step of bacterial folate synthesis. They act as PABA mimics and competitively inhibit dihydropteroate synthase, the enzyme that combines PABA with the pteridine precursor to form dihydropteroate. Because this is the first step in making dihydrofolate and ultimately tetrahydrofolate, blocking it prevents the downstream production of essential one-carbon units needed for thymidine (DNA) synthesis. This is why these drugs are effective: bacteria must synthesize folate de novo, whereas humans obtain folate from their diet. The later step, reduction of dihydrofolate to tetrahydrofolate, is blocked by another drug (trimethoprim), not by sulfonamides or PAS.

Sulfonamides and PAS block the initial step of bacterial folate synthesis. They act as PABA mimics and competitively inhibit dihydropteroate synthase, the enzyme that combines PABA with the pteridine precursor to form dihydropteroate. Because this is the first step in making dihydrofolate and ultimately tetrahydrofolate, blocking it prevents the downstream production of essential one-carbon units needed for thymidine (DNA) synthesis. This is why these drugs are effective: bacteria must synthesize folate de novo, whereas humans obtain folate from their diet. The later step, reduction of dihydrofolate to tetrahydrofolate, is blocked by another drug (trimethoprim), not by sulfonamides or PAS.