Question

Sulphonamides serve to inhibit the

• A. Growth of cancer cells, since they inhibit mammalian dihydrofolate formation
• B. Folate biosynthesis in bacteria
• C. Folate biosynthesis in animals
• D. Growth of cancer cells since they inhibit thymidylate synthase

Hint:

Sulphonamide Selectivity: Competitively inhibits bacterial Dihydropteroate Synthase (PABA mimic). Animals lack this enzyme and absorb dietary folate.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Sulfonamides exhibit selective antibacterial toxicity as structural analogues of para-aminobenzoic acid (PABA), competitively inhibiting bacterial dihydropteroate synthase.
Key Formula or Approach:
\[ \text{PABA} + \text{Pteridine} \xrightarrow[\text{Inhibited by Sulphonamides}]{\text{Dihydropteroate Synthase}} \text{Dihydropteroic Acid} \longrightarrow \text{Tetrahydrofolate (THF)} \]

Step 2: Detailed Explanation:
Mechanism of action of Sulphonamides:
- Bacteria must synthesize folic acid de novo from para-aminobenzoic acid (PABA), pteridine, and glutamate.
- Sulphonamides are structural analogues of PABA that competitively inhibit bacterial Dihydropteroate Synthase, completely blocking de novo folate biosynthesis in bacteria.
- Because animal/mammalian cells cannot synthesize folate de novo and instead obtain preformed dietary folate via specific membrane transporters (lacking dihydropteroate synthase), sulphonamides selectively kill bacteria without harming host mammalian cells.

Step 3: Final Answer:
Hence, sulphonamides inhibit Folate biosynthesis in bacteria, corresponding to option (B).