Paper II — 2015 September (Supplementary) (2010 Scheme) — Q9

Paper II

2015 September (Supplementary) (2010 Scheme) · 40 marks · 120 min

Question

Mechanism of action of co-trimoxazole.

Q91 marksShort Answers★✓

Answer

Co-trimoxazole is a fixed-dose combination of trimethoprim and sulfamethoxazole, which produces sequential blockade of bacterial folate synthesis: sulfamethoxazole competitively inhibits dihydropteroate synthase (blocking conversion of PABA to dihydrofolic acid), while trimethoprim inhibits dihydrofolate reductase (blocking conversion of dihydrofolic acid to tetrahydrofolic acid, needed for nucleic acid synthesis). This sequential blockade at two steps of the same pathway produces a synergistic bactericidal effect greater than either drug alone.

Related Concepts

1. 1Sulfonamides and Cotrimoxazole

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