Final answer:
Trimethoprim and methotrexate inhibit different steps in the folic acid synthesis pathway, preventing the production of coenzymes required for nucleic acid synthesis.
Step-by-step explanation:
Trimethoprim and methotrexate inhibit different steps in the folic acid synthesis pathway.
Methotrexate inhibits folic acid reductase, which is responsible for the conversion of folic acid to tetrahydrofolic acid. This inhibition prevents the production of coenzymes required for one-carbon transfer reactions in the biosynthesis of purines and pyrimidines.
On the other hand, trimethoprim is a structural analog of dihydrofolic acid and inhibits a later step in the metabolic pathway. It competitively inhibits the synthesis of tetrahydrofolic acid, blocking the production of coenzymes needed for nucleic acid synthesis.