40. Which of the following is the mechanism of action of 5-fluorouracil? (A) competitive inhibitor of dihydrofolate reductase, (B) suicide substrate for thymidylate synthase, (C) competitive inhibitor of ribonucleotide reductase, (D) non-competitive inhibitor of thioredoxin reductase, (E) none are true.

39. The coenzyme for two single carbon additions in purine biosynthesis is: (A) THF, (B) biotin, (C) SAM, (D) TPP, (E) none are true.

38. Glutamine synthetase (GS) belongs to what class of enzymes? (A) isomerases, (B) oxidoreductase, (C) ligase, (D) lyase, (E) transferase.