Question

Where is thymidylate synthase's active site when it binds with Fluorouracil?

Answer 1

Thymidylate synthase (TS) is the enzyme which catalyses formation of dTMP (2′-deoxythymidine-5′-monophosphate) from dUMP (2′-deoxyuridine-5′-monophosphate). The CH3  (methyl) donor is CH2H4 F (5,10-methylene-tetrahydrofolate). 5-fluorouracil (5FU) is an inhibitor of the TS, which is a target in chemotherapy of colon cancer.

TS acts in 2 stages:

1. dUMP binding to receptor site by which a configurational change is induced opening an adjacent binding site for CH2H4F. 1 carbon group of folate transfers to uridine ring, which yields dihydrofolate and dTMP. dTMP is subsequently phosphorylated by a kinase to dTDP and dTTP, one of the bases for DNA synthesis.
2. The 5FU metabolite, known as FdUMP (fluorodeoxyuridine monophosphate), is a potential inhibiter of TS. FdUMP binds at the same site and with the same affinity as dUMP, but the fluorine atom at the 5′ cannot be displaced, unlike hydrogen. A ternary complex is formed subsequently by covalent bonding of TS and reduced folate, where at 6′ position of FdUMP a cysteine thiol of TS is attached with 1-C group of folate adjacent to F at the 5′ position.