Thymidylate synthase: Structure, inhibition, and strained conformations during catalysis

Thymidylate synthase (TS) is a long-standing target for chemotherapeutic agents because of its central role in DNA synthesis, and it is also of interest because of its rich mechanistic features. The reaction catalyzed by TS is the methylation of dUMP, with the transferred methyl group provided by the cofactor methylenetetrahydrofolate (CH₂THF). Recently, several crystal structure determinations and mechanistic studies have led to a deeper understanding of the TS reaction mechanism, and address the role of conformational change in TS catalysis and inhibition. Included among these structures are complexes of TS bound to substrate dUMP; cofactor CH₂THF; the nucleotide analogs 5-fluoro-dUMP, 5-nitro-dUMP and dGMP; and the promising antifolates BW1843, ZD1694, and AG337. From these studies, a picture of TS emerges where ligand-induced conformational changes play key roles in catalysis by straining the thiol adduct that occurs during the reaction; by protecting the highly reactive reaction intermediates; and by providing a means to stabilize a high-energy conformer of the cofactor after initial binding of a low-energy conformer. The best inhibitors of TS also induce and stabilize a conformational change in TS. One inhibitor, BW1843, distorts the active site on binding, and intercalates into a hydrophobic patch between two mobile subdomains in the protein. Also discussed are recent developments in the cell biology and regulation of eukaryotic TS and the use of structure- based drug design in the development of the antifolates currently in clinical trial for the treatment of cancer.