Structural Analysis of Dihydrofolate Reductase and Thymidylate Synthase from Mammalian and Pathogenic Organisms

The ubiquitous enzyme dihydrofolate reductase (DHFR) is responsible for the reduction of 5,6-dihydrofolate to 5,6,7,8-tetrahydrofolate in an NADPH-dependent manner. The enzymes DHFR and thymidylate synthase (TS), which converts deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), are coupled in the folate pathway as the product of TS (dihydrofolate) is the substrate for DHFR. Because of their crucial roles in the production of the precursors of RNA and DNA for protein synthesis in every organism, DHFR and TS are key pharmacological targets for the treatment of cancer, as well as bacterial and opportunistic infections. The effectiveness of antifolates lies in their ability to selectively disrupt folate pathways that ultimately lead to cell death. However, in many instances the efficacy of clinically available antifolates is limited by their ineffectiveness against many pathogenic organisms or by the increase in drug-resistance, mainly due to the rise of mutations observed in clinical isolates. This review surveys more than 300 DHFR structures and over 200 TS structures representing 28 species of enzyme. Novel antifolates continue to be synthesized in an effort to enhance species selectivity and to increase potency without added toxicity. The focus of this review will be on DHFR and TS enzymes from mammalian and pathogenic organisms that have become the target of bioterrorism or have become a major medical concern as drug-resistance has overtaken the efficacy of many current treatments.