InterPro domain: IPR012262

This group represents a bifunctional dihydrofolate reductase/thymidylate synthase found in some plant species and protozoal parasites including malarial species and trypanosomes. In other species dihydrofolate reductase and thymidilate synthase are encoded on separate polypeptides.

Thymidylate synthase ( 2.1.1.45 ) [ 1 ] catalyzes the reductive methylation of dUMP to dTMP with concomitant conversion of 5,10-methylenetetrahydrofolate to dihydrofolate: 5,10-methylenetetrahydrofolate + dUMP = dihydrofolate + dTMP This provides the sole de novo pathway for production of dTMP and is the only enzyme in folate metabolism in which the 5,10-methylenetetrahydrofolate is oxidised during one-carbon transfer [ 2 ]. The enzyme is important for regulating the balanced supply of the 4 DNA precursors in normal DNA replication: defects in the enzyme activity affecting the regulation process can cause various biological and genetic abnormalities. A cysteine residue is involved in the catalytic mechanism (it covalently binds the 5,6-dihydro-dUMP intermediate). The sequence around the active site of this enzyme is conserved from phages to vertebrates.

Dihydrofolate reductase (DHFR) ( 1.5.1.3 ) catalyses the NADPH-dependent reduction of dihydrofolate to tetrahydrofolate: 5,6,7,8-tetrahydrofolate + NADP+ = 7,8-dihydrofolate + NADPH + H+ This is an essential step in de novo synthesis both of glycine and of purines and deoxythymidine phosphate (the precursors of DNA synthesis) [ 3 ], and important also in the conversion of deoxyuridine monophosphate to deoxythymidine monophosphate. Although DHFR is found ubiquitously in prokaryotes and eukaryotes, and is found in all dividing cells, maintaining levels of fully reduced folate coenzymes, the catabolic steps are still not well understood [ 4 ].

As this enzyme is essential in both nucleic acid and amino acid biosynthesis, it is an important target of antiparasitic drugs. Resistance to antimalarial drugs that target this enzyme is often due to mutations that prevent drug binding but maintain enzyme activity. The structure of the wild-type and drug resistant malarial enzymes provides insights into the development of resistance and suggests approaches for the design of new drugs against this target [ 5 ].

1. On the mechanism of action of folate- and biopterin-requiring enzymes. Annu. Rev. Biochem. 49, 227-51
2. Atomic structure of thymidylate synthase: target for rational drug design. Science 235, 448-55
3. A gene for dihydrofolate reductase in a herpesvirus. Science 239, 1145-7
4. Crystal structure of human dihydrofolate reductase complexed with folate. Eur. J. Biochem. 174, 377-85
5. Insights into antifolate resistance from malarial DHFR-TS structures. Nat. Struct. Biol. 10, 357-65