<p>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.</p><p>Thymidylate synthase (<db_xref db="EC" dbkey="2.1.1.45"/>) [<cite idref="PUB00000031"/>] catalyzes the reductive methylation of dUMP to dTMP with concomitant conversion of 5,10-methylenetetrahydrofolate to dihydrofolate:<reaction>5,10-methylenetetrahydrofolate + dUMP = dihydrofolate + dTMP</reaction>This provides the sole <i>de novo</i> 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 [<cite idref="PUB00005100"/>]. 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.</p><p>Dihydrofolate reductase (DHFR) (<db_xref db="EC" dbkey="1.5.1.3"/>) catalyses the NADPH-dependent reduction of dihydrofolate to tetrahydrofolate:<reaction>5,6,7,8-tetrahydrofolate + NADP+ = 7,8-dihydrofolate + NADPH + H+</reaction>This is an essential step in <i>de novo</i> synthesis both of glycine and of purines and deoxythymidine phosphate (the precursors of DNA synthesis) [<cite idref="PUB00005107"/>], 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 [<cite idref="PUB00001361"/>].</p><p>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 [<cite idref="PUB00021881"/>]. </p> Bifunctional dihydrofolate reductase/thymidylate synthase