Dihydrofolate reductases (DHFRs) (EC 1.5.1.3) [1] are ubiquitous enzymes which catalyze the NADPH-linked reduction of 7,8-dihydrofolate to 5,6,7,8-tetrahydrofolate. DHFRs are also capable of catalyzing the NADPH-linked reduction of folate to 7,8-dihydrofolate, but at a lesser rate, which varies among species. They can be inhibited by a number of antagonists such as trimethroprim and methotrexate which are used as antibacterial or anticancerous agents.

Thymidylate synthase (TS) (see <PDOC00086>) and DHFR catalyze sequential reactions in the thymidylate cycle, which supplies cells with their sole de novo source of 2'-deoxythymidylate (dTMP) for DNA synthesis. TS catalyzes a reductive methylation of 2'deoxyuridylate (dUMP) to form dTMP in which the cofactor for the reaction, 5,10-methylenetetrahydrofolate is converted to dihydrofolate (FH(2)). DHFR then reduces FH(2) to tetrahydrofolate (FH(4)) in a reaction requiring NADPH. In sources as diverse as bacteriophage, prokaryotes, fungi, mammalian viruses, and vertebrates, TS and DHFR are distinct monofunctional enzymes. Protozoa and at least some plants are unusual in having a joined bifunctional polypetide that catalyzes both reactions [2,3].

An eight-stranded β sheet consisting of seven parallel strands and a carboxy-terminal antiparallel strand composes the core of the DHFR domain. The β-sheet core is flanked by α-helices (see <PDB:1DRH>) [2,3,4,5,6].

We have derived a signature pattern from a region in the N-terminal part of the DHFR domain, which includes a conserved Pro-Trp dipeptide; the tryptophan has been shown [7] to be involved in the binding of substrate by the enzyme. We have also developed a profile, which covers the entire DHFR domain.