Enzymes that participate in the transfer of one-carbon units require the coenzyme tetrahydrofolate (THF).Various reactions generate one-carbon derivatives of THF, which can be interconverted between differentoxidation states by methylene-THF dehydrogenase (1.5.1.5), methenyl-THF cyclohydrolase (3.5.4.9)and formyl-THF synthetase (6.3.4.3) [1, 2]. The dehydrogenase and cyclohydrolaseactivities are expressed by a variety of multifunctional enzymes, including the tri-functional eukaryoticC1-tetrahydrofolate synthase [3]; a bifunctional eukaryotic mitochondrial protein; and thebifunctional Escherichia coli folD protein [3, 3]. Methylene-tetrahydrofolate dehydrogenase andmethenyltetrahydrofolate cyclo-hydrolase share an overlapping active site [3], and as such areusually located together in proteins, acting in tandem on the carbon-nitrogen bonds of substrates otherthan peptide bonds.

The sequence of the dehydrogenase/cyclohydrolase domain is highly conserved in all forms of the enzyme. This entry contains two conserved signature patterns; the first one is located in the N-terminal part of these enzymes and contains three acidic residues. The second pattern is a highly conserved sequence of 9 amino acids, which is located in the C-terminal section.

1. Site-directed mutagenesis of yeast C1-tetrahydrofolate synthase: analysis of an overlapping active site in a multifunctional enzyme. Biochemistry 28, 2099-106
2. The isolation and characterization of a Drosophila gene encoding a putative NAD-dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase. Biochim. Biophys. Acta 1173, 94-8

Enzymes that participate in the transfer of one-carbon units require the coenzyme tetrahydrofolat
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