Methylenetetrahydrofolate dehydrogenase/methenyltetrahydrofolate cyclohydrolase catalyzes the formation of 5,10-methenyl-tetrahydrofolate (5,10-methenyl-THF) from 5,10-methylene-THF and subsequent formation of 10-formyl-THF from 5,10-methenyltetrahydrofolate. 5,10-methylene-THF is a substrate for thymidylate synthetase, while 10-formyl-THF is a formyl group donor in purine and formylmethionine-tRNA syntheses. A variety of human pathologies, including conditions such as spina bifida and hyperhomocysteinemia, are related to metabolic or dietary folate deficiencies.
5,10-methylene-THF dehydrogenase and cyclohydrolase activities are generally found on a single, bifunctional enzyme in bacteria and in most mitochondria. The bacterial enzymes and cytosolic enzymes of eukaryotes are predominantly NADP dependent, whereas most mitochondrial enzymes and monofunctional dehydrogenases use NAD. These enzymes bind a single folate substrate and a single NADP coenzyme per subunit.