Accepted name: methenyltetrahydrofolate cyclohydrolase
Reaction: 5,10-methenyltetrahydrofolate + H2O = 10-formyltetrahydrofolate
Other name(s): Citrovorum factor cyclodehydrase; cyclohydrolase; formyl-methenyl-methylenetetrahydrofolate synthetase (combined); 5,10-methenyltetrahydrofolate 5-hydrolase (decyclizing)
Systematic name: 5,10-methenyltetrahydrofolate 5-hydrolase (ring-opening)
Comments: In eukaryotes, the enzyme occurs as a trifunctional enzyme that also has methylenetetrahydrofolate dehydrogenase (NADP+) (EC and formate—tetrahydrofolate ligase (EC activity. In some prokaryotes, it occurs as a bifunctional enzyme that also has dehydrogenase (EC activity or formimidoyltetrahydrofolate cyclodeaminase (EC activity.
1.  Rabinowitz, J.C. and Pricer, W.E. The enzymatic synthesis of N10-formyltetrahydrofolic acid and its role in ATP formation during formiminoglycine degradation. J. Am. Chem. Soc. 78 (1956) 4176–4178.
2.  Tabor, H. and Wyngarden, L. The enzymatic formation of formiminotetrahydrofolic acid, 5,10-methenyltetrahydrofolic acid, and 10-formyltetrahydrofolic acid in the metabolism of formiminoglutamic acid. J. Biol. Chem. 234 (1959) 1830–1849. [PMID: 13672973]
[EC created 1961]