5-formyltetrahydrofolate cyclo-ligase or methenyl-THF synthetase EC:6.3.3.2 catalyses the interchange of 5-formyltetrahydrofolate (5-FTHF) to 5-10-methenyltetrahydrofolate, this requires ATP and Mg2+ [1]. 5-FTHF is used in chemotherapy where it is clinically known as Leucovorin [2].

1: Cloning and characterization of the human 5,10-methenyltetrahydrofolate synthetase-encoding cDNA. Dayan A, Bertrand R, Beauchemin M, Chahla D, Mamo A, Filion M, Skup D, Massie B, Jolivet J; Gene 1995;165:307-311. PMID:8522195

2: Primary structure and tetrahydropteroylglutamate binding site of rabbit liver cytosolic 5,10-methenyltetrahydrofolate synthetase. Maras B, Stover P, Valiante S, Barra D, Schirch V; J Biol Chem 1994;269:18429-18433. PMID:8034591