5-formyltetrahydrofolate cyclo-ligase or methenyl-THF synthetase EC:6.3.3.2 catalyzes the interchange of 5-formyltetrahydrofolate (5-FTHF) to 5-10-methenyltetrahydrofolate, this requires ATP and Mg2+. 5-FTHF is used in chemotherapy where it is clinically known as Leucovorin.
Jiyao W et al.(2023). "The conserved domain database in 2023.",