Contributes to tetrahydrofolate metabolism. Helps regulate carbon flow through the folate-dependent one-carbon metabolic network that supplies carbon for the biosynthesis of purines, thymidine and amino acids. Catalyzes the irreversible conversion of 5-formyltetrahydrofolate (5-FTHF) to yield 5,10-methenyltetrahydrofolate. (6S)-5-formyl-5,6,7,8-tetrahydrofolate + ATP = 5,10-methenyltetrahydrofolate + ADP + phosphate Monomer. The disease is caused by variants affecting the gene represented in this entry. Belongs to the 5-formyltetrahydrofolate cyclo-ligase family.