(6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + NADP(+) = 5,10-methenyltetrahydrofolate + NADPH 5,10-methenyltetrahydrofolate + H2O = (6S)-10-formyltetrahydrofolate + H(+) (6S)-5,6,7,8-tetrahydrofolate + ATP + formate = (6S)-10-formyltetrahydrofolate + ADP + phosphate One-carbon metabolism; tetrahydrofolate interconversion. Homodimer. This trifunctional enzyme consists of two major domains: an N-terminal part containing the methylene-THF dehydrogenase and cyclohydrolase activities and a larger C-terminal part containing formyl-THF synthetase activity. In the N-terminal section; belongs to the tetrahydrofolate dehydrogenase/cyclohydrolase family. In the C-terminal section; belongs to the formate--tetrahydrofolate ligase family.