Catalyzes the oxidation of 5,10-methylenetetrahydrofolate to 5,10-methenyltetrahydrofolate and then the hydrolysis of 5,10-methenyltetrahydrofolate to 10-formyltetrahydrofolate. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + NADP(+) = 5,10-methenyltetrahydrofolate + NADPH 5,10-methenyltetrahydrofolate + H2O = (6S)-10-formyltetrahydrofolate + H(+) One-carbon metabolism; tetrahydrofolate interconversion. Homodimer. Probably essential, it was not disrupted in a global transposon mutagenesis study. Belongs to the tetrahydrofolate dehydrogenase/cyclohydrolase family.