An nadp-dependent oxidoreductase that catalyses the conversion of 5,10-methyleneterahydrofolate to 5,10-methenyl-tetrahydrofolate. In higher eukaryotes a trifunctional enzyme exists with additional methenyltetrahydrofolate cyclohydrolase and formate-tetrahydrofolate ligase activity. The enzyme plays an important role in the synthesis of 5-methyltetrahydrofolate, the methyl donor for the VITAMIN B12-dependent remethylation of homocysteine to methionine via methionine SYNTHETASE.