Required for both C1 and C2 metabolism. 5,10-methenyltetrahydrofolate + H2O = (6S)-10-formyltetrahydrofolate + H(+) One-carbon metabolism; formaldehyde assimilation via serine pathway. Homodimer. Mutants are unable to grow on either methanol or ethylamine in the presence of glycolate. Glyoxylate restores growth, but only on C2 compounds. Belongs to the cyclodeaminase/cyclohydrolase family.