N(1)-(5-phospho-beta-D-ribosyl)glycinamide + (6R)-5,10-methenyltetrahydrofolate + H2O = N(2)-formyl-N(1)-(5-phospho-beta-D-ribosyl)glycinamide + (6S)-5,6,7,8-tetrahydrofolate + 2 H(+) — reaction