The dark-operative version is a completely different protein, consisting of three subunits that exhibit significant sequence similarity to the three subunits of nitrogenase, which catalyzes the formation of ammonia from dinitrogen.[6] This enzyme might be evolutionary older but (being similar to nitrogenase) is highly sensitive to free oxygen and does not work if its concentration exceeds about 3%.[7] Hence, the alternative, light-dependent version needed to evolve.