x1 = FeIII x2 = FeII x3 = FeII_O2 x4 = FeIII_star x5 = FeII_star x6 = FeII_star_O2 x7 = FeIII_NO x8 = FeII_NO x9 = NADPH x10 = O2 x11 = citrulline x12 = NO3 x13 = NO x14 = NADPplus x15 = FeIII_t