LAUSR

CYP154C8 catalyzes the hydroxylation of diverse steroids, as has previously been demonstrated, by using an NADH‐dependent system including putidaredoxin and putidaredoxin reductase as redox partner proteins carrying electrons from NADH. In other reactions, CYP154C8 reconstituted with spinach ferredoxin and NADPH‐dependent ferredoxin reductase displayed catalytic activity different from that of the NADH‐dependent system. The NADPH‐dependent system showed multistep oxidation of progesterone and other substrates including androstenedione, testosterone, and nandrolone. (Diacetoxyiodo)benzene was employed to generate compound I (FeO3+), actively supporting the redox reactions catalyzed by CYP154C8. In addition to 16α‐hydroxylation, progesterone and 11‐oxoprogesterone also underwent hydroxylation at the 6β‐position in reactions supported by (diacetoxyiodo)benzene. CYP154C8 was active in the presence of high concentrations (>10 mm) of H2O2, with optimum conversion surprisingly being achieved at ≈75 mm H2O2. More importantly, H2O2 tolerance by CYP154C8 was evident in the very low heme oxidation rate constant (K) even at high concentrations of H2O2. Our results demonstrate that alternative redox partners and oxidizing agents influence the catalytic efficiency and product distribution of a cytochrome P450 enzyme. More importantly, these choices affected the type and selectivity of reaction catalyzed by the P450 enzyme.