We are investigating the redox chemistry of wild-type (WT) and mutant (1-12G) cytochrome P450 BM3. Absorption spectra in solution feature the Fe(III) Soret at 418 nm for WT and a split Soret for 1-12G at 390 and 418 nm. Voltammetry of the proteins within DDAPSS films on the surface of carbon electrodes reveal nearly identical Fe(III/II) potentials (approximately -200 mV vs Ag/AgCl), but significant differences in k degrees , 250 vs 30 s(-)(1), and Fe(III/II)-CO potentials, -140 vs -115 mV, for WT vs 1-12G. Catalytic reduction of dioxygen by the proteins on rotating-disk electrodes was analyzed using Levich and Koutecky-Levich treatments. The data reveal 1-12G n and k(obs) values that are, respectively, 1.7 and 0.07 times those of WT, suggesting that the two proteins differ strikingly in their reactions with dioxygen.