The thermodynamics and kinetics of electron transfer in the cytochrome P450cam enzyme system

In anaerobic environments the first electron transfer in substrate‐free P450cam is known to be thermodynamically unfavourable, but in the presence of dioxygen the reduction potential for the reaction shifts positively to make electron transfer thermodynamically favourable. Nevertheless a slower rate of electron transfer is observed in the substrate‐free P450cam compared to substrate‐bound P450cam. The ferric haem centre in substrate‐free P450cam changes from six co‐ordinate to five co‐ordinate when reduced whereas in substrate‐bound P450cam the iron centre remains five co‐ordinate in both oxidation states. The slower rate of electron transfer in the substrate‐free P450cam is therefore attributed to a larger reorganisation energy as predicted by Marcus theory.