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

In anaerobic environments the first electron transfer in substrate-free P450(cam) 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 P450(cam) compared to substrate-bound P450(cam). The ferric haem centre in substrate-free P450(cam) changes from six co-ordinate to five coordinate when reduced whereas in substrate-bound P450(cam) the iron centre remains five co-ordinate in both oxidation states. The slower rate of electron transfer in the substrate-free P450(cam) is therefore attributed to a larger reorganisation energy as predicted by Marcus theory. (C) 1999 Federation of European Biochemical Societies.