Conformational switching at cytochrome a during steady-state turnover of cytochrome c oxidase.

As an electron transfer-driven proton pump, cytochrome c oxidase (ferrocytochrome-c:oxygen oxidoreductase, EC 1.9.3.1) must alternate between two conformations in each valence state of the redox element associated with ion translocation. Using second derivative absorption spectroscopy, the conformation of the cytochrome a cofactor has been investigated during steady-state turnover of this enzyme. Resting cytochrome c oxidase displays a transition for ferric cytochrome a at 430 nm. During aerobic steady-state turnover, this band is replaced by a ferrous cytochrome a transition at 450 nm. When anaerobicity is achieved, the transition occurs at 444 nm. The 450-nm-absorbing species is thus the dominant form during turnover, suggesting that conformational transitions of cytochrome a direct electron transfer during catalysis and may direct as well proton translocation in the last step of the respiratory electron transfer chain.