Conformational changes in cytochrome c and cytochrome oxidase upon complex formation: a resonance Raman study

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Abstract
The fully oxidized complex of cytochrome c and cytochrome oxidase formed at low ionic strength was studied by resonance Raman spectroscopy. The spectra of the complex and of the indivudal components were compared over a wide frequency range using Soret band excitation. In both partners of the complex, structural changes occurs in the heme groups and in their immediate protein environment. The spectra of the complex in the 1600-1700 cm-1 frequency range were dominated by bands from the cytochrome oxidase component, whereas those in the 300-500 cm-1 range were dominated by bands from the cytochrome c component, hence allowing separation of the contributions from the two individual species. For cytochrome c, spectral changes were observed which correspond to the induction of the conformational state I and the six-coordinated low-spin configuration of state II on binding to cytochrome oxidase. While in state I the structure of cytochrome c is essentially the same as in solution, state II is characterized by a structural rearrangement of the heme pocket, leading to a weakening of the axial iron-methionine bond and an opening of the heme crevice which is situated in the center of the binding domain for cytochrome oxidase. The relative contributions of the two cytochrome c states were estimated to be approximately in the ratio 1:1 in the complex. It is proposed that the electron transfer from cytochrome c to cytochrome oxidase is controlled by the conformational equilibrium between the states I and II which exhibit different redox potentials. The spectral changes assignable to heme a are interpreted in terms of alterations in the interaction of the cytochrome oxidase protein matrix with the peripheral substituents of the heme group, on binding cytochrome c. It is suggested that, in particular, the mobility of the formyl group is reduced. These structural changes may reflect the establishment of the proper alignment of the heme a group for the electron transfer. Subtle effects on the resonance Raman bands of heme a3 are attributed to modified interactions of the .pi.-electron systems of the porphyrin with adjacent amino acid side chains, suggesting that there is a structural communication between the cytochrome c binding site and the heme a3 pocket. Spectra of the complex between the reduced cytochrome c and the partially oxidized cyanide-ligated cytochrome oxidase also indicated that conformational changes take place in these redox partners.

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