Electrochemical and spectroscopic investigations of the cytochrome bc(1) complex from Rhodobacter capsulatus

The cytochrome bc(1) complex from Rhodobacter capsulatus was investigated b y protein electrochemistry and visible/IR spectroscopy. Infrared difference spectra, which represent redox-induced conformational changes of cofactors and their protein environments, show signals of the hemes, the quinone Q(i ), and small conformational changes of the protein backbone. Furthermore, b and features were tentatively assigned to protonated aspartic or glutamic a cids involved in the redox transition of each of the b hemes, a proline in that of the [2Fe-2S] protein, and an arginine in that of cytochrome b(H). T he midpoint potential of the [2Fe-2S] protein was determined for the first time at physiological temperature to be +290 mV at pH 8.7. The reduced minu s oxidized difference extinction coefficients of the alpha-bands of the cyt ochromes were calculated as 11.5, 19, and 6.7 mM(-1) cm(-1) for cytochromes c(1), b(H), and b(L), respectively. A novel method has been developed to q uantify protonation reactions of the complex during the redox reactions of its cofactors by evaluation of the buffer signals in the midinfrared region . Values will be discussed in relation to the pH dependence of the midpoint potentials.