The mechanism of the electron transfer process from cytochrome c to cytochrome c oxidase studied by resonance Raman spectroscopic techniques

Abstract

In the terminal step of the respiratory chain of aerobic organisms, the membrane-bound enzyme cytochrome c oxidase (CcO) reduces dioxygen to water [1]. The required electrons are delivered stepwise by the soluble electron carrier cytochrome c (Cyt) [2]. Prior to these interprotein electron transfer reactions, Cyt binds to CcO via electrostatic interactions between the positively charged lysine-rich region on the surface of Cyt and a negatively charged binding domain of CcO [3]. Although the crystal structures of the individual redox proteins have been determined (e. g., [4, 5]), the structure of the Cyt-CcO protein complex is not known. On the other hand, a variety of spectroscopic results indicate that conformational changes are induced in the active sites of both partner proteins upon complex formation [6 – 8]. In our previous resonance Raman (RR) studies [7, 8], we could demonstrate that upon formation of the fully oxidized Cyt-CcO complex, a conformational equilibrium of the bound Cyt is established which involves two conformers, B1 and B2. These species are also formed upon binding to negatively charged model systems (electrodes, phospholipid vesicles, polyanions) which mimic the interaction domain of CcO [9, 10]. In contrast to Bl, which can be regarded as essentially identical to the unbound Cyt, the spectrum of state B2 reveals significant structural changes in the heme pocket of this conformer which most likely include the exchange of the Met-80 axial ligand presumably by a hydroxide [11] and are accompanied by a drastic lowering of the reduction potential [9]. These findings have been suggested to be of functional relevance for interprotein electron transfer to CcO [7, 8].