A Kinetic Model Of Proton Transport In A Multi-Redox Center Protein: Cytochrome C Oxidase

Simulations of electrochemical measurements are presented, making use of a model system comprising cytochrome c oxidase (CcO) immobilized in a strict orientation on an electrode. This allows studying direct electron transfer (ET) to CcO by electrochemistry. The advantage of this system is that modeling studies can be directly compared with actual measurements using a combination of electrochemical and spectroscopic techniques. Previous modeling studies of fast-scan voltammetry and time-resolved FTIR studies have shown that ET from the electrode into the enzyme highly likely occurs via CuA. Thereafter electrons are transferred to the remaining redox centers in exactly the same sequence that is natural to the enzyme. In the present investigation we extend this four-electron transfer model in two steps. In the first step we consider protonation equilibria of the oxidized and reduced species for each of the four centers. In the second step we add oxygen/H2O as the terminal (fifth) redox couple including protonation of reduced oxygen to water. Finally we arrive at a kinetic model comprising five redox couples describing a string of bimolecular reactions with protonations. Protons pumped by the enzyme arrive at the electrode, where they are reduced to molecular hydrogen. Hence both electron and proton currents can be observed. Different parameter settings are employed for the modeling, in which known standard redox potentials of the redox centers are maintained while protonation constants are varied. The model is shown to simulate cyclovoltammetry data of the CcO in the absence and presence of oxygen.