A Kinetic Approach of Proton Transport in a Multi-Redox Center Protein: Cytochrome c Oxidase

A kinetic model of the cytochrome c oxidase (CcO) is presented taking into account four redox centers with individual protonation constants for the reduced and oxidized state. This approach opens the possibility to probe the effect of protonations on redox kinetics. In this context, we make use of a model system developed earlier, which allows to study direct electron transfer (ET) to redox enzymes by electrochemistry. Electrons are transferred between the electrode and the enzyme, but only if CuA is directed toward the electrode. Hence, we employ electrochemical kinetics to CuA coupled to a sequence of second order chemical reactions including protonations to simulate cyclovoltammetry data. Modeling studies are directly compared with experimental data showing that electrochemical excitation of CcO is dominated by direct ET to CuA, occurring in the ms time scale. Catalytic ET as well as proton transfer currents measured in the presence of oxygen are simulated considering oxygen O2 /H2O as a fifth redox couple. Proton uptake and release can be explained by the difference in protonation constants of consecutive redox couples. We conclude from these results that proton pumping can be modeled in terms of protonation-dependent redox kinetics.