Physical contact between cytochrome c₁ and cytochrome c increases the driving force for electron transfer

In oxidative phosphorylation, the transfer of electrons from reduced cofactors to molecular oxygen via the electron transport chain (ETC) sustains the electrochemical transmembrane potential needed for ATP synthesis. A key component of the ETC is complex III (CIII, cytochrome bc₁), which transfers electrons from reduced ubiquinone to soluble cytochrome c (Cc) coupled to proton translocation into the mitochondrial intermembrane space. One electron from every two donated by hydroquinone at site P is transferred to Cc via the Rieske-cytochrome c₁ (Cc₁) pathway. According to recent structural analyses of CIII and its transitory complex with Cc, the interaction between the Rieske subunit and Cc₁ switches intermittently during CIII activity. However, the electrochemical properties of Cc₁ and their function as a wire between Rieske and Cc are rather unexplored. Here, temperature variable cyclic voltammetry provides novel data on the thermodynamics and kinetics of interfacial electron transfer of immobilized Cc₁. Findings reveal that Cc₁ displays two channels for electron exchange, with a remarkably fast heterogeneous electron transfer rate. Furthermore, the electrochemical properties are strongly modulated by the binding mode of the protein. Additionally, we show that electron transfer from Cc₁ to Cc is thermodynamically favored in the immobilized Cc₁-Cc complex. Nuclear Magnetic Resonance, HADDOCK, and Surface Plasmon Resonance experiments provide further structural and functional data of the Cc₁-Cc complex. Our data supports the Rieske-Cc₁-Cc pathway acting as a unilateral switch thyristor in which redox potential modulation through protein-protein contacts are complemented with the relay-like Rieske behavior.