Electrostatically Driven Second-Sphere Ligand Switch between High and Low Reorganization Energy Forms of Native Cytochrome <i>c</i>

We have employed a combination of protein film voltammetry, time-resolved vibrational spectroelectrochemistry and molecular dynamics simulations to evaluate the electron-transfer reorganization free energy (λ) of cytochrome <i>c</i> (Cyt) in electrostatic complexes that mimic some basic features of protein–protein and protein–lipid interactions. The results reveal the existence of two native-like conformations of Cyt that present significantly different λ values. Conversion from the high to the low λ forms is triggered by electrostatic interactions, and involves the rupture of a weak H-bond between first- (M80) and second-sphere (Y67) ligands of the heme iron, as a distinctive feature of the conformational switch. The two flexible Ω loops operate as transducers of the electrostatic signal. This fine-tuning effect is abolished in the Y67F Cyt mutant, which presents a λ value similar to the WT protein in electrostatic complexes. We propose that interactions of Cyt with the natural redox partner proteins activate a similar mechanism to minimize the reorganization energy of interprotein electron transfer