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

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 (lambda) of cytochrome c (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 lambda values. Conversion from the high to the low lambda 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 Omega loops operate as transducers of the electrostatic signal. This fine-tuning effect is abolished in the Y67F Cyt mutant, which presents a lambda 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.