Medium and temperature effects on the redox chemistry of cytochrome c

Cytochromes c (cytc) are ubiquitous heme-containing metalloproteins that shuttle electrons in a variety of electron-transport chains, most often central to the production of the chemical energy necessary for cell life. The reduction potential (E degrees') of the Fe3+/2+ couple is central to the physiological role of these species in that it influences the thermodynamic and kinetic features of electron-exchange reactions with redox partners. In the last two decades, voltammetric techniques exploiting the heterogeneous electron exchange between cytc and solid electrodes have proved to be particularly valuable for the determination of E degrees' values for these species and for characterizing the mechanistic and kinetic aspects of the redox process for the various cytc conformers under a variety of solution conditions. The understanding of how, and to what extent, different molecular factors control the E degrees' value in these species has been the subject of much debate. First coordination sphere effects on the heme iron and the interactions of the heme group with the surrounding polypeptide chain and the solvent are the main factors affecting E degrees' in cytc. These interactions are sensitive to medium effects such as the pH and the nature and ionic composition of the solvent. E degrees' is also strongly affected by the temperature, This article summarizes the authors' work on the effects on the selective stabilization of the two redox states of class I cytochromes c exerted by acid-base equilibria, general ionic strength effects, specific anion binding, the presence of nonaqueous solvents, and the temperature. The temperature dependence of E degrees' allows the determination of the enthalpy and entropy changes that accompany protein reduction. These parameters have proved to be informative with regard to the interplay between first coordination sphere effects and electrostatics at the heme-protein interface, including solvent dipoles, which mainly affect the reduction enthalpy, and solvent reorganization effects and differences in protein dynamics between the two oxidation states, which control the reduction entropy instead.