Cytochrome <i>c</i>: A Thermodynamic Study of the Relationships among Oxidation State, Ion‐Binding and Structural Parameters

Abstract

The standard redox potential at I = 0.01, 25°C and pH 7.0 has been determined for the following species of cytochrome c : horse heart, baker's yeast isoenzyme‐1, Candida species yeast, tuna heart and turkey heart. The thermodynamic parameters of the redox reaction were evaluated and found to be identical, within experimental error, in the five species investigated. The effect of pH on the standard redox potential of horse heart cytochrome c was studied in the pH range 7.0–11.2. A single heme‐linked ionization of the oxidized protein was observed in this pH range, with a dissociation constant of 3 × 10 −9 at I = 0 and 25°C. The effects of the electrostatic media on the standard redox potential of horse heart cytochrome c depend on the nature of the anions employed. For non‐binding medium at 25°C, the observed potential dependence on the ionic strength is given by the equation: E 0 obs = 0.274–0.336 √I/(1 + 6√I). In binding medium, specific binding of ions to the protein takes place.