The redox couple of the cytochrome c cyanide complex: the contribution of heme iron ligation to the structural stability, chemical reactivity, and physiological behavior of horse cytochrome c

Abstract

Contrary to most heme proteins, ferrous cytochrome c does not bind ligands such as cyanide and CO. In order to quantify this observation, the redox potential of the ferric/ferrous cytochrome c-cyanide redox couple was determined for the first time by cyclic voltammetry. Its E0' was -240 mV versus SHE, equivalent to -23.2 kJ/mol. The entropy of reaction for the reduction of the cyanide complex was also determined. From a thermodynamic cycle that included this new value for the cyt c cyanide complex E0', the binding constant of cyanide to the reduced protein was estimated to be 4.7 x 10(-3) L M(-1) or 13.4 kJ/mol (3.2 kcal/mol), which is 48.1 kJ/mol (11.5 kcal/mol) less favorable than the binding of cyanide to ferricytochrome c. For coordination of cyanide to ferrocytochrome c, the entropy change was earlier experimentally evaluated as 92.4 J mol(-1) K(-1) (22.1 e.u.) at 25 K, and the enthalpy change for the same net reaction was calculated to be 41.0 kJ/mol (9.8 kcal/mol). By taking these results into account, it was discovered that the major obstacle to cyanide coordination to ferrocytochrome c is enthalpic, due to the greater compactness of the reduced molecule or, alternatively, to a lower rate of conformational fluctuation caused by solvation, electrostatic, and structural factors. The biophysical consequences of the large difference in the stabilities of the closed crevice structures are discussed.

Figure 1.

Cyclic voltammogram of 1 mM horse cytochrome c in the presence of 0.1 M KCN at pH 7.0, Tris buffer, 25°C at a scan rate of 50 mV/sec. The circles (○) are the experimental data; the solid line, the computer simulation of the CV with Digisim using the 2 × 2 mechanism shown in Scheme 1.

Figure 2.

Temperature dependence of the equilibrium reduction potential (E⁰′) of the cyanide complex of horse cytochrome c.

Scheme 1.

The 2 × 2 square mechanism for the redox driven ligand exchange of cytochrome c in the presence of cyanide ion. Pr represents the native protein, and Pr^* symbolizes the protein after undergoing the structural changes concomitant with the displacement of the native ligand Met-80 away from the iron by CN⁻.