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

• Published in: Protein science 2006-02, Vol.15 (2), p.234-241
• Main Authors: Schejter, Abel, Ryan, Michael D., Blizzard, Erica R., Zhang, Chongyao, Margoliash, Emanuel, Feinberg, Benjamin A.
• Format: Article
• Language: English
• Subjects: Animals; cyanide coordination; Cyanides - metabolism; cyt c–cyanide complex; cytochrome c; Cytochromes c - chemistry; Cytochromes c - metabolism; Entropy; Heme - metabolism; Horses; Iron - metabolism; Oxidation-Reduction; Protein Conformation; protein stability; Static Electricity; Thermodynamics
• ISSN: 0961-8368; 1469-896X
• DOI: 10.1110/ps.051825906

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 × 10−3 LM−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 Jmol−1K−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.