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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

¹ Sackler Institute of Molecular Medicine, Sackler Medical School, Tel-Aviv University, Tel-Aviv 69978, Israel
² Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233, USA
³ Department of Chemistry, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin 53211-3029, USA
⁴ Department of Biochemistry, Molecular Biology, and Cell Biology, Northwestern University, Evanston, Illinois 60208, USA

(RECEIVED September 9, 2005; FINAL REVISION November 17, 2005; ACCEPTED November 17, 2005)

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 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.

Keywords: cytochrome c; cyanide coordination; cyt c–cyanide complex; thermodynamics; protein stability

Article and publication are at http://www.proteinscience.org/cgi/doi/10.1110/ps.051825906.

Reprint requests to: Abel Schejter, Sackler Institute of Molecular Medicine, Sackler Medical School, Tel-Aviv University, Tel-Aviv 69978, Israel; e-mail: molec03{at}post.tau.ac.il; fax: +972-3-547-2818.

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