title: 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.
authors.affiliation: Sackler Institute of Molecular Medicine, Sackler Medical School, Tel-Aviv University, Tel-Aviv 69978, Israel. molec03@post.tau.ac.il
authors.forename: Abel
authors.initials: A
authors.lastname: Schejter
authors.forename: Michael D
authors.initials: MD
authors.lastname: Ryan
authors.forename: Erica R
authors.initials: ER
authors.lastname: Blizzard
authors.forename: Chongyao
authors.initials: C
authors.lastname: Zhang
authors.forename: Emanuel
authors.initials: E
authors.lastname: Margoliash
authors.forename: Benjamin A
authors.initials: BA
authors.lastname: Feinberg
mesh_terms: D000818:Animals; D003486:Cyanides / Q000378:metabolism*; D045304:Cytochromes c / Q000737:chemistry* / Q000378:metabolism; D019277:Entropy; D006418:Heme / Q000378:metabolism*; D006736:Horses; D007501:Iron / Q000378:metabolism*; D010084:Oxidation-Reduction; D011487:Protein Conformation; D055672:Static Electricity; D013816:Thermodynamics
chemical_list: D003486:Cyanides; D006418:Heme; D045304:Cytochromes c; D007501:Iron