title: Determination of the redox properties of the Rieske [2Fe-2S] cluster of bovine heart bc1 complex by direct electrochemistry of a water-soluble fragment.
abstract: The redox potential of the Rieske [2Fe-2S] cluster of the bc1 complex from bovine heart mitochondria was determined by cyclic voltammetry of a water-soluble fragment of the iron/sulfur protein. At the nitric-acid-treated bare glassy-carbon electrode, the fragment gave an immediate and stable quasireversible response. The midpoint potential at pH 7.2, 25 degrees C and I of 0.01 M was Em = +312 +/- 3 mV. This value corresponds within 20 mV to results of an EPR-monitored dye-mediated redox titration. With increasing ionic strength, the midpoint potential decreased linearly with square root of I up to I = 2.5 M. From the cathodic-to-anodic peak separation, the heterogeneous rate constant, k degrees, was calculated to be approximately 2 x 10(-3) cm/s at low ionic strength; the rate constant increased with increasing ionic strength. From the temperature dependence of the midpoint potential, the standard reaction entropy was calculated as delta S degrees = -155 J.K-1.mol-1. The pH dependence of the midpoint potential was followed over pH 5.5-10. Above pH 7, redox-state-dependent pK changes were observed. The slope of the curve, -120 mV/pH above pH9, indicated two deprotonations of the oxidized protein. The pKa values of the oxidized protein, obtained by curve fitting, were 7.6 and 9.2, respectively. A group with a pKa,ox of approximately 7.5 could also be observed in the optical spectrum of the oxidized protein. Redox-dependent pK values of the iron/sulfur protein are considered to be essential for semiquinone oxidation at the Qo center of the bc1 complex.
authors.affiliation: Universitätsklinikum Frankfurt, Zentrum für Biologische Chemie, Therapeutische Biochemie, Federal Republic of Germany.
authors.forename: T A
authors.initials: TA
authors.lastname: Link
authors.forename: W R
authors.initials: WR
authors.lastname: Hagen
authors.forename: A J
authors.initials: AJ
authors.lastname: Pierik
authors.forename: C
authors.initials: C
authors.lastname: Assmann
authors.forename: G
authors.initials: G
authors.lastname: von Jagow
mesh_terms: D000818:Animals; D002417:Cattle; D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy; D014450:Electron Transport Complex III / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D008929:Mitochondria, Heart / Q000201:enzymology*; D009994:Osmolar Concentration; D010446:Peptide Fragments / Q000737:chemistry; D012995:Solubility; D013816:Thermodynamics
chemical_list: D007506:Iron-Sulfur Proteins; D010446:Peptide Fragments; C037989:Rieske iron-sulfur protein; D014450:Electron Transport Complex III