Determination of the redox properties of the Rieske [2Fe‐2S] cluster of bovine heart bc₁ complex by direct electrochemistry of a water‐soluble fragment

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.