The location, orientation and stoichiometry of the Rieske iron-sulfur cluster in membranes from Rhodopseudomonas sphaeroides

Abstract

Neutral and negatively charged dysprosium complexes are able to enhance the spin relaxation rate of the Rieske iron-sulfur cluster only when added from the cytochrome c 2 side of the photosynthetic membrane, indicating that the Rieske cluster is asymmetrically placed in the membrane, nearer the cytochrome c 2 side. The g z -axis of the Rieske cluster, taken to be the iron-iron axis of this binuclear cluster, lies in the membrane plane, as does the g y -axis. Appropriately, the g x -axis is orthogonal to the membrane plane. A comparison with a mammalian mitochondrial standard indicates that there are 0.65 ± 0.1 Rieske cluster per reaction center. This is in excellent agreement with previously determined estimates of the number of antimycin-binding sites, and binding sites for what is known phenomenologically as Q Z, suggesting that there is one of each per ubiquinol-cytochrome c 2 oxidoreductase.