Evidence for a two proton dependent redox equilibrium in an archaeal Rieske iron-sulfur cluster

The recently detected Rieske iron-sulfur center in the membrane of the thermoacidophilic archaeon Sulfolobus acidocaldarius (Anemüller et al., 1993, FEBS Lett. 318, 61-64) was further characterized by EPR spectroscopy, coupled to redox-potentiometry and functional studies. The reduction potential is pH-dependent above pH 6, revealing the influence of two ionization equilibria in the oxidized form, with pKaox-values of 6.2 and 8.5. Above pH 9, the slope of the curve is--120 mV/pH-unit. A partially purified fraction exerted a ubiquinol-cytochrome c oxidoreductase activity. To our knowledge, for the first time, in a membrane bound Rieske iron-sulfur protein, unequivocal evidence for a two proton dependent redox equilibrium is presented.