Redox components of cytochrome bc-type enzymes in acidophilic prokaryotes II. The Rieske protein of phylogenetically distant acidophilic organisms

The Rieske proteins of two phylogenetically distant acidophilic organisms, i.e. the proteobacterium Thiobacillus ferrooxidans and the crenarchaeon Sul folobus acidocaldarius, were studied by EPR. Redox titrations at a range of pH values showed that the Rieske centers of both organisms are characteriz ed by redox midpoint potential-versus-pH curves featuring a common pK value of 6.2. This pK value is significantly more acidic (by almost 2 pH units) than that of Rieske proteins in neutrophilic species. The orientations of t he Rieske center's g tensors with respect to the plane of the membrane were studied between pH 4 and 8 using partially ordered samples. At pH 4, the S ulfolobus Rieske cluster was found in the "typical" orientation of chemical ly reduced Rieske centers, whereas this orientation changed significantly o n going toward high pH values. The Thiobacillus protein, by contrast, appea red to be in the "standard" orientation at both low and high pH values. The results are discussed with respect to the molecular parameters conveying a cid resistance and in light of the recently demonstrated long-range conform ational movement of the Rieske protein during enzyme turnover in cytochrome bc(1) complexes.