CD-monitored redox titration of the Rieske Fe-S protein of Rhodobacter sphaeroides: pH dependence of the midpoint potential in isolated bc(1) complexand in membranes

The redox potential of the Rieske Fe-S protein has been investigated using circular dichroism (CD)-spectroscopy, The CD features characteristic of the purified bc(1) complex and membranes of Rhodobacter sphaeroides were found in the region between 450 and 550 nm, The difference between reduced and o xidized CD-spectra shows a negative band at about 500 nm with a half of wid th 30 nm that corresponds to the specific dichroic absorption of the reduce d Rieske protein (Fee, J.A. et al, (1984) J, Biol, Chem, 259, 124-133; Degl i Esposti, M. et al, (1987) Biochem, J, 241, 285-290; Rich, P.R. and Wiggin s, T.E. (1992) Biochem, Sec. Trans. 20, 241S), It was found that the redox potential at pH 7.0 for the Rieske center in the isolated bc(1) complex and in chromatophore membranes from the R-26 strain of Rb, sphaeroides is 300 +/- 5 mV, In chromatophores from the BC17C strain of Rb. sphaeroides, the E -m value measured for the Rieske iron-sulfur protein (ISP) was higher (315 +/- 5 mV), but the presence of carotenoids made measurement less accurate. The E-m varied with pH in the range above pH 7, and the pH dependence was w ell fit either by one pK at similar to 7.5 in the range of titration, or by two pK values, pK(1) = 7.6 and pK(2) = 9.8. Similar titrations and pK valu es were found for the Rieske Fe-S protein in the isolated bc(1) complex and membranes from the R-26 strain of Rb, sphaeroides. The results are discuss ed in the context of the mechanism of quinol oxidation by the bc(1) complex , and the role of the iron sulfur protein in formation of a reaction comple x at the Q(0)-site. (C) 1998 Federation of European Biochemical Societies.