Abundance, subunit composition, redox properties, and catalytic activity of the cytochrome bc sub(1) complex from alkaliphilic and halophilic, photosynthetic members of the family Ectothiorhodospiraceae

Ubiquinol-cytochrome c oxidoreductase (cytochrome bc sub(1)) complexes were demonstrated to be present in the membranes of the alkaliphilic and halophilic purple sulfur bacteria Ectothiorhodospira halophila, Ectothiorhodospira mobilis), and Ectothiorhodospira shaposhnikovii by protoheme extraction, immunoblotting, and electron paramagnetic resonance spectroscopy. The g sub(y) values of the Rieske (2F2-2S) clusters observed in membranes of E. mobilis) and E. halophila were 1.895 and 1.910, respectively. In E. mobilis membranes, the cytochrome bc sub(1) complex was present in a stoichiometry of approximately 0.2 per reaction center. This complex was isolated and characterized. It contained four prosthetic groups: low-potential cytochrome b (ctyochrome b sub(L); E sub(m) = -142 mV), high-potential cytochrome b (cytochrome b sub(H); E sub(m) = 116 mV), cytochrome c sub(1) (E sub(m) = 341 mV), and a Rieske iron-sulfur cluster. The absorbance spectrum of cytochrome b sub(L) displayed an asymmetric alpha -band with a maximum at 564 nm and a shoulder at 559 nm. The alpha bands of cytochrome b sub(H) and cytochrome c sub(1) peaked at 559.5 and 553 nm, respectively. These prosthetic groups were associated with three different polypeptides: cytochrome b, cytochrome c sub(1), and the Rieske iron-sulfur protein, with apparent molecular masses of 43, 30, and 21 kDa, respectively. No evidence for the presence of a fourth subunit was obtained. Maximal ubiquinol-cytochrome c oxidoreductase activity of the purified complex was observed at pH 8; the turnover rate was 57 mol of cytochrome c reduced/(mol of cytochrome c sub(1))/s. The complex showed a strikingly low sensitivity towards typical inhibitors of cytochrome bc sub(1) complexes.