The Rhodospirillum rubrum cytochrome bc₁ complex: pep tide composition, prosthetic group content and quinone binding

A cytochrome bc₁ complex, essentially free of bacteriochlorophyll, has been purified from the photosynthetic purple non-sulfur bacterium Rhodospirillum rubrum. The complex catalyzes electron flow from quinol to cytochrome c (turnover number = 75 s^-1) that is inhibited by low concentrations of antimycin A and myxothiazol. The complex contains only three peptide subunits: cytochrome b (M_r = 35000); cytochrome c₁ (M_r = 31000) and the Rieske iron-sulfur protein (M_r = 22400). E_m values (pH 7.4) were measured for cytochrome c, (+ 320 mV) and the two hemes of cytochrome b (- 33 and -90 mV). Electron flow from quinol to cytochrome c is inhibited when the complex is pre-illuminated in the presence of a ubiquinone photoaffinity analog (azido-Q). During illumination, the azido-Q becomes covalently attached to the cytochrome b peptide and, to a lesser extent, to cytochrome c₁.