The cytochrome bc1 complex of Rhodobacter capsulatus: ubiquinol oxidation in a dimeric Q-cycle?

We studied the cytochrome bc1 complex (hereafter bc) by flash excitation of Rhodobacter capsulatis chromatophores. The reduction of the high-potential heme b(h), of cytochrome b (at 561 nm) and of cytochromes c (at 552 nm) and the electrochromic absorption transients (at 524 nm) were monitored after the first and second flashes of light, respectively. We kept the ubiquinone pool oxidized in the dark and concerned for the ubiquinol formation in the photosynthetic reaction center only after the second flash. Surprisingly, the first flash caused the oxidation of about one ubiquinol per bc dimer. Based on these and other data we propose a dimeric Q-cycle where the energetically unfavorable oxidation of the first ubiquinol molecule by one of the bc monomers is driven by the energetically favorable oxidation of the second ubiquinol by the other bc monomer resulting in a pairwise oxidation of ubiquinol molecules by the dimeric bc in the dark. The residual unpaired ubiquinol supposedly remains on the enzyme and is then oxidized after the first flash.