THE CYTOCHROME BC(1) COMPLEX OF RHODOBACTER-CAPSULATUS - UBIQUINOL OXIDATION IN A DIMERIC Q-CYCLE

We studied the cytochrome bc(1) complex (hereafter be) by flash excita tion of Rhonobacter capsulatus chromatophores. The reduction of the hi gh-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, respectiv ely. We kept the uhiquinone pool oxidized in the dark and concerned fo r the ubiquinol formation in the photosynthetic reaction center only a fter the second flash. Surprisingly; the first flash caused the oxidat ion of about one ubiquinol per bc dimer, Based on these and other data we propose a dimeric Q-cycle where the energetically unfavorable oxid ation of the first ubiquinol molecule by one of the be monomers is dri ven by the energetically favorable oxidation of the second ubiquinol b y the other be monomer resulting in a pairwise oxidation of ubiquinol molecules by the dimeric be in the dark. The residual unpaired ubiquin ol supposedly remains on the enzyme and is then oxidized after the fir st flash. (C) 1998 Federation of European Biochemical Societies.