The effect of the size of the quinone pool on the electrogenic reactions in the ubiquinol-cytochrome c2 oxidoreductase of Rhodobacter capsulatus. Pool behaviour at the quinone reductase site

Abstract

(1) Electron transfer within the ubiquinol-cytochrome c 2 oxidoreductase ( bc 1) complex produces a charge separation across the membrane which, in some bacterial chromatophores, can be monitored by the electrochromic band shift of carotenoids. The electrogenic steps are believed to coincide with the electron transfer through the cytochrome b chain to the quinone reductase site (Q c) of the complex. (2) The kinetics of these electrogenic reactions have been studied in chromatophores from Rhodobacter capsulatus, activated by a single turnover flash, as a function of the ambient redox potential, and of the size of the ubiquinone pool in isooctane-extracted membranes. (3) In chromatophores containing the native ubiquinone (UQ) pool the rate of charge transfer, monitored by the electrochromic red shift of caretenoids, is progressively increased by lowering the E h below 160 mV. At 100 mV the rate, when normalized to the number of reaction centers turning over, reaches a maximum and declines significantly when the E h is lowered further (down to 40 mV). This rate remains constant for E h values between 40 mV and −40 mV. (4) When the size of the UQ pool is decreased by isoctane extraction, the stimulation in the rate of charge separation takes place only at more negative E h values and the value of the maximum rate is decreased. Also at negative E h values, however, when no pre-oxidized quinone from the pool is present in the membrane, the rate does not decline significantly from its maximum value. (5) The size of the UQ pool affects also the number of charges translocated per flash by the bc 1 complex, since it affects the probability of rapid multiple turnover of the complex, which occurs at E h < 160 mV in unextracted membranes. (6) These data are interpreted as evidence that oxidized UQ from the pool interacts collisionally with the Q c site. At E h < 40 mV, when the pool is totally reduced before the flash, the oxidized UQ molecule produced at every turnover at the Q z site of the bc 1 complex is sufficient for sustaining a rather high reaction rate at the Q c site. The apparent K m estimated for quinone reduction at the Q c site is approx. 1 mM, less than one order of magnitude smaller than that estimated for QH 2 at the Q z site. The rate of transfer of ubiquinone from Q z to Q c required by such a Q cycle mechanism does not appear incompatible with the experimental data.