Differential extraction and structural specificity of specialized ubiquinone molecules in secondary electron transfer in chromatophores from Rhodopseudomonas sphaeroides, Ga

In chromatophores from the facultative photosynthetic bacterium, Rhodopseudomonas sphaeroides, Ga, the function of ubiquinone-10 (UQ-10) at two specialized binding sites (Q_B and Q_Z) has been determined by kinetic criteria. These were the rate of rereduction of flash-oxidized [BChl]₂⁺ through the back reaction, or the binary pattern of cytochrome b₅₆₁ (for the Q_b site), and the rapid rate of rereduction of flash-oxidized cytochrome c, or the relative amplitude of the antimycin-sensitive Phase III ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{~ 1.5}\text{ms}$) of the carotenoid spectral shift induced by a single turnover flash at E_h ~ 100 mV (for the Q_Z site). The phenomenon associated with the two binding sites behaved differently on extraction of UQ from lyophilized chromatophores using isooctane. By this selective extraction procedure it has been possible to show that UQ-10 molecules are required at different concentrations in the membrane for specific redox events in secondary electron transfer. The reduction of cytochrome b occurs in particles which no longer show the phenomena associated with Q_Z, but still possess a large proportion of Q_b, while rapid rereduction of flash-oxidized cytochrome c requires an additional complement of UQ-10 (Q_Z). Extracted particles lacking Q_Z and a large amount of Q_B have been reconstituted with different UQ homologs (UQ-1, UQ-3, and UQ-10). Specific redox events have been studied in reconstituted particles. All UQ homologs act as secondary acceptors from the reaction center; UQ-3 and UQ-10, but not UQ-1, are also able to reconstitute the function of Q_Z as electron donor to cytochrome c. Only UQ-10, however, is able to restore normal rates of the overall cyclic electron transfer induced by a train of flashes, and maximal rates of the light-induced ATP synthesis. The results are interpreted in terms of Q-cycle mechanisms in which quinone and quinol at both the Q_Z and Q_b sites are in rapid equilibrium with the quinone pool.