‘Backlash’ and the coupling between electron transport and proton translocation in bacterial chromatophores

Abstract

At the onset of continuous illumination of chromatophores from Rhodopseudomonas capsulata there is a burst of H +-disappearance from the external medium, lasting about 100 ms, which coincides with the period of membrane potential (Δψ) development. Three ‘backlash’ factors contribute to the burst. (1) A period of rapid electron transport through the photosynthetic reaction centre and early donors and acceptors, with accompanying charge translocation, preceeds the steady state in cyclic electron transport whose rate is determined by the slower reaction in the cytochrome b c 1 complex. Inhibition of the b c 1 complex with antimycin A reduces by about 50%, the rapid phase of H +-disappearance and Δψ generation. (2) During the early turnovers of electron transport, before Δψ has reached a maximum, the rate of electron transport is relatively unrestricted by thermodynamic back-pressure from Δψ and proceeds rapidly. The rate of electron transport decreases as Δψ rises. The rate of electron transport at high Δψ in steady-state is shown to have a sensitive dependence on the magnitude of Δψ. (3) As Δψ rises at the onset of illumination, the rate of passive ion flux across the chromatophore membranes increases. Because of the non-ohmic conductance of the membrane, the rate at which Δψ is dissipated increases disproportionately as Δψ develops. Consequently, the effect of the dissipative reactions becomes more pronounced as the burst progresses. Valinomycin and K + lead to an enhanced rate of light-induced H + disappearance during the period in which the development of Δψ is depressed. The stimulation of the apparent H + uptake is a consequence of both an increase in the rate of electron transport and a decrease in the rate of Δψ-driven H + efflux. These results are consistent with energy coupling hypotheses in which electron transport is coupled to the translocation of protons between the bulk aqueous phases. In particular, it is shown that electron transport is kinetically tightly coupled to bulk phase proton translocation and Δψ generation, and that Δψ formation restricts electron transport and the appearance of protons in the bulk phase.