Redox potential dependence of photophosphorylation and electron transfer in continuous illumination of Rhodopseudomonas sphaeroides chromatophores

Abstract

The dependence on redox potential ( E h) of the steady-state photophosphorylation rate in chromatophores of Rhodopseudomonas sphaeroides Ga was measured using slowly equilibrating (and hence less interfering) redox mediators. The remaining interference of the mediators was taken into account by extrapolating to zero mediator concentration. The extents of cytochrome redox reactions (in the presence of antimycin) and of the carotenoid shift were similarly measured. The redox titration of cytochrome c oxidation is consistent with a requirement for prior cytochrome c 2 reduction and prior Q I (primary electron acceptor) oxidation, while the titration of cytochrome b reduction is consistent with a requirement for prior (BChl) 2 reduction and prior cytochrome b 560 oxidation. The steady-state carotenoid shift extent is a much more broadly peaked function of E h than is the extent following a single-turnover flash, indicating that the transmembrane electrical potential difference can be built up to significant levels by minimal rates of electron flow. The photophosphorylation rate, in contrast, is much more strongly E h dependent, supporting the concept of a threshold membrane energization below which phosphorylation cannot occur. Earlier work by others showing a requirement for equilibrium cytochrome c 2 reduction and Q I oxidation is clearly confirmed. A much greater enhancement in phosphorylation rate was found at low E h than has heretofore been reported. This threefold enhancement is discussed in relation to the equilibrium redox states of the ubiquinone-10 complement of the chromatophore membrane.