A study of the kinetic properties of the stable semiquinone of the reaction-center secondary acceptor in chromatophores of non-sulfur purple bacteria |
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Institution: | 1. Heinrich Pette Institute, Leibniz Institute for Experimental Virology, Hamburg, Germany;2. European XFEL GmbH, Schenefeld, Germany |
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Abstract: | Flash-induced absorption changes at 450 nm were investigated in isolated chromatophores of Rhodopseudomonas sphaeroides and Rhodospirillum rubrum non-sulfur purple bacteria to follow the redox changes of the semiquinone species of the secondary quinone acceptor of the photosynthetic reaction center. Excitation of a dark-adapted chromatophore suspension by a series of successive flashes in the presence of electron donors capable of rapidly reducing the photooxidized reaction-center pigment causes the formation of a stable semiquinone species (Q⨪B) with a lifetime which is shown to be proportional to the amount of the oxidized redox mediator in the incubation medium. It is shown that the disappearance of the flash-induced absorption changes at 450 nm on lowering the ambient redox potential (Eh) to 200–300 mV is the result of increasing the lifetime of Q⨪B, as the amount of the oxidized mediator diminishes; consequently, in these circumstances, the 2–5 min dark interval between the flash cycles appears insufficient for Q⨪B recovery. After the addition of redox mediators with a low midpoint potential, acting as an oxidant for Q⨪B, the flash-induced redox changes of Q⨪B were observed at low Eh values unless Eh reached a value at which QB underwent reduction at equilibrium to form QBH2. The data provide evidence that reaction centers with a fully oxidized secondary acceptor can donate electrons to the cyclic electron-transport chain only after two turnovers, leading to the formation of the doubly reduced ubiquinone species (QBH2) of the secondary acceptor. |
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