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Study of kinetic and thermodynamic properties of secondary quinone acceptor of photosynthetic reaction center in chromatophore membranes of nonsulfur purple bacteria

Study of kinetic and thermodynamic properties of secondary quinone acceptor of photosynthetic reaction center in chromatophore membranes of nonsulfur purple bacteria

Biologicheskie Membrany 2(7): 725-737

Flash-induced absorption changes at 450 nm, reflecting redox changes of semiquinone form of the secondary electron acceptor (Qb.-) of the photosynthetic reaction centre (RC), have been studied in chromatophores of nonsulphur purple bacteria Rhodospirillum rubrum and Rhodopseudomonas sphaeroides. In the presence of redox mediators, fat reducing the flash-oxidized primary donor of RC, the lifetime of the flash-generated QB.- molecules is reversely porportional to the concentration of the oxidized molecules of redox-mediator. The decrease of flash-induced absorption changes at 450 nm with the decrease of redox potential of the medium is due to the increase of QB.- lifetime because of the decrease of concentration of oxidized form of redox mediator and therefore unsufficient time of dark adaptation. A simple kinetical model, making possible to determine the necessary time interval of dark adaptation from known value of the rate constant of QB.- oxidation by redox mediator, redox-mediator concentration and redox potential, is proposed. Redox titration of the flash-induced abdsorption changes at 450 nm in chromatophores of R. sphaeroides, which was held on in the presence of low-potential redox-mediators, completely oxidizing the QB.- during the time of dark adaptation, has shown that the midpoint potential of redox couple QB/QBH2 is 80 .+-. 10 mV (pH 7.0). From the low value of the stability constant of QB.- it can be concluded that long lifetime of QB.- is determined by kinetical rather than by thermodynamical factors. From the data obtained it is concluded that electron transfer from RC to the electron carriers of the cyclic electron transport chain occurs only after two turnovers of the RC and formation of the double-reduced ubiquinol form of the secondary electron acceptor of RC.

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