The general kinetic model of electron transfer in photosynthetic reaction centers activated by multiple flashes.

Abstract

A new general kinetic model for the functioning of photosynthetic reaction centers (RC) of purple bacteria, under multiple flash activation, has been developed. The model includes the primary electron donor (P870) as well as the primary (QA) and secondary (QB) acceptor quinones. The new features of this general model include: (1) consideration of four different states of the QB binding site (vacant, occupied by QB, by QB- and by QBH2), (2) incorporation of the dark relaxation of the RC between flashes, (3) the assumption of fast exchange of quinones between the RC and quinone pool in detergent micelles or chromatophore membrane, (4) description of the kinetics of electron transfer in both oxidized (no donor for P870+) and reduced (in the presence of donor for P870+) conditions simultaneously, (5) the consideration of both single and multiple flash activation of the RC of purple bacteria and (6) consideration of the cumulative effects of all previous flashes of the series in the response induced by the current flash. This model is used to calculate and predict (1) flash-induced binary oscillations of the secondary acceptor semiquinone (QB-), (2) flash-induced behavior of P870+ in the presence and absence of electron donor and (3) the apparent equilibrium constant of electron transfer between QA and QB and others. Different characteristics of RC are analyzed as a function of flash intensity, time between flashes, concentration of electron donor, redox-potential of the medium, concentration of pool quinone and quinol, association and dissociation equilibrium constants for quinone and quinol at the QB binding site, equilibrium constants of electron transfer between QA- and QB and between QA- and QB-, as well as the rate constants of oxidation of QA- and QB- by redox mediators. The proposed model can be used as a basis for assays of kinetic behavior of native and mutant RC of purple bacteria and for determination of the factors influencing the release of QH2 from RC. The latter is needed for analysis of factors controlling light-activated electron transport in the cytochrome bc1 complexes of purple bacteria by quinol molecules released from RC. The developed general approach for parallel consideration of flash-induced transitions of RC and its following dark relaxation between flashes can also be used for kinetic description of photosynthetic RC of oxygenic photosynthesis.