THE GENERAL KINETIC-MODEL OF ELECTRON-TRANSFER IN PHOTOSYNTHETIC REACTION CENTERS ACTIVATED BY MULTIPLE FLASHES

A new general kinetic model for the functioning of photosynthetic reac tion centers (RC) of purple bacteria, under multiple hash activation, has been developed. The model includes the primary electron donor (P87 0) as well as the primary (Q(A)) and secondary (Q(B)) acceptor quinone s, The new features of this general model include: (1) consideration o f four different states of the Q(B) binding site (vacant, occupied by Q(B), by Q(B)(-) and by Q(B)H(2)), (2) incorporation of the dark relax ation of the RC between flashes, (3) the assumption of fast exchange o f quinones between the RC and quinone pool in detergent micelles or ch romatophore membrane, (4) description of the kinetics of electron tran sfer in both oxidized (no donor for P870(+)) and reduced (in the prese nce of donor for P870(+)) conditions simultaneously, (5) the considera tion of both single and multiple flash activation of the RC of purple bacteria and (6) consideration of the cumulative effects of all previo us hashes of the series in the response induced by the current hash, T his model is used to calculate and predict (1) flash-induced binary os cillations of the secondary acceptor semiquinone (Q(B)(-)), (2) flash- induced behavior of P870(+) in the presence and absence of electron do nor and (3) the apparent equilibrium constant of electron transfer bet ween Q(A) and Q(B) and others. Different characteristics of RC are ana lyzed as a function of hash intensity, time between hashes, concentrat ion of electron donor, redox-potential of the medium, concentration of pool quinone and quinol, association and dissociation equilibrium con stants for quinone and quinol at the Q(B) binding site, equilibrium co nstants of electron transfer between Q(A)(-) and Q(B) and between Q(A) (-) and Q(B)(-), as well as the rate constants of oxidation of Q(A)(-) and Q(B)(-) by redox mediators. The proposed model can be used as a b asis for assays of kinetic behavior of native and mutant RC of purple bacteria and for determination of the factors influencing the release of QH(2) from RC, The latter is needed for analysis of factors control ling light-activated electron transport in the cytochrome bc(1) comple xes of purple bacteria by quinol molecules released from RC. The devel oped general approach for parallel consideration of flash-induced tran sitions of RC and its following dark relaxation between flashes can al so be used for kinetic description of photosynthetic RC of oxygenic ph otosynthesis.