Electron transfer in reaction centers of Rhodopseudomonas sphaeroides. II. Free energy and kinetic relations between the acceptor states Q(-)A Q(-)B and QAQ(2-)B.

Thermodynamic equilibria and electron transfer kinetics involving the quinone acceptor complex in reaction centers from Rhodopseudomonas sphaeroides were investigated. We focussed on reactions involving the two-electron states QA Qn and QAQ~-, described by the scheme DQAQa~-D +X,~~A- , ~~a- ~k~ .~D+ "r~~ AK~'La2- - k~2~ k~lk O~ (2)D+~D The equilibrium partitioning between QA Q n and QAQ 2n- was determined spectroscopically from either the concentration of oxidized cytochrome c or the concentration of semiquinone after successive flashes of light.At pH < 9.5, QAQ2n - is stabilized relative to QAQn, while for pH > 9.5, QAQB is energetically favored.The reduction of QA, to form QAQ~, is not associated with a protonation step (pK< 8). However, the reduction of Q~, to form the final state QAQ~-, is accompanied by an uptake of a proton (pK >/10.7). The preferential interaction of a proton with QAQ2n - provides the driving force for the forward electron transfer.The shift toward the photochemically inactive state QAQa with increasing pH may serve as a feedback mechanism in photosynthetic organisms to limit the rise in intracellular pH. The electron-transfer rate constants were determined from the observed kinetics and the equilibria between the states QAQ2n - and QA Q n. The forward rate constant z-.A~2n~ was approximately proportional to the proton concentration, whereas kta2A~ depended only weakly on pH. The recombination kinetics of D +QAQ2n- was biphasic. The slow rate agreed with the predicted charge recombination via the intermediate state D +QAQff; the fast rate may be due to the recombination from a separate (conformational) state. The results of this work were combined with those of a previous study on reactions involving the one-electron precursor states QAQa and QAQn(Kleinfeld, D., Okamura, M.Y., and Feher, G. (1984) Biochim. Biophys. Acta 766, 126-140). The overall sequence for the protonation of the reaction center in response to successive reductions of the accept or complex involves the uptake of one proton for each electron transferred to QB- This sequential uptake initiates the formation of a proton gradient across the cell membrane.