Electron transfer through the quinone acceptor complex of Photosystem II in bicarbonate-depleted spinach thylakoid membranes as a function of actinic flash number and frequency

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In this paper four major points with respect to HCO 3 −-reversible inhibition in spinach thylakoid membranes, depleted of HCO 3 − in the presence of inhibitory anions, are established. (1) The oxidation of Q A − (Q A is the primary quinone acceptor of Photosystem II) by Q B (Q B is the secondary quinone acceptor of Photosystem II) or Q B −, following one or two actinic flashes, respectively, exhibits a smaller t 50 time at which [Q A −] is 50% of maximum [Q A −]) after a flash at pH 7.5 than at pH 6.5 (2) The characteristic oscillations, due to differential rates of Q A − oxidation by Q B or Q B −, observed in the fluorescence flash pattern, generated by assaying the chlorophyll a fluorescence at specific times after an actinic flash and plotting these data as a function of flash number, are lost (i.e., the turnover of two electron gate is hampered). (3) At 1 Hz, the slowest oxidation of Q A −, as indicated by f 50 values, depends on both the pH and flash number: at pH 6.5, t 50 for Q A − decay reaches a maximum value after only three flashes (one turnover of the two-electron gate), whereas at pH 7.5, the t 50 is increased further until after five flashes (two turnovers of the two-electron gate). (4) The t 50 values of Q A − oxidation also depend on the actinic flash frequency: at 5 Hz, [Q A −] reaches its maximum after flash 5 even at pH 6.5. The increase observed in the t 50 values of Q A − oxidation in treated membranes is accompanied by the presence of slow components of Q A − oxidation in the 0.1–10 s range which can achieve an amplitude of more than 70%. These components are suggested to be related to protonation steps involved in the quinone acceptor complex of Photosystem II and support the conclusion that the rate-limiting step in electron transfer in HCO 3 −-depleted thylakoids may be the protonation of Q B − and possibly Q B 2−. A working hypothesis is presented that explains the flash, frequency and pH dependence of Q A − decay observed in this paper.