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

In this paper four major points with respect to HCO3- -reversable inhibition in spinach thylakoid membranes, depleted of HCO3- in the presence of inhibitory anions, are established. (1) The oxidation of QA- (QA is the primary quinone acceptor of Photosystem II) by QB (QB is the secondary quinone acceptor of Photosystem II) or QB-, following one or two actinic flashes, respectively, exhibits a smaller t50 (time at which [QA-] is 50% of maximum [QA-]) after a flash at pH 7.5 than at pH 6.5. (2) The characteristic oscillations, due to differential rates of QA- oxidation by QB or QB-, 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 QA-, as indicated by t50 values, depends on both the pH and flash number: at pH 6.5, t50 for QA- decay reaches a maximum value after only three flashes (one turnover of the two-electron gate), whereas at pH 7.5, the t50 is increased further until after five flashes (two turnovers of the two-electron gate). (4) The t50 values of QA- oxidation also depend on the actinic flash frequency: at 5 Hz, [QA-] reaches its maximum after flash 5 even at pH 6.5. The increase observed in the t50 values of QA- oxidation in treated membranes is accompanied by the presence of slow components of QA- 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 HCO3- -depleted thylakoids may be the protonation of QB- and possibly QB-(2). A working hypothesis is presented that explains the flash, frequency and pH dependence of QA- decay observed in this paper.