Kinetics and Pathways of Charge Recombination in Photosystem II

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Abstract
The mechanism of charge recombination of the S2QA- state in photosystem II was investigated by modifying the free energy gap between the quinone acceptor QA and the primary pheophytin acceptor Ph. This was done either by changing the midpoint potential of Ph (using mutants of the cyanobacterium Synechocystis with a modified hydrogen bond to this cofactor), or that of QA (using different inhibitors of the QB pocket). The results show that the recombination rate is dependent on the free energy gap between Ph and QA, which confirms that the indirect recombination pathway involving formation of Ph- has a significant contribution. In the mutant with the largest free energy gap, direct electron transfer from QA- to P+ predominates. The temperature dependence of the recombination rate was investigated, showing a lower activation enthalpy in this mutant compared with the WT. The data allow the determination of the rate of the direct route and of its relative weight in the various strains. The set of currently accepted values for the midpoint potentials of the QA/QA-, Ph/Ph-, and P+/P* couples is not consistent with the relatively rapid rate of the indirect recombination pathway found here, nor with the 3% yield of delayed fluorescence as previously estimated by de Grooth and van Gorkom (1981, Biochim. Biophys. Acta 635, 445−456). It is argued that a likely explanation is that the midpoint potentials of the two latter couples are more positive than believed due to electrostatic interactions. If such is the case, the estimation of the midpoint potential of the P+/P and S2/S1 couples must also be revised upward, with values of 1260 and 1020 mV, respectively.

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