Effects of Ca2+ and EGTA on P680*+ reduction kinetics and O2 evolution of Photosystem II.

We report for the first time significant changes in the P680*+ reduction kinetics of Photosystem II (PS II) in which the 17 and 23 kDa extrinsic polypeptides are intact, in the presence of Ca(2+) or ethylene glycol bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) which were added to vary the Ca(2+) concentration from 5 microM to 30 mM. The decrease in the extent of normal P680*+ reduction decay with lifetimes of 40-370 ns and a corresponding increase in the extent of kinetics with lifetimes of 20-220 micros was interpreted as being due to electron transfer from Y(Z) to P680*+ being replaced by slow forward conduction and by processes including P680*+/Q(A)(-) recombination. The question of whether changes in P680*+ reduction kinetics were caused by loss of Ca(2+) from PS II or by direct interaction of EGTA with PS II was addressed by lowering the free-Ca(2+) concentration of suspensions of PS II core complexes by serial dilution in the absence of EGTA. Despite a significant decrease in the rate of O(2) evolution after this treatment, only small changes in the P680*+ reduction kinetics were observed. Loss of Ca(2+) did not affect P680*+ reduction associated with electron transfer from Y(Z). Since much larger changes in the P680*+ reduction kinetics of intact PS II occurred at comparable free-Ca(2+) concentrations in the presence of EGTA, we conclude that EGTA influenced the P680*+ reduction kinetics by directly interacting with PS II rather than by lowering the free Ca(2+) concentration of the surrounding media. Notwithstanding these effects, we show that useful information about Ca(2+) binding to PS II can be obtained when direct interaction of EGTA is taken into account.