Effects of Ca 2+ and EGTA on P680 [rad]+ reduction kinetics and O 2 evolution of Photosystem II

Abstract

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 (β-aminoethyl ether)- N, N, N′, N′-tetraacetic acid (EGTA) which were added to vary the Ca 2+ concentration from 5 μM 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 μs 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.