Towards understanding the role of calcium in O2 evolution of PS II

Abstract

Calcium is a necessary cofactor for optimal O2 evolution of thylakoid membrane fragments. The reduction of P680+ by electron transfer from the O2 evolving centre (OEC) is also affected by changes in Ca2+ concentration. Because the rate of P680+ reduction varies with the S-state of PS II in a five-flash cycle, it is assumed to be correlated with the rate of O2 evolution. We investigated the P680+ reduction kinetics of PS II enriched thylakoids in the presence of CaCl2 and the Ca2+-specific chelator ethylene glycol bis (b-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). As expected, the extent of P680+ decay with 20-200 ns decay times decreased as the free-Ca2+ concentration decreased. Plots of the amplitudes of these decay components as a function of Ca2+ concentration indicate non-cooperative binding of Ca2+ to a site with a dissociation constant of 19.1 mM and to another site with a lower affinity for Ca2+. Although the amplitudes of the 20-200 ns P680+ decay kinetics decreased as the EGTA concentration was increased, the O2 evolution rate of continuously illuminated thylakoid membranes remained constant. Possible reasons for this were explored using an extended reversible radical-pair model of PS II kinetics. The modelling demonstrates a poor correlation between flash-induced P680+ decay with 20-200 ns kinetics and O2 evolution under continuous illumination. We conclude that this effect is due to a rate-limiting step for O2 evolution at the PS II acceptor side and a rapid exchange of free Ca2+ at a site that controls electron transfer from the OEC to P680