Modification of the thermoluminescence properties of Ca²⁺ depleted Photosystem II membranes by the 23 kDa extrinsic polypeptide and by oligocarboxylic acids

Abstract

Photosystem II membranes were isolated from chloroplasts of pokeweed (Phytolacca americana) and rendered deficient in Ca²⁺, an inorganic cofactor of photosynthetic water oxidation. The thermoluminescence properties of such membranes were found to depend on the Ca²⁺-depleting method used. This feature was analyzed with respect to the thermoluminescence emission that accompanied the recombination reaction between the reduced acceptor Q_A ⁻ and the oxidant of the S₂ state. It was determined that the differences observed among various preparations of Ca²⁺-depleted membranes were attributable to the presence or absence of the extrinsic 23 kDa polypeptide on the membranes. The binding of this polypeptide to Ca²⁺-depleted membranes devoid of the 17 and 23 kDa extrinsic polypeptides caused the thermoluminescence to be emitted at a higher temperature due to a further stabilization of an already abnormally stable S₂ state. Addition of the chelators EDTA or EGTA and of citrate brought about a similar response. The conditions required for the upshift of the emission temperature of thermoluminescence strongly resembled those identified by Boussac et al. (FEBS Lett. 277 (1990) 69–74) as responsible for modifying the EPR multiline signal from the S₂ state of Ca²⁺-depleted PS II membranes. Consistent with the authors' interpretation of the reason for this modification, we conclude that the elevated emission temperature of the thermoluminescence emission reflects an abnormal ligand environment of the Mn-center in PS II that may be created by a direct ligation of the added agents to Mn. Evidence is also presented that the return to a normal S₂ after an addition of Ca²⁺ occurs via yet another condition of S₂ which, in terms of its thermoluminescence properties, resembles that of Ca²⁺-depleted membranes before addition of modifying agents, but is not identical to it.