Abstract
Analyses were made of the effects of extraction of the 17,24 kilodalton extrinsic proteins from spinach versus wheat photosystem II (PSII) membranes on Ca abundance and O2 evolution capacity determined in the absence and presence of either Cl− or Ca2+. Extraction of these proteins from spinach PSII routinely diminished steady state O2 evolution by about 70% when assayed in the presence of sufficient Cl−. Additionally, O2 evolution of 17,24 kilodalton-less spinach PSII membranes showed about 2-fold more enhancement by Ca2+ than by Cl− during assay. When the same extraction and assay procedures were applied to wheat PSII membranes, we observed, in contrast to 17,24 kilodalton-less spinach PSII, only about 50% inhibition of O2 evolution and about 2-fold greater enhancement by Cl− than by Ca2+. Irrespective of differences in the magnitude of enhancement of O2 evolution by Ca2+versus Cl− in spinach versus wheat, the Km values for Cl− (about 1.7 millimolar) and Ca2+ (about 1.5 millimolar) were similar for both type preparations. The abundance of Ca specifically associated with fully functional PSII (about 2 and about 3 Ca/200 chlorophyll for spinach and wheat, respectively) was diminished to about 1 per 200 chlorophyll upon 17.24 kilodalton protein depletion. Further treatment of wheat 17,24 kilodalton-less PSII in darkness with 2 molar NaCl/1 millimolar ethyleneglycol-bis(β-aminoethyl ether)-N,N′-tetraacetic acid/20 micromolar A231872 made O2 evolution highly dependent on Ca2+ addition, much like the 17,24 kilodalton-less spinach PSII. Analyses of this Ca2+ effect on O2 evolution revealed both high (Km about 65 micromolar) and low (Km about 1.5 millimolar) affinity Ca2+ sites in wheat 17,24 kilodalton-less PSII. The results suggest that during 17,24 kilodalton extraction by NaCl, spinach PSII is more susceptible than wheat PSII to loss of high affinity Ca and irreversible inhibition of O2 evolution.
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