A Calcium-Selective Site in Photosystem II of Spinach Chloroplasts

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Abstract
After acid-treatment of spinach (Spinacia oleracea) chloroplasts, various partial electron transport reactions are inactivated from 25 to 75%. Divalent cations in concentrations from 10 to 50 millimolar can partially restore electron transport rates. Two cation-specific sites have been found in photosystem II: one on the 3-(3,4-dichlorophenyl)-1, 1-dimethylurea-insensitive silicomolybdate pathway, which responds better to restoration by Mg2+ than by Ca2+ ions, the other on the forward pathway to photosystem I, located on the 2,5-dimethylbenzoquinone pathway. This site is selectively restored by Ca2+ ions. When protonated chloroplasts are treated with N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)aziridine, a carboxyl group modifying reagent, presumed to react with glutamic and aspartic acid residues of proteins, restoration of electron transport at the Ca2+-selective site on the 2,5-dimethylbenzoquinone pathway is impaired, while no difference in restoration is seen at the Mg2+ site on the 3-(3,4-dichlorophenyl)-1,1-dimethylurea-insensitive silicomolybdate pathway. Trypsin treatment of chloroplasts modifies the light-harvesting pigment-protein complex, destroys the dibromothymoquinone-insensitive 2,5-dimethyl-benzoquinone reduction, but does not interfere with the partial restoration of activity of this pathway by Ca2+ ions, implying that the selective Ca2+ effect on photosystem II (selective Ca2+ site) is different from its effects as a divalent cation on the light-harvesting pigment-protein complex involved in the excitation energy distribution between the two photosystems.

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