Calcium retards NH2OH inhibition of O2 evolution activity by stabilization of Mn2+ binding to photosystem II.
Calcium is required for oxidation of water to molecular oxygen by photosystem II; the Ca2+ demand of the reaction increases upon removal of 23- and 17-kDa extrinsic polypeptides from detergent-derived preparations of the photosystem. Employing the manganese reductant NH2OH as a probe to examine the function of Ca2+ in photosystem II reveals that (1) Ca2+ slows the rate of NH2OH inhibition of O2 evolution activity, but only in photosystem II membranes depleted of extrinsic proteins, (2) other divalent cations (Sr2+, Cd2+) that compete for the Ca2+ site also slow NH2OH inhibition, (3) Ca2+ is noncompetitive with respect to NH2OH, (4) in order to slow inhibition, Ca2+ must be present prior to the initiation of NH2OH reduction of manganese, and (5) Ca2+ appears not to interfere with NH2OH reduction of manganese. We conclude that the ability of Ca2+ to slow the rate of NH2OH inhibition arises from the site in photosystem II where Ca2+ normally stimulates O2 evolution and that the mechanism of this phenomenon arises from the ability of Ca2+ or certain surrogate metals to stabilize the ligation environment of the manganese complex.
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Department of Biology, University of Michigan, Ann Arbor 48109-1048.
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D002118:Calcium / Q000494:pharmacology*; D004492:Edetic Acid / Q000494:pharmacology; D019811:Hydroxylamine; D006898:Hydroxylamines / Q000494:pharmacology*; D007700:Kinetics; D046911:Macromolecular Substances; D008345:Manganese / Q000378:metabolism*; D008970:Molecular Weight; D010100:Oxygen / Q000378:metabolism*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D045332:Photosystem II Protein Complex; D010944:Plants / Q000378:metabolism*; D011485:Protein Binding
D006898:Hydroxylamines; D046911:Macromolecular Substances; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D019811:Hydroxylamine; D008345:Manganese; D004492:Edetic Acid; D010100:Oxygen; D002118:Calcium