Inorganic Ions Affect Reductant-Mediated Inhibition of the Manganese Cluster of PSII
Although Ca+2 and Cl− are essential cofactors in the oxygen evolving complex, the precise functions and ligation sites of these cofactors in the linear 4-electron oxidation sequence of the Mn cluster in PSII are still unclear. Research on the roles of the extrinsic polypeptides in the oxygen evolving complex has showed that removal of the 17 and 23 kDa proteins enhances the Ca+2 and Cl− requirements for oxygen evolution activity and creates an altered structure in the PSII complex so that Mn can be inactivated by bulky reductants such as hydroquinone (1) or N, N, N’, N’-tetramethyl-p-phenylenediamine (2). These bulky reductants have no inhibitory effect on intact PSII preparations. However, small reductants such as NH2OH can inactivate native as well as salt-washed PSII membranes in which the 17 and 23 kDa species are released. We have examined the interactions among the extrinsic polypeptides, Ca+2, and Cl−, which may influence the ability of a reductant to inhibit oxygen evolution activity. In this communication, we present evidence to show that inorganic ions such as Ca+2 and Cl− can protect against reductant inactivation of oxygen evolution activity.
KeywordsPSII Complex Essential Cofactor PSII Membrane Oxygen Evolution Activity Manganese Cluster
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