Inactivation of the Water-Oxidizing Complex by Exogenous Reductants in PS II Membranes Depleted of Extrinsic Proteins

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We have studied the inactivation of the water-oxidizing complex by exogenous, 'general' reductants in various types of PS II membrane. Extraction of the 33, 23 and 17 kDa proteins from PS II membranes rendered the functional Mn susceptible to rapid solubilization by reductants such as hydroquinone, benzidine and ascorbate, while water analogs, such as NH_2OH, inactivated the complex regardless of the presence of PS II extrinsic proteins. The extent of the inactivation was dependent on the hydrophobicity of the reductants examined. Diphenylcarbazide, an efficient electron donor to Z^+ and D^+, did not inactivate the Mn complex. As reported earlier [Ghanotakis et al. (1984) Biochim. Biophys. Acta 767: 524] , weak illumination decelerated the inactivation of the complex by the various reductants. Kinetic analyses of the flash-induced protection provided evidence about the nature of the light state that was not susceptible to the reductants. This state was generated and decayed with half times of approximately 0.5 and 9 s, respectively. However, such light protection was diminished under Cl^--depleted conditions, at slightly alkaline pH, or when ascorbate was employed as a reductant. Furthermore, we observed that the oxidation of N,N,N',N'-tetramethyl-p-phenylenediamine, which reacts with the Mn complex, was accomplished as a biphasic reaction. The amount of the fast phase, which was almost eliminated after the reconstitution of the 33 kDa protein and Ca^<2+^gt; , was approximately 7 electron equivalents per 200 Chl. From these results, it is likely that the bulky, 'general' reductants reduce the functional Mn directly by solubilizing Mn from the complex in the same way as do the water analogs. The effectiveness of these reductants in the photoactivation of the apo-water-oxidizing complex is also discussed.
日本植物生理学会の論文

Inactivation of the Water-Oxidizing Complex by Exogenous Reductants in PS II Membranes Depleted of Extrinsic Proteins

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