The Oxygen-Evolving Complex Requires Chloride To Prevent Hydrogen Peroxide Formation

Illumination of PSII core preparations can cause the production of H₂O₂at rates which approach 60 µmol of H₂O₂(mg of Chl·h)^-1. The rate of peroxide production is maximal at pH 7.2 at low sucrose concentrations and at concentrations of Cl^-(1.5-3.0 mM) that limit the rate of the oxidation of water to O₂. The rate of H₂O₂production increased with pH from pH 6.8 to 7.2 and was inversely proportional to the oxidation of water to O₂from pH 6.8 to 7.5. While EDTA does not inhibit H₂O₂production, this reaction is abolished by 5 mM NH2OH and inhibited by the same concentrations of NH3 that affect water oxidation which indicates that the oxygen-evolving complex is responsible for the production of peroxide generated upon illumination of PSII core preparations. These results support a mechanism in which bound Cl^-in the S₂state is displaced by OH^-ions which are then oxidized by the OEC to form H₂O₂. Thus, the OEC requires Cl^- to prevent access to the active site of the OEC until four oxidizing equivalents can be generated to allow the oxidation of water to O₂.