Abstract
The interaction of CaCl2, and H2O2, was studied in photosystem II (PSII) enriched submembrane preparations depleted in the extrinsic polypeptides associated with oxygen evolution. In PSII preparations, depleted of 16 and 23 kDa polypeptides, the addition of exogenous CaCl2; substantially stimulated the rate of H2O oxidation but had no effect on the rate of H2O2; oxidation. In PSII preparations depleted of 16, 23 and 33 kDa polypeptides, addition of CaCl2, strongly inhibited the H2O2, oxidation when mediated via exogenous Mn2+. The inhibition kinetics of H2O2, oxidation by CaCl2; in these PSII preparations were negatively correlated with the retention of native Mn‐atoms in the PSII core complex. These results suggest that removal of 16 and 23 kDa extrinsic polypeptides from the PSII oxygen‐evolving complex causes disorganisation of Ca2+ and Cl and allows H2O2, to undergo oxidation and to donate electrons to P680 via the native Mn‐cluster and/or exogenous Mn2+. However, readdition of Ca2+ and Cl to the depleted preparations restores the native conformation of the PSII core complex, consequently inhibiting H2O2; oxidation.

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