Calcium limits substrate accessibility or reactivity at the manganese cluster in photosynthetic water oxidation

The structural changes which the tetramanganese cluster responsible for catalyzing photosynthetic water oxidation undergoes upon calcium depletion of photosystem II membranes via the citrate extraction method has been further characterized. The modified multiline EPR signal which forms has been further identified with an S2' oxidation state. The increased number of hyperfine lines (at least 26) and 25% narrower hyperfine splittings from 55Mn versus the normal S2-state signal indicate a redistribution of spin density, most likely within a spin-coupled tetranuclear Mn cluster. A simpler binuclear Mn description for this signal can be eliminated. Slow conformational changes occur over 30 min which cause subtle changes in the hyperfine structure. Comparison to the modified multiline signals produced by Sr2+ replacement of Ca2+ and NH3-treated PSII reveal similarities suggestive of formation of the same spin S = 1/2 state. Substrate accessibility in the dark S1' state, measured as Mn2+ release upon incubation with NH2OH, is increased by 10-fold over calcium-containing PSII centers. Diphenylcarbazide, an efficient electron donor to Tyr-Z+ only in PSII membranes in which Mn is removed or dislocated, was found to donate electrons in Ca(2+)-depleted PSII, indicating altered accessibility or reactivity. These results suggest a possible "gatekeeper" role for Ca2+ in limiting access of substrate water to the Mn cluster. These changes are not due to release of the three extrinsic polypeptides of PSII which remain bound. Citrate treatment also causes partial air oxidation of the reaction-center Fe2+ ion, associated with the quinone electron acceptors. The resulting Fe3+ possesses an EPR signal at g = 4.37 arising from conversion to a rhombic symmetry ligand field.