The Basic Properties of the Electronic Structure of theOxygen-evolving Complex of Photosystem II Are Not Perturbed byCa2+ Removal

摘要

Ca²⁺ is an integral component of the Mn4O5Ca cluster of the oxygen-evolving complex in photosystem II (PS II). Its removal leads to the loss of the water oxidizing functionality. The S2′ state of the Ca²⁺-depleted cluster from spinach is examined by X- and Q-band EPR and ⁵⁵Mn electron nuclear double resonance (ENDOR) spectroscopy. Spectral simulations demonstrate that upon Ca²⁺ removal, its electronic structure remains essentially unaltered, i.e. that of a manganese tetramer. No redistribution of the manganese valence states and only minor perturbation of the exchange interactions between the manganese ions were found. Interestingly, the S2′ state in spinach PS II is very similar to the native S2 state of Thermosynechococcus elongatus in terms of spin state energies and insensitivity to methanol addition. These results assign the Ca²⁺ a functional as opposed to a structural role in water splitting catalysis, such as (i) being essential for efficient proton-coupled electron transfer between YZ and themanganese cluster and/or (ii) providing an initial binding site for substratewater. Additionally, a novel ⁵⁵Mn²⁺ signal,detected by Q-band pulse EPR and ENDOR, was observed inCa²⁺-depleted PS II. Mn²⁺ titration,monitored by ⁵⁵Mn ENDOR, revealed a specific Mn²⁺binding site with a submicromolar KD.Ca²⁺ titration of Mn²⁺-loaded,Ca²⁺-depleted PS II demonstrated that the site isreversibly made accessible to Mn²⁺ by Ca²⁺depletion and reconstitution. Mn²⁺ is proposed to bind at oneof the extrinsic subunits. This process is possibly relevant for the formationof the Mn4O5Ca cluster during photoassembly and/or D1repair.