New structure model of oxygen-evolving center and mechanism for oxygen evolution in photosynthesis

So far, many important questions and problems concerning the structure and mechanism of photosynthetic oxygen evolution are still unsolved. On the basis of recent achievements in this field, a new structure model is proposed whereby two H2O molecules bind asymmetrically to two manganese ions (Mn-1(II) and Mn-4(III)) at the open end of "C" shaped cluster and keep rather large distance. Two histidine residues coordinate to the other two manganese ions in higher oxidation state (Mn-2(IV) and Mn-3(IV)) through their nitrogen atoms of the imidazole. CI bound as terminal ligand to Mn-4(III) is connected to Ca, and the latter is needed to maintain the special configuration of two Mn2O2 units by bridged-ore and bridged-carboxylate ligands. The whole structure of oxygen evolution center is asymmetry. A new mechanism for oxygen evolution invokes predictions of asymmetric oxidation of two H2O molecules, dynamic structural changes of oxygen evolving center and indirect proton transport, etc. Only in S-2 state, could Mn-1(IV) = O. intermediate with high oxidation potential be formed. The S-2 --> S-3 process occurs with significant structural changes, as well as intramolecular and intermolecular hydrogen transfer. The S-3 state corresponds to intermediate of Mn-1(IV)-O ... H ... O-Mn-4(IV). During S-3 --> [ S-4 ] --> S-0 , the O-O bond is formed only in S-4 state. The change of nucleophilic interaction between CI and manganese ions different oxidation states has consequence for the significant structural changes in H2O oxidation process.