Molecular mechanism of water oxidation in photosynthesis based on the functioning of manganese in two different environments

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Abstract
We present a model of photosynthetic water oxidation that utilizes the property of higher-valent Mn ions in two different environments and the characteristic function of redox-active ligands to explain all known aspects of electron transfer from H 2 O to Z, the electron donor to P680, the photosystem II reaction center chlorophyll a . There are two major features of this model. ( i ) The four functional Mn atoms are divided into two groups of two Mn each: [Mn] complexes in a hydrophobic cavity in the intrinsic 34-kDa protein; and (Mn) complexes on the hydrophilic surface of the extrinsic 33-kDa protein. The oxidation of H 2 O is carried out by two [Mn] complexes, and the protons are transferred from a [Mn] complex to a (Mn) complex along the hydrogen bond between their respective ligand H 2 O molecules. ( ii ) Each of the two [Mn] ions binds one redox-active ligand (RAL), such as a quinone (alternatively, an aromatic amino acid residue). Electron transfer occurs from the reduced RAL to the oxidized Z. When the experimental data concerning atomic structure of the water-oxidizing center (WOC), electron transfer between the WOC and Z, the electronic structure of the WOC, the proton-release pattern, and the effect of Cl - are compared with the predictions of the model, satisfactory qualitative and, in many instances, quantitative agreements are obtained. In particular, this model clarifies the origin of the observed absorption-difference spectra, which have the same pattern in all S-state transitions, and of the effect of Cl - -depletion on the S states.

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