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

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 H₂O 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 H₂O 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 H₂O 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.