The mechanism of photosynthetic water oxidation

Abstract

Photosynthetie water oxidation is unique to plants and cyanobacteria, it occurs in thylakoid membranes. The components associated with this process include: a reaction center polypeptide, having a molecular weight (Mr) of 47–50 kilodaltons (kDa), containing a reaction center chlorophyll a labeled as P680, a plastoquinol(?)-electron donor Z, a primary electron acceptor pheophytin, and a quinone electron acceptor Q_A; three ‘extrinsic’ polypeptides having Mr of approximately 17 kDa, 23 kDa, and 33 kDa; and, in all likelihood, an approximately 34 kDa ‘intrinsic’ polypeptide associated with manganese (Mn) atoms. In addition, chloride and calcium ions appear to be essential components for water oxidation. Photons, absorbed by the so-called photosystem II, provide the necessary energy for the chemical oxidation-reduction at P680; the oxidized P680 (P680⁺), then, oxidizes Z, which then oxidizes the water-manganese system contained, perhaps, in a protein matrix. The oxidation of water, leading to O₂ evolution and H⁺ release, requires four such independent acts, i.e., there is a charge accumulating device (the so-called S-states). In this minireview, we have presented our current understanding of the reaction center P680, the chemical nature of Z, a possible working model for water oxidation, and the possible roles of manganese atoms, chloride ions, and the various polypeptides, mentioned above. A comparison with cytochrome c oxidase, which is involved in the opposite process of the reduction of O₂ to H₂O, is stressed. This minireview is a prelude to the several minireviews, scheduled to be published in the forthcoming issues of Photosynthesis Research, including those on photosystem II (by H.J. van Gorkom); polypeptides of the O₂-evolving system (by D.F. Ghanotakis and C.F. Yocum); and the role of chloride in O₂ evolution (by S. Izawa).