Water oxidation in photosystem II: from radical chemistry to multielectron chemistry.

Abstract

This review article describes the progress that has been made in understanding the photosystem II/oxygen-evolving complex (PSII/OEC). Photosystem II forms the photochemical core of the system: upon light absorption PSII generates oxidizing equivalents or holes at sufficiently high potential to oxidize water. The OEC can exist in five redox states depending on the number of stored oxidizing equivalents. These redox states are designated S{sub 0}-S{sub 4}, with S{sub 4} being the most oxidizing and capable of oxidizing water. The rapid progress in researching this important membrane protein has been due to several factors. First, oxygen evolution has proven to be quite stable to biochemical manipulation. Second, the crystallization of the bacterial reaction center, the realization of the analogies between it and PSII, and the demonstration of the relevance of constructing experiments on the basis of the analogy have provided a shortcut to dissecting the oxygen-evolving system. Finally, because the water-splitting process, like all photosynthetic reactions, can be initiated by light, kinetic studies over the picosecond to second time regime have become possible. The majority of the current work is aimed at determining the structure of the complex.