Photosystem II:: Structure and mechanism of the water:plastoquinone oxidoreductase

This mini-review briefly summarizes our current knowledge on the reaction pattern of light-driven water splitting and the structure of Photosystem II that acts as a water:plastoquinone oxidoreductase. The overall process comprises three types of reaction sequences: (a) light-induced charge separation leading to formation of the radical ion pair P680(+center dot)Q(A)(-center dot); (b) reduction of plastoquinone to plastoquinol at the Q(B) site via a two-step reaction sequence with Q(A)(-center dot) as reductant and (c) oxidative water splitting into O-2 and four protons at a manganese-containing catalytic site via a four-step sequence driven by P680(+center dot) stop as oxidant and a redox active tyrosine Y-Z acting as mediator. Based on recent progress in X-ray diffraction crystallographic structure analysis the array of the cofactors within the protein matrix is discussed in relation to the functional pattern. Special emphasis is paid on the structure of the catalytic sites of PQH(2) formation (Q(B)-site) and oxidative water splitting (Mn4OxCa cluster). The energetics and kinetics of the reactions taking place at these sites are presented only in a very concise manner with reference to recent up-to-date reviews. It is illustrated that several questions on the mechanism of oxidative water splitting and the structure of the catalytic sites are far from being satisfactorily answered.