Photosystem II: The machinery of photosynthetic water splitting

Abstract

This review summarizes our current state of knowledge on the structural organization and functional pattern of photosynthetic water splitting in the multimeric Photosystem II (PS II) complex, which acts as a light-driven water: plastoquinone-oxidoreductase. The overall process comprises three types of reaction sequences: (1) photon absorption and excited singlet state trapping by charge separation leading to the ion radical pair ${\text{P}}680^{ + \bullet } {\text{Q}}_{\text{A}}^{ - \bullet } \left( { \overset{\wedge}{=}{\text{P}}_{\text{D1}}^{ + \bullet } {\text{Q}}_{\text{A}}^{ - \bullet } } \right)$ formation, (2) oxidative water splitting into four protons and molecular dioxygen at the water oxidizing complex (WOC) with ${\text{P}}680^{ + \bullet }$ as driving force and tyrosine Y_Z as intermediary redox carrier, and (3) reduction of plastoquinone to plastoquinol at the special Q_B binding site with ${\text{Q}}_{\text{A}}^{ - \bullet }$ acting as reductant. Based on recent progress in structure analysis and using new theoretical approaches the mechanism of reaction sequence (1) is discussed with special emphasis on the excited energy transfer pathways and the sequence of charge transfer steps: $^{1} \left( {\text{RC-PC}} \right)^{ *} {\text{Q}}_{\text{A}} \to {\text{P}}_{{{\text{D}}2}} {\text{P}}_{{{\text{D}}1}} {\text{Chl}}_{{{\text{D}}1}}^{ + \bullet } {\text{Pheo}}_{{{\text{D}}1}}^{ - \bullet } {\text{Q}}_{\text{A}} \to {\text{P}}_{{{\text{D}}2}} {\text{P}}_{{{\text{D}}1}}^{ + \bullet } {\text{Chl}}_{{{\text{D}}1}} {\text{Pheo}}_{{{\text{D}}1}}^{ - \bullet } {\text{Q}}_{\text{A}} \to {\text{P}}_{{{\text{D}}2}} {\text{P}}_{{{\text{D}}1}}^{ + \bullet } {\text{Chl}}_{\text{D1}} {\text{Pheo}}_{\text{D1}} {\text{Q}}_{\text{A}}^{ - \bullet } ,$ where ¹(RC-PC)* denotes the excited singlet state ¹P680* of the reaction centre pigment complex. The structure of the catalytic Mn₄O _X Ca cluster of the WOC and the four step reaction sequence leading to oxidative water splitting are described and problems arising for the electronic configuration, in particular for the nature of redox state S₃, are discussed. The unravelling of the mode of O–O bond formation is of key relevance for understanding the mechanism of the process. This problem is not yet solved. A multistate model is proposed for S₃ and the functional role of proton shifts and hydrogen bond network(s) is emphasized. Analogously, the structure of the Q_B site for PQ reduction to PQH₂ and the energetic and kinetics of the two step redox reaction sequence are described. Furthermore, the relevance of the protein dynamics and the role of water molecules for its flexibility are briefly outlined. We end this review by presenting future perspectives on the water oxidation process.