Light induced oxidative water splitting in photosynthesis: Energetics, kinetics and mechanism

The essential steps of photosynthetic water splitting take place in Photosystem II (PS II) and comprise three different reaction sequences: (i) light induced formation of the radical pair P680(+center dot) Q(A)(-center dot) (ii) P680(+center dot) driven oxidative water splitting into O-2 and four protons, and (iii) two step plastoquinone reduction to plastoquinol by Q(A)(-center dot). This mini-review briefly summarizes our state of knowledge on energetics, kinetics and mechanism of oxidative water splitting. Essential features of the two types of reactions involved are described: (a) P680(+center dot) reduction by the redox active tyrosine Y-z, and (b) sequence of oxidation steps induced by Y-z(ox) in the water-oxidizing complex (WOC). The rate of the former reaction is limited by the non-adiabatic electron transfer (NET) step and the multi-phase kinetics shown to originate from a sequence of relaxation processes. In marked contrast, the rate of the stepwise oxidation by Y-z(ox) of the WOC up to the redox level S-3 is not limited by NET but by trigger reactions which probably comprise proton shifts and/or conformational changes. The overall rate of the final reaction sequence leading to formation and release of O-2 is assumed to be limited by the electron transfer step from the S-3 state of WOC to Y-z(ox) due to involvement of an endergonic redox equilibrium. Currently discussed controversial ideas on possible pathways are briefly outlined. Several crucial points of the mechanism of oxidative water splitting, like O-O bond formation, role of local proton shift(s), details of hydrogen bonding, are still not clarified and remain a challenging topic of future research. (C) 2011 Elsevier B.V. All rights reserved.