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Photosynth Res. 2005 Jun;84(1-3):325-31.

Considerations on the mechanism of photosynthetic water oxidation - dual role of oxo-bridges between Mn ions in (i) redox-potential maintenance and (ii) proton abstraction from substrate water.

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  • 1Freie Universit├Ąt Berlin, FB Physik Arnimallee 14, 14195 Berlin, Germany.


Two mechanistic problems of photosynthetic water oxidation at the Mn complex of Photosystem II (PS II) are considered. (I) In the four Mn-oxidizing transitions, any pure Mn oxidation is predicted to cause an increase in redox potential that renders subsequent oxidation steps impossible (redox-potential problem). Formation of unprotonated oxo-bridges may counteract the potential increase. (II) The O-O formation step without any high-pK bases acting as proton acceptors is energetically unfavorable (acceptor-base problem). The pK of oxides in a bridging position between Mn ions may increase drastically upon reduction of Mn in the water-oxidation step (>10 units), thus rendering them favorable proton acceptors. It is proposed that in PS II, in the course of the four oxidizing transitions at least two unprotonated oxo-bridges are formed. Thereby (i) a redox potential increase is prevented and (ii) proton acceptors are prepared for the O-O formation step. Water oxidation in the O-O bond formation step is facilitated by simultaneous Mn reduction and proton transfer to bridging oxides amounting to hydrogen atom or hydride transfer from substrate water to the Mn-oxo core of the Mn complex of PS II.

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