Photosynthesis Research

, Volume 84, Issue 1–3, pp 325–331

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

Article

DOI: 10.1007/s11120-004-7080-2

Cite this article as:
Dau, H. & Haumann, M. Photosynth Res (2005) 84: 325. doi:10.1007/s11120-004-7080-2

Abstract

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.

Keywords

manganese complex oxygen evolution Photosystem II X-ray absorption spectroscopy 

Abbreviations

EPR

electron paramagnetic resonance

EXAFS

extended X-ray absorption fine-structure

PS

Photosystem

TyrZ

Tyr 161 of the D1 protein of PS II

XANES

X-ray absorption near-edge structure

XAS

X-ray absorption spectroscopy

Copyright information

© Springer 2005

Authors and Affiliations

  1. 1.Freie Universität BerlinBerlinGermany

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