期刊
BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS
卷 1857, 期 1, 页码 23-33出版社
ELSEVIER SCIENCE BV
DOI: 10.1016/j.bbabio.2015.09.011
关键词
Photosystem II; Water-oxidizing complex (WOC); Oxygen-evolving complex (OEC); Equilibrium constant for S-4 -> S-0 transition
资金
- Energimyndigheten [36648-1]
- Strong Research Environment Solar Fuels Umea (Umea University)
- Artificial Leaf Project Umea (K&A Wallenberg Foundation) [KAW 2011.0055]
- COST Action CARISMA (Catalytic Routines for Small Molecule Activation) [CM1205]
- CNRS
- Initiative d'Excellence program from the French State [ANR-11-LABX-0011-01]
- HelioBiotec platform
- EU
- Region PACA
- French Ministry of Research
- CEA
Photosynthetic water oxidation to molecular oxygen is carried out by photosystem II (PSII) over a reaction cycle involving four photochemical steps that drive the oxygen-evolving complex through five redox states S-i (i = 0, ... , 4). For understanding the catalytic strategy of biological water oxidation it is important to elucidate the energetic landscape of PSII and in particular that of the final S-4 --> S-0 transition. In this short-lived chemical step the four oxidizing equivalents accumulated in the preceding photochemical events are used up to form molecular oxygen, two protons are released and at least one substrate water molecule binds to the Mn4CaO5 cluster. In this study we probed the probability to form S-4 from S-0 and O-2 by incubating YD-less PSII in the S-0 state for 2-3 days in the presence of O-18(2) and (H2O)-O-16. The absence of any measurable O-16,18(2) formation by water-exchange in the S-4 state suggests that the S-4 state is hardly ever populated. On the basis of a detailed analysis we determined that the equilibrium constant K of the S-4 --> S-0 transition is larger than 1.0 x 10(7) so that this step is highly exergonic. We argue that this finding is consistent with current knowledge of the energetics of the S-0 to S-4 reactions, and that the high exergonicity is required for the kinetic efficiency of PSII. (C) 2015 The Authors. Published by Elsevier B.V.
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