Detection of an intermediate of photosynthetic water oxidation

Autor(en): Clausen, J
Junge, W 
Stichwörter: CRYSTAL-STRUCTURE; ELECTRON-TRANSFER; EVOLUTION; KINETICS; MECHANISM; Multidisciplinary Sciences; OXYGEN-EVOLVING COMPLEX; PHOTOSYSTEM-II; PROTON-TRANSFER; REDOX TRANSITIONS; Science & Technology - Other Topics; SPLITTING ENZYME
Erscheinungsdatum: 2004
Herausgeber: NATURE PUBLISHING GROUP
Journal: NATURE
Volumen: 430
Ausgabe: 6998
Startseite: 480
Seitenende: 483
Zusammenfassung: 
The oxygen that we breathe is produced by photosystem II of cyanobacteria and plants. The catalytic centre, a Mn4Ca cluster, accumulates four oxidizing equivalents before oxygen is formed, seemingly in a single reaction step1-8 2H(2)O(<--)(-->) O-2 = 4H(+) 4e(-). The energy and cycling of this reaction derives solely from light. No intermediate oxidation product of water has been detected so far. Here, we shifted the equilibrium of the terminal reaction backward by increasing the oxygen pressure and monitoring ( by absorption transients in the near-ultraviolet spectrum) the electron transfer from bound water into the catalytic centre. A tenfold increase of ambient oxygen pressure (2.3 bar) half-suppressed the full progression to oxygen. The remaining electron transfer at saturating pressure ( 30 bar) was compatible with the formation of a stabilized intermediate. The abstraction of four electrons from water was probably split into at least two electron transfers: mildly endergonic from the centre's highest oxidation state to an intermediate, and exergonic from the intermediate to oxygen. There is little leeway for photosynthetic organisms to push the atmospheric oxygen concentration much above the present level.
ISSN: 00280836
DOI: 10.1038/nature02676

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