Detection of an intermediate of photosynthetic water oxidation


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 step1,2,3,4,5,6,7,8 2H2OO2 + 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.

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Figure 1: Ultraviolet absorption transients at a wavelength of 360 nm of dark-adapted PSII core particles under excitation with five short laser flashes (arrows).
Figure 2: Ultraviolet absorption transients as induced by the second and third flash under variation of the gas composition and pressure.
Figure 3: Electron transfer into the catalytic Mn4Ca centre at three different oxygen pressures, and corrected ultraviolet absorption transients at the third flash.
Figure 4: Titration of the corrected absorption transients by elevated oxygen pressure (see text and equation (4)).


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We thank H. Kenneweg for technical assistance, H. Heine for advice on the construction of the pressure cell, R. Debus for cooperation on Synechocystis, and R. Ahlbrink and A. Mulkidjanian for discussions and help. This work was financially supported by the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie and the Land Niedersachsen.

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Correspondence to Wolfgang Junge.

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Clausen, J., Junge, W. Detection of an intermediate of photosynthetic water oxidation. Nature 430, 480–483 (2004).

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