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Time-resolved oxygen production by PSII: chasing chemical intermediates
Jürgen Clausen1, Richard J Debus, Wolfgang Junge
1Abteilung Biophysik, Fachbereich Biologie/Chemie, Barbarastr. 11, Universität Osnabrück, D-49069 Osnabrück, Germany.
Biochimica Et Biophysica Acta
|April 22, 2004
Summary
Researchers investigated the oxygen-evolving process in Photosystem II (PSII). They found evidence for a short-lived intermediate in oxygen release, challenging previous assumptions about its existence and duration.
Area of Science:
- Biochemistry
- Photosynthesis research
- Plant physiology
Background:
- Photosystem II (PSII) catalyzes water oxidation, producing dioxygen through a four-step mechanism.
- The S(3)=>S(0) transition is the final step where oxygen is liberated.
- The existence and nature of chemical intermediates in this process remain elusive, with some studies suggesting their presence based on Arrhenius plot deviations.
Purpose of the Study:
- To investigate the temperature dependence of oxygen evolution in PSII.
- To identify and characterize potential chemical intermediates in the water-to-dioxygen pathway.
- To clarify the kinetics and activation energies of oxygen release and associated steps.
Main Methods:
- Monitoring UV-absorption transients related to Mn-cluster and tyrosine Z reduction.
- Employing polarographic measurements with a specialized Pt-electrode to detect oxygen release.
- Analyzing Arrhenius plots of oxygen release and UV-absorption transients across a temperature range of -2 to 32°C.
Main Results:
- No deviation from a linear Arrhenius plot for oxygen release was observed, indicating no sufficiently long-lived intermediate detectable by this method.
- Oxygen release kinetics varied significantly between wild-type and mutant strains (Synechocystis WT* vs. D1-D61N).
- A temperature-dependent lag-phase in polarographic transients, indicative of a short-lived intermediate, was identified and characterized (0.45 ms at 20°C, 31 kJ/mol activation energy).
- This intermediate was not detected in UV-transients, suggesting it is transiently formed and bound.
Conclusions:
- The study provides evidence for a short-lived, transiently bound oxygen intermediate in the water-oxidation cycle of PSII.
- Previous evidence for a long-lived intermediate based on Arrhenius plot breaks is not supported by this high-resolution polarographic approach.
- The findings refine our understanding of the final steps in oxygen evolution, highlighting the dynamic nature of intermediates.