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Proton evolution from photosystem II. Stoichiometry and mechanistic considerations
Biochimica Et Biophysica Acta
|November 17, 1977
Summary
Dark-adapted chloroplasts show oscillating proton release with flashes, not matching oxygen yield patterns. Four protons are released during water oxidation, with specific amounts at each step.
Area of Science:
- Photosynthesis research
- Chloroplast biophysics
- Water oxidation mechanisms
Background:
- Understanding the intricate process of water oxidation in photosynthesis is crucial for comprehending energy conversion in plants.
- The relationship between proton release and oxygen evolution during the catalytic cycle of photosystem II remains an area of active investigation.
Purpose of the Study:
- To precisely quantify proton release during the flash-induced catalytic cycle of water oxidation in dark-adapted chloroplasts.
- To elucidate the specific proton release pattern across the four steps of the water oxidation cycle.
- To compare the oscillation patterns of proton release and oxygen evolution.
Main Methods:
- Utilizing a sequence of light flashes on dark-adapted chloroplasts to monitor proton release.
- Employing ferricyanide reduction as a calibration method to quantify proton release associated with oxygen evolution.
- Analyzing the influence of pre-flashes on the oscillation patterns of proton and oxygen yields.
Main Results:
- Proton release oscillates with a period of 4, exhibiting a pattern distinct from oxygen evolution oscillations.
- Two protons are released during the terminal oxygen-liberating step, with the remaining two released during precursor reactions.
- Specific proton release amounts were determined for each transition: S1→S2 (0 protons), S0→S1 (0.75 protons), and S2→S3 (1.25 protons).
- The observed proton release pattern excludes a simple 1, 0, 1, 2 distribution across the four steps of water oxidation.
Conclusions:
- The study provides a detailed quantitative analysis of proton release during the water oxidation cycle in chloroplasts.
- The findings refine our understanding of the proton stoichiometry and timing within the catalytic cycle of photosystem II.
- A model is proposed to interpret the complex flash yield patterns of proton release, offering insights into the mechanism of water oxidation.