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Quantitative relationships between phosphorylation, electron flow, and internal hydrogen ion concentrations in
The Journal of Biological Chemistry
|March 25, 1976
Summary
This study confirms that proton (H+) gradients drive ATP synthesis in chloroplasts. Precise measurements show proton translocation is directly linked to ATP production, with three protons moving for each ATP molecule formed.
Area of Science:
- Plant Physiology
- Bioenergetics
- Photosynthesis Research
Background:
- Understanding the mechanism coupling electron transport to ATP synthesis in photosynthesis is crucial.
- Proton (H+) gradients across thylakoid membranes are hypothesized to drive photophosphorylation.
- Precise measurement of these gradients and their relationship to ATP production requires robust methodologies.
Purpose of the Study:
- To provide further evidence for the role of light-induced H+ concentration gradients (deltapH) in photophosphorylation.
- To investigate the proportionality between internal H+ concentration ([H+]in) and the rate of electron flow.
- To determine the phosphorylation efficiency (P/e2 ratio) and its dependence on H+ concentration.
Main Methods:
- Utilized [14C]hexylamine uptake, measured by centrifugal filtration, to estimate deltapH in spinach chloroplast thylakoids.
- Manipulated light intensity and gramicidin D concentrations to vary H+ concentrations and electron flow rates.
- Analyzed the relationship between phosphorylation efficiency and internal H+ concentration under varying conditions.
Main Results:
- Hexylamine uptake provided precise and reliable estimations of deltapH, independent of thylakoid amount or hexylamine concentration.
- Internal H+ concentration ([H+]in) showed a direct proportionality to electron flow rate when varied by light intensity.
- Calculated phosphorylation efficiency (P/e2) was constant when basal electron flow was accounted for, yielding a value of approximately 1.3.
Conclusions:
- The study provides strong evidence that an electrochemical H+ gradient is the key coupling mechanism for photophosphorylation.
- The rate of ATP formation is directly proportional to the internal H+ concentration raised to the third power ([H+]in3).
- Three protons (H+) are translocated out of thylakoids for every molecule of adenosine triphosphate (ATP) synthesized.