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Reaction cycle and thermodynamics in bacteriorhodopsin.
1Department of Physiology and Biophysics, University of California, Irvine 92717.
Acta Physiologica Scandinavica. Supplementum
|January 1, 1992
Summary
Light-induced proton transport in bacteriorhodopsin involves retinal isomerization and a key switch step. This study details the photocycle
Area of Science:
- Biophysics
- Photochemistry
- Protein Dynamics
Background:
- Bacteriorhodopsin utilizes light energy to transport protons across membranes.
- The photocycle involves retinal isomerization and proton transfer steps.
- Understanding energy coupling in ion pumps is a fundamental biophysical question.
Purpose of the Study:
- To elucidate the reaction sequence and thermodynamics of the bacteriorhodopsin photocycle.
- To investigate the role of specific proton transfer steps in energy coupling.
- To analyze the protein's conformational changes during the photocycle.
Main Methods:
- Time-resolved absorption spectroscopy using a gated multichannel analyzer.
- Measurements conducted over a range of temperatures (5-30°C) and timescales (100 ns to 100 ms).
- Analysis based on a proposed single-cycle model with reversible reactions.
Main Results:
- Confirmed the existence of the M1-->M2 reaction as the proton transfer switch.
- Calculated thermodynamic parameters pinpointed energy exchange at the switch step.
- Observed a significant entropy decrease at the switch, indicating protein conformational change.
Conclusions:
- The M1-->M2 reaction is crucial for proton transfer and energy coupling in bacteriorhodopsin.
- Protein conformational changes at the switch step conserve free energy for the photocycle.
- Findings offer insights into the mechanism of ion pumps and energy transduction.