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Bacteriorhodopsin in ice. Accelerated proton transfer from the purple membrane surface
1Dept. of Physics, Freie Universität, Berlin, Germany.
FEBS Letters
|December 17, 1990
Summary
Proton transfer in bacteriorhodopsin is faster in ice than in water. This suggests proton translocation occurs through an ice-like structure within the protein.
Area of Science:
- Biophysics
- Membrane Protein Dynamics
- Proton Transfer Mechanisms
Background:
- Bacteriorhodopsin (BR) is a light-driven proton pump crucial for energy transduction.
- Understanding proton translocation kinetics and surface/bulk transfer is key to BR function.
Purpose of the Study:
- To investigate the photocycle and proton pumping kinetics of BR in both water and ice.
- To determine the rate of proton transfer from the membrane surface to the aqueous bulk phase.
Main Methods:
- Utilized purple membranes containing bacteriorhodopsin.
- Employed optical pH indicators (pyranine and fluorescein) to monitor proton release in aqueous and frozen states.
- Analyzed photocycle intermediates (L550 to M412) and M formation rates.
Main Results:
- In water, proton release to the bulk phase (monitored by pyranine) lagged behind surface release (monitored by fluorescein).
- In ice, pyranine and fluorescein responded simultaneously, indicating the surface/bulk transfer is not rate-limiting.
- Proton ejection occurs during the L550 to M412 photocycle transition.
- Arrhenius analysis of M formation suggests an ice-like structure facilitates proton translocation.
Conclusions:
- The rate-limiting step for proton transfer changes significantly between aqueous and frozen states.
- Proton translocation through bacteriorhodopsin likely involves an ice-like pathway within the protein structure.
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