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Cation binding by bacteriorhodopsin.

C H Chang1, J G Chen, R Govindjee

  • 1University of Illinois, Department of Physiology and Biophysics, 407 South Goodwin Avenue, 524 Burrill Hall, Urbana, IL 61801.

Proceedings of the National Academy of Sciences of the United States of America
|January 1, 1985
PubMed
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Removing divalent cations like calcium and magnesium from bacteriorhodopsin changes its color to blue and disrupts its photochemical cycle. Adding cations restores the purple color and function, highlighting the role of ions in bacteriorhodopsin structure and activity.

Area of Science:

  • Biochemistry
  • Biophysics
  • Membrane Protein Research

Background:

  • Bacteriorhodopsin, a light-driven proton pump in purple membranes, plays a crucial role in cellular energy transduction.
  • The native purple membrane contains bound divalent cations, primarily calcium and magnesium, associated with bacteriorhodopsin molecules.
  • The precise function of these bound cations in bacteriorhodopsin's structure and photocycle remains incompletely understood.

Purpose of the Study:

  • To investigate the impact of divalent cation removal on bacteriorhodopsin's spectral properties and photochemical activity.
  • To explore the role of cations in maintaining the purple membrane's native structure and function.
  • To characterize the cation-binding sites and their influence on the bacteriorhodopsin photocycle.

Main Methods:

Related Experiment Videos

  • Extensive washing of purple membranes to remove endogenous divalent cations.
  • Spectroscopic analysis (UV-Vis absorption) to monitor spectral shifts.
  • Photochemical assays to study the bacteriorhodopsin photocycle intermediates.
  • Reconstitution experiments with various cations to restore native properties.

Main Results:

  • Extensively washed purple membrane exhibits a spectral shift from purple to blue (λmax ≈ 600 nm) upon removal of bound Ca2+ and Mg2+ ions.
  • The blue membrane intermediate is similar to that observed at low pH and can be reverted to purple by adding various cations.
  • Divalent and trivalent cations are more effective than monovalent cations in restoring the purple color, with efficiency approaching one cation per pigment molecule.
  • Removal of divalent cations abolishes the unprotonated Schiff base (M-type) intermediate and alters the photochemical cycle.
  • Lanthanum treatment displaces native cations and significantly reduces the decay rate of the M412 intermediate and proton uptake.

Conclusions:

  • Divalent cations are essential for maintaining the purple color and native photochemical cycle of bacteriorhodopsin.
  • Cation binding sites within the purple membrane are critical for the protein's structural integrity and functional activity.
  • The study provides insights into the ion-binding mechanisms and their regulatory role in proton pumping by bacteriorhodopsin.