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Bacteriorhodopsin can function without a covalent linkage between retinal and protein

U Schweiger1, J Tittor, D Oesterhelt

  • 1Max-Planck-Institute for Biochemistry, Martinsried, FRG.

Biochemistry
|January 18, 1994
PubMed
Summary

The covalent bond in bacteriorhodopsin is not essential for its function as a light-driven proton pump. This study explored a mutant lacking this bond, revealing its continued, albeit reduced, proton transport capabilities.

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Area of Science:

  • Biochemistry
  • Molecular Biology
  • Biophysics

Background:

  • Bacteriorhodopsin (BR) is a light-driven proton pump that uses retinal covalently linked to lysine 216.
  • Photon absorption by retinal triggers protein conformational changes essential for proton translocation.

Purpose of the Study:

  • To investigate the necessity of the covalent bond between retinal and lysine 216 for bacteriorhodopsin function.
  • To characterize the proton pumping activity and spectral properties of a mutant lacking this covalent linkage.

Main Methods:

  • Expression of a lysine 216 to alanine mutant in Halobacterium salinarium.
  • Reconstitution of the mutant apoprotein with various retinylidene Schiff bases.
  • Spectroscopic analysis (absorbance maxima) and proton pumping measurements (flash photolysis).

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Main Results:

  • The mutant protein could be reconstituted, with retinylideneethylamine forming the most stable chromoprotein.
  • Proton pump activity was observed in whole cells (30 mol H+/mol BR/min), though lower than wild-type.
  • Spectra showed distinct absorbance maxima (475 nm, 620 nm, 568 nm) dependent on pH.
  • Flash photolysis revealed M-intermediate formation only from the 568-nm species, with a decay half-time of 17 ms.

Conclusions:

  • The covalent linkage between retinal and the protein is not fundamentally required for bacteriorhodopsin's light-driven proton pump activity.
  • Mutant bacteriorhodopsin retains partial proton transport function, indicating alternative mechanisms or reduced efficiency without the covalent bond.