939. Bacteriorhodopsin

Nomenclature

BR.

Description and references

The only protein of the purple membrane of Halobacterium halobium and other halophilic bacteria. It is a rhodopsin-like pigment containing retinal linked to lysine through a Schiff's base and has a mol wt of approx 26,000. Bacteriorhodopsin functions as an energy transducer or “proton pump”; unlike animal rhodopsins, it uses light energy to generate an electrochemical gradient and this stored energy is used by the cell for ATP synthesis and other important energy-requiring functions. Discovery in the purple membrane of Halobacterium halobium: D. Oesterhelt, W. Stoekenius, Nature New Biol. 233, 149 (1971); A. E. Blaurock, W. Stoekenius, ibid. 152. Isoln and identification in H. cutirubrum: S. C. Kushwaha, M. Kates, Biochim. Biophys. Acta 316, 235 (1973). Description of functions: D. Oesterhelt, W. Stoekenius, Proc. Natl. Acad. Sci. USA 70, 2853 (1973). Proposed mechanism of the “proton pump”: K. Schulten, P. Tavan, Nature 272, 85 (1978). Structural elucidation: Y. A. Ovchinnikov et al., FEBS Lett. 100, 219 (1979); H. G. Khorana et al., Proc. Natl. Acad. Sci. USA 76, 5046 (1979). Three-dimensional crystallographic study: H. Michel, D. Oesterhelt, ibid. 77, 1283 (1980). Series of articles on structure, biosynthesis, and energy transduction: Photochem. Photobiol. 33, 417-608 (1981). Review of energy transduction: H. V. Westerhoff, Z. Dancshazy, Trends Biochem. Sci. 9, 112 (1984). Comprehensive reviews: W. Stoekenius et al., Biochim. Biophys. Acta 505, 215-278 (1979); W. Stoekenius, R. A. Bogomolni, Annu. Rev. Biochem. 51, 587-616 (1982). Review of crystallization, structure and function: J. Deisenhoffer, H. Michel, Science 245, 1463-1473 (1989). Review of photophysical properties and optimization for use in bioelectronic devices: K. J. Wise et al., Trends Biotechnol. 20, 387-394 (2002).

Properties

Bacteriorhodopsin is not bleached by light in the same fashion as animal rhodopsins; only small spectral changes are caused by transfer from light to dark. When exposed to light, BR has a broad light absorption maximum at 568 nm that slowly shifts back to 558 nm in the dark. The 558 and 568 nm states are known as the dark-adapted and light-adapted states, respectively. Light-adapted BR, upon absorption of a light quantum, undergoes a series of reversible changes, having intermediate forms with absorption maxima of 590, 550, and 412 nm. uv max (2M HCl): 565 nm. Slow bleaching occurs when membrane suspensions are illuminated in the presence of hydroxylamine or sodium borohydride. When hydroxylamine is used, the uv max is shifted to 360 nm. The bleached membrane is called the “apomembrane”; the protein after loss of the chromophore is termed bacterio-opsin.

Use

As a tool in biological energy transduction research.