Bacteriorhodopsin

• UniProt: P02945

Bacteriorhodopsin (BR ) is an integral membrane protein in the cell membrane of Halobacterium salinarum extremophile organism ( halobacteria ). The protein BR is the light energy converter of phototrophic energy of Halobacterium salinarum. The Halobacterium salinarum accomplished phototrophy differs fundamentally from Photosynthesen, for example, of the oxygenic photosynthesis of plants. The light energy is not used here for the cleavage of water, but is used on the energy converter BR to establish a proton concentration difference between the inside ( cytoplasm ) and the outer space of the bacterium. This concentration difference is the energy source for the ATP synthase. Due to its exceptional functionality, the protein is of great scientific interest, also a number of technical applications has been proposed for BR.

History

The name " bacteriorhodopsin " for a protein of the purple membrane (purple membrane) of Halobacterium salinarum ( halobacteria ) proposed in 1971 the biochemist Dieter Oesterhelt and Stoeckenius Walther electron microscopists. Together with the biophysicist Allen E. Blaurock they had demonstrated that the purple membrane of Halobacterium contains a crystalline vorliegendes retinal protein. Two years later Oesterhelt and Stoeckenius characterized the function of the purple membrane as a light-dependent proton transport, and interpreted this as a simple form of photosynthesis. At the latest after Richard Henderson and Nigel Unwin had created in 1975 by electron microscopy, a structural model of the protein, bacteriorhodopsin became a model object of bioenergetics, membrane and structural biology. From the mid- 1970s appeared on the subject of more than one hundred publications per year, and related proteins such as halorhodopsin or Sensorhodopsine were described. Plans for the technical use of the bacteriorhodopsin -containing purple membrane can be found since the late 1970s.

Structure of the protein

The protein consists of 248 amino acids of BR, which, arranged in seven approximately parallel alpha helices that traverse the cell membrane and form a pore. In this pore is a protein bound to the retinal molecule. Retinal, the chromophore of the molecule and usually called a imine bond in this connection as a Schiff base, attached to the amine function of the amino acid Lys216. Under physiological conditions, the chromophore is present only as all-trans and 13- cis isomer. The isomerization is effected under exposure to light.

BR is the cell membrane of Halobacterium salinarum in, assembled into trimers, two-dimensional crystalline regions. Current up to five micron areas where BR trimers are present in two-dimensional hexagonal arrangement in the lipid bilayer, hot purple membrane (PM). The embedding of the BR in the purple membrane leads to a remarkable stability of the protein against physico- chemical influences. So stay color and photochemical activity of the PM in the presence of oxygen as well as get in the dry state.

Function of the protein

BR can serve as a light -driven energy molecular machine are considered, the pumping protons. In a multi-step process initiated by the light-induced isomerization of the chromophore and is driven by changes in the proton affinities of amino acid functions, protons are displaced from the cytoplasmic to the extracellular side of the pore of the protein. Inducing for the directed proton shift is the isomerization of the retinal chromophore due to light absorption. The chromophore is in the unexposed state, as a mixture of all-trans and 13 -cis-retinal after exposure only in the 13-cis -configuration. This has, due to the embedding of the chromophore structural changes of the protein sequence, which has a direct effect on the first of the protonated state of the Schiff base. This proton is located after isomerization in an energetically unfavorable environment and is discharged to the direct interaction partner of the Schiff base, Asp85, in extracellular direction. It is a sequence of four other unidirectional proton shifts linked before finally the output state of the protein is restored and a new cycle can be traversed. This light-driven proton pumping is linked to a cyclic sequence spectroscopically distinguishable states of the protein. This sequence is called the photocycle. The passage of the photo cycle is due to exposure to yellow ( M state, absorption maximum 410 nm ) is connected to a reversible color change from purple ( 570 nm B-stage, the absorption maximum ).

Technical Applications

Security printing pigment

First practical application was Bacteriorhodopsin as a photochromic pigment in security inks.

Protein stores with 50 terabytes of capacity

This is made possible this enormous space by the light sensitivity of the BR protein. When light falls on the protein, so it turns into a series of different molecules, each of which has a characteristic shape and color, before it returns to the initial state. This intermediate stage is for the production of chemical energy and keeps in nature about an hour. By modifying the DNA of the bacterium, this condition can be maintained for several years. Applied to the binary system of computer technology, means the ground state 0 and the modified state, a first