Membrane-bound protein, bacteriorhodopsin

Light is indispensable for life. Green plants and some bacteria use solar energy for the energy source in their photosynthesis [1-3]. Archeal bacteriorhodopsin is a membrane bound protein and works as a light-driven proton pump [4, 5]. Another role of light is information carrier that is recognized in vision and photo-sensors. 

Bacteriorhodopsin (BR) is the only membrane-bound protein in the purple membrane of Halobacterium halobium [12-16]. It is a single-chain polypeptide... 

A nice example of an indirect structure determination using the trNOE method is the study of the conformation of a loop of the membrane protein bacteriorhodopsin (BR) [28]. Antibodies were raised against BR, and subsequently the complex of a heptapetide derived from BR was studied in complex with the antibody by trNOE. The bound conformation is a reasonably good representation of the conformation of the peptide in its native state in BR. 

The membrane-bound bacteriorhodopsin system also provides an example of the illustration of electrochemical aspects of enzymic machineries which were not discussed in the context of this symposium. Membrane-bound enzyme systems are of special interest because they are oriented yielding a vectorial property which was stressed by Peter Mitchell already in the early sixties. The structural mobility of intramembrane molecules is restricted due to the lipid protein interaction. There might be controlled translational motion or rotational motion or in cases such as the purple membrane no gross motion at all. 

Membrane protein side chains (for example, Lys- CH2 Val- CH2) have been shown to possess fast ( is) motions from measurements of specifically deuterated residues in bacteriorhodopsin, even though the membrane environment was relatively rigid and crystalline. Similar approaches showed that the a-CH group rotation is fast (t (is) for membrane-bound, specifically deuterated retinal in the same protein, even at —60°C. Somewhat slower motions (in the ms range) occur in peptide backbones of membrane proteins. 

The CP MAS NMR spectroscopy has been also extensively used for studies of proteins containing retinylidene chromophore like proteorhodopsin or bacteriorhodopsin. Bacteriorhodopsin is a protein component of purple membrane of Halobacterium salinarium.71 7 This protein contains 248 amino acids residues, forming a 7-helix bundle and a retinal chromophore covalently bound to Lys-216 via a Schiff base linkage. It is a light-driven proton pump that translocates protons from the inside to the outside of the cell. After photoisomerization of retinal, the reaction cycle is described by several intermediate states (J, K, L, M, N, O). Between L and M intermediate states, a proton transfer takes place from the protonated Schiff base to the anionic Asp85 at the central part of the protein. In the M and/or N intermediate states, the global conformational changes of the protein backbone take place. 

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