Halobacterium halobium bacteriorhodopsin from

Ethanol and choline glycerolipids were isolated from calf brain and beef heart lipids by PTLC using silica gel H plates. Pure ethanol amine and choline plasmalogens were obtained with a yield of 80% [74]. Four phosphohpid components in the purple membrane (Bacteriorhodopsin) of Halobacterium halobium were isolated and identified by PTLC. Separated phosphohpids were add-hydrolyzed and further analyzed by GC. Silica gel G pates were used to fractionate alkylglycerol according to the number of carbon atoms in the aliphatic moiety [24]. Sterol esters, wax esters, free sterols, and polar lipids in dogskin hpids were separated by PTLC. The fatty acid composition of each group was determined by GC. 

Fig. 5.4. Structure of the bacteriorhodopsin from Halobacterium halobium. Ribbon diagram of bacteriorhodopsin and retinal as a ball-and-stick model. Bacteriorhodopsin crosses the membrane with seven a-helices that are arranged in a bundle form with the chromophore retinal bound in the interior. According to Kimura et al. (1997), with per-...

A model for the structure of bacteriorhodopsin, a membrane protein from Halobacterium halobium. The protein has seven membrane-spanning segments connected by shorter stretches of hydrophilic amino acid residues. 

The photoinduction ion flux derives from the similarity of vesicle systems to the proton flux in halobacterium halobium cell envelopes in the bacteriorhodopsin photocycle [126]. Liposome permeability to glucose can similarly be induced by photoexdtation in vesicles containing polyacetylene or thiophene as ion mediators [127]. As in planar bilayers, the surface charge [128] of the vesicle and the chain length of the component surfactant [129] influence assodation between the donor-acceptor pairs, and hence the distance of separation of components inside and outside the vesicle walls. 

So far, no protein has been found as a common constituent of all membranes (compare the almost universal existence of the lipids PC and PE), even from the same species. Thus, it seems unlikely that there is a universal structural protein in membranes. The numbers of different proteins in a membrane vary widely according to membrane type. The plasma membrane of the bacterium Halobacterium halobium contains only 1 protein (bacteriorhodopsin), whereas the membrane of another bacterium, Escherichia coli, contains about 100. The plasma membrane of the human red blood cell contains at least 17 different proteins. 

From the bioenergetic point of view, Halobacterium halobium has been studied much more extensively than other extremely halophilic archaea it is in this species that bacteriorhodopsin was first found. (Later it has also been observed in closely related species H. cutirubrum and H. salinarium [2].) In H. halobium, three types of energy-supplying processes have been identified, namely, (i) respiration, (ii) light-dependent ion pumping and (hi) arginine fermentation (Fig. 1). 

Fig. 1. Sequence alignment for three bacteriorhodopsins and two halorhodopsins. For brevity the single-letter amino acid code is used. Alignment and helical assignments are as in ref. [17]. Designations AR-1, a bacteriorhodopsin from Halobacterium sp. aus-1 [44] AR-2, a bacteriorhodopsin from Halobacterium sp. aus-2 [45] BR, bacteriorhodopsin from H. halobium [5] FIR, halorhodopsin from H. halobium [43] PHR, a halorhodopsin from Natronobacterium pharaonis[ l]. The patterns of dots and asterisks indicate either identity in all five sequences (dots only) or identity among the bacteriorhodopsins and halorhodopsins only (dots and asterisks).

Independent support for the validity of the foregoing component analysis is provided by experiments carried out with a mutant bacteriorhodopsin. Purple membranes were isolated from a mutant strain of Halobacterium halobium in which a point mutation at residue 212 (aspartic acid replaced by asparagine) was carried out by a new method of site-directed mutagenesis and expression (43, 44). The photosignal was found to be pH-independent in the range of pH 4-11 (45, 46). This photosignal was found to be a pure B1 component because its time course could be superimposed, after normalization, with that of the pure B1 component observed in a multilayered mutant bacteriorhodopsin film. Thus, Nature does indeed decompose the photosignal in accordance with the outlined component analysis. In other words, the B1 component as defined is indeed a natural entity. 

The membrane protein which has been most extensively studied by CD is bacteriorhodopsin (bR) from Halobacterium halobium. Two aspects of the CD spectrum of bR have been controversial, both of which have been reviewed by the principal protagonists of the opposing viewpoints the analysis of the far-UV CD for secondary structure and the interpretation of the visible CD spectrum arising from the retinal chromo-phore. ... 

Purple membrane(PM) of the extreme halophile Halobacterium halobium is another efficient photosynthetic system.Upon the absorption of visible light the unique protein of this membrane,bacteriorhodopsin (BR),undergoes a complicated photocycle and extrudes protons from the cell interior against their concentration gradient across the membrane.The free energy associated with this electrochemical gradient is used to transform ADP into ATP in the final step of photosynthesis (1,2). 

Moreover, the chromoprotein bacteriorhodopsin from Halobacterium halobium has been incorporated into liposomes of the polymerizable sulfolipid 61. Bacteriorhodopsin was found to be active as a light-driven proton pump in the polymerized liposomes... 

Experimental synthesis of ATP at the octane/water interface was carried out [11, 12, 20]. The proton flow through the ATP-synthetase complex from octane to water was provided by creating an excess (relative to equilibrium) concentration of undissociated (or Lewis) acid in the octane phase (Fig. 37). This was achieved in three ways by direct addition of add-pentachlorphenol to octane, by the action of NADH-ferricyanidreductase of the respiratory chain of submitochondrial particles, and also through the action of the H -pump of bacteriorhodopsin sheets from Halobacterium halobium. 

The purple membrane was one of the very first membranes to be characterised by AFM [169] here trimers of bacteriorhodopsin (BR) were observed on membranes of Halobacterium halobium. BR serves as a light-driven proton (H ) pump. It contains eight typtophan residues and is composed of seven heUces [170]. Both the membrane and BR have been the subject of a great deal of research [83,171-177] into the detailed structure of the membrane, which consists of approximately 75% BR and 25% lipids [178]. Worcester et al. [179] presented a study of the purple membrane, using platinum/palladium (80/20) wire cantilevers to study the membrane in air at a controlled relative humidity (55-75%). This study enabled the observation of unidirectional parallel rows, spaced approximately 5 nm apart from each other. It was suggested that they directly represented the smface structure of the purple membrane. 

There are two ways in which membranes of diacetylenic lipids containing intrinsic membrane proteins can be obtained either proteins extracted from natural membranes with detergent can be reconstituted into synthetic diacetylenic phosphatidylcholines or the growth medium of micro-organisms incapable of synthesizing their own fatty acids can be enriched with diacetylenic fatty acid. In this laboratory, Ca2+-ATPase from sarcoplasmic reticulum and bacteriorhodopsin from the purple membrane of Halobacterium halobium have been reconstituted into diacetylenic phosphatidylcholines. Provided the more reactive mixed-chain lipids are used polymerisation can be achieved before the protein is denatured by the UV irradiation. Both proteins remain active within polymeric bilayers. 

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