Halobacterium halobium, bacteriorhodopsin

ITowever, membrane proteins can also be distributed in nonrandom ways across the surface of a membrane. This can occur for several reasons. Some proteins must interact intimately with certain other proteins, forming multisubunit complexes that perform specific functions in the membrane. A few integral membrane proteins are known to self-associate in the membrane, forming large multimeric clusters. Bacteriorhodopsin, a light-driven proton pump protein, forms such clusters, known as purple patches, in the membranes of Halobacterium halobium (Eigure 9.9). The bacteriorhodopsin protein in these purple patches forms highly ordered, two-dimensional crystals. 

FIGURE 10.20 A schematic drawing of Halobacterium halobium. The purple patches ( tain bacteriorhodopsin (bR). 

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 three-dimensional structure also has been elucidated for bacteriorhodopsin, an integral membrane protein of the halophilic (salt-loving) bacterium Halobacterium halobium. This protein has been studied intensively because of its remarkable activity as a light-driven proton pump (see chapter 14). It forms well-ordered arrays in two-dimensional sheets that can be studied by electron diffraction. Measurements of the diffraction patterns show clearly that bacteriorhodopsin has seven transmembrane helices (fig. 17.12). 

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. 

Bacteriorhodopsin, is a retinal-containing protein in the purple membrane of a halophilic, (salt-loving) archaebacterium, Halobacterium halobium, which pumps protons out of the cell on activation by light.The three-dimensional structure of bacteriorhodopsin resembles that of rhodopsin in the eye. 

Bacteriorhodopsin, a light-driven proton pump, is a large (27000 Dalton) membrane protein, located in the purple membrane of halobacterium halobium (for a recent review see Lanyi, 1993). It spans the membrane by seven-a-helices (see Fig 6.6-7). The chro-mophore retinal is embedded inside the protein, shielded by the helices. Retinal connects to the Lys 216 of the protein via a protonated Schiff s base (Fig. 6.6-7). 

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). 

Sequencing has shown that the repeated sequences are insertion sequences similar to those widely distributed in the bactena and in eucarya. The first of these characterized was ISHl, an insertion in the bacteriorhodopsin gene of Halobacterium halobium S9. The... 

---