Bacteriorhodopsin A Light-Driven Proton Pump

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. 

The incorporation of a membrane protein into a polymerizable liposome from (22) was demonstrated by R. Pabst n9). The chromoprotein bacteriorhodopsin — a light-driven proton pump from halophilic bacteria — was incorporated into monomeric sulfolipid liposomes by ultrasonication. The resulting proteoliposomes were poly-... 

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). 

Figure 18.26. Testing the Chemiosmotic Hypothesis. ATP is synthesized when reconstituted membrane vesicles containing bacteriorhodopsin (a light-driven proton pump) and ATP synthase are illuminated. The orientation of ATP synthase in this reconstituted membrane is the reverse of that in the mitochondrion.

Previous studies with bacteriorhodopsin, a light-driven proton pump, have demonstrated that this protein can be... 

We summarise recent work on computer modelling and simulation of proteins involved in bioenergetic processes and in peptide-membrane interactions. Homology modelling, electrostatic calculations and conformational analysis of a photosynthetic reaction centre protein are described. Bacteriorhodopsin, a light-driven proton pump protein is examined from several aspects, including its hydration and conformational thermodynamics. Finally, we present results on lipid perturbation on interaction with a cyclic decapeptide antibiotic, gramicidin S. 

When Mitchell first described his chemiosmotic hypothesis in 1961, little evidence existed to support it, and it was met with considerable skepticism by the scientific community. Eventually, however, considerable evidence accumulated to support this model. It is now clear that the electron transport chain generates a proton gradient, and careful measurements have shown that ATP is synthesized when a pH gradient is applied to mitochondria that cannot carry out electron transport. Even more relevant is a simple but crucial experiment reported in 1974 by Efraim Racker and Walther Stoeckenius, which provided specific confirmation of the Mitchell hypothesis. In this experiment, the bovine mitochondrial ATP synthasereconstituted in simple lipid vesicles with bac-teriorhodopsin, a light-driven proton pump from Halobaeterium halobium. As shown in Eigure 21.28, upon illumination, bacteriorhodopsin pumped protons... 

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. 

Proton gradients can be built up in various ways. A very unusual type is represented by bacteriorhodopsin (1), a light-driven proton pump that various bacteria use to produce energy. As with rhodopsin in the eye, the light-sensitive component used here is covalently bound retinal (see p. 358). In photosynthesis (see p. 130), reduced plastoquinone (QH2) transports protons, as well as electrons, through the membrane (Q cycle, 2). The formation of the proton gradient by the respiratory chain is also coupled to redox processes (see p. 140). In complex III, a Q,cycle is responsible for proton translocation (not shown). In cytochrome c oxidase (complex IV, 3), trans-... 

D. Oesterhett, Bacteriorhodopsin as an example of a light driven proton pump , Angew Chem. int. Edit, 15, 1976, 17. 

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). 

Birge, R. R., Nature of the primary photochemical events in rhodopsin and bacteriorhodopsin. Biochim. Biophys. Acta 1016 293, 1990. A review covering rhodopsin s structure, spectroscopic properties and responses to light. This article also discusses the closely related protein, bacteriorhodopsin, which serves as a light-driven proton pump in halo-philic bacteria. 

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. 

Fig. 1.2. The idealized bacteriorhodopsin liposome containing a light-driven proton pump in a membrane with some proton, K, Cl, HCl conductance and allowing some exchange.

These results show that, for practical purposes, we can treat bacteriorhodopsin as a converter that utilizes light of a certain thermodynamic potential to create a gradient of protons of a certain electrochemical potential, and that the rate at which this converter operates is sensitive to a sort of respiratory control phenomenon it is inhibited by the proton gradient which it generates (for review see Ref. 36). Such apparently orthodox behaviour of a light-driven proton pump has been held improbable [12], because it would contradict the idea that photochemical reactions are irreversible . At least (but see also Refs. 29, 30] in this sense the application of MNET to bacteriorhodopsin liposomes has had heuristic value. 

If bacteriorhodopsin is to be a light-driven proton pump, the photochemical reactions of the retinal chromophore will be accompanied by changes in residues not immediately adjacent to the retinal. The individual steps of the photochemical... 

The function of bacteriorhodopsin as a light-driven proton pump is well established from studies [14,70,83-85,323] of whole H. halobium cells, cell envelope vesicles prepared from the cells [78,324], and liposomes [17,18,135,191,325-327] as well as planar films [328-339] into which purple membrane was incorporated. In all of these cases light-dependent net translocation of protons across the membrane is observed, whose magnitude exceeds the number of bacteriorhodopsin molecules in the system by up to two orders of magnitude. 

Experiments on bacteriorhodopsin (BR), which is the basis for a light-driven proton pump in halobacteria, were recently reported [96], The primary photoreaction is believed to be a trans-to-cis isomerization. Absorption of a 620-nm pulse by BR in membranes was followed by measurements of stimulated emission at various probe wavelengths between 695 and 930 nm. The rapid (ti 200 fs) decay of stimulated emission itensity at the bluer wavelengths, slower decay (T2 500 fs) at redder wavelengths, and biexponential decay at intermediate wavelengths were interpreted in terms of partially coherent rotational motion along the Sj potential surface. 

Bacteriorhodopsin was cloned in 1981 (45) and was found to contain seven putative transmembrane (TM) regions composed of hydrophobic amino acids, thus making it the first known seven TM protein (46) however, it is a light-driven proton pump, not a G protein-coupled receptor. The opsin apoprotein of bovine... 

Bacteriorhodopsin is the pigment present in the membranes of the Halobacterium halobium (halophilic bacteria), a light-driven proton pump that synthesizes ATP... 

Bacteriorhodopsin is the sole membrane protein of seven a-helical transmembrane chains present in the purple membrane of Halobacterium salinarum. This is active as a light-driven proton pump through the photoisomerization of retinal (Fig. 2) from the aW-trans, 15-anti to the 13-cis, 15-anti form covalently linked to Lys216 (helix G) of a single-chain polypeptide of 248 amino acid... 

Very recently first SERRS results about bacteriorhodopsin have been communicated by Nabiev et al. Bacteriorhodopsin is a membrane protein found in bacteria which functions as a light driven proton pump. Using the short-range mechanism of SERS (Chapt. 4.1) the active site (retinal chromophore) position of the protein in the membrane has been estimated with high accuracy It is interesting to note, that adsorption of bacteriorhodopsin on silver colloids seems to fix light-induced cyclic transformations in the protein active sites. 

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... 

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