Light-driven proton pump

Bacteriorhodopsin contains seven transmembrane a helices Bacteriorhodopsin is 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. 

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... 

When light-driven proton pumping across the thylakoid membrane occurs, a concomitant efflux of Mg ions from vesicles into the stroma is observed. This efflux of Mg somewhat counteracts the charge accumulation due to H ... 

BR from H. salinarum is a light-driven proton pump, which is triggered by the photoisomerization of retinal covalently linked to its Lys216. It consists of a single polypeptide of 248 amino-acid residues, including seven a-helical TM chains A-G and interconnecting loops, as schematically illustrated in Figure 23. BR is one of the most intensively studied membrane proteins. A variety of experimental techniques have shown it to be... 

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. 

These significant findings form the basis of a set of design principles for the construction of molecular photovoltaic cells and other nanoscale electronic devices in which the control of both the rate and directionality of ET processes is an essential requirement. The successful construction of an artificial light-driven proton pump, based on principles of long-range ET processes illustrates the promise of this approach.1501... 

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-... 

Light-driven proton pump (bacterio-rhodopsin)... 

Figure 4.32 Light driven proton pump using a donor-chromophore-acceptor triad and quinone proton transporter across a phospholipid bilayer membrane.

As an attempt to connect the first discussion, which was concerned with diffusion-reaction coupling, with Dr. Williams presentation of enzymes as dynamic systems, I wanted to direct attention to a number of specific systems. These are the energy-transducing proteins that couple scalar chemical reactions to vectorial flow processes. For example, I am thinking of active transport (Na-K ATPase), muscular contraction (actomyosin ATPase), and the light-driven proton pump of the well-known purple... 

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

FIGURE 19-59 Light-driven proton pumping by bacteriorhodopsin. 

A crucially important finding is that submitochon-drial particles or vesicles from broken chloroplasts will synthesize ATP from ADP and P , when an artificial pH gradient is imposed.172186 Isolated purified FjF0 ATPase from a thermophilic Bacillus has been coreconstituted into liposomes with the light-driven proton pump bacteiiorhodopsin (Chapter 23). Illumination induced ATP synthesis.187 These observations support Mitchell s proposal that the ATP synthase is both spatially separate from the electron carriers in the membrane and utilizes the protonmotive force to make ATP. Thus, the passage of protons from the outside of the mitochondria back in through the ATP synthase induces the formation of ATP. What is the stoichiometry of this process ... 

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-... 

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. 

Another unique property of at least some of the halobacteria is the ability to grow phototrophically by employing the light-driven proton pump bacteriorhodopsin. The proton gradient that is produced is used directly to generate ATP (Hartmann et al., 1980 Oesterhelt and Kripphal, 1983). Photoassimilation of CO2 by halobacteria was shown by Danon and Caplan (1977) and Oren (1983). In vivo C02 fixation was demonstrated by Javor (1988) and the existence of the enzyme ribulose-bisphosphate carboxylase activity in several halobacteria was shown by Altekar and Rajagopalan (1990). 

Many of the early genetic studies were done on H. halobium and its related strains. The popularity of these strains stemmed from the fact that several interesting spontaneous mutations could be readily detected. Among the most characterized mutations are those that affect the production of the protein part of the purple membrane, the heavily studied light-driven proton pump bacteriorhodopsin. Spontaneous mutations occur at a frequency of lO 4. Analysis of these mutations showed that in almost every case a foreign DNA sequence was introduced into the bacterioopsin (bop) structural gene or into sequences surrounding it. 

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. 

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