Ion pumping, bacteriorhodopsin

The electrochemical potential across the membrane drives protons back into the cell through a membrane ATP synthase complex very similar to that of mitochondria and chloroplasts. Thus, when 02 is limited, halobacteria can use light to supplement the ATP synthesized by oxidative phosphorylation. Halobacteria do not evolve 02, nor do they carry out photoreduction of NADP+ their phototransducing machinery is therefore much simpler than that of cyanobacteria or plants. Nevertheless, the proton-pumping mechanism used by this simple protein may prove to be prototypical for the many other, more complex, ion pumps. Bacteriorhodopsin ... 

Membranes contain many largely a-helical proteins. Cell surface receptors often appear to have one, two, or several membrane-spanning helices (see Chapter 8). The single peptide chain of the bacterial light-operated ion pump bacteriorhodopsin (Fig. 23-45) folds back upon itself to form seven helical rods just long enough to span the bacterial membrane in which it functions.189 Photosynthetic reaction centers contain an a helix bundle which is formed from two different protein subunits (Fig. 23-31).190 A recently discovered a,a barrel contains 12 helices. Six parallel helices form an inner barrel and 6 helices antiparallel to the first 6 form an outer layer (see Fig. 2-29).191-193... 

In the 1970s and early 1980s, four rhodopsins were discovered in the cytoplasmic membrane of the archaeon Halobacterium salinarum the light-driven ion pumps bacteriorhodopsin (BR ) and halor-hodopsin and the phototaxis receptors sensory rhodopsin 1 (SRP), and sensory rhodopsin 11... 

Thus, the important question of the secondary structure of the transmembrane elements can only be addressed with models and by structural comparison with other transmembrane proteins for which the structure has been resolved. Detailed information on the structure of transmembrane elements is available for the photoreaction center of Rhodopseudomonas viridis (review Deisenhofer and Michel, 1989), cytochrome c oxidase (Iwata et al., 1995) and the OmpF porin of E. coli (Cowan et al., 1992 Fig. 5.3), amongst others. In addition, high resolution electron microscopic investigations and X-ray studies of bacteriorhodopsin, a light-driven ion pump with seven transmembrane elements, have yielded valuable information on the structure and configuration of membrane-spaiming elements (Henderson et al., 1990 Kimura et al., 1997 Pebay-Peyrula et al., 1997 Fig. 5.4). With the successful crystallization of the photoreaction center of Rhodopseudomonas viridis, a membrane protein was displayed at atomic resolution for the first time (Deisenhofer et al., 1985). The membrane-... 

Luecke, H. (2000) Atomic resolution structures of bacteriorhodopsin photocycle intermediates the role of discrete water molecules in the function of this light-driven ion pump. Biochim. Biophys. Acta 1460, 133-156. 

Halorhodopsiti. In addition to bacteriorhodopsin there are three other retinal-containing proteins in membranes of halobacteria. From mutant strains lacking bacteriorhodopsin the second protein, halorhodopsin, has been isolated. It acts as a light-driven chloride ion pump, transporting Cl from outside to inside. Potassium ions follow, and the pump provides a means for these bacteria to accumulate KC1 to balance the high external osmotic pressure of the environment in which they live.578 The amino acid sequences of halorhodopsins from several species are very similar to those of bacteriorhodopsin as is the three-dimensional structure.589 However, the important proton-carrying residues D85 and D96 of bacteriorhodopsin are replaced by threonine and alanine, respectively, in halorhodopsin.590 Halorhodopsin (hR)... 

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). 

Halobacteria contain four rhodopsins bacteriorhodopsin, halorhodopsin, sensor-yrhodopsin, and phoborhodopsin (Fig. 4.2A) [11-17]. Bacteriorhodopsin and halorhodopsin are light-driven ion pumps, which act as an outward proton pump and an inward Ch pump, respectively. Sensoryrhodopsin and phoborhodopsin are photoreceptors that act to produce attractant and repellent responses in phototaxis, respectively. These four archaeal rhodopsins have similar structures seven helices constitute the transmembrane portion of the protein, and a retinal chromophore is bound to a lysine residue of the seventh helix via a protonated SchifF base linkage (Fig. 4.1). A negatively charged counterion is present to stabilize the positive charge inside the protein the counterion is an aspartate except for in halorhodopsin, which possesses a chloride ion. In sensoryrhodopsin, interaction with a transmembrane transducer protein raises the pKa of the aspartate, so that the aspartate is protonated at neutral pH. 

Bacteriorhodopsin is a light-driven ion pump that is found in the purple membrane of the archaebacterium Halobacterium salinarum. Bacteriorhodopsin monomers consist of a bundle of seven transmembrane a-helices that are connected by short interhelical loops and enclose a molecule of retinal that is buried in the protein interior, approximately half way... 

Bacteriorhodopsin is unique among membrane proteins because of its central role in many different scientific disciplines and its use as the proving ground for many different scientific approaches such as laser spectroscopy, membrane biophysics, and molecular biology (1-6). In addition, bacteriorhodopsin has attained a new prominence, not only because it is used as a model of membrane ion pumps, but also because it is increasingly important for such technological applications as advanced photonic and photoelectronic... 

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