Bacteriorhodopsin, function

To some extent this has indeed happened. Results so far suggest that bacteriorhodopsin functions in an unexpectedly simple way while incorporating all of what one would expect from an effective energy-transducing system proton transfer across the internal hydrophobic barrier generates a proton-motive force independent of external proton concentration it is essentially irreversible and resists proton back-pressure yet proceeds with minimal thermal losses. The mechanism of chloride transport in halorhodopsin is not yet as clear, but there is reason to believe there is a far-reaching analogy between this system and bacteriorhodopsin. 

Different aspects of bacteriorhodopsin functioning have been studied early [35-37, 223, 233-249]. However, as a rule, the researchers have described the mechanism of proton transport phenomenologically, though the analysis of the results taken from the FT-IR difference spectra yielded detailed descriptions of the proton state in a proton pathway in all of the above-mentioned intermediates (see, e.g., Zundel [6]). 

The mechanism of the polariton absorption by strong symmetric hydrogen bonds has successfully been applied for the description of anomalous of the infrared spectra of the a-KICVHIC crystal. A complex investigation has been done at the disclosing of the molecular mechanism of the bacteriorhodopsin functioning. 

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. 

Fig. 8.19 SEM images of mesolamellar thin films produced by intercalation of nanosheets of (A) aminopropyl-functionalized silica or (B) AMP between stacked purple membrane fragments containing bacteriorhodopsin (scale bars= 10pm).

Bromley, KM., Patil, A.J., Seddon, A. M., Booth, P. and Mann, S. (2007) Bio-functional mesolamellar nanocomposite based on inorganic/ polymer intercalation in purple membrane (Bacteriorhodopsin) films. Advanced Materials, 19, 2433—2438. 

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. 

Wavepacket motion is now routinely observed in systems ranging from the very simple to the very complex. In the latter category, we note that coherent vibrational motion on functionally significant time scales has been observed in the photosynthetic reaction center [15], bacteriorhodopsin [16], rhodopsin [17], and light-harvesting antenna of purple bacteria (LH1) [18-20]. Particularly striking are the results of Zadoyan et al. [21] on the... 

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... 

The dynamics of proton binding to the extra cellular and the cytoplasmic surfaces of the purple membranes were measured by the pH jump methods [125], The purple membranes selectively labeled by fluorescein Lys-129 of bacteri-orhodopsin were pulsed by protons released in the aqueous bulk from excited pyranine and the reaction of the protons with the indicators was measured. Kinetic analysis of the data implied that the two faces of the membrane differ in then-buffer capacities and in their rates of interaction with bulk protons. The extracellular surfaces of the purple membrane contains one anionic proton binding site per protein molecule with pA" 5.1. This site is within a Coulomb cage radius from Lys-129. The cytoplasmic surface of the purple membrane bears four to five pro-tonable moieties that, due to close proximity, function as a common proton binding site. The reaction of the proton with this cluster is at a very fast rate (3 X 1010 M-1 sec ). The proximity between the elements is sufficiently high that even in 100 mM NaCl, they still function as a cluster. Extraction of the chromophore retinal from the protein has a marked effect on the carboxylates of the cytoplasmic surface, and two to three of them assume positions that almost bar their reaction with bulk protons. Quantitative evaluation of the dynamics of proton transfer from photoactivated bacteriorhodopsin to the bulk has been done by using numerical... 

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