Light driven H+-pump

However, subsequent studies on halobacterial retinal proteins seemed to argue against the photoreceptor function of bacteriorhodopsin. It was found that (i) bacteriorhodopsin operates as a light-driven H pump and (ii) in the same halobacteria there are two other retinal proteins, i.e., sensory rhodopsin I and sensory rhodopsin II (also known as photorhodopsin) which are specialized in photoreception rather than in H pumping, being present in much smaller amounts than bacteriorhodopsin [36-43]. It was shown that... 

A thermodynamic assessment of light-driven H -pumps requires the estimation of the conjugate force of the flow of H -ions which is the difference in electrochemical potential of these ions. Ay, between the inner thylakoidal space and the suspending medium. The partitioning of suitable markers is used to determine the two constituents of this quantity, e.g. methylamine for the chemical part and thiocyanate for the electric part, respectively (Rottenberg et al., 1972). The analysis of such data is based on the assumption that the concentration of markers in the whole space outside the thylakoids is constant and equal to that in the suspending medium. 

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... 

Kolbe, M., Besir, H., Essen, L. and Oesterhelt, D. (2000). Structure of the light-driven chloride pump halorhodopsin at 1.8 A resolution, Science, 288, 1390-1396. 

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

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. 

It is instructive to compare this result with that obtained by considering the bacteriorhodopsin liposomes as a black box, with gradients of H, K, Cl and their associated forces, plus the light-driven proton pump with its associated force. According to phenomenological thermodynamics, we would described such a system by the following set of equations in matrix form ... 

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. 

Seki A, Kubo I, Sasabe H and Tomioka H 1994 A new anion-sensitive biosensor using an ion-sensitive field effect transistor and a light-driven chloride pump, halorhodopsin Appl. Biochem. Biotechnol. 48 205-11 Fuller B E, Okajima T L and Hong F T 1995 Analysis of the d.c. photoelectric signal from model bacteriorhodopsin membranes d.c. photoconductivity determination by means of the null current method and the effect of proton ionophores Bioelectrochem. Bioenerget. 37 109-24 Cone R A 1967 Early receptor potential photoreversible charge displacement in rhodopsin Science 155 1128-31... 

The plasma membrane of the archaebacteria Halobacterium sali-narium [H. salinarium) consists of 75% BR and 25% lipids (w/w). As a light-driven proton pump containing seven transmembrane a -helices, BR absorbs light (T- ax = 560 nm) and builds up an electrochemical potential across the inner cell membrane of H. salinarium. BR forms a 2D trigonal crystal in purple membranes (Fig. 17.2A). 

Bjorklund, J., H. Biverstahl, A. Graslund, L. Maler, and P. Brzezinski (2006) Real.time transmembrane translocation of penetratin driven by light-generated proton pumping. Biophys. J. Biophys. Lett. 91, L29-L31. 

The purple membrane was one of the very first membranes to be characterised by AFM [169] here trimers of bacteriorhodopsin (BR) were observed on membranes of Halobacterium halobium. BR serves as a light-driven proton (H ) pump. It contains eight typtophan residues and is composed of seven heUces [170]. Both the membrane and BR have been the subject of a great deal of research [83,171-177] into the detailed structure of the membrane, which consists of approximately 75% BR and 25% lipids [178]. Worcester et al. [179] presented a study of the purple membrane, using platinum/palladium (80/20) wire cantilevers to study the membrane in air at a controlled relative humidity (55-75%). This study enabled the observation of unidirectional parallel rows, spaced approximately 5 nm apart from each other. It was suggested that they directly represented the smface structure of the purple membrane. 

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