Bacteriorhodopsin liposomes

According to the scheme of MNET we can now list the processes that can take place in the bacteriorhodopsin liposomes and write down the flux-force relation for each of the elemental processes. By adding the fluxes of each of the chemical species, we arrive at a set of equations, represented in matrix form  

The parameter contains the free-energy of the absorbed photon, as well as the extra constant that may have to be added according to Eqn. 28 [24]. 

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 number of different proportionality constants in the latter set of equations is reduced by Onsager s reciprocity relation, which states that Ljj = L--. Such a symmetry is also present in these (though not in all) MNET equations. 

In practice, it is not feasible to test the derived equations experimentally by varying all the forces and fluxes independently. Usually some simplification is gained by allowing the system to develop to a specific steady state. A useful steady state for illuminated bacteriorhodopsin liposomes is that of electroneutral total flow, i.e., the condition in which the net movement across the membrane of all chemical species adds up to no charge movement. It can be derived and shown that this condition is attained within seconds, considering the membrane resistance and electrical capacity in the usual salt media [28], Electroneutral total flow is mathematically expressed as  

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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 equations predict the effect of addition of ionophores on the rate of light-driven proton uptake by the bacteriorhodopsin liposomes. Such predictions have been experimentally tested and verified [23,28-30],... 

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. 

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-... 

The photoinduction ion flux derives from the similarity of vesicle systems to the proton flux in halobacterium halobium cell envelopes in the bacteriorhodopsin photocycle [126]. Liposome permeability to glucose can similarly be induced by photoexdtation in vesicles containing polyacetylene or thiophene as ion mediators [127]. As in planar bilayers, the surface charge [128] of the vesicle and the chain length of the component surfactant [129] influence assodation between the donor-acceptor pairs, and hence the distance of separation of components inside and outside the vesicle walls. 

Neutron diffraction of samples with added deuterated water [130], as well as hydrogen exchange studies [118], suggest that no aqueous channels exist between the protein molecules in purple membranes, or between the helices, i.e., the lipids completely fill the spaces. As expected from the crystalline structure and the relatively low lipid/protein ratio, the packing of protein in purple membrane is quite rigid, and the mobility of the protein in the lattice appears low, as determined by flash dichroism [131,132]. On the other hand, bacteriorhodopsin incorporated into liposomes at high lipid/protein ratios exists as a monomeric molecule [133], and shows rapid rotation by this criterion (relaxation time 15 /us) above the phase... 

Because the chromophores in the bacteriorhodopsin trimer are in close proximity, favorable orientation of the retinal absorption vectors would produce electronic interactions within the trimer. The CD spectrum of bacteriorhodopsin is indeed different from that of vertebrate rhodopsin, which exists as a monomer. The observed splitting of the CD spectrum around 574 nm into a negative and a positive band is interpreted, accordingly, as exciton interaction among the three retinals [45-47,217]. Detergent treatment or disruption of the crystalline lattice by thermal motion in liposomes causes the disappearance of the CD splitting and the appearance of a single CD peak. 

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. 

Many different methods [340] are available for the reconstitution of bacteriorhodopsin into liposomes (a) sonication of purple membrane with dried, dispersed lipids [341] (b) co-precipitation of lipids and bacteriorhodopsin from organic solvents, such as dimethylsulfoxide [340] (c) dispersion of purple membrane sheets or monomeric bacteriorhodopsin and lipids in detergents, such as Triton X-100 [340], cholate [17,325], deoxycholate [170], or octyl-glucoside [327,342], followed by dialysis or removal of the detergent with Bio-Beads. 

Dissociation of the purple membrane lattice, followed by incorporation of the bacteriorhodopsin into liposomes [327,356,357], yields a functional proton transport system. Thus, bacteriorhodopsin monomers will translocate protons upon illumination. 

Fig. 24. A schematic representation of light-induced proton translocation and ATP synthesis in liposomes reconstituted by incorporating bacteriorhodopsin and ATP synthase.

It should be mentioned that BLMs and liposomes have been employed to aid the understanding of H. halobium at the molecular level. Incorporation of bacteriorhodopsin (BR) into planar BLMs has been carried out successfully for electrical measurements by several groups of investigators since 1976. Karvaly s group (60, 61) appears to be the first to report both photovoltaic effects and photoconductivity in BR-containing BLMs (for earlier references, see reference 44 and reviews by Hong and Dratz in this volume). 

A crucially important finding is that submitochon-drial particles or vesicles from broken chloroplasts will synthesize ATP from ADP and Pj, when an artificial pH gradient is imposed. Isolated purified FjEg ATPase from a thermophilic Bacillus has been coreconstituted into liposomes with the light-driven proton pump bacteriorhodopsin (Chapter 23). Illumination induced ATP synthesis. 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 ... 

Fig.l Membrane-mimicking environments, (a) Bacteriorhodopsin spin labeled at position 36 and 46 with the software MMM. (b) Schematic drawings of bacteriorhodopsin inserted in micelle, liposome, membrane bilayer and nanodisc. The liposome is not drawn to scale... 

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... 

Reconstitution. The reconstitution of membranes from purified protein and lipid components is a well-known method for the study of membrane protein functionality. Membrane proteins that have been reconstituted into liposomes of polymerizable lipids include ATP ssmthetase (230), bacteriorhodopsin (231,232), rhodopsin (233,234), and porins. 

Freisleben, H. J. Zwicker, K. Jezek, R John, G. Bettin-Bogutzki, A. Ring, K. Nawroth, T. Reconstimtion of bacteriorhodopsin and ATP synthase from Micrococcus luteus into liposomes of the purified main tetraether lipid from Thermoplasma acidophilum proton conductance and light-driven ATP synthesis. Chem. Phys. Lipids 1995, 78, 137-147. 

Hellingwerf, K. J., Arents, J. C., Scholte, B. J., Westerhoff, H. V. Bacteriorhodopsin in liposomes. II. Experimental evidence in support of a theoretical model. Biochim Biophys Acta. 1979, 547, 561-582. 

Seigneuret, M. Rigaud, J. L. Use of the fluorescent pH probe pyranine to detect heterogeneous directions of proton movement in bacteriorhodopsin reconstituted large liposomes. FEES Lett. 1985, 188,101-106. 

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