Bacteriorhodopsin proton translocation

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-... 

Dencher, N. A., Dresselhaus, D., Zaccai, G., and Bueldt, G. (1989). Structural changes in bacteriorhodopsin during proton translocation revealed by neutron diffraction. Proc. Natl. Acad. Sci. USA 86, 7876—7879. 

Sass, H.J., Buldt, G., Gessenich, R., Hehn, D., Neff, D., Schlesinger, R., Berendzen, J., and Ormos, P. (2000). Structural alterations for proton translocation in the M state of wild-type bacteriorhodopsin. Nature 406, 649-653. 

Subramaniam, S., and Henderson, R. (2000b). Molecular mechanism of vectorial proton translocation by bacteriorhodopsin. Nature 406, 653-657. 

Figure 1 Functional residues and bound water in the extracellular region of bacteriorhodopsin. The all-frons retinal is shown in purple and the hydrogen bonds in gold. The direction of overall proton translocation is indicated with an arrow. The Schiff base (NZ) and the Ci 3 atom of the retinal are labeled. Coordinates from Reference 3.

The N decay step completes the transport cycle even though the recovery of bacterio-rhodopsin has not yet taken place. Until this point in the photocycle, absorption of a second photon (e.g. by M in the blue-light effect ) defeats the transport process as it recovers bacteriorhodopsin without net proton translocation [137,138]. Absorption of a second photon by N, however, results in completion of the transport cycle and this takes place more rapidly than it does by the thermal route [139]. It appears then that the reactions after N serve simply to recover the chromophore. 

The 13-c/j retinal-chromophore in dark-adapted bacteriorhodopsin exhibits a very different photocycle, whose predominant intermediate has an absorption maximum at 610 nm [199], and which contains no intermediate [202,238] analogous to M. The 610 nm intermediate will decay to either the 13-c/s chromophore or the dW-trans form, the latter pathway being responsible for the phenomenon of light-adaptation [199]. This pathway does not explain, however, why monomeric bacteriorhodopsin shows poor light-adaptation [168,239]. The chromophore in the 13-c/s configuration is not associated with proton translocation [240]. Indeed, reconstitution of bacterio-opsin with 13-demethyl retinal, which traps the retinal moiety in the 13-c/s configuration, results [241] in a non-transporting photocycle. 

Instead relies on the vectorial transport of proteins which leads to the establishment of an electrochemical proton gradient across the cell membrane. The energy for proton translocation Is provided by light which Is absorbed by the chromophore of bacteriorhodopsin. 

Further evidence for the light-driven proton translocation by bacteriorhodopsin was obtained by Racker and Stoeckenius who carried out reconstitution ofthe purple membrane into phospholipid vesicles. In the light, the reconstituted vesicles took up protons from the exterior at a rate of 50-200 ng per mg of bacteriorhodopsin, and released them in the dark. The rate of proton uptake in the fight and release in the dark was accelerated by the addition ofvalinomycin, while uncouplers of oxidative phosphorylation abolished the uptake of protons altogether. Note that the direction of proton transport from outside to the inside of the vesicle reported by the authors was opposite to that observed in intact cells, the possibility that bacteriorhodopsin might be oppositely oriented in cells and vesicles, was subsequently confirmed by freeze-etch electron microscopy. 

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

Bacteriorhodopsin is the quintessential transmembrane ion pun ). It consists of a small, seven-helix protein where proton transport across the membrane is driven by photoisomerization of retinal from the all trans to the 13-cis,l5-anti configuration. A number of high-resolution crystal structures of the protein and its photointermediates have been used to propose several competing mechanisms describing proton translocation to fhe extracellular surface. Unresolved issues include understanding how conformational changes couple to proton transfer and the role played by water molecules in the proton transfer process. ... 

The third alternative is proton exchange along hydrogen-bonded water molecules (33-35). In bacteriorhodopsin, for example, a recent structural model at 3.5-A resolution strongly suggests that water molecules form a narrow channel and are involved in proton delivery to the chromophore (36). The remainder of this review will discuss chains of hydrogen-bonded water molecules as potential proton translocators and describe some initial tests of the concept. 

Our analysis of the possible role of the ERP suggests another attribute of intelligent materials that is, modular design of molecular functions. The ability of bacteriorhodopsin to bind protons from the cytoplasm upon light stimulation serves as a critical step in proton translocation. The same event in rhodopsin, however, may serve an entirely different function it triggers the cyclic GMP cascade. Thus, Nature could well have utilized a common design for vision and for photosynthesis (85). The same design principle may be implemented with completely different types of molecules or materials. On the other hand, the same molecular event may be exploited for different purposes. 

Various models for the mechanism of proton translocation by bacteriorhodopsin have been postulated. The essential features of these models are in the ground state the Q-ring of the aW-trans retinal group is tightly bound in a hydrophobic pocket... 

The hght-induced proton translocation by bacteriorhodopsin at the planar interface of octane/water [10,12,19,20] and in octane-water emulsions [64] has been studied. A retinotoxin thought to form a stable shift base with retinol in rhodopsin, inhibited the light-actived proton transport [64]. 

There is now abundant evidenee that bacteriorhodopsin is a light-activated vectorial ion pump that translocates protons across the bacterial plasma membrane. The mechanism of proton translocation is unknown, although the process is linked to a complex photocycle that involves a number of intermediates [Mowery and Stoeckenius (1981) and Stoeckenius and Bogomolni (1982) see also articles in Packer (1982)] in which the Schiff base is protonated in the BR 570 state and deprotonated in the MR 412 state (Lewis et al., 1974 Aton et al., 1977 Rothschild et al., 1981). The electrochemical gradient generated in this way is used by the cell for ATP synthesis. 

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