Methoxyretinals in bacteriorhodopsin. Absorption maxima, cis-trans isomerization and retinal protein interaction

Photocurrent generation in a light-harvesting system with multifunctional artificial nanochannels

Inspired by the biological light-driven proton pump, a light-harvesting system based on multifunctional artificial nanochannels is developed for photocurrent generation.

Bacteriorhodopsin wildtype and variant aspartate-96 --> aspargine as reversible holographic media

Air dried films of purple membranes (PM) from Halobacterium halobium containing the photochromic protein bacteriorhodopsin (BR) were prepared and the BR-photocycle of this material analyzed. The absorption maxima of the initial state B (lambda(max) = 570 nm) and the photochemical intermediate M (lambda(max) = 412 nm), which is the longest living intermediate in suspension (tau approximately 10 ms), were spectrally well separated. Light-induced population gratings between B and M were used for reversible holographic recording in these dry PM films. The resolution (>5,000 lines/mm) of PM films was comparable to the corresponding values of conventional photochromic recording materials. The longterm stability toward photochemical degradation of PM films is excellent (> 100.000 recording cycles). The spectral bandwidth (400-680 nm) of such films covers nearly the whole visible spectrum. Both the photochemical transition from B --> M with wavelengths in the green-red range and from M --> B with blue light were utilized for holographic recording. The latter possibility (M --> B) seems to be advantageous for several applications because the holographic grating is only formed during reconstruction. Higher reading intensities lead to higher population of the M-state and result in an increase of the fringe contrast instead of decreasing it. New possibilities for the further development of holographic media based on bacteriorhodopsin are raised by the availability of PM variants with modified optical properties. By the use of the variant BR-326, which differs from the wildtype PM by a single amino acid exchange (aspartate-96 --> asparagine), the sensitivity of PM films is increased by approximately 50% from 12 cm(2)/J to 19 cm(2)/J for recording with 568 nm. The sensitivity for recording with 413 nm (33 cm(2)/J) is not influenced by the amino acid exchange. The observed diffraction efficiency eta of PM films with BR-326 is twice that of BR-wildtype (BR-WT) films and is in the range of conventional organic photochromics ( approximately 1%). In dried films of both BR-WT and BR-326 the M-decay was shown to be at least biexponential.

The chromophore induces a correct folding of the polypeptide chain of bacteriorhodopsin

The pK values of the Schiff bases of several bacteriorhodopsin (BR) preparations have been determined by titration. While for the native protein a high pK of 13 has been reported [Druckmann et al. (1982) Biochemistry 21, 4953], we find that a BR reconstituted from retinal and the apoprotein obtained from the retinal-deficient strain JW5 exhibits a low pK value, 8.5. When the retinal chromophore is added to growing JW5 cells leading to in vivo BR formation, this BR shows a high Schiff base pK, >/=10.2. A value of 9.3 was determined when BR was reconstituted from retinal and BO, obtained from bleaching BR with hydroxylamine. A low pK value of 8.1 was found when 13-trifluoro(CF3)-retinal was used as chromophore for in vitro reconstitution [Sheves et al. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 3262], which is confirmed in this study. When we add CF3-retinal to growing JW5 cells, this low pK shifts to 9.1. Besides wild-type protein, the apoprotein from the mutant D96N (from the chromophore-deficient strain L-07) was also used for in vitro reconstitution with either chromophore, retinal or CF3-retinal. Irrespective of the chromophore used, both mutant BRs exhibit low pK values of their Schiff bases of 8.1. Flash photolysis with respect to the rise and decay of the M-photocycle intermediate of wild-type and D96N-mutated BR carrying retinal and CF3-retinal revealed that in both proteins the incorporation of the trifluororetinal leads to a faster rise of the M-intermediate and to a slower decay. Since the apoprotein from the chromophore-deficient JW5 strain of H. salinarium, despite its lower boyant density, is arranged into trimers (according to CD measurements), we propose that the high pK value of the BR Schiff base is induced by long-distance interactions between BR molecules in the purple membrane patches which control the pK of the chromophore.

Bacteriorhodopsin: a biological material for information processing

Technology which makes use of biological materials has advanced dramatically in the last few decades. Production of specific biochemicals by selected microbial strains, the use of enzymes for stereospecific biosynthesis of materials and gene technological production of biologically important macromolecules are a few examples of these developments.

Factors affecting the absorption maxima of acidic forms of bacteriorhodopsin. A study with artificial pigments

The absorption maximum (568 nm) of light-adapted bacteriorhodopsin bR568 undergoes reversible changes after acidification. At pH 2.9, the absorption shifts to 605 nm (forming bR605) and it blue shifts to 565 nm, after further acidification to pH approximately 0.5 (forming bR565). Molecular models accounting for such acid-induced changes are relevant to the structure and function of bacteriorhodopsin. In the present study we approached the problem by applying artificial bR pigments based on selectively modified synthetic retinals. This may allow direct identification of the specific regions in the retinal binding site where the above changes in the protein-retinal interactions take place. We investigated the spectroscopic effects of acid in a variety of artificial pigments, including cyaninelike retinals, retinals bearing bulky groups at C4, short polyenes, and retinals in which the beta-ionone ring was substituted by aromatic rings. The results provide direct evidence for the hypothesis that the generation of bR605 is due to changes in polyene-opsin interactions in the vicinity of the Schiff base linkage. The second transition (to bR565) was not observed in artificial pigments bearing major changes in the ring structure of the retinal. Two approaches accounting for this observation are presented. One argues that the generation of bR565 is associated with acid-induced changes in retinal-protein interactions in the vicinity of the retinal ring. The second involves changes in polyene-opsin interactions in the vicinity of the Schiff base linkage. For both bRw and bRN5, our results do not discriminate between the direct titration of negative or dipolar protein groups in the binding site and changes in the retinal-protein interactions induced by changes in the protein structure outside of the binding site.