Conformational retinal

Figure 12.4 The light-absorbing pigment retinal undergoes a conformational change called isomerization, when it absorbs light.

Fi re 12.6 Schematic diagram Illustrating the proton movements in the photocycle of bacteriorhodopsin. The protein adopts two main conformational states, tense (T) and relaxed (R). The T state binds trans-tetinal tightly and the R state binds c/s-retinal. (a) Stmcture of bacteriorhodopsin in the T state with hflus-retinal bound to Lys 216 via a Schiff base, (b) A proton is transferred from the Schiff base to Asp 85 following isomerization of retinal and a conformational change of the protein. 

The CP MAS NMR spectroscopy has been also extensively used for studies of proteins containing retinylidene chromophore like proteorhodopsin or bacteriorhodopsin. Bacteriorhodopsin is a protein component of purple membrane of Halobacterium salinarium.71 7 This protein contains 248 amino acids residues, forming a 7-helix bundle and a retinal chromophore covalently bound to Lys-216 via a Schiff base linkage. It is a light-driven proton pump that translocates protons from the inside to the outside of the cell. After photoisomerization of retinal, the reaction cycle is described by several intermediate states (J, K, L, M, N, O). Between L and M intermediate states, a proton transfer takes place from the protonated Schiff base to the anionic Asp85 at the central part of the protein. In the M and/or N intermediate states, the global conformational changes of the protein backbone take place. 

The active compound within the bacillary layer is retinal. To simplify the photo-physics within the rods and cones hugely, absorption of a photon initiates a series of conformational changes that lead ultimately to photo-isomerization of retinal from the 11-cis isomer to the 11-trans isomer see Figure 9.20. The uncoiling of the molecule following photo-excitation triggers a neural impulse, which is detected and deconvoluted by the brain. The photochemical reaction is breakage and, after rotation, re-formation of the C=C bond. 

FIGURE 45. Orientation and conformation of retinal in bR, constructed from the individual methyl group orientations that have been determined by solid-state 2H NMR. The angles 6 of the C—CD3 bond vectors with respect to the membrane normal (N) were evaluated for Cis (37°), C49 (40°) and C20 (32°) from the zero-tilt spectra shown in Figure 44 and with the aid of line-shape simulation of the tilt series in Figure 42 and 43. Reprinted with permission from Reference 57. Copyright (1994) American Chemical Society... 

FIGURE 47. Three-dimensional structure of the cyclohexene ring of retinal in bR as determined by 2H NMR, relative to the membrane surface in the x-y plane. Analysis of the orientations of the three deuterium labeled methyl groups on the puckered ring (skew around C1-C6) indicates that the chromophore has a 6s-trans conformation around the C6-C7 bond. Reprinted with permission from Reference 60. Copyright (1997) American Chemical Society... 

The absorption of light by CM-retinal converts it to the trans-form which induces a conformational change in opsin. One photon catalyses the isomerisation of one molecule of... 

In such a way we were able to conclude that the illumination of suspensions of photoreceptor outer segments by 450 nm light at 77°K, which was known to result in the rhodopsin— prelumirhodopsin transition (corresponding to 11-cis-retinal— transretinal photoisomerization of chromophore), leads also to the appearance of some reduction centers and to the conformational change of membrane. 

In a crystal structure47111-ds-retinal has the 12-s-cis conformation shown at the top in Eq. 23-36 rather than the 12-s-frans conformation at the center and in which there is severe steric hindrance between the 10-H and 13-CH3. Nevertheless, H-and 13C-NMR spectroscopy suggest that the retinal in rhodopsin is in a twisted 12-s-frans conformation.472 4723 The Schiff base of 11-ds-retinal with N-butylamine has an absorption maximum at -360 nm but N-protonation, as in the structure in Eq. 23-36, shifts the maximum to 440 nm with emax = 40,600 M 1 cm 1 (Fig. 23-42). This large shift in the wavelength of the absorption maximum (the opsin shift) indicates that binding to opsin stabiliz-... 

What are the chemical structures of the intermediates in Eq. 23-37, and why are there so many of them The answer to the last question is that the initial photochemical process is very fast. Subsequent conformational rearrangments and movement of protons are slower, occur in distinct steps, and give rise to the observed series of intermediates. To shed light on these processes many experiments have been done with analogs of retinal,502,505 508 often using very rapid spectroscopic techniques.37,508 These studies have shown that the isomerization of the Schiff base from... 

There ensues a series of dark reactions or conformational changes that have the effect of greatly activating the imine linkage of the all-frans-rhodopsin towards hydrolysis. On hydrolysis, all-frawj-retina] is released and is unable to recombine with opsin until it is reconverted to the 11-cis isomer. The trans-to-cis rearrangement is a thermal rather than a photochemical reaction and is catalyzed by the enzyme retinal isomerase. The cycle of reactions is summarized in Figure 28-13. 

Isomerization of the retinal Schiff s base can occur when the molecule is excited with light, because the C-l 1-C-12 bond loses much of its double-bond character in the excited state. The valence bond diagrams of figure S2.7 illustrate this point. In the ground state of rhodopsin, the potential energy barrier to rotation about the C-l 1-C-l2 bond is on the order of 30 kcal/mol. This barrier essentially vanishes in the excited state. In fact, the energy of the excited molecule probably is minimal when the C-11 -C-l2 bond is twisted by about 90° (fig. S2.8). The excited molecule oscillates briefly about this intermediate conformation, and when it decays back to a ground state it usually settles into the ail-trans isomer, bathorhodopsin. 

Chen, C.-K., Wieland, T., and Simon, M. I. (1996). RGS-r, a retinal specific RGS protein, binds an intermediate conformation of transducin and enhances recycling. Proc. Natl. Acad. Sci. USA 93, 12885-12889. 

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