Opsin conformation

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 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-... 

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. 

Studies of retinal isomers and retinal analogs in solution were useful in that they permitted to speculate on the conformation of the opsin-bound 1 l-c/s-retinal, an important problem in the stereochemistry of vision. Although there was a priori no reason to believe that the conformation of 1 l-cis-retinal when bound to opsin will be identical either to its solution conformation or to its conformation in the crystalline... 

Data is also shown for bovine rhodopsin, 63, for comparison. The hydroretinals 66-68 presumably assume 9-cis or 11 -cis like conformations when bound to opsin. Retinals 67 and 68 form non-bleachable pigments, i.e., no change in their A, occurs upon exposure to room light irradiation by UV light leads to decomposition products instead of separation of the chromophore from opsin. a In MeOH. In case of split chromophores the absorption maxima of the enal moieties are given. b Protonated Schiff base with n-butylamine in MeOH. ... 

In rhodopsin, substitution of Glul34 " (the E in E/DRY) with Gin enhances the basal activity of retinal-free opsin suggesting that the mutation causes the receptor to adopt a conformation that resembles the light activated wild type protein (Gohen et al., 1993). The acidic functional group of Glul34 is proposed to form a salt bridge with a primary amine... 

The absorption of light by rhodopsin results in a change in the configuration of the retinaldehyde from the 11 -cis to the all- trans isomer, together with a conformational change in opsin. This results in both the release of retinaldehyde from the Schiff base and the initiation of a nerve impulse. The overall process is known as bleaching, because it results in the loss of the color of rhodopsin. 

VKainin A (retinol) is present in many cells of the body. In the retina it is part of the rod photoreceptor pigment rhodopsin (=cis-retina] + a protein called opsin) and cone photoreceptor pigment iodopsin (= cis-retinal + a different opsin protein). When light strikes the cis-retinal part of the rhodopsin or iodopsin molecule, the "cis form changes to trans-retinal, a conformational in (he t i04 culv ti.c CIjUaU... 

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