11-c/s-Retinal

FIGURE 17.11 Imine formation between the aldehyde function of 11-c/s-retinal and an amino group of a protein (opsin) is involved in the chemistry of vision. The numbering scheme in retinal is specifically developed for carotenes and related compounds. 

Rhodopsin is a transmembrane protein linked to 11-c/s-retinal, which, on photoabsorption, decomposes to opsin and all-f/a/75-retinal. Rhodopsin has a molecular weight of about 40,000. Its C-terminus is exposed on the cytoplasmic surface of the disk, and its sugar-containing... 

The all-trams forms of the vitamins A predominate but 11-c/s-retinal is the lightabsorbing chromophore of the visual pigments... 

Figure 23-40 (A) Diagram of a vertebrate rod cell. From Abrahamson and Fager.446 (B) Electron micrograph of a longitudinal section of the outer segment of a retinal rod of a rat. There are 600-2000 discs per rod and 2 x 104 and 8 x 10s rhodopsin molecules per disc. Courtesy of John E. Dowling. (C) Enlarged section from Dratz and Hargrave.451 (D) Schematic drawing of rhodopsin. The two a helices in the front have been partly cut away to reveal the 11-c/s retinal in protonated Schiff base linkage to lysine 296. From Nathans.448 Courtesy of Jeremy Nathans.

P. Du and E. R. Davidson, /. Phys. Chem., 94, 7013 (1990). Ab-Initio Study on the Excitation-Energies of the Protonated Schiff-Base of 11-c/s-Retinal. 

When rhodopsin absorbs light in the vision process, the cis double bond between carbons 11 and 12 isomerizes to a trans double bond. This isomerization triggers a nerve impulse telling the brain that light has been absorbed by the eye. The imine of the isomerized product is unstable and is hydrolyzed to opsin and the all-trans form of retinal (also known as vitamin A aldehyde). All-trans retinal is converted back to 11 -c/s-retinal by enzymes so that it can be used again in rhodopsin formation. 

The human eye uses a c/s-alkene, 11-c/s-retinal, to detect light, and a cis-trans isomerism reaction is at the heart of the chemical mechanism by Which we see. The light-sensitive pigment in the cells of the retina is an imine, formed by reaction of ll-ci ... 

Photochemical c/s-trans isomerization in a conjugated polyene system is thought to be the crucial primary process in vision. The visual pigment (rhodopsin) is derived from 11-c/s-retinal by reaction of the aldehyde group with an amino substituent in a protein (opsin). There is considerable distortion in the geometry ofthischromophoric group anyway, because of the spatial requirements of the protein... 

Fig. 5.10 Representation of the conformation of all-fransretinal, and 11-c/s-retinal as bound to rhodopsin.58.59 Reproduced from ref. 58 with permission of Elsevier Science.

In the body, vitamin A is converted to 11 -c/s-retinal, the light-sensitive compound responsible for vision in all vertebrates (Section 21.11B). It is also needed for healthy mucous membranes. 

Figure 32.23 Atomic motion in retinal. The Schiff-base nitrogen atom moves 5 A as a consequence of the light-induced isomerization of 11-c/s-retinal to all-trons-retinal by rotation about the bond shown in red.

Fig. 4.6 The fluorescence decay kinetics of bovine rhodopsin (A), protonated Schiff base of 11-c/s-retinal (PSB11) in methanol (B), and protonated Schiff base of5-membered locked 11 -c/s retinal (5m-PSBll) in methanol (C). The data in (A) are from Kandori et al. [52], while those in (B) and (C) are from Kandori...

Slowly, arrestin dissociates, rhodopsin is dephosphorylated, and all-iraws-retinal is replaced with 11-c/s-retinal. Rhodopsin is ready for another phototransduction cycle. 

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