The properties of visual pigments

In 1933 G. Wald discovered the presence of vitamin A in the retina [3]. Subsequently, it was also found that it is the 11-c/s isomer of vitamin A aldehyde, 

The opsin consists of protein (ca. 80-85% of which is rhodopsin), phospholipids and carbohydrates and contains very little cholesterol (1-3%) (for a review, see [6]). While the molecular weight (e.g., 40000 for bovine rhodopsin) [7], carbohydrate [8,9], lipid and amino acid [10-12] composition have been established for some rhodopsins, there is as yet no example of a visual pigment for which the full amino acid sequence is known. Only a quarter of about the 300 residues of rhodopsin have been sequenced [13,14], 39 residues at the N-terminus and 40 residues at the C-terminus. The structure of the moiety containing retinal, i.e., retinal-lysine-alanine, which is located in the carboxy-terminal region has, however, been elucidated ([15] see also [78] and references therein). The N-terminal residue was identified as acetylmethionine [16]. 

Visual pigments display characteristic absorption spectra which result from the very specific interactions between protein and chromophore in the binding site, i.e., the absorbance spectrum of retinal at ca. 380 nm is red-shifted to ca. 500 nm in bovine rhodopsin. However, depending on species, rhodopsins absorb from 440 to ca. 600 nm. Porphyropsins show a similar spread in their absorption maxima, absorb at longer wavelengths than the corresponding rhodopsins, and have lower extinction coefficients (ca. 75%) than rhodopsins (e.g., bovine rhodopsin e, ca. 40500) [17] as shown in Fig. 2. 

Bovine ROS membranes show a CD band at ca. 280 nm attributed to tt-tt transitions of aromatic residues and n-ir transitions of cysteine, as well as two maxima in the visible region at ca. 340 nm and 490 nm (the / and a bands) corresponding to the cis and main peaks of the absorption spectrum. The intensity of the a band in the CD is species-dependent, but is always somewhat blue-shifted relative to the Xmax in all species, and this shift is promoted by detergent solubilization. Strong micellar effects have been observed in the intensity of both a and / CD bands [44] (Fig. 4). 

Further, a suspension of bovine ROS displayed an a CD peak 2.3 times more intense than the corresponding extract [40,41]. It is the conformational stability of rhodopsin which is affected by detergent solubilization, the smallest divergence from ROS being observed in digitonin. 

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