Bovine visual pigments

Cartoon representation of the three-dimensional structures of three different photoresponsive proteins. Left GFP with its HBDI anionic chromophore. Center the bovine visual pigment rhodopsin together with the cationic PSB11 chromophore. Right bacteriorhodopsin and its PSBAT chromophore... 

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. 

A series of hydroretinals were synthesized and were bound to bovine opsin to form visual pigment analogs, the hydrorhodopsins [92]. Model protonated Schiff bases were also prepared from each of these retinals, and the magnitudes of the opsin shifts were determined. Table 1 summarizes the data obtained and shows the opsin shift for bovine rhodopsin for comparative purposes. Binding studies were also carried out to ascertain that the hydroretinals occupy the same binding site as 11-cK-retinal in rhodopsin. This is required to show the relevance of the data to the natural system. 

Nathans J. Determinants of visual pigment absorbance identification of the retinylidene Schiff s base counterion in bovine rhodopsin. Biochemistry 1990 29 9746-9752. 

FIGURE 6.10 Cartoon representations of protein structures for (a) bovine catalase (Protein Data Bank [PDB] entry SCAT) and (b) bacteriorhodopsin (PDB entry 3WJK), a proton pump similar to the human visual pigment and found in bacterial cells. In these two examples of protein molecules, we can see the amino acid chain coiled and folded into different structures. Alpha-helical structures are shown in red and beta sheets in yellow. Diagrams such as this are commonly used to depict the complex hierarchical structure of proteins. 

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