Retinoids and the Basis of Color Vision

The absorption maxima of free 11-cis-retinal is in the near-ultraviolet range (360-380 nm). However, the absorption maxima of the 11-cis-retinylidene moiety in the context of apro-tonated Schiff base linkage with rod opsins is shifted to approximately 500 nm, squarely in the middle of the visual range. Two factors contribute to this dramatic red shift in the absorption maxima of 11-cis-retinylidene group relative to that of free 11-cis-retinal: (1)pro-tonation of the Schiff base and (2) the existence of intramolecular negative charges contributed by the opsin component of rhodopsin (22). One of these negative charges is derived from Glu113, a residue that is conserved among all known vertebrate visual pigments. Glu113 is located in the highly hydrophobic environment of opsin transmembrane domain 2 and provides the counterion for the positively charged Schiff base (22). The existence of this counterion in such a hydrophobic environment shifts the pifa of the protonated Schiff base by 7 log units, effectively preventing spontaneous hydrolysis of the 11-cis-retinyli-dene group.

The recent elucidation of the crystal structure of rhodopsin bound by 11-cis-retinal at intracellular

Figure 7.15. Rhodopsin crystal structure. Three-imensional structure of rhodopsin based on X-ray crystallography (186). Note that all-trans-retinal is rotected from the intradiscal side by multiple structural elements, including several /3 strands. Carbohydrates are in blue, 11-cis-retinal is in green, nd helices are in gray. Red shadows are added for esthetic reasons. This figure was generated by C. Behnke (Universityof Washington) and is reprinted with permission from Prog. Retin. Eye Res., 20, 469-521 (2001) .See color insert.

Figure 7.15. Rhodopsin crystal structure. Three-imensional structure of rhodopsin based on X-ray crystallography (186). Note that all-trans-retinal is rotected from the intradiscal side by multiple structural elements, including several /3 strands. Carbohydrates are in blue, 11-cis-retinal is in green, nd helices are in gray. Red shadows are added for esthetic reasons. This figure was generated by C. Behnke (Universityof Washington) and is reprinted with permission from Prog. Retin. Eye Res., 20, 469-521 (2001) .See color insert.

2.8A allows direct visualization (Fig. 7.15) of the spatial arrangements of the transmembrane domains of rhodopsin and the location of 11-cis-retinal in the pocket (85). These investigators found that 11-cis-retinal contributes to the inactive conformation of rhodopsin y holding the transmembrane regions of the protein together. Although the structure of bleached rhodopsin, i.e., that containing 11-runs-retinal, was not reported, one can easily kraep the magnitude of the rhodopsin conformational change that must accompany light-iinduced isomerization of the chromophore, 1-cis-retinal.

The above-described structural studies onfirmed that interactions of the 11-czs-reti-

nylidene group with a cluster of amino acids of the opsin component dictate the absorption maxima of rod-derived rhodopsin and also provide a basis for the visual perception of color in cone cells of the retina. Although the chromophore is attached to cone opsins through an identical, protonated Schiff base linkage and likely occupies a similar position in the opsin pocket, the absorption maxima of the cone rhodopsins differ significantly from that of rod rhodopsin. However, in contrast to those expressed in monochromatic rod cells, cone visual pigments have absorption maximas that cover the visual spectrum. This is believed to result largely from differential interactions of group with the side chains of amino acids lining the cavity in which the chromophore resides (22, 85). As a result of these differential interactions, visual pigments present in cones are activated by light of varying wavelengths giving rise to the perception of color.

Was this article helpful?

0 0
How to Stop Your Depression Now

How to Stop Your Depression Now

Finally, Retired Clinical Counsellor Reveals the Secrets Successful Psychiatrists and Psychologists Don't Want You to Know. How to Stop Your Depression Now Reclaim Yourself and Live Again Get the Depression Busting Tools You Need To Win the War Against Depression. Depression is an illness that many people often sweep under a rug. However if depression is left untreated... Your life can become a living nightmare. Depression is a growing epidemic in the US, but it never gets the urgent attention it deserves. You need help and you need it now.

Get My Free Ebook


Post a comment