Opsins

Literature References: Broad class of species-specific proteins which form the basis of the visual pigments and of bacteriorhodopsin, q.v. Structurally integrated into the rods and cones of the retina of the eye or into the photoreceptor membranes of certain bacteria. Each of these cell types produces a genetically specified opsin which has been classified on the basis of cellular source: scotopsins (rods), photopsins (cones), and bacteriopsins (bacteria). Methods for purification, prepn and assay: R. Hubbard et al., Methods Enzymol. 18, 615-653 (1971). Series of articles on photoreceptor biosynthesis: ibid. 81, 763-815 (1982). Review of biosynthetic process: D. S. Papermaster, B. G. Schneider in Cell Biology of the Eye, D. McDevitt, Ed. (Academic Press, New York, 1982) p 475. The photoreceptor activity of visual pigments is due to a carotenoid chromophore, retinal or 3-dehydroretinal, q.q.v., bound as a protonated Schiff base to a lysine moiety in the opsin portion of the molecule: A. R. Osenoff, R. Callender, Biochemistry 13, 4243 (1974). Each pigment has unique physicochemical properties. The most significant is the absorption spectrum which is regulated by electrostatic interactions between the chromophore and the charged or dipolar groups on the opsin: R. Hubbard, L. Sperling, Exp. Eye Res. 17, 581 (1973); B. Honig et al., J. Am. Chem. Soc. 101, 7084 (1979). A visual system, a set of pigments spanning the light sensitivity range of a particular species, is generally based on one type of chromophore combined with various opsins. Visual systems based on retinal are the most widespread in nature. The 11-cis isomer is utilized by rhodopsin, q.v., the most common pigment of rod cells, and by the corresponding trichromatic cone pigments (see Iodopsin). Bacteriorhodopsin is composed of bacterioopsin and trans retinal. Visual systems based on 3-dehydroretinal, exemplified by the pigments porphyropsin and cyanopsin, q.q.v., have been found to occur only in certain fish and amphibians. Visual pigments utilizing both types of chromophore have been found to coexist in the retina of some of these species: T. E. Reuter et al., J. Gen. Physiol. 58, 351 (1971). Exposure to light initiates the bleaching of the pigment through a series of distinct intermediates involving the isomerization and ultimate dissociation of the chromophore from the opsin: R. Hubbard, A. Kropf, Proc. Natl. Acad. Sci. USA 44, 140 (1958); B. Honig et al., ibid. 76, 2503 (1979). This process initiates the mechanism of energy transduction and visual excitation: T. G. Ebrey, B. Honig, Q. Rev. Biophys. 8, 129-184 (1975); B. Honig, Annu. Rev. Phys. Chem. 29, 31-57 (1978); R. Uhl, E. W. Abrahamson, Chem. Rev. 81, 291 (1981). Review of energy transduction in invertebrate photoreceptors: P. Hillman et al., Physiol. Rev. 63, 668-772 (1983); in bacteriorhodopsin: H. V. Westerhoff, Z. Dancshazy, Trends Biochem. Sci. 9, 112 (1984). General reviews: G. Wald, Science 162, 230-239 (1968); D. F. O'Brien, ibid. 218, 961-966 (1982); A. Maeda, T. Yoshizawa, Photochem. Photobiol. 35, 891-898 (1983); P. S. Zurer, Chem. Eng. News 61, 24-35 (Nov. 28, 1983). See also: Methods Enzymol. 88, 1-836 (1982).