opsin

thumb|150px|Three-dimensional structure of cattle rhodopsin. The seven transmembrane domains are shown in varying colors. The chromophore is shown in red. thumb|right|400px|The retinal molecule inside an opsin protein absorbs a photon of light. Absorption of the photon causes retinal to change from its 11-cis-retinal isomer into its all-trans-retinal isomer. This change in shape of retinal pushes against the outer opsin protein to begin a signal cascade, which may eventually result in chemical signaling being sent to the brain as visual perception. The retinal is re-loaded by the body so that signaling can happen again. Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most prominently, they are found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in vision. Humans have in total nine opsins. Beside vision and light perception, opsins may also sense temperature, sound, or chemicals.

== Structure and function == Animal opsins are molecules that absorb light from the environment to allow for vision in animals. Opsins are G-protein-coupled receptors (GPCRs), which are chemoreceptors and have seven transmembrane domains forming a binding pocket for a ligand. The ligand for opsins is the vitamin A-based chromophore 11-cis-retinal, which is covalently bound to a lysine residue in the seventh transmembrane domain through a Schiff-base. When the 11-cis-retinal absorbs a photon of light and isomerizes to all-trans-retinal this causes a conformal change in the opsin, and activates a phototransduction cascade. Thus, a chemoreceptor is converted to a light or photo(n)receptor.