Opsin

An opsin is a light-sensitive protein that changes shape after absorbing photons, initiating signaling in photoreceptor cells. Combined with a retinal chromophore, opsins form the visual pigments of rods and cones.

Retinal: A vitamin A–derived molecule that absorbs light and changes shape within visual pigments. Light converts opsin-bound 11-cis-retinal to all-trans-retinal, triggering opsin activation.
Seven-transmembrane domain: A protein architecture in which one polypeptide crosses a membrane seven times. This membrane-spanning scaffold forms the characteristic structure of animal opsins.
Optogenetics: A set of methods that use genetically encoded light-sensitive proteins to control or monitor cells. Engineered opsins let researchers activate or silence selected neurons with light.
Photopsin: A cone-cell opsin that combines with retinal to form a color-sensitive visual pigment. Photopsins are the cone opsins underlying color vision in many vertebrates.
Phototransduction: The biochemical process by which photoreceptor cells convert light into electrical signals. Activated opsin starts the signaling cascade that produces the photoreceptor's electrical response.
Chromophore: The light-absorbing part of a molecule that gives it its optical properties. Retinal is opsin's chromophore, supplying the photon-sensitive component of the visual pigment.
Channelrhodopsin: A light-gated ion channel from microbial opsins that can depolarize cells. Its discovery enabled direct optical activation of neurons in optogenetic experiments.
Melanopsin: A light-sensitive opsin found in retinal ganglion cells and some other tissues. Unlike rod and cone opsins, melanopsin supports non-image-forming responses such as circadian regulation.
Rhodopsin: The light-sensitive pigment in rod cells, consisting of opsin bound to retinal. It is the best-studied vertebrate opsin complex and mediates dim-light vision.
Schiff base: A chemical linkage formed between a carbonyl compound and a primary amine. A protonated Schiff base attaches retinal to a lysine residue in many visual opsins.

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