Gn-x protein-coupled receptor

Photopsins (also known as Cone opsins) are the photoreceptor proteins found in the cone cells of the retina that are the basis of color vision. Iodopsin, the cone pigment system in chicken retina, is a close analog of the visual purple rhodopsin that is used in night vision. Iodopsin consists of the protein component and a bound chromophore, retinal.

Opsins are Gn-x protein-coupled receptors of the retinylidene protein family. Isomerization of 11-cis-retinal into all-trans-retinal by light induces a conformational change in the protein that activates photopsin and promotes its binding to G protein transducin, which triggers a second messenger cascade.

Different opsins differ in a few amino acids and absorb light at different wavelengths as retinal-bound pigments.

In humans there are 3 different iodopsins (rhodopsin analogs) that contain the protein-pigment complexes photopsin I, II, and III.

The 3 types of iodopsins are called erythrolabe(photopsin I + retinal), chlorolabe(photopsin II + retinal), and cyanolabe(photopsin III + retinal).

These photopsins have absorption maxima for red ["erythr"-red] (photopsin I), green ["chlor"-green] (photopsin II), and bluish-violet light ["cyan"-bluish violet] (photopsin III).

George Wald received the 1967 Nobel Prize in Physiology or Medicine for his experiments in the 1950s that showed the difference in absorbance by these photopsins (see image).

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