Photopsin

Overview
Photopsins (also known as iodopsins) are the photoreceptor proteins found in the cone cells of the retina that are the basis of color vision. Photopsins are very close analogs of the visual purple rhodopsin that is used in night vision. Photopsins consist of a protein called opsin and a bound chromophore, the retinal.

Function
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.

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

In humans there are three different iodopsins (rhodopsin analogs) that form the protein-pigment complexes photopsin I, II, and III. They are called erythrolabe, chlorolabe, and cyanolabe, respectively. These photopsins have absorption maxima for yellowish-green (photopsin I), green (photopsin II), and bluish-violet light (photopsin III).

History
George Wald got 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).