Photopsine

Photopsin, sometimes referred to as photopsine, is a light-sensitive protein pigment in cone photoreceptor cells of the retina. It belongs to the opsin family, G protein-coupled receptors that bind 11-cis-retinal to form a photopigment capable of initiating phototransduction when struck by light.

In humans there are three cone photopigments: S-cone photopsin (blue sensitive, peak around 420 nm), M-cone photopsin (green sensitive, ~533 nm), L-cone photopsin (red sensitive, ~565 nm). Each photopsin is encoded by a separate gene: OPN1SW for S; OPN1MW and OPN1LW for M and L. M and L arose by duplication on the X chromosome; variation in these genes underlies common forms of color vision deficiency.

The photopigments are located in the outer segments of cone cells and contribute to daylight color vision. Absorption of a photon causes isomerization of the bound retinal chromophore (11-cis to all-trans), activating the opsin and the phototransduction cascade via the G protein transducin, lowering cGMP, closing cGMP-gated channels, and hyperpolarizing the cell. Re-synthesis via the retinal pigment epithelium regenerates 11-cis-retinal.

Cones regenerate their photopigments more rapidly than rods. The spectral sensitivities of photopsins may vary among species, and many animals possess additional cone opsins, enabling broader color vision, including tetrachromacy in some birds and fish.