fotopszin

Fotopsin is a light-sensitive pigment in the cone cells of the retina. It is an opsin protein bound to 11-cis-retinal that absorbs light and triggers a cascade of signals leading to vision.

In humans, three main photopsins mediate color vision: S-cone photopsin (blue, peak around 420 nm), M-cone photopsin (green, around 534 nm), and L-cone photopsin (red, around 564 nm). They are encoded by the genes OPN1SW, OPN1MW, and OPN1LW, respectively. The combination of their responses across the retina enables color discrimination.

Photopsins are located in the outer segments of cone photoreceptors; light triggers isomerization of the bound chromophore and activation of the G protein transducin, leading to a decrease in cGMP, closure of cGMP-gated channels, and hyperpolarization of the cell. This initiates the neural signaling that is interpreted as color by the brain.

Differences from rhodopsin: photopsins support daylight vision and color perception, whereas rhodopsin in rods is optimized for low-light conditions and black-and-white vision.

Variations in cone opsin genes can cause color vision deficiencies, particularly red-green color blindness, due to altered spectral sensitivity or gene rearrangements. Evolutionarily, photopsins have arisen through gene duplication and diversification of an ancestral opsin, with different species featuring varying complements and peak sensitivities.