Photopsin

Photopsin (also known as cone opsins) are a type of opsin which are found in the cone cells of the retina that are responsible for color vision. Photopsins are very closely related to the visual purple rhodopsin that is used in night vision. Photopsins have the highest absorption maxima of any known opsins.

Structure and Function

Photopsins are composed of a protein called opsin and a bound chromophore, either 11-cis retinal or 11-cis 3,4-didehydroretinal. The opsin protein covalently binds to the chromophore through a Schiff base linkage to a lysine residue in the seventh transmembrane alpha helix. The absorption of light by the retinal chromophore leads to its isomerization to all-trans retinal. This isomerization induces a conformational change in the opsin protein, leading to the activation of the photopsin and, thus, the cone cell.

Types of Photopsins

There are three different types of photopsins, each with a different absorption maximum, that allow for trichromatic color vision. Each type of photopsin is specialized for a certain range of wavelengths of light. The three types are:

• Photopsin I (short-wavelength sensitive, SWS), also known as blue opsin, which absorbs light best around 420 nm.
• Photopsin II (middle-wavelength sensitive, MWS), also known as green opsin, which absorbs light best around 534 nm.
• Photopsin III (long-wavelength sensitive, LWS), also known as red opsin, which absorbs light best around 564 nm.

Role in Color Vision

The differential response of the three photopsins to light of different wavelengths allows the eye to perceive a broad spectrum of colors, through a process known as color opponency. For example, moderate stimulation of a short-wavelength cone cell, strong stimulation of a middle-wavelength cone cell, and weak stimulation of a long-wavelength cone cell would result in the perception of a greenish color.

See Also

• Rhodopsin
• Color vision
• Cone cell
• Opsin