Rhodopsin, also known as visual purple, is expressed in vertebrate photoreceptor cells. It is a pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the class of G-protein coupled receptors. It is the chemical that allows night-vision, and is extremely sensitive to light. Exposed to white light, the pigment immediately bleaches, and it takes about 30 minutes to regenerate fully in humans.
Rhodopsin consists of two building blocks, an opsin protein called scotopsin and a reversibly covalently bound cofactor, retinal (retinaldehyde). Retinal derives from Vitamin A and is made in the retina. Isomerization of 11-cis-retinal into all-trans-retinal by light induces a conformational change in the opsin that activates the associated G protein and triggers a second messenger cascade.
Rhodopsin of the rods most strongly absorbs green-blue light and therefore appears reddish-purple, which is why it is also called "visual purple". It is responsible for the monochromatic vision in the dark.
Several closely related opsins, the photopsins, exist that differ only in a few amino acids and in the wavelengths of light that they absorb most strongly. These pigments are found in the different types of the cone cells of the retina and are the basis of color vision. Humans have three different other opsins beside rhodopsin, with absorption maxima for yellowish-green (photopsin I), green (photopsin II), and bluish-violet (photopsin III) light.
Some archaea express a proton pump called bacteriorhodopsin to carry out photosynthesis. Like rhodopsin, bacteriorhodopsin contains retinal and has seven transmembrane alpha helices; however it is not coupled to a G protein. An alga is known to have an opsin that contains its own monolithic light-gated ion channel, channelrhodopsin-2.
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