Rhodopsin, also known as visual purple, is a vertebrate photoreceptor protein. It is a pigment of the retina that is responsible for the first events in the perception of light. Rhodopsins belong to the class of G-protein coupled receptors. There exist variants of rhodopsin that are sensitive for different wavelengths of light, the photopsins.
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 11-trans-retinal by light induces a conformational change in rhodopsin 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-bacteria express a proton pump called bacteriorhodopsin to carry out photosynthesis. An alga is known to have an opsin that contains its own monolithic light-gated ion channel, channelrhodopsin-2.
Normalised absorption spectra of human rhodopsin (dashed) and the three photopsins.
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