Rhodopsin

Rhodopsin, also known as visual purple, is expressed in metazoan 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. Rhodopsin is extremely sensitive to light, and enables night-vision. Exposed to white light, the pigment immediately bleaches, and it takes about 30 minutes to regenerate fully in humans.

Structure
Rhodopsin consists of two building blocks, an opsin protein called scotopsin and a reversibly covalently bound cofactor, retinal (retinaldehyde). The structure of rhodopsin consists of a bundle of seven transmembrane helices that surround the photoreactive chromophore, 11-cis retinal. Retinal, the chromophore portion of rhodopsin, is made in the retina from Vitamin A. 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.

The photoisomerization of rhodopsin has been studied in detail via x-ray crystallography on rhodopsin crystals. A first photoproduct called photorhodopsin forms within 200 femtoseconds after irradiation followed within picoseconds by a second one called bathorhodopsin with distorted all-trans bonds. This intermediate can be trapped and studied at cryogenic temperatures. Several models (a.o. the bicycle-pedal mechanism, hula-twist mechanism) attempt to explain how the retinal group can change its conformation without clashing with the enveloping rhodopsin protein pocket.

Rhodopsin and retinal disease
Mutation of the rhodopsin gene is a major contributor to various retinopathies such as retinitis pigmentosa. The disease causing protein generally aggregates with ubiquitin in inclusion bodies, disrupts the intermediate filament network and impairs the ability of the cell to degrade non-functioning proteins which leads to photoreceptor apoptosis. Other mutations on rhodopsin lead to congential stationary night blindness, mainly due to constitutive activation, when the mutations occur around the chromophore binding pocket of rhodopsin (Mendes et al., 2005). Several other pathological states relating to rhodopsin have been discovered including poor post-Golgi trafficing, dysregulative activation, rod outer segment instability and arrestin binding.

Microbial rhodopsins
Some prokaryotes express proton pumps called bacteriorhodopsin, proteorhodopsin, xanthorhodopsin to carry out phototrophy. Like rhodopsin, these contain retinal and have seven transmembrane alpha helices; however they are not coupled to a G protein. Bacterial halorhodopsin is a light-activated chloride pump. Finally, an alga is known to have an opsin that contains its own monolithic light-gated ion channel, channelrhodopsin. While bacteriorhodopsin, halorhodopsin, and channelrhodopsin all have significant sequence homology to one another, they have no detectable sequence identity to G-protein coupled receptor (GPCR) family where rhodopsins belong. Nevertheless, bacterial rhodopsins and GPCR are possibly evolutionary related, based on similarity of their three-dimensional structures. Therefore, they have been assigned to the same superfamily in Structural Classification of Proteins