- Retinylidene protein
Retinylidene proteins are a family of proteins that use
retinalas chromophorefor lightreception. This group of proteins is also commonly referred to as rhodopsins. They are the molecular basis for a variety of light-sensing systems from phototaxisin flagellates to eyesightin animals.
All rhodopsins consist of two building blocks, a
proteinmoiety and a reversibly covalently bound non-protein cofactor, retinal(retinaldehyde). The protein structure of rhodopsin consists of a bundle of seven transmembrane helices that form an internal pocket binding the photoreactive chromophore. They form a superfamily with other membrane-bound receptors containing seven transmembrane domains, for example odorand chemokinereceptors.cite journal |author=Sakmar T |title=Structure of rhodopsin and the superfamily of seven-helical receptors: the same and not the same |journal=Curr Opin Cell Biol |volume=14 |issue=2 |pages=189–95 |year=2002 |pmid=11891118 |doi=10.1016/S0955-0674(02)00306-X]
Mechanism of light reception
Instead of being activated by binding chemical
ligands like their relatives, rhodopsins contain retinal which changes conformation in reaction to light via photoisomerizationand thus are activated by light. The retinal molecule can take on several different cis-trans isomeric forms, such as all-"trans", 11-"cis" and 13-"cis". Photoisomerization( light-dependent isomerization) of retinal from "cis" to "trans" or vice versa induces a conformational change in the receptor protein. This change acts as a molecular switch to activate a signal transductionmechanism within the cell. Depending on the type of rhodopsin, it either opens an ion channel(for example in bacteria) or activates an associated G proteinand triggers a second messengercascade (for example in animal eyes).
Types of rhodopsins
Retinylidene proteins or rhodopsins are present in many species from bacteria to algae and animals. They can be divided into two distinct groups based on their sequence as well as the retinal isomer they contain at the ground state and their signal transduction mechanisms.
Ion channels and pumps
Rhodopsins found in prokaryotes and algae commonly contain an all-"trans" retinal isomer at the ground state that isomerizes to 13-"cis" upon light activation, also known as microbial-type chromophore. Examples are bacterial sensory rhodopsins,
channelrhodopsin, bacteriorhodopsin, halorhodopsin, and proteorhodopsin. They act as light-gated ion channels and can be further distinguished by the type of ion they channel. Bacteriorhodopsin functions as a proton pump, whereas halorhodopsin act as a chloride channel. Their functions range from bacterial photosynthesis(bacteriorhodopsin) to driving phototaxis(channelrhodopsins in flagellates). Signal transduction in phototaxis involves depolarizationof the cell membrane.cite journal |author=Nagel G, Szellas T, Kateriya S, Adeishvili N, Hegemann P, Bamberg E |title=Channelrhodopsins: directly light-gated cation channels |journal=Biochem Soc Trans |volume=33 |issue=Pt 4 |pages=863–6 |year=2005 |pmid=16042615 |doi=10.1042/BST0330863]
G protein-coupled receptors
The retinylidene proteins of the animal kingdom are also referred to as
opsins. Vertebrates contain five subfamilies of (rhod)opsins and arthropods three subfamilies [ [http://www.gpcr.org/7tm/seq/001_004/001_004.html G Protein-Coupled Receptor Data Base] ] . Opsins belong to the class of G protein-coupled receptors and bind an 11-"cis" isomer of retinal at the ground state that photoisomerizes to an all-"trans" retinal upon light activation. They are commonly found in the light-sensing organs, for example in the photoreceptor cells of vertebrate retinawhere they facilitate eyesight. Animal opsins can also be found in the skinof amphibians, the pineal glands of lizards and birds, the hypothalamusof toads, and the human brain. They can be categorized into several distinct classes including:
*visual opsins (classical
The "visual purple"
rhodopsin(opsin-2) of the rod cells in the vertebrate retina absorbs green-blue light. The photopsins of the cone cells of the retina differ in a few amino acids resulting in a shift of their light absorption spectra. The three human photopsins absorb yellowish-green (photopsin I), green (photopsin II), and bluish-violet (photopsin III) light and are the basis of color vision, whereas the more light-sensitive "visual purple" is responsible for the monochromatic vision in the dark. Light signal transduction involves an enzyme cascade of G-proteins ( transducin), cGMP phosphodiesterase, closure of a cationchannel and ultimately hyperpolarization of the visual photoreceptor cell.cite journal |author=Terakita A |title=The opsins |journal=Genome Biol |volume=6 |issue=3 |pages=213 |year=2005 |pmid=15774036 |doi=10.1186/gb-2005-6-3-213]
The visual rhodopsins of
arthropods and molluscs differ from the vertebrate proteins in their signal transduction cascade involving G-proteins, phospholipase C, and ultimately depolarizationof the visual photoreceptor cell.
opsins found in humans include encephalopsin (or panopsin, opsin-3), melanopsin (opsin-4), neuropsin (opsin-5) and peropsin. Melanopsinis involved in the light entrainment of the circadian clockin vertebrates. Encephalopsins and neuropsins are highly expressed in nerve cells and brain tissue, but so far their function is unknown. Peropsin binds all-"trans" retinal (microbial-type chromophore) and might function as a photoisomeraseto return retinal to the 11-"cis" isomer form needed in visual perception.
* [http://opm.phar.umich.edu/families.php?superfamily=6 Calculated positions of retinylidene proteins in the lipid bilayer]
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