Rhodopsin

From Self-sufficiency
Jump to: navigation, search
edit
Rhodopsin (opsin 2, rod pigment) (retinitis pigmentosa 4, autosomal dominant)
250px
Sensory rhodopsin II (rainbow colored) embedded in a lipid bilayer (heads red and tails blue) with Transducin below it. Gtα is colored red, Gtβ blue, and Gtγ yellow. There is a bound GDP molecule in the Gtα-subunit and a bound retinal (black) in the rhodopsin. The N-terminus terminus of rhodopsin is red and the C-terminus blue. Presumed anchoring of transducin to the membrane has been drawn in black.
Identifiers
SymbolsRHO; MGC138309; MGC138311; OPN2; RP4
External IDsOMIM180380 MGI97914 HomoloGene68068 GeneCards: RHO Gene
RNA expression pattern
250px
250px
More reference expression data
Orthologs
SpeciesHumanMouse
Entrez6010212541
EnsemblENSG00000163914ENSMUSG00000030324
UniProtP08100Q8K0D8
RefSeq (mRNA)NM_000539NM_145383
RefSeq (protein)NP_000530NP_663358
Location (UCSC)Chr 3:
130.73 - 130.74 Mb
Chr 6:
115.9 - 115.9 Mb
PubMed search[1][2]

Rhodopsin, also known as visual purple, 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 G-protein coupled receptor family and are extremely sensitive to light, enabling vision in low-light conditions.[1] Exposed to light, the pigment immediately photobleaches, and it takes about 30 minutes[2] to regenerate fully in humans.


Structure

Rhodopsin consists of the protein moiety opsin and a reversibly covalently bound cofactor, retinal. Opsin, a bundle of seven transmembrane helices connected to each other by protein loops, totaling, binds retinal, a photoreactive chromophore, in a central pocket on the seventh helix at a lysine residue, which lies horizontally with relation to the membrane. Each outer segment disc, of course, contains many (thousands) visual pigment molecules. About half the opsin is within the lipid bilayer. Retinal is produced in the retina from Vitamin A, from dietary beta-carotene. Isomerization of 11-cis-retinal into all-trans-retinal by light induces a conformational change (bleaching) in opsin continuing with metarhodopsin II, which activates the associated G protein transducin and triggers a second messenger cascade.[2][3] [4]


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 monochromatic vision in the dark.[2]

File:Bovine rhodopsin.png
Bovine rhodopsin

Several closely related opsins exist that differ only in a few amino acids and in the wavelengths of light that they absorb most strongly. Humans have four different other opsins beside rhodopsin. The photopsins are found in the different types of the cone cells of the retina and are the basis of color vision. They have absorption maxima for yellowish-green (photopsin I), green (photopsin II), and bluish-violet (photopsin III) light. The remaining opsin (melanopsin) is found in photosensitive ganglion cells and absorbs blue light most strongly.

The structure of rhodopsin has been studied in detail via x-ray crystallography on rhodopsin crystals. The photoisomerization dynamics has been investigated with time-resolved IR spectroscopy and UV/Vis spectroscopy. 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 (e.g. 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.[5][6][7]

Recent data supports that it is a functional monomer as opposed to a dimer, which was the paradigm of G-coupled protein receptors for many years. [8]

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.[9] Other mutations on rhodopsin lead to X-linked congenital stationary night blindness, mainly due to constitutive activation, when the mutations occur around the chromophore binding pocket of rhodopsin.[10] Several other pathological states relating to rhodopsin have been discovered including poor post-Golgi trafficking, dysregulative activation, rod outer segment instability and arrestin binding.[10]

Microbial rhodopsins

Some prokaryotes express proton pumps called bacteriorhodopsin, proteorhodopsin, xanthorhodopsin to carry out phototrophy.[11] 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.[11] 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.[12]

References

Cite error: Invalid <references> tag; parameter "group" is allowed only.

Use <references />, or <references group="..." />

Further reading

External links

ar:رودوبسين

be:Радапсін ca:Rodopsina cs:Rodopsin da:Rhodopsin de:Rhodopsin et:Rodopsiin es:Rodopsina fr:Rhodopsine hr:Rodopsin it:Rodopsina he:אופסין#רודופסין lt:Rodopsinas nl:Rodopsine ja:ロドプシン pl:Rodopsyna pt:Rodopsina ru:Родопсин fi:Rodopsiini

uk:Родопсин
  1. Litmann BJ, Mitchell DC (1996). "Rhodopsin structure and function". In Lee AG. Rhodopsin and G-Protein Linked Receptors, Part A (Vol 2, 1996) (2 Vol Set). Greenwich, Conn: JAI Press. pp. 1–32. ISBN 1-55938-659-2. 
  2. 2.0 2.1 2.2 Stuart JA, Brige RR (1996). "Characterization of the primary photochemical events in bacteriorhodopsin and rhodopsin". In Lee AG. Rhodopsin and G-Protein Linked Receptors, Part A (Vol 2, 1996) (2 Vol Set). Greenwich, Conn: JAI Press. pp. 33–140. ISBN 1-55938-659-2. 
  3. Hofmann KP, Heck M (1996). "Light-induced protein-protein interactions on the rod photoreceptor disc membrane". In Lee AG. Rhodopsin and G-Protein Linked Receptors, Part A (Vol 2, 1996) (2 Vol Set). Greenwich, Conn: JAI Press. pp. 141–198. ISBN 1-55938-659-2. 
  4. Kolb H, Fernandez E, Nelson R, Jones BW (2010-03-01). "Webvision: Photoreceptors". University of Utah. 
  5. Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
  6. Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
  7. Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
  8. http://pubs.acs.org/doi/abs/10.1021/bi050720o
  9. Saliba RS, Munro PM, Luthert PJ, Cheetham ME (15 July 2002). "The cellular fate of mutant rhodopsin: quality control, degradation and aggresome formation". J. Cell. Sci. 115 (Pt 14): 2907–18. PMID 12082151. 
  10. 10.0 10.1 Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
  11. 11.0 11.1 Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
  12. http://scop.mrc-lmb.cam.ac.uk/scop/data/scop.b.g.e.b.html.