Editing Opsin (section)

== Structure and function ==
Animal opsins detect light and are the molecules that allow us to see. Opsins are [[G-protein-coupled receptor]]s (GPCRs),<ref name=Casey1988>{{cite journal | vauthors = Casey PJ, Gilman AG | title = G protein involvement in receptor-effector coupling | journal = The Journal of Biological Chemistry | volume = 263 | issue = 6 | pages = 2577–2580 | date = February 1988 | pmid = 2830256 | doi = 10.1016/s0021-9258(18)69103-3 | s2cid = 38970721 | doi-access = free }}</ref><ref name=Attwood1994>{{cite journal | vauthors = Attwood TK, Findlay JB | title = Fingerprinting G-protein-coupled receptors | journal = Protein Engineering | volume = 7 | issue = 2 | pages = 195–203 | date = February 1994 | pmid = 8170923 | doi = 10.1093/protein/7.2.195 }}</ref> which are [[chemoreceptor]]s and have seven transmembrane [[alpha helix|domains]] forming a [[binding pocket]] for a ligand.<ref>{{cite journal | vauthors = Dixon RA, Kobilka BK, Strader DJ, Benovic JL, Dohlman HG, Frielle T, Bolanowski MA, Bennett CD, Rands E, Diehl RE, Mumford RA, Slater EE, Sigal IS, Caron MG, Lefkowitz RJ, Strader CD | display-authors = 6 | title = Cloning of the gene and cDNA for mammalian beta-adrenergic receptor and homology with rhodopsin | journal = Nature | volume = 321 | issue = 6065 | pages = 75–79 | date = May 1986 | pmid = 3010132 | doi = 10.1038/321075a0 | s2cid = 4324074 | bibcode = 1986Natur.321...75D }}</ref><ref>{{cite journal | vauthors = Dixon RA, Sigal IS, Rands E, Register RB, Candelore MR, Blake AD, Strader CD | title = Ligand binding to the beta-adrenergic receptor involves its rhodopsin-like core | journal = Nature | volume = 326 | issue = 6108 | pages = 73–77 | date = March 1987 | pmid = 2881211 | doi = 10.1038/326073a0 | s2cid = 4352920 | bibcode = 1987Natur.326...73D }}</ref> The [[ligand]] for opsins is the [[vitamin A]]-based [[chromophore]] 11-''cis''-retinal,<ref>{{cite journal | vauthors = Wald G |title=Carotenoids and the Vitamin A Cycle in Vision |journal=Nature |date=July 1934 |volume=134 |issue=3376 |pages=65 |doi=10.1038/134065a0|bibcode=1934Natur.134...65W |s2cid=4022911 |doi-access=free }}</ref><ref>{{cite journal | vauthors = Wald G, Brown PK, Hubbard R, Oroshnik W | title = Hindered Cis Isomers of Vitamin a and Retinene: The Structure of the Neo-B Isomer | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 41 | issue = 7 | pages = 438–451 | date = July 1955 | pmid = 16589696 | pmc = 528115 | doi = 10.1073/pnas.41.7.438 | doi-access = free | bibcode = 1955PNAS...41..438W }}</ref><ref>{{cite journal | vauthors = Brown PK, Wald G | title = The neo-b isomer of vitamin A and retinene | journal = The Journal of Biological Chemistry | volume = 222 | issue = 2 | pages = 865–877 | date = October 1956 | pmid = 13367054 | doi = 10.1016/S0021-9258(20)89944-X | doi-access = free }}</ref><ref>{{cite journal | vauthors = Oroshnik W |title = The Synthesis and Configuration of Neo-B Vitamin A and Neoretinine b | journal = Journal of the American Chemical Society | date = June 1956 | volume = 78 | issue = 11 | pages = 2651–2652 | doi = 10.1021/ja01592a095}}</ref><ref>{{cite journal | vauthors = Oroshnik W, Brown PK, Hubbard R, Wald G | title = HINDERED CIS ISOMERS OF VITAMIN A AND RETINENE: THE STRUCTURE OF THE NEO-b ISOMER | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 42 | issue = 9 | pages = 578–580 | date = September 1956 | pmid = 16589909 | pmc = 534254 | doi = 10.1073/pnas.42.9.578 | doi-access = free | bibcode = 1956PNAS...42..578O }}</ref> which is covalently bound to a [[lysine]] residue<ref>{{cite journal | vauthors = Bownds D | title = Site of attachment of retinal in rhodopsin | journal = Nature | volume = 216 | issue = 5121 | pages = 1178–1181 | date = December 1967 | pmid = 4294735 | doi = 10.1038/2161178a0 | s2cid = 1657759 | bibcode = 1967Natur.216.1178B }}</ref> in the seventh transmembrane domain<ref>{{cite journal | vauthors = Hargrave PA, McDowell JH, Curtis DR, Wang JK, Juszczak E, Fong SL, Rao JK, Argos P | display-authors = 6 | title = The structure of bovine rhodopsin | journal = Biophysics of Structure and Mechanism | volume = 9 | issue = 4 | pages = 235–244 | date = 1983 | pmid = 6342691 | doi = 10.1007/BF00535659 | s2cid = 20407577 }}</ref><ref name=Palczewski2000>{{cite journal | vauthors = Palczewski K, Kumasaka T, Hori T, Behnke CA, Motoshima H, Fox BA, Le Trong I, Teller DC, Okada T, Stenkamp RE, Yamamoto M, Miyano M | display-authors = 6 | title = Crystal structure of rhodopsin: A G protein-coupled receptor | journal = Science | volume = 289 | issue = 5480 | pages = 739–745 | date = August 2000 | pmid = 10926528 | doi = 10.1126/science.289.5480.739 | citeseerx = 10.1.1.1012.2275 | bibcode = 2000Sci...289..739P }}</ref><ref name=Murakami2008>{{cite journal | vauthors = Murakami M, Kouyama T | title = Crystal structure of squid rhodopsin | journal = Nature | volume = 453 | issue = 7193 | pages = 363–367 | date = May 2008 | pmid = 18480818 | doi = 10.1038/nature06925 | s2cid = 4339970 | bibcode = 2008Natur.453..363M }}</ref> through a [[Schiff-base]].<ref>{{cite journal | vauthors = Collins FD | title = Rhodopsin and indicator yellow | journal = Nature | volume = 171 | issue = 4350 | pages = 469–471 | date = March 1953 | pmid = 13046517 | doi = 10.1038/171469a0 | s2cid = 4152360 | bibcode = 1953Natur.171..469C }}</ref><ref>{{cite journal | vauthors = Pitt GA, Collins FD, Morton RA, Stok P | title = Studies on rhodopsin. VIII. Retinylidenemethylamine, an indicator yellow analogue | journal = The Biochemical Journal | volume = 59 | issue = 1 | pages = 122–128 | date = January 1955 | pmid = 14351151 | pmc = 1216098 | doi = 10.1042/bj0590122 }}</ref> However, 11-''cis''-retinal only blocks the binding pocket and does not activate the opsin. The opsin is only activated when 11-''cis''-retinal absorbs a [[photon]] of light and [[Isomerization|isomerizes]] to all-''trans''-retinal,<ref>{{cite journal | vauthors = Hubbard R, Kropf A | title = The Action of Light on Rhodopsin | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 44 | issue = 2 | pages = 130–139 | date = February 1958 | pmid = 16590155 | pmc = 335377 | doi = 10.1073/pnas.44.2.130 | doi-access = free | bibcode = 1958PNAS...44..130H }}</ref><ref>{{cite journal | vauthors = Kropf A, Hubbard R | title = The mechanism of bleaching rhodopsin | journal = Annals of the New York Academy of Sciences | volume = 74 | issue = 2 | pages = 266–280 | date = November 1959 | pmid = 13627857 | doi = 10.1111/j.1749-6632.1958.tb39550.x | bibcode = 1959NYASA..74..266K | s2cid = 45830716 }}</ref> the receptor activating form,<ref name=Choe2011>{{cite journal | vauthors = Choe HW, Kim YJ, Park JH, Morizumi T, Pai EF, Krauss N, Hofmann KP, Scheerer P, Ernst OP | display-authors = 6 | title = Crystal structure of metarhodopsin II | journal = Nature | volume = 471 | issue = 7340 | pages = 651–655 | date = March 2011 | pmid = 21389988 | doi = 10.1038/nature09789 | s2cid = 4302421 | bibcode = 2011Natur.471..651C }}</ref><ref name=Wald1968>{{cite journal | vauthors = Wald G | title = Molecular basis of visual excitation | journal = Science | volume = 162 | issue = 3850 | pages = 230–239 | date = October 1968 | pmid = 4877437 | doi = 10.1126/science.162.3850.230 | bibcode = 1968Sci...162..230W }}</ref> causing conformal changes in the opsin,<ref name=Choe2011 /> which activate a [[phototransduction cascade]].<ref>{{cite journal | vauthors = Terakita A, Kawano-Yamashita E, Koyanagi M |title=Evolution and diversity of opsins |journal=Wiley Interdisciplinary Reviews: Membrane Transport and Signaling |date=January 2012 |volume=1 |issue=1 |pages=104–111 |doi=10.1002/wmts.6|doi-access=free }}</ref> Thus, a chemoreceptor is converted to a [[Photoreceptor protein|light or photo(n)receptor]].<ref name=Guehmann2022>{{cite journal | vauthors = Gühmann M, Porter ML, Bok MJ | title = The Gluopsins: Opsins without the Retinal Binding Lysine | journal = Cells | volume = 11 | issue = 15 | pages = 2441 | date = August 2022 | pmid = 35954284 | pmc = 9368030 | doi = 10.3390/cells11152441 | doi-access = free }} [[File:CC-BY icon.svg|50px]] Material was copied and adapted from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>

In the vertebrate photoreceptor cells, all-''trans''-retinal is released and replaced by a newly synthesized 11-''cis''-retinal provided from the retinal epithelial cells. 
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Beside 11-''[[Cis-trans isomerism|cis]]''-retinal (A1), 11-''cis''-3,4-didehydroretinal (A2) is also found in [[vertebrate]]s as ligand such as in freshwater fishes.<ref name=Wald1968 /> A2-bound opsins have a shifted ''λ''<sub>max</sub> and absorption spectrum compared to A1-bound opsins.<ref>{{cite journal | vauthors = Amora TL, Ramos LS, Galan JF, Birge RR | title = Spectral tuning of deep red cone pigments | journal = Biochemistry | volume = 47 | issue = 16 | pages = 4614–4620 | date = April 2008 | pmid = 18370404 | pmc = 2492582 | doi = 10.1021/bi702069d }}</ref>