retinal

{{Chembox

| Watchedfields = changed

| verifiedrevid = 444085949

| Name = All-trans-retinal

| ImageFile = All-trans-Retinal.svg

| ImageSize = 250

| ImageAlt = Skeletal formula of retinal

| ImageFile1 = Retinal 3D ball.png

| ImageSize1 = 260

| ImageAlt1 = Ball-and-stick model of the retinal molecule

| IUPACName = Retinal

| SystematicName = (2E,4E,6E,8E)-3,7-Dimethyl-9-(2,6,6-trimethylcyclohex-1-en-1-yl)nona-2,4,6,8-tetraenal

| OtherNames = {{Bulleted list|Retinene|Retinaldehyde|Vitamin A aldehyde|RAL}}

|Section1={{Chembox Identifiers

| CASNo_Ref = {{cascite|correct|CAS}}

| CASNo = 116-31-4

| UNII_Ref = {{fdacite|correct|FDA}}

| UNII = RR725D715M

| PubChem = 638015

| ChemSpiderID = 553582

| ChEBI = 17898

| StdInChI = 1S/C20H28O/c1-16(8-6-9-17(2)13-15-21)11-12-19-18(3)10-7-14-20(19,4)5/h6,8-9,11-13,15H,7,10,14H2,1-5H3/b9-6+,12-11+,16-8+,17-13+

| StdInChIKey = NCYCYZXNIZJOKI-OVSJKPMPSA-N

| SMILES = CC1=C(C(CCC1)(C)C)/C=C/C(=C/C=C/C(=C/C=O)/C)/C}}

|Section2={{Chembox Properties

| C=20 | H=28 | O=1

| Appearance = Orange crystals from petroleum etherMerck Index, 13th Edition, 8249

| Density =

| MeltingPtC = 61 to 64

| MeltingPt_ref =

| BoilingPt =

| Solubility = Nearly insoluble

| SolubleOther = Soluble

| Solvent = fat}}

|Section7={{Chembox Hazards

| MainHazards =

| FlashPt =

| AutoignitionPt = }}

|Section8={{Chembox Related

| OtherFunction =

| OtherFunction_label =

| OtherCompounds = retinol; retinoic acid; beta-carotene; dehydroretinal; 3-hydroxyretinal; 4-hydroxyretinal}}

}}

Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision).

Some microorganisms use retinal to convert light into metabolic energy. One study suggests that approximately three billion years ago, most living organisms on Earth used retinal, rather than chlorophyll, to convert sunlight into energy. Because retinal absorbs mostly green light and transmits purple light, this gave rise to the Purple Earth hypothesis.{{Cite journal |last1=DasSarma |first1=Shiladitya |last2=Schwieterman |first2=Edward W. |date=2018 |title=Early evolution of purple retinal pigments on Earth and implications for exoplanet biosignatures |journal=International Journal of Astrobiology |language=en |publication-date=2018-10-11 |volume=20 |issue=3 |pages=241–250 |doi=10.1017/S1473550418000423 |s2cid=119341330 |issn=1473-5504|doi-access=free |arxiv=1810.05150 }}

Retinal itself is considered to be a form of vitamin A when eaten by an animal. There are many forms of vitamin A, all of which are converted to retinal, which cannot be made without them. The number of different molecules that can be converted to retinal varies from species to species. Retinal was originally called retinene,{{cite journal |last1=Wald |first1=George |title=Carotenoids and the Vitamin A Cycle in Vision |journal=Nature |date=14 July 1934 |volume=134 |issue=3376 |pages=65 |doi=10.1038/134065a0 |bibcode=1934Natur.134...65W |s2cid=4022911|doi-access=free }} and was renamed{{cite journal |last1=Wald |first1=G. |title=Molecular basis of visual excitation |journal=Science |date=11 October 1968 |volume=162 |issue=3850 |pages=230–9 |pmid=4877437 |doi=10.1126/science.162.3850.230 |bibcode=1968Sci...162..230W}} after it was discovered to be vitamin A aldehyde.{{cite journal |last1=MORTON |first1=R. A. |last2=GOODWIN |first2=T. W. |title=Preparation of Retinene in Vitro |journal=Nature |date=1 April 1944 |volume=153 |issue=3883 |pages=405–406 |doi=10.1038/153405a0 |bibcode=1944Natur.153..405M |s2cid=4111460}}{{cite journal |last1=Ball |first1=S. |last2=Goodwin |first2=T. W. |last3=Morton |first3=R. A. |title=Retinene1-vitamin A aldehyde. |journal=The Biochemical Journal |date=1946 |volume=40 |issue=5–6 |pages=lix |pmid=20341217}}

Vertebrate animals ingest retinal directly from meat, or they produce retinal from carotenoids – either from α-carotene or β-carotene – both of which are carotenes. They also produce it from β-cryptoxanthin, a type of xanthophyll. These carotenoids must be obtained from plants or other photosynthetic organisms. No other carotenoids can be converted by animals to retinal. Some carnivores cannot convert any carotenoids at all. The other main forms of vitamin A – retinol and a partially active form, retinoic acid – may both be produced from retinal.

Invertebrates such as insects and squid use hydroxylated forms of retinal in their visual systems, which derive from conversion from other xanthophylls.

Vitamin A metabolism

Living organisms produce retinal by irreversible oxidative cleavage of carotenoids.{{cite journal |last1=von Lintig |first1=Johannes |last2=Vogt |first2=Klaus |year=2000 |title=Filling the Gap in Vitamin A Research: Molecular Identification of An Enzyme Cleaving Beta-carotene to Retinal |journal=Journal of Biological Chemistry |volume=275 |issue=16 |pages=11915–11920 |pmid=10766819 |doi=10.1074/jbc.275.16.11915 |doi-access=free}}

For example:

catalyzed by a beta-carotene 15,15'-monooxygenase{{cite journal |last=Woggon |first=Wolf-D. |year=2002 |title=Oxidative cleavage of carotenoids catalyzed by enzyme models and beta-carotene 15,15'-monooxygenase |journal=Pure and Applied Chemistry |volume=74 |issue=8 |pages=1397–1408 |doi=10.1351/pac200274081397 |doi-access=free}} or a beta-carotene 15,15'-dioxygenase.{{cite journal |last1=Kim |first1=Yeong-Su |last2=Kim |first2=Nam-Hee |last3=Yeom |first3=Soo-Jin |last4=Kim |first4=Seon-Won |last5=Oh |first5=Deok-Kun |year=2009 |title=In Vitro Characterization of a Recombinant Blh Protein from an Uncultured Marine Bacterium as a β-Carotene 15,15′-Dioxygenase |journal=Journal of Biological Chemistry |volume=284 |issue=23 |pages=15781–93 |pmid=19366683 |doi=10.1074/jbc.M109.002618 |pmc=2708875|doi-access=free }}

Just as carotenoids are the precursors of retinal, retinal is the precursor of the other forms of vitamin A. Retinal is interconvertible with retinol, the transport and storage form of vitamin A:

{{block indent|retinal + NADPH + H⁺ {{eqm}} retinol + NADP⁺}}

{{block indent|retinol + NAD⁺ {{eqm}} retinal + NADH + H⁺,}}

catalyzed by retinol dehydrogenases (RDHs){{cite journal |last1=Lidén |first1=M |last2=Eriksson |first2=U |year=2006 |title=Understanding Retinol Metabolism: Structure and Function of Retinol Dehydrogenases |journal=Journal of Biological Chemistry |volume=281 |issue=19 |pages=13001–04 |doi=10.1074/jbc.R500027200 |pmid=16428379 |doi-access=free}} and alcohol dehydrogenases (ADHs).{{cite journal |last1=Duester |first1=G |title=Retinoic Acid Synthesis and Signaling during Early Organogenesis |journal=Cell |volume=134 |issue=6 |pages=921–31 |date=September 2008 |pmid=18805086 |pmc=2632951 |doi=10.1016/j.cell.2008.09.002}}

Retinol is called vitamin A alcohol or, more often, simply vitamin A. Retinal can also be oxidized to retinoic acid:

{{block indent|retinal + NAD⁺ + H₂O → retinoic acid + NADH + H⁺ (catalyzed by RALDH)}}

{{block indent|retinal + O₂ + H₂O → retinoic acid + H₂O₂ (catalyzed by retinal oxidase),}}

catalyzed by retinal dehydrogenases{{cite journal |last1=Lin |first1=Min |last2=Zhang |first2=Min |last3=Abraham |first3=Michael |last4=Smith |first4=Susan M. |last5=Napoli |first5=Joseph L. |year=2003 |title=Mouse Retinal Dehydrogenase 4 (RALDH4), Molecular Cloning, Cellular Expression, and Activity in 9-cis-Retinoic Acid Biosynthesis in Intact Cells |journal=Journal of Biological Chemistry |volume=278 |issue=11 |pages=9856–9861 |doi=10.1074/jbc.M211417200 |pmid=12519776 |doi-access=free}} also known as retinaldehyde dehydrogenases (RALDHs) as well as retinal oxidases.{{cite web |url=https://www.genome.jp/dbget-bin/www_bget?enzyme+1.2.3.11 |title=KEGG ENZYME: 1.2.3.11 retinal oxidase |access-date=2009-03-10}}

Retinoic acid, sometimes called vitamin A acid, is an important signaling molecule and hormone in vertebrate animals.

Vision

Retinal is a conjugated chromophore. In the Vertebrate eyes, retinal begins in an 11-cis-retinal configuration, which — upon capturing a photon of the correct wavelength — straightens out into an all-trans-retinal configuration. This configuration change pushes against an opsin protein in the retina, which triggers a chemical signaling cascade, which results in perception of light or images by the brain. The absorbance spectrum of the chromophore depends on its interactions with the opsin protein to which it is bound, so that different retinal-opsin complexes will absorb photons of different wavelengths (i.e., different colors of light).

=Opsins=

File:1415_Retinal_Isomers.jpg

File:Rhodopsin-transducin.png (rainbow-colored) embedded in a lipid bilayer (heads red and tails blue) with transducin below it. G_tα is colored red, G_tβ blue, and G_tγ yellow. There is a bound GDP molecule in the G_tα-subunit and a bound retinal (black) in the rhodopsin. The N-terminus terminus of rhodopsin is red and the C-terminus blue. Anchoring of transducin to the membrane has been drawn in black.]]

Retinal is bound to opsins, which are G protein-coupled receptors (GPCRs).{{cite journal |last1=Casey |first1=P J |last2=Gilman |first2=A G |title=G protein involvement in receptor-effector coupling. |journal=Journal of Biological Chemistry |date=February 1988 |volume=263 |issue=6 |pages=2577–2580 |doi=10.1016/s0021-9258(18)69103-3 |pmid=2830256|s2cid=38970721 |doi-access=free }}{{cite journal |last1=Attwood |first1=T. K. |last2=Findlay |first2=J. B. C. |title=Fingerprinting G-protein-coupled receptors |journal=Protein Engineering, Design and Selection |date=1994 |volume=7 |issue=2 |pages=195–203 |doi=10.1093/protein/7.2.195|pmid=8170923 }} Opsins, like other GPCRs, have seven transmembrane alpha-helices connected by six loops. They are found in the photoreceptor cells in the retina of eye. The opsin in the vertebrate rod cells is rhodopsin. The rods form disks, which contain the rhodopsin molecules in their membranes and which are entirely inside of the cell. The N-terminus head of the molecule extends into the interior of the disk, and the C-terminus tail extends into the cytoplasm of the cell. The opsins in the cone cells are OPN1SW, OPN1MW, and OPN1LW. The cones form incomplete disks that are part of the plasma membrane, so that the N-terminus head extends outside of the cell. In opsins, retinal binds covalently to a lysine{{cite journal |last1=Bownds |first1=Deric |title=Site of Attachment of Retinal in Rhodopsin |journal=Nature |date=December 1967 |volume=216 |issue=5121 |pages=1178–1181 |doi=10.1038/2161178a0 |pmid=4294735|bibcode=1967Natur.216.1178B |s2cid=1657759 }} in the seventh transmembrane helix{{cite journal |last1=Hargrave |first1=P. A. |last2=McDowell |first2=J. H. |last3=Curtis |first3=Donna R. |last4=Wang |first4=Janet K. |last5=Juszczak |first5=Elizabeth |last6=Fong |first6=Shao-Ling |last7=Mohana Rao |first7=J. K. |last8=Argos |first8=P. |title=The structure of bovine rhodopsin |journal=Biophysics of Structure and Mechanism |date=1983 |volume=9 |issue=4 |pages=235–244 |doi=10.1007/BF00535659 |pmid=6342691|s2cid=20407577 }}{{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 = 739–45 | date = August 2000 | pmid = 10926528 | doi = 10.1126/science.289.5480.739 | citeseerx = 10.1.1.1012.2275 | bibcode = 2000Sci...289..739P }}{{cite journal | vauthors = Murakami M, Kouyama T | title = Crystal structure of squid rhodopsin | journal = Nature | volume = 453 | issue = 7193 | pages = 363–7 | date = May 2008 | pmid = 18480818 | doi = 10.1038/nature06925 | bibcode = 2008Natur.453..363M | s2cid = 4339970 }} through a Schiff base.{{cite journal |last1=Collins |first1=F. D. |title=Rhodopsin and Indicator Yellow |journal=Nature |date=March 1953 |volume=171 |issue=4350 |pages=469–471 |doi=10.1038/171469a0 |pmid=13046517|bibcode=1953Natur.171..469C |s2cid=4152360 }}{{cite journal |last1=Pitt |first1=G. A. J. |last2=Collins |first2=F. D. |last3=Morton |first3=R. A. |last4=Stok |first4=Pauline |title=Studies on rhodopsin. 8. Retinylidenemethylamine, an indicator yellow analogue |journal=Biochemical Journal |date=1 January 1955 |volume=59 |issue=1 |pages=122–128 |doi=10.1042/bj0590122 |pmid=14351151|pmc=1216098 }} Forming the Schiff base linkage involves removing the oxygen atom from retinal and two hydrogen atoms from the free amino group of lysine, giving H₂O. Retinylidene is the divalent group formed by removing the oxygen atom from retinal, and so opsins have been called retinylidene proteins.

Opsins are prototypical G protein-coupled receptors (GPCRs).{{cite journal |last=Lamb |first=T D |year=1996 |title=Gain and kinetics of activation in the G-protein cascade of phototransduction |journal=Proceedings of the National Academy of Sciences |volume=93 |issue=2 |pages=566–570 |pmid=8570596 |doi=10.1073/pnas.93.2.566 |pmc=40092 |bibcode=1996PNAS...93..566L|doi-access=free }} Cattle rhodopsin, the opsin of the rod cells, was the first GPCR to have its amino acid sequence{{cite journal |last1=Ovchinnikov |first1=Yu.A. |title=Rhodopsin and bacteriorhodopsin: structure-function relationships |journal=FEBS Letters |date=8 November 1982 |volume=148 |issue=2 |pages=179–191 |doi=10.1016/0014-5793(82)80805-3 |pmid=6759163|s2cid=85819100 |doi-access=free |bibcode=1982FEBSL.148..179O }} and 3D-structure (via X-ray crystallography) determined. Cattle rhodopsin contains 348 amino acid residues. Retinal binds as chromophore at Lys²⁹⁶. This lysine is conserved in almost all opsins, only a few opsins have lost it during evolution.{{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 | doi = 10.3390/cells11152441 | pmc = 9368030 | doi-access = free }} Opsins without the retinal binding lysine are not light sensitive.{{cite journal |last1=Katana |first1=Radoslaw |last2=Guan |first2=Chonglin |last3=Zanini |first3=Damiano |last4=Larsen |first4=Matthew E. |last5=Giraldo |first5=Diego |last6=Geurten |first6=Bart R.H. |last7=Schmidt |first7=Christoph F. |last8=Britt |first8=Steven G. |last9=Göpfert |first9=Martin C. |title=Chromophore-Independent Roles of Opsin Apoproteins in Drosophila Mechanoreceptors |journal=Current Biology |date=September 2019 |volume=29 |issue=17 |pages=2961–2969.e4 |doi=10.1016/j.cub.2019.07.036 |pmid=31447373|s2cid=201420079 |doi-access=free |bibcode=2019CBio...29E2961K }}{{cite journal |last1=Leung |first1=Nicole Y. |last2=Thakur |first2=Dhananjay P. |last3=Gurav |first3=Adishthi S. |last4=Kim |first4=Sang Hoon |last5=Di Pizio |first5=Antonella |last6=Niv |first6=Masha Y. |last7=Montell |first7=Craig |title=Functions of Opsins in Drosophila Taste |journal=Current Biology |date=April 2020 |volume=30 |issue=8 |pages=1367–1379.e6 |doi=10.1016/j.cub.2020.01.068 |pmid=32243853|pmc=7252503 |bibcode=2020CBio...30E1367L }}{{cite journal | vauthors = Kumbalasiri T, Rollag MD, Isoldi MC, Castrucci AM, Provencio I | title = Melanopsin triggers the release of internal calcium stores in response to light | journal = Photochemistry and Photobiology | volume = 83 | issue = 2 | pages = 273–279 | date = March 2007 | pmid = 16961436 | doi = 10.1562/2006-07-11-RA-964 | s2cid = 23060331 }} Such opsins may have other functions.

Although mammals use retinal exclusively as the opsin chromophore, other groups of animals additionally use four chromophores closely related to retinal: 3,4-didehydroretinal (vitamin A₂), (3R)-3-hydroxyretinal, (3S)-3-hydroxyretinal (both vitamin A₃), and (4R)-4-hydroxyretinal (vitamin A₄). Many fish and amphibians use 3,4-didehydroretinal, also called dehydroretinal. With the exception of the dipteran suborder Cyclorrhapha (the so-called higher flies), all insects examined use the (R)-enantiomer of 3-hydroxyretinal. The (R)-enantiomer is to be expected if 3-hydroxyretinal is produced directly from xanthophyll carotenoids. Cyclorrhaphans, including Drosophila, use (3S)-3-hydroxyretinal.{{cite journal |last1=Seki |first1=Takaharu |last2=Isono |first2=Kunio |last3=Ito |first3=Masayoshi |last4=Katsuta |first4=Yuko |year=1994 |title=Flies in the Group Cyclorrhapha Use (3S)-3-Hydroxyretinal as a Unique Visual Pigment Chromophore |journal=European Journal of Biochemistry |volume=226 |issue=2 |pages=691–696 |doi=10.1111/j.1432-1033.1994.tb20097.x |pmid=8001586 |doi-access=}}{{cite journal |last1=Seki |first1=Takaharu |last2=Isono |first2=Kunio |last3=Ozaki |first3=Kaoru |last4=Tsukahara |first4=Yasuo |last5=Shibata-Katsuta |first5=Yuko |last6=Ito |first6=Masayoshi |last7=Irie |first7=Toshiaki |last8=Katagiri |first8=Masanao |year=1998 |title=The metabolic pathway of visual pigment chromophore formation in Drosophila melanogaster: All-trans (3S)-3-hydroxyretinal is formed from all-trans retinal via (3R)-3-hydroxyretinal in the dark |journal=European Journal of Biochemistry |volume=257 |issue=2 |pages=522–527 |doi=10.1046/j.1432-1327.1998.2570522.x |pmid=9826202 |doi-access=free}} Firefly squid have been found to use (4R)-4-hydroxyretinal.

=Visual cycle=

File:Visual cycle.svg

The visual cycle is a circular enzymatic pathway, which is the front-end of phototransduction. It regenerates 11-cis-retinal. For example, the visual cycle of mammalian rod cells is as follows:

all-trans-retinyl ester + H₂O → 11-cis-retinol + fatty acid; RPE65 isomerohydrolases;{{cite journal |last1=Moiseyev |first1=Gennadiy |last2=Chen |first2=Ying |last3=Takahashi |first3=Yusuke |last4=Wu |first4=Bill X. |last5=Ma |first5=Jian-xing |year=2005 |title=RPE65 is the isomerohydrolase in the retinoid visual cycle |journal=Proceedings of the National Academy of Sciences |volume=102 |issue=35 |pages=12413–12418 |doi=10.1073/pnas.0503460102 |pmid=16116091 |pmc=1194921 |bibcode=2005PNAS..10212413M|doi-access=free }}
11-cis-retinol + NAD⁺ → 11-cis-retinal + NADH + H⁺; 11-cis-retinol dehydrogenases;
11-cis-retinal + aporhodopsin → rhodopsin + H₂O; forms Schiff base linkage to lysine, -CH=N⁺H-;
rhodopsin + hν → metarhodopsin II (i.e., 11-cis photoisomerizes to all-trans):
:(rhodopsin + hν → photorhodopsin → bathorhodopsin → lumirhodopsin → metarhodopsin I → metarhodopsin II);
metarhodopsin II + H₂O → aporhodopsin + all-trans-retinal;
all-trans-retinal + NADPH + H⁺ → all-trans-retinol + NADP⁺; all-trans-retinol dehydrogenases;
all-trans-retinol + fatty acid → all-trans-retinyl ester + H₂O; lecithin retinol acyltransferases (LRATs).{{cite journal |last1=Jin |first1=Minghao |last2=Yuan |first2=Quan |last3=Li |first3=Songhua |last4=Travis |first4=Gabriel H. |year=2007 |title=Role of LRAT on the Retinoid Isomerase Activity and Membrane Association of Rpe65 |journal=Journal of Biological Chemistry |volume=282 |issue=29 |pages=20915–20924 |doi=10.1074/jbc.M701432200 |pmid=17504753 |pmc=2747659|doi-access=free }}

Steps 3, 4, 5, and 6 occur in rod cell outer segments; Steps 1, 2, and 7 occur in retinal pigment epithelium (RPE) cells.

RPE65 isomerohydrolases are homologous with beta-carotene monooxygenases; the homologous ninaB enzyme in Drosophila has both retinal-forming carotenoid-oxygenase activity and all-trans to 11-cis isomerase activity.{{cite journal |last1=Oberhauser |first1=Vitus |last2=Voolstra |first2=Olaf |last3=Bangert |first3=Annette |last4=von Lintig |first4=Johannes |last5=Vogt |first5=Klaus |year=2008 |title=NinaB combines carotenoid oxygenase and retinoid isomerase activity in a single polypeptide |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=48 |pages=19000–5 |doi=10.1073/pnas.0807805105 |pmid=19020100 |pmc=2596218 |bibcode=2008PNAS..10519000O|doi-access=free }}

Microbial rhodopsins

All-trans-retinal is also an essential component of microbial opsins such as bacteriorhodopsin, channelrhodopsin, and halorhodopsin, which are important in bacterial and archaeal anoxygenic photosynthesis. In these molecules, light causes the all-trans-retinal to become 13-cis retinal, which then cycles back to all-trans-retinal in the dark state. These proteins are not evolutionarily related to animal opsins and are not GPCRs; the fact that they both use retinal is a result of convergent evolution.{{cite journal |doi=10.1016/S1011-1344(02)00245-2 |pmid=11960728 |title=All-trans to 13-cis retinal isomerization in light-adapted bacteriorhodopsin at acidic pH |year=2002 |last1=Chen |first1=De-Liang |last2=Wang |first2=Guang-yu |last3=Xu |first3=Bing |last4=Hu |first4=Kun-Sheng |journal=Journal of Photochemistry and Photobiology B: Biology |volume=66 |issue=3 |pages=188–194|bibcode=2002JPPB...66..188C }}

History

The American biochemist George Wald and others had outlined the visual cycle by 1958. For his work, Wald won a share of the 1967 Nobel Prize in Physiology or Medicine with Haldan Keffer Hartline and Ragnar Granit.[https://www.nobelprize.org/prizes/medicine/1967/summary/ Nobel Prize in Physiology or Medicine 1967]

References

External links

[http://www.accessexcellence.org/AE/AEC/CC/vision_background.html First Steps of Vision] - National Health Museum
[http://www.chemistry.wustl.edu/~edudev/LabTutorials/Vision/Vision.html Vision and Light-Induced Molecular Changes]
[https://palaeo-electronica.org/2000_1/retinal/vision.htm Retinal Anatomy and Visual Capacities]
[https://www.ch.ic.ac.uk/vchemlib/mim/bristol/retinal/retinal_text.htm Retinal], Imperial College v-chemlib

Category:Aldehydes

Category:Apocarotenoids

Category:Cyclohexenes

Category:Photosynthetic pigments

Category:Signal transduction

Category:Vision

Category:Vitamin A

he:אופסין#רטינל