Retinal

Last updated
All-trans-retinal
All-trans-Retinal.svg
Retinal 3D ball.png
Names
IUPAC name
Retinal
Systematic IUPAC name
(2E,4E,6E,8E)-3,7-Dimethyl-9-(2,6,6-trimethylcyclohex-1-en-1-yl)nona-2,4,6,8-tetraenal
Other names
  • Retinene
  • Retinaldehyde
  • Vitamin A aldehyde
  • RAL
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.003.760 OOjs UI icon edit-ltr-progressive.svg
PubChem CID
UNII
  • InChI=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+
    Key: NCYCYZXNIZJOKI-OVSJKPMPSA-N
  • CC1=C(C(CCC1)(C)C)/C=C/C(=C/C=C/C(=C/C=O)/C)/C
Properties
C20H28O
Molar mass 284.443 g·mol−1
AppearanceOrange crystals from petroleum ether [1]
Melting point 61 to 64 °C (142 to 147 °F; 334 to 337 K) [1]
Nearly insoluble
Solubility in fatSoluble
Related compounds
Related compounds
 retinol; retinoic acid; beta-carotene; dehydroretinal; 3-hydroxyretinal; 4-hydroxyretinal
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

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).

Contents

Some microorganisms use retinal to convert light into metabolic energy. In fact, a recent study suggests most living organisms on our planet ~3 billion years ago used retinal to convert sunlight into energy rather than chlorophyll. Since retinal absorbs mostly green light and transmits purple light, this gave rise to the Purple Earth Hypothesis. [2]

There are many forms of vitamin A — all of which are converted to retinal, which cannot be made without them. Retinal itself is considered to be a form of vitamin A when eaten by an animal. The number of different molecules that can be converted to retinal varies from species to species. Retinal was originally called retinene , [3] and was renamed [4] after it was discovered to be vitamin A aldehyde . [5] [6]

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. [7]

For example:

beta-carotene + O2 → 2 retinal,

catalyzed by a beta-carotene 15,15'-monooxygenase [8] or a beta-carotene 15,15'-dioxygenase. [9]

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:

retinal + NADPH + H+⇌ retinol + NADP+
retinol + NAD +⇌ retinal + NADH + H+,

catalyzed by retinol dehydrogenases (RDHs) [10] and alcohol dehydrogenases (ADHs). [11]

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

retinal + NAD+ + H2O → retinoic acid + NADH + H+ (catalyzed by RALDH)
retinal + O2 + H2O → retinoic acid + H2O2 (catalyzed by retinal oxidase),

catalyzed by retinal dehydrogenases [12] also known as retinaldehyde dehydrogenases (RALDHs) [11] as well as retinal oxidases. [13]

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 human eye, 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 human 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

An opsin protein surrounds a molecule of 11-cis retinal, awaiting the arrival of a photon. Once the retinal molecule captures a photon, its configuration change causes it to push against the surrounding opsin protein which may cause the opsin to send a chemical signal to the human brain indicating that light has been detected. Retinal is then converted back to its 11-cis configuration by ATP phosphorylation, and the cycle begins again. 1415 Retinal Isomers.jpg
An opsin protein surrounds a molecule of 11-cis retinal, awaiting the arrival of a photon. Once the retinal molecule captures a photon, its configuration change causes it to push against the surrounding opsin protein which may cause the opsin to send a chemical signal to the human brain indicating that light has been detected. Retinal is then converted back to its 11-cis configuration by ATP phosphorylation, and the cycle begins again.
Animal GPCR rhodopsin (rainbow-colored) embedded in a lipid bilayer (heads red and tails blue) with transducin below it. Gta is colored red, Gtb blue, and Gtg yellow. There is a bound GDP molecule in the Gta-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. Rhodopsin-transducin.png
Animal GPCR rhodopsin (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. Anchoring of transducin to the membrane has been drawn in black.

Retinal is bound to opsins, which are G protein-coupled receptors (GPCRs). [14] [15] 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 [16] in the seventh transmembrane helix [17] [18] [19] through a Schiff base. [20] [21] 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). [22] Cattle rhodopsin, the opsin of the rod cells, was the first GPCR to have its amino acid sequence [23] and 3D-structure (via X-ray crystallography) determined. [18] Cattle rhodopsin contains 348 amino acid residues. Retinal binds as chromophore at Lys296. [18] [23] This lysine is conserved in almost all opsins, only a few opsins have lost it during evolution. [24] Opsins without the retinal binding lysine are not light sensitive. [25] [26] [27] Such opsins may have other functions. [26] [24]

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. [28] [29] Firefly squid have been found to use (4R)-4-hydroxyretinal.

Visual cycle

Visual cycle Visual cycle.svg
Visual cycle

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:

  1. all-trans-retinyl ester + H2O → 11-cis-retinol + fatty acid; RPE65 isomerohydrolases; [30]
  2. 11-cis-retinol + NAD+ → 11-cis-retinal + NADH + H+; 11-cis-retinol dehydrogenases;
  3. 11-cis-retinal + aporhodopsin → rhodopsin + H2O; forms Schiff base linkage to lysine, -CH=N+H-;
  4. rhodopsin + hν → metarhodopsin II (i.e., 11-cis photoisomerizes to all-trans):
    (rhodopsin + hν → photorhodopsin → bathorhodopsin → lumirhodopsin → metarhodopsin I → metarhodopsin II);
  5. metarhodopsin II + H2O → aporhodopsin + all-trans-retinal;
  6. all-trans-retinal + NADPH + H+ → all-trans-retinol + NADP+; all-trans-retinol dehydrogenases;
  7. all-trans-retinol + fatty acid → all-trans-retinyl ester + H2O; lecithin retinol acyltransferases (LRATs). [31]

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; [7] the homologous ninaB enzyme in Drosophila has both retinal-forming carotenoid-oxygenase activity and all-trans to 11-cis isomerase activity. [32]

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. [33]

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. [34]

See also

Related Research Articles

<span class="mw-page-title-main">Vitamin A</span> Essential nutrient

Vitamin A is a fat-soluble vitamin and an essential nutrient for animals. The term "vitamin A" encompasses a group of chemically related organic compounds that includes retinol, retinal, retinoic acid, and several provitamin (precursor) carotenoids, most notably beta-carotene. Vitamin A has multiple functions: it is essential for embryo development and growth, for maintenance of the immune system, and for vision, where it combines with the protein opsin to form rhodopsin – the light-absorbing molecule necessary for both low-light and color vision.

<span class="mw-page-title-main">Retinol</span> Chemical compound

Retinol, also called vitamin A1, is a fat-soluble vitamin in the vitamin A family that is found in food and used as a dietary supplement. Retinol or other forms of vitamin A are needed for vision, cellular development, maintenance of skin and mucous membranes, immune function and reproductive development. Dietary sources include fish, dairy products, and meat. As a supplement it is used to treat and prevent vitamin A deficiency, especially that which results in xerophthalmia. It is taken by mouth or by injection into a muscle. As an ingredient in skin-care products, it is used to reduce wrinkles and other effects of skin aging.

<span class="mw-page-title-main">Rhodopsin</span> Light-sensitive receptor protein

Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduction in rods. Rhodopsin mediates dim light vision and thus is extremely sensitive to light. When rhodopsin is exposed to light, it immediately photobleaches. In humans, it is regenerated fully in about 30 minutes, after which the rods are more sensitive. Defects in the rhodopsin gene cause eye diseases such as retinitis pigmentosa and congenital stationary night blindness.

In visual physiology, adaptation is the ability of the retina of the eye to adjust to various levels of light. Natural night vision, or scotopic vision, is the ability to see under low-light conditions. In humans, rod cells are exclusively responsible for night vision as cone cells are only able to function at higher illumination levels. Night vision is of lower quality than day vision because it is limited in resolution and colors cannot be discerned; only shades of gray are seen. In order for humans to transition from day to night vision they must undergo a dark adaptation period of up to two hours in which each eye adjusts from a high to a low luminescence "setting", increasing sensitivity hugely, by many orders of magnitude. This adaptation period is different between rod and cone cells and results from the regeneration of photopigments to increase retinal sensitivity. Light adaptation, in contrast, works very quickly, within seconds.

β-Carotene Red-orange pigment of the terpenoids class

β-Carotene (beta-carotene) is an organic, strongly coloured red-orange pigment abundant in fungi, plants, and fruits. It is a member of the carotenes, which are terpenoids (isoprenoids), synthesized biochemically from eight isoprene units and thus having 40 carbons. Among the carotenes, β-carotene is distinguished by having beta-rings at both ends of the molecule. β-Carotene is biosynthesized from geranylgeranyl pyrophosphate.

<span class="mw-page-title-main">Proteorhodopsin</span> Family of transmembrane proteins

Proteorhodopsin is a family of transmembrane proteins that use retinal as a chromophore for light-mediated functionality, in this case, a proton pump. pRhodopsin is found in marine planktonic bacteria, archaea and eukaryotes (protae), but was first discovered in bacteria.

<span class="mw-page-title-main">Opsin</span> Class of light-sensitive proteins

Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most prominently, they are found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in vision. Humans have in total nine opsins. Beside vision and light perception, opsins may also sense temperature, sound, or chemicals.

Visual phototransduction is the sensory transduction process of the visual system by which light is detected to yield nerve impulses in the rod cells and cone cells in the retina of the eye in humans and other vertebrates. It relies on the visual cycle, a sequence of biochemical reactions in which a molecule of retinal bound to opsin undergoes photoisomerization, initiates a cascade that signals detection of the photon, and is indirectly restored to its photosensitive isomer for reuse. Phototransduction in some invertebrates such as fruit flies relies on similar processes.

<span class="mw-page-title-main">Retinoic acid</span> Metabolite of vitamin A

Retinoic acid (used simplified here for all-trans-retinoic acid) is a metabolite of vitamin A1 (all-trans-retinol) that mediates the functions of vitamin A1 required for growth and development. All-trans-retinoic acid is required in chordate animals, which includes all higher animals from fish to humans. During early embryonic development, all-trans-retinoic acid generated in a specific region of the embryo helps determine position along the embryonic anterior/posterior axis by serving as an intercellular signaling molecule that guides development of the posterior portion of the embryo. It acts through Hox genes, which ultimately control anterior/posterior patterning in early developmental stages.

<span class="mw-page-title-main">Carotenoid oxygenase</span>

Carotenoid oxygenases are a family of enzymes involved in the cleavage of carotenoids to produce, for example, retinol, commonly known as vitamin A. This family includes an enzyme known as RPE65 which is abundantly expressed in the retinal pigment epithelium where it catalyzed the formation of 11-cis-retinol from all-trans-retinyl esters.

Rhodopsin kinase is a serine/threonine-specific protein kinase involved in phototransduction. This enzyme catalyses the following chemical reaction:

Retinylidene proteins, or rhodopsins in a broad sense, are proteins that use retinal as a chromophore for light reception. They are the molecular basis for a variety of light-sensing systems from phototaxis in flagellates to eyesight in animals. Retinylidene proteins include all forms of opsin and rhodopsin. While rhodopsin in the narrow sense refers to a dim-light visual pigment found in vertebrates, usually on rod cells, rhodopsin in the broad sense refers to any molecule consisting of an opsin and a retinal chromophore in the ground state. When activated by light, the chromophore is isomerized, at which point the molecule as a whole is no longer rhodopsin, but a related molecule such as metarhodopsin. However, it remains a retinylidene protein. The chromophore then separates from the opsin, at which point the bare opsin is a retinylidene protein. Thus, the molecule remains a retinylidene protein throughout the phototransduction cycle.

The visual cycle is a process in the retina that replenishes the molecule retinal for its use in vision. Retinal is the chromophore of most visual opsins, meaning it captures the photons to begin the phototransduction cascade. When the photon is absorbed, the 11-cis retinal photoisomerizes into all-trans retinal as it is ejected from the opsin protein. Each molecule of retinal must travel from the photoreceptor cell to the RPE and back in order to be refreshed and combined with another opsin. This closed enzymatic pathway of 11-cis retinal is sometimes called Wald's visual cycle after George Wald (1906–1997), who received the Nobel Prize in 1967 for his work towards its discovery.

In enzymology, a retinol dehydrogenase (RDH) (EC 1.1.1.105) is an enzyme that catalyzes the chemical reaction

<span class="mw-page-title-main">RRH</span> Protein-coding gene in the species Homo sapiens

Peropsin, a visual pigment-like receptor, is a protein that in humans is encoded by the RRH gene. It belongs like other animal opsins to the G protein-coupled receptors. Even so, the first peropsins were already discovered in mice and humans in 1997, not much is known about them.

<span class="mw-page-title-main">Retinal G protein coupled receptor</span> Protein-coding gene in the species Homo sapiens

RPE-retinal G protein-coupled receptor also known as RGR-opsin is a protein that in humans is encoded by the RGR gene. RGR-opsin is a member of the rhodopsin-like receptor subfamily of GPCR. Like other opsins which bind retinaldehyde, it contains a conserved lysine residue in the seventh transmembrane domain. RGR-opsin comes in different isoforms produced by alternative splicing.

<span class="mw-page-title-main">RPE65</span> Protein-coding gene in humans

Retinal pigment epithelium-specific 65 kDa protein, also known as retinoid isomerohydrolase, is an enzyme of the vertebrate visual cycle that is encoded in humans by the RPE65 gene. RPE65 is expressed in the retinal pigment epithelium and is responsible for the conversion of all-trans-retinyl esters to 11-cis-retinol during phototransduction. 11-cis-retinol is then used in visual pigment regeneration in photoreceptor cells. RPE65 belongs to the carotenoid oxygenase family of enzymes.

<span class="mw-page-title-main">CYP27C1</span> Protein-coding gene in the species Homo sapiens

CYP27C1 is a protein that in humans is encoded by the CYP27C1 gene. The Enzyme Commission number (EC) for this protein is EC 1.14.19.53. The full accepted name is all-trans-retinol 3,4-desaturase and the EC number 1 classifies CYP27C1 as a oxidoreductase that acts on paired donor by reducing oxygen. It is also identifiable by the UniProt code Q4G0S4.

<span class="mw-page-title-main">Emixustat</span> Chemical compound

Emixustat is a small molecule notable for its establishment of a new class of compounds known as visual cycle modulators (VCMs). Formulated as the hydrochloride salt, emixustat hydrochloride, it is the first synthetic medicinal compound shown to affect retinal disease processes when taken by mouth. Emixustat was invented by the British-American chemist, Ian L. Scott, and is currently in Phase 3 trials for dry, age-related macular degeneration (AMD).

<span class="mw-page-title-main">Vertebrate visual opsin</span>

Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to opsin, as they were the first opsins discovered and are still the most widely studied opsins.

References

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  28. ↑ Seki, Takaharu; Isono, Kunio; Ito, Masayoshi; Katsuta, Yuko (1994). "Flies in the Group Cyclorrhapha Use (3S)-3-Hydroxyretinal as a Unique Visual Pigment Chromophore". European Journal of Biochemistry. 226 (2): 691–696. doi:10.1111/j.1432-1033.1994.tb20097.x. PMID   8001586.
  29. ↑ Seki, Takaharu; Isono, Kunio; Ozaki, Kaoru; Tsukahara, Yasuo; Shibata-Katsuta, Yuko; Ito, Masayoshi; Irie, Toshiaki; Katagiri, Masanao (1998). "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". European Journal of Biochemistry. 257 (2): 522–527. doi: 10.1046/j.1432-1327.1998.2570522.x . PMID   9826202.
  30. ↑ Moiseyev, Gennadiy; Chen, Ying; Takahashi, Yusuke; Wu, Bill X.; Ma, Jian-xing (2005). "RPE65 is the isomerohydrolase in the retinoid visual cycle". Proceedings of the National Academy of Sciences. 102 (35): 12413–12418. Bibcode:2005PNAS..10212413M. doi: 10.1073/pnas.0503460102 . PMC   1194921 . PMID   16116091.
  31. ↑ Jin, Minghao; Yuan, Quan; Li, Songhua; Travis, Gabriel H. (2007). "Role of LRAT on the Retinoid Isomerase Activity and Membrane Association of Rpe65". Journal of Biological Chemistry. 282 (29): 20915–20924. doi: 10.1074/jbc.M701432200 . PMC   2747659 . PMID   17504753.
  32. ↑ Oberhauser, Vitus; Voolstra, Olaf; Bangert, Annette; von Lintig, Johannes; Vogt, Klaus (2008). "NinaB combines carotenoid oxygenase and retinoid isomerase activity in a single polypeptide". Proceedings of the National Academy of Sciences. 105 (48): 19000–5. Bibcode:2008PNAS..10519000O. doi: 10.1073/pnas.0807805105 . PMC   2596218 . PMID   19020100.
  33. ↑ Chen, De-Liang; Wang, Guang-yu; Xu, Bing; Hu, Kun-Sheng (2002). "All-trans to 13-cis retinal isomerization in light-adapted bacteriorhodopsin at acidic pH". Journal of Photochemistry and Photobiology B: Biology. 66 (3): 188–194. doi:10.1016/S1011-1344(02)00245-2. PMID   11960728.
  34. ↑ Nobel Prize in Physiology or Medicine 1967

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