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Retinal (also known as retinaldehyde) is a polyene
chromophore A chromophore is the part of a molecule responsible for its color. The color that is seen by our eyes is the one not absorbed by the reflecting object within a certain wavelength spectrum of visible light. The chromophore is a region in the mo ...
. Retinal, bound to proteins called
opsin 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 ...
s, is the chemical basis of visual phototransduction, the light-detection stage of
visual perception Visual perception is the ability to interpret the surrounding environment through photopic vision (daytime vision), color vision, scotopic vision (night vision), and mesopic vision (twilight vision), using light in the visible spectrum ref ...
(vision). 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 Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to ...
. Since retinal absorbs mostly green light and transmits purple light, this gave rise to the
Purple Earth Hypothesis The Purple Earth hypothesis is an astrobiological hypothesis that photosynthetic life forms of early Earth were based on the simpler molecule retinal rather than the more complex chlorophyll, making Earth appear purple rather than green. An examp ...
. There are many forms of
vitamin A Vitamin A is a fat-soluble vitamin and an essential nutrient for humans. It is a group of organic compounds that includes retinol, retinal (also known as retinaldehyde), retinoic acid, and several provitamin A carotenoids (most notably ...
— 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 The retinenes (retinene1 and retinene2) are chemical derivatives of vitamin A (see retinol) formed through oxidation reactions. Retinene1 is better known as retinal and is fundamental in the transduction of light into visual signals in the photo ...
, and was renamed after it was discovered to be
vitamin A Vitamin A is a fat-soluble vitamin and an essential nutrient for humans. It is a group of organic compounds that includes retinol, retinal (also known as retinaldehyde), retinoic acid, and several provitamin A carotenoids (most notably ...
aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl gro ...
.
Vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
animals ingest retinal directly from meat, or they produce retinal from
carotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, ...
s — either from α-carotene or β-carotene — both of which are
carotene The term carotene (also carotin, from the Latin ''carota'', "carrot") is used for many related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but in general cannot be made by animals (with the ex ...
s. They also produce it from β-cryptoxanthin, a type of xanthophyll. These carotenoids must be obtained from plants or other
photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
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 Retinol, also called vitamin A1, is a fat-soluble vitamin in the vitamin A family found in food and used as a dietary supplement. As a supplement it is used to treat and prevent vitamin A deficiency, especially that which results in xeroph ...
and a partially active form, retinoic acid — may both be produced from retinal.
Invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chorda ...
s such as
insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pa ...
s and
squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fittin ...
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. For example: : beta-carotene + O2 → 2 retinal, catalyzed by a beta-carotene 15,15'-monooxygenase or a beta-carotene 15,15'-dioxygenase. Just as carotenoids are the precursors of retinal, retinal is the precursor of the other forms of vitamin A. Retinal is interconvertible with
retinol Retinol, also called vitamin A1, is a fat-soluble vitamin in the vitamin A family found in food and used as a dietary supplement. As a supplement it is used to treat and prevent vitamin A deficiency, especially that which results in xeroph ...
, the transport and storage form of vitamin A: :retinal +
NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require NA ...
+ H+ retinol + NADP+ :retinol + NAD+ retinal + NADH + H+, catalyzed by retinol dehydrogenases (RDHs) and alcohol dehydrogenases (ADHs). Retinol is called vitamin A
alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...
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 dehydrogenase In enzymology, a retinal dehydrogenase, also known as retinaldehyde dehydrogenase (RALDH), catalyzes the chemical reaction converting retinal to retinoic acid. This enzyme belongs to the family of oxidoreductases, specifically the class acting on a ...
s also known as retinaldehyde dehydrogenases (RALDHs) as well as retinal oxidases. Retinoic acid, sometimes called vitamin A
acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a se ...
, is an important signaling molecule and hormone in vertebrate animals.


Vision

Retinal is a conjugated chromophore. In the
human eye The human eye is a sensory organ, part of the sensory nervous system, that reacts to visible light and allows humans to use visual information for various purposes including seeing things, keeping balance, and maintaining circadian rhythm. ...
, retinal begins in an 11-''cis''-retinal configuration, which — upon capturing a
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
of the correct wavelength — straightens out into an all-''trans''-retinal configuration. This configuration change pushes against an opsin protein in the
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...
, which triggers a chemical signaling cascade, which results in
perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous syste ...
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

Retinal is bound to
opsin 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 ...
s, which are
G protein-coupled receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...
s (GPCRs). Opsins, like other GPCRs, have seven transmembrane alpha-helices connected by six loops. They are found in the
photoreceptor cell A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiat ...
s in the
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...
of eye. The opsin in the vertebrate
rod cell Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in pe ...
s is
rhodopsin 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 phototransductio ...
. The rods form disks, which contain the rhodopsin molecules in their membranes and which are entirely inside of the cell. The
N-terminus The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...
head of the molecule extends into the interior of the disk, and the
C-terminus The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein i ...
tail extends into the cytoplasm of the cell. The opsins in the
cone cell Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cone ...
s are OPN1SW,
OPN1MW Green-sensitive opsin is a protein that in humans is encoded by the ''OPN1MW'' gene. OPN1MW2 is a similar opsin. See also * Opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, ...
, and OPN1LW. The cones form incomplete disks that are part of the
plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...
, so that the N-terminus head extends outside of the cell. In opsins, retinal binds covalently to a
lysine Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. It contains an α-amino group (which is in the protonated form under biological conditions), an α-carboxylic acid group (which is in the deprotonated − ...
in the seventh transmembrane helix through a Schiff base. 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 protein Retinylidene proteins, 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 ...
s. Opsins are prototypical
G protein-coupled receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...
s (GPCRs). Cattle rhodopsin, the opsin of the rod cells, was the first GPCR to have its amino acid sequence and 3D-structure (via
X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
) determined.
Cattle Cattle (''Bos taurus'') are large, domesticated, cloven-hooved, herbivores. They are a prominent modern member of the subfamily Bovinae and the most widespread species of the genus '' Bos''. Adult females are referred to as cows and adult ...
rhodopsin contains 348
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
residues. Retinal binds as chromophore at Lys296. This lysine is conserved in almost all opsins, only a few opsins have lost it during
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
. Opsins without the retinal binding lysine are not light sensitive. 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), (3''R'')-3-hydroxyretinal, (3''S'')-3-hydroxyretinal (both vitamin A3), and (4''R'')-4-hydroxyretinal (vitamin A4). Many fish and amphibians use 3,4-didehydroretinal, also called
dehydroretinal Dehydroretinal (3,4-dehydroretinal) is a derivative metabolite of retinal belonging to the group of vitamin A2 as a retinaldehyde form, besides the endogenously present 3,4-dehydroretinol and 3,4-dehydroretinoic acid. The livers of some freshwat ...
. With the exception of the
diptera Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced ...
n suborder
Cyclorrhapha Cyclorrhapha is an unranked taxon within the infraorder Muscomorpha. They are called "Cyclorrhapha" ('circular-seamed flies') with reference to the circular aperture through which the adult escapes the puparium. This is a circumscriptional name t ...
(the so-called higher flies), all
insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pa ...
s examined use the (''R'')-
enantiomer In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical anti ...
of 3-hydroxyretinal. The (''R'')-enantiomer is to be expected if 3-hydroxyretinal is produced directly from xanthophyll carotenoids. Cyclorrhaphans, including ''
Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many speci ...
'', use (3''S'')-3-hydroxyretinal. Firefly squid have been found to use (4''R'')-4-hydroxyretinal.


Visual cycle

The visual cycle is a circular
enzymatic pathway In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence of chemical reac ...
, 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 In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, f ...
; RPE65 isomerohydrolases; # 11-''cis''-retinol + NAD+ → 11-''cis''-retinal + NADH + H+; 11-''cis''-retinol dehydrogenases; # 11-''cis''-retinal +
aporhodopsin 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 i ...
rhodopsin 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 phototransductio ...
+ H2O; forms Schiff base linkage to
lysine Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. It contains an α-amino group (which is in the protonated form under biological conditions), an α-carboxylic acid group (which is in the deprotonated − ...
, -CH=N+H-; #rhodopsin + 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 acyltransferase Lecithin retinol acyltransferase is an enzyme that in humans is encoded by the ''LRAT'' gene. Function Lecithin retinol acyltransferase is a microsomal enzyme that catalyzes the esterification of all-trans-retinol into all-trans-retinyl ester ...
s (LRATs). 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.


Microbial rhodopsins

All-''trans''-retinal is also an essential component of microbial opsins such as
bacteriorhodopsin Bacteriorhodopsin is a protein used by Archaea, most notably by haloarchaea, a class of the Euryarchaeota. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. The resulting ...
, channelrhodopsin, and halorhodopsin. 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 Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
.


History

The American biochemist
George Wald George Wald (November 18, 1906 – April 12, 1997) was an American scientist who studied pigments in the retina. He won a share of the 1967 Nobel Prize in Physiology or Medicine with Haldan Keffer Hartline and Ragnar Granit. In 1970, Wald pr ...
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 The Nobel Prize in Physiology or Medicine is awarded yearly by the Nobel Assembly at the Karolinska Institute for outstanding discoveries in physiology or medicine. The Nobel Prize is not a single prize, but five separate prizes that, accordi ...
with Haldan Keffer Hartline and Ragnar Granit.Nobel Prize in Physiology or Medicine 1967
/ref>


See also

*
Purple Earth hypothesis The Purple Earth hypothesis is an astrobiological hypothesis that photosynthetic life forms of early Earth were based on the simpler molecule retinal rather than the more complex chlorophyll, making Earth appear purple rather than green. An examp ...
* Sensory nervous system *
Visual perception Visual perception is the ability to interpret the surrounding environment through photopic vision (daytime vision), color vision, scotopic vision (night vision), and mesopic vision (twilight vision), using light in the visible spectrum ref ...
* Visual phototransduction


References


Further reading

* * * * * * * * * Good historical review. * * * * * * * * * The oceans are full of type 1 rhodopsin. * * * * *


External links


First Steps of Vision
- National Health Museum

Imperial College v-chemlib {{Authority control Aldehydes Apocarotenoids Cyclohexenes Photosynthetic pigments Signal transduction Vision Vitamin A he:אופסין#רטינל