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 thanchlorophyll, to convert sunlight into energy. Because retinal absorbs mostly green light and transmits purple light, this gave rise to thePurple Earth hypothesis.[2]
Retinal itself is considered to be a form ofvitamin 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 calledretinene,[3] and was renamed[4] after it was discovered to bevitamin Aaldehyde.[5][6]
Vertebrate animals ingest retinal directly from meat, or they produce retinal fromcarotenoids – either fromα-carotene orβ-carotene – both of which arecarotenes. They also produce it fromβ-cryptoxanthin, a type ofxanthophyll. These carotenoids must be obtained from plants or otherphotosynthetic 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 asinsects andsquid use hydroxylated forms of retinal in their visual systems, which derive from conversion from otherxanthophylls.
Just as carotenoids are the precursors of retinal, retinal is the precursor of the other forms of vitamin A. Retinal is interconvertible withretinol, the transport and storage form of vitamin A:
Retinal is aconjugated chromophore. In theVertebrate eyes, retinal begins in an 11-cis-retinal configuration, which — upon capturing aphoton of the correct wavelength — straightens out into an all-trans-retinal configuration. This configuration change pushes against an opsin protein in theretina, which triggers a chemical signaling cascade, which results inperception 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).
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 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 GPCRrhodopsin (rainbow-colored) embedded in alipid bilayer (heads red and tails blue) withtransducin below it. Gtα is colored red, Gtβ blue, and Gtγ yellow. There is a boundGDP molecule in the Gtα-subunit and a boundretinal (black) in the rhodopsin. TheN-terminus terminus of rhodopsin is red and theC-terminus blue. Anchoring of transducin to the membrane has been drawn in black.
Retinal is bound toopsins, which areG protein-coupled receptors (GPCRs).[14][15] Opsins, like other GPCRs, have seven transmembranealpha-helices connected by six loops. They are found in thephotoreceptor cells in theretina of eye. The opsin in the vertebraterod cells isrhodopsin. The rods form disks, which contain the rhodopsin molecules in their membranes and which are entirely inside of the cell. TheN-terminus head of the molecule extends into the interior of the disk, and theC-terminus tail extends into the cytoplasm of the cell. The opsins in thecone cells areOPN1SW,OPN1MW, andOPN1LW. The cones form incomplete disks that are part of theplasma membrane, so that the N-terminus head extends outside of the cell. In opsins, retinal binds covalently to alysine[16] in the seventh transmembrane helix[17][18][19] through aSchiff 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 calledretinylidene proteins.
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 calleddehydroretinal. With the exception of thedipteran suborderCyclorrhapha (the so-called higher flies), allinsects examined use the (R)-enantiomer of 3-hydroxyretinal. The (R)-enantiomer is to be expected if 3-hydroxyretinal is produced directly fromxanthophyll carotenoids. Cyclorrhaphans, includingDrosophila, use (3S)-3-hydroxyretinal.[28][29]Firefly squid have been found to use (4R)-4-hydroxyretinal.
The visual cycle is a circularenzymatic 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:
RPE65 isomerohydrolases arehomologous with beta-carotene monooxygenases;[7] the homologous ninaB enzyme inDrosophila has both retinal-forming carotenoid-oxygenase activity and all-trans to 11-cis isomerase activity.[32]
All-trans-retinal is also an essential component ofmicrobial opsins such asbacteriorhodopsin,channelrhodopsin, andhalorhodopsin, which are important inbacterial andarchaealanoxygenic 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 ofconvergent evolution.[33]
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