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]
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,[3] and was renamed[4] after it was discovered to be vitamin Aaldehyde.[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]
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:
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
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.
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.
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:
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]
All-trans-retinal is also an essential component of
microbial opsins such as
bacteriorhodopsin,
channelrhodopsin, and
halorhodopsin, which are important in
bacterial and
archaealanoxygenic 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]
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