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Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called , 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.
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, and was renamed after it was discovered to be vitamin A aldehyde.
Vertebrate animals ingest retinal directly from meat, or they produce retinal from – either from alpha-Carotene or β-carotene – both of which are . 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.
such as and squid use hydroxylated forms of retinal in their visual systems, which derive from conversion from other xanthophylls.
For example:
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, the transport and storage form of vitamin A:
catalyzed by retinol dehydrogenases (RDHs) and alcohol dehydrogenases (ADHs).
Retinol is called vitamin A alcohol or, more often, simply vitamin A. Retinal can also be oxidized to retinoic acid:
catalyzed by retinal dehydrogenases also known as retinaldehyde dehydrogenases (RALDHs) as well as .
Retinoic acid, sometimes called vitamin A carboxylic acid, is an important signaling molecule and hormone in vertebrate animals.
Opsins are prototypical G protein-coupled receptors (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) 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. 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. With the exception of the suborder Cyclorrhapha (the so-called higher flies), all 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 (3 S)-3-hydroxyretinal. Firefly squid have been found to use (4 R)-4-hydroxyretinal.
Steps 3, 4, 5, and 6 occur in rod cell; 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.
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