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. Opsins An opsin protein surrounds a molecule of 11- cis retinal, awaiting the arrival of a photon. 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). 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. 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. Retinoic acid, sometimes called vitamin A acid, is an important signaling molecule and hormone in vertebrate animals. Retinal + NAD + + H 2O → retinoic acid + NADH + H + (catalyzed by RALDH) retinal + O 2 + H 2O → retinoic acid + H 2O 2 (catalyzed by retinal oxidase),Ĭatalyzed by retinal dehydrogenases also known as retinaldehyde dehydrogenases (RALDHs) as well as retinal oxidases. Retinal can also be oxidized to retinoic acid: Retinol is called vitamin A alcohol or, more often, simply vitamin A. Retinal + NADPH + H + ⇌ retinol + NADP + retinol + NAD + ⇌ retinal + NADH + H +,Ĭatalyzed by retinol dehydrogenases (RDHs) and alcohol dehydrogenases (ADHs). Retinal is interconvertible with retinol, the transport and storage form of vitamin A: Just as carotenoids are the precursors of retinal, retinal is the precursor of the other forms of vitamin A. Ĭatalyzed by a beta-carotene 15,15'-monooxygenase or a beta-carotene 15,15'-dioxygenase. Living organisms produce retinal by irreversible oxidative cleavage of carotenoids. Invertebrates such as insects and squid use hydroxylated forms of retinal in their visual systems, which derive from conversion from other xanthophylls. The other main forms of vitamin A - retinol and a partially active form, retinoic acid - may both be produced from retinal. Some carnivores cannot convert any carotenoids at all. No other carotenoids can be converted by animals to retinal. These carotenoids must be obtained from plants or other photosynthetic organisms. They also produce it from β-cryptoxanthin, a type of xanthophyll. 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. Retinal was originally called retinene, and was renamed after it was discovered to be vitamin A aldehyde. The number of different molecules that can be converted to retinal varies from species to species. 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. Since retinal absorbs mostly green light and transmits purple light, this gave rise to the Purple Earth Hypothesis. 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. Some microorganisms use retinal to convert light into metabolic energy. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Retinal (also known as retinaldehyde) is a polyene chromophore.
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