Retinal is also known as retinaldehyde. It was originally called retinene, and renamed after it was discovered to be vitamin A aldehyde. Retinal is one of the many forms of vitamin A (the number of which varies from species to species). Retinal is a polyene chromophore, bound to proteins called opsins, and is the chemical basis of animal vision.Retinal allows certain microorganisms to convert light into metabolic energy. Retinal is also known as retinaldehyde. It was originally called retinene, and renamed after it was discovered to be vitamin A aldehyde. Retinal is one of the many forms of vitamin A (the number of which varies from species to species). Retinal is a polyene chromophore, bound to proteins called opsins, and is the chemical basis of animal vision.Retinal allows certain microorganisms to convert light into metabolic energy. Vertebrate animals ingest retinal directly from meat, or they produce retinal from carotenoids, either from one of two carotenes (α-carotene, β-carotene) or 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, and 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. Living organisms produce retinal (RAL) by irreversible oxidative cleavage of carotenoids.For example, catalyzed by a beta-carotene 15,15'-monooxygenaseor 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 (ROL), 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 (RA) catalyzed by retinal dehydrogenases also known as retinaldehyde dehydrogenases (RALDHs)as well as retinal oxidases.Retinoic acid, sometimes called vitamin A acid, is an important signaling molecule and hormone in vertebrate animals. 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 can result 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 different wavelengths (different colors of light). Opsins are proteins and the retinal-binding visual pigments found in the photoreceptor cells in the retinas of eyes. An opsin is arranged into a bundle of seven transmembrane alpha-helices connected by six loops. In rod cells the opsin molecules are embedded in the membranes of the disks 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. In cone cells the disks are defined by the cell's plasma membrane so that the N-terminus head extends outside of the cell. Retinal binds covalently to a lysine on the transmembrane helix nearest the C-terminus of the protein through a Schiff base linkage. Formation of 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.