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. 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). 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 in the seventh transmembrane helix through a Schiff base. 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. 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