Retinal

retinaldehydeall-trans-retinalretinylideneall-''trans''-retinalall-trans retinalretina
Retinal is also known as retinaldehyde.wikipedia
153 Related Articles

Vitamin A

Avitamins ARAE
Retinal is also known as retinaldehyde. It was originally called retinene, and renamed after it was discovered to be vitamin A aldehyde.
Vitamin A is a group of unsaturated nutritional organic compounds that includes retinol, retinal, retinoic acid, and several provitamin A carotenoids (most notably beta-carotene).

Retinene

Retinal is also known as retinaldehyde. It was originally called retinene, and renamed after it was discovered to be vitamin A aldehyde.
Retinene 1 is better known as retinaldehyde or simply retinal and is fundamental in the transduction of light into visual signals in the photoreceptor level of the retina (known as the visual cycle).

Aldehyde

aldehydesformyldialdehyde
Retinal is also known as retinaldehyde. It was originally called retinene, and renamed after it was discovered to be vitamin A aldehyde.
From the left: (1) formaldehyde and (2) its trimer 1,3,5-trioxane, (3) acetaldehyde and (4) its enol vinyl alcohol, (5) glucose (pyranose form as α--glucopyranose), (6) the flavorant cinnamaldehyde, (7) the visual pigment retinal, and (8) the vitamin pyridoxal.

Carotene

β-carotenecarotenesbeta carotene
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.
β-Carotene is composed of two retinyl groups, and is broken down in the mucosa of the human small intestine by β-carotene 15,15'-monooxygenase to retinal, a form of vitamin A.

Retinol

vitamin Aretinol metabolismA
The other main forms of vitamin A, retinol, and a partially active form, retinoic acid, may both be produced from retinal.
It is converted in the body to retinal and retinoic acid through which it acts.

Chromophore

chromophoreschromophoricchromophore group
Retinal is a polyene chromophore, bound to proteins called opsins, and is the chemical basis of animal vision. When the 11-cis-retinal chromophore absorbs a photon it isomerizes from the 11-cis state to the all-trans state.
Common examples include retinal (used in the eye to detect light), various food colorings, fabric dyes (azo compounds), pH indicators, lycopene, β-carotene, and anthocyanins.

Retinol dehydrogenase

catalyzed by retinol dehydrogenases (RDHs) and alcohol dehydrogenases (ADHs).
Thus, the two substrates of this enzyme are retinol and NAD +, whereas its 3 products are retinal, NADH (or NADPH in the case where NADP + is a cofactor), and H +.

Retinal dehydrogenase

catalyzed by retinal dehydrogenases also known as retinaldehyde dehydrogenases (RALDHs)
The 3 substrates of this enzyme are retinal, NAD +, and H 2 O, whereas its 3 products are retinoic acid, NADH, and H +.

Retinylidene protein

retinal-binding proteins
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 H 2 O. Retinylidene is the divalent group formed by removing the oxygen atom from retinal, and so opsins have been called retinylidene proteins.
Retinylidene protein, is a family of proteins that use retinal as a chromophore for light reception.

Retinal oxidase

as well as retinal oxidases.
The 3 substrates of this enzyme are retinal, O 2, and H 2 O, whereas its two products are retinoic acid and H 2 O 2.

11-cis retinal

11-cis-retinal11-''cis''-retinal11-cis''-retinal
When the 11-cis-retinal chromophore absorbs a photon it isomerizes from the 11-cis state to the all-trans state. 3) 11-cis-retinal + aporhodopsin → rhodopsin + H 2 O; forms Schiff base linkage to lysine, -CH=N + H-,
The 11-cis Retinal is an isomer of retinal.

Xanthophyll

xanthophyllsxanthophyll cycle '''Xanthophylls'''
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 two specific xanthophylls do not function in the mechanism of sight, since they cannot be converted to retinal (also called retinaldehyde or vitamin A aldehyde).

Beta-carotene 15,15'-dioxygenase

beta-carotene 15,15'-monooxygenaseb-carotene 15,15'-monooxygenaseβ-carotene 15,15'-monooxygenase
catalyzed by a beta-carotene 15,15'-monooxygenase
This is a cleavage reaction which cleaves β-carotene, utilizes molecular oxygen, is enhanced by the presence of bile salts and thyroxine, and generates two molecules of retinal.

Photoreceptor cell

photoreceptorsphotoreceptorphotoreceptor cells
Opsins are proteins and the retinal-binding visual pigments found in the photoreceptor cells in the retinas of eyes.
The membranous photoreceptor protein opsin contains a pigment molecule called retinal.

Photoisomerization

photoisomerizable
Vision begins with the photoisomerization of retinal.
In fact, photoisomerization of the molecule retinal in the eye is the mechanism that allows for vision.

Retinoic acid

9-''cis''-retinoic acidRAretinoic acid gradient
The other main forms of vitamin A, retinol, and a partially active form, retinoic acid, may both be produced from retinal.
Retinoic acid is responsible for most of the activity of vitamin A, save visual pigment effects that require retinal (retinaldehyde), and cell metabolism effects that may require retinol itself.

Retina

retinal diseasesretinasretinal
Opsins are proteins and the retinal-binding visual pigments found in the photoreceptor cells in the retinas of eyes.
The photon causes the retinal bound to the receptor protein to isomerise to trans-retinal.

Dehydroretinal

3-dehydroretinaldehydroretinaldehyde
Many fish and amphibians use 3,4-didehydroretinal, also called dehydroretinal.
Dehydroretinal is a derivative (metabolite) of retinal.

Schiff base

azomethineSchiff base linkageSchiff base reaction
Retinal binds covalently to a lysine on the transmembrane helix nearest the C-terminus of the protein through a Schiff base linkage. 3) 11-cis-retinal + aporhodopsin → rhodopsin + H 2 O; forms Schiff base linkage to lysine, -CH=N + H-,
Similarly, the cofactor retinal forms a Schiff base in rhodopsins, including human rhodopsin (via Lysine 296), which is key in the photoreception mechanism.

Rhodopsin

visual purple
3) 11-cis-retinal + aporhodopsin → rhodopsin + H 2 O; forms Schiff base linkage to lysine, -CH=N + H-,
Rhodopsin consists of two components, a protein molecule also called scotopsin and a covalently-bound cofactor called retinal.

Photon

photonslight quantalight
When the 11-cis-retinal chromophore absorbs a photon it isomerizes from the 11-cis state to the all-trans state.
A classic example is the molecular transition of retinal (C 20 H 28 O), which is responsible for vision, as discovered in 1958 by Nobel laureate biochemist George Wald and co-workers.

Channelrhodopsin

channelrhodopsin-2channelrhodopsins
All-trans-retinal is also an essential component of type I, or microbial, opsins such as bacteriorhodopsin, channelrhodopsin, and halorhodopsin.
They are seven-transmembrane proteins like rhodopsin, and contain the light-isomerizable chromophore all-trans-retinal (an aldehyde derivative of vitamin A).

Lysine

Lyslysine degradationlysine biosynthesis
Retinal binds covalently to a lysine on the transmembrane helix nearest the C-terminus of the protein through a Schiff base linkage. 3) 11-cis-retinal + aporhodopsin → rhodopsin + H 2 O; forms Schiff base linkage to lysine, -CH=N + H-,
In opsins like rhodopsin and the visual opsins (encoded by the genes OPN1SW, OPN1MW, and OPN1LW), retinaldehyde forms a Schiff base with a conserved lysine residue, and interaction of light with the retinylidene group causes signal transduction in color vision (See visual cycle for details).

Bacteriorhodopsin

bacteriorhodopsinsArchaerhodopsin 3
All-trans-retinal is also an essential component of type I, or microbial, opsins such as bacteriorhodopsin, channelrhodopsin, and halorhodopsin.
Each chain has seven transmembrane alpha helices and contains one molecule of retinal buried deep within, the typical structure for retinylidene proteins.

Retinal pigment epithelium

retinal pigmented epitheliumpigment epitheliumretinal pigment epithelial
Steps 3,4,5,6 occur in rod cell outer segments; Steps 1, 2, and 7 occur in retinal pigment epithelium (RPE) cells.
4) Visual cycle: The visual cycle fulfills an essential task of maintaining visual function and needs therefore to be adapted to different visual needs such as vision in darkness or lightness. For this, functional aspects come into play: the storage of retinal and the adaption of the reaction speed. Basically vision at low light intensities requires a lower turn-over rate of the visual cycle whereas during light the turn-over rate is much higher. In the transition from darkness to light suddenly, large amount of 11-cis retinal is required. This comes not directly from the visual cycle but from several retinal pools of retinal binding proteins which are connected to each other by the transportation and reaction steps of the visual cycle.