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Retinal, also called retinaldehyde or vitamin A aldehyde
Aldehyde

An aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double bond to an oxygen atom , is called the aldehyde group....
, is one of the three forms of vitamin A. Retinal is a polyene
Polyene

Polyenes are poly-Saturation organic compounds that contain one or more sequences of alternating double and single carbon-carbon bonds. These double carbon-carbon bonds interact in a process known as Conjugated system, which results in an overall lower energy state of the molecule....
 chromophore
Chromophore

A chromophore is part of a molecule responsible for its color.When a molecule absorbs certain wavelengths of visible spectrum and transmits or reflects others, the molecule has a color....
, and bound to proteins called opsins, is the chemical basis of animal
Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
 vision. Bound to proteins called type 1 rhodopsins, retinal allows certain microorganisms to convert light into metabolic energy. Animals produce retinal from carotenoids which they must obtain from plants or other photosynthetic organisms. In turn, animals produce the other two forms of vitamin A, retinol
Retinol

Retinol, the animal form of vitamin A, is a fat-soluble vitamin important in visual system and bone growth. It is also a Terpenoid. Retinol is among the most useable forms of vitamin A, which also include Retinal , Retinoic acid and retinyl ester ....
 and retinoic acid
Retinoic acid

Retinoic acid is the oxidized form of Vitamin A. It functions in determining position along embryonic anterior/posterior axis in chordates. It acts through Hox genes, which ultimately controls anterior/posterior patterning in early developmental stages....
, from retinal.

example

beta-carotene
Beta-carotene

?-Carotene is an organic compound - a terpenoid, a red-orange pigment abundant in plants and fruits. As a carotene with ?-rings at both ends, it is the most common form of carotene....
 + O2 2 retinal


catalyzed by a beta-carotene 15,15'-monooxygenase
Beta-carotene 15,15'-monooxygenase

In enzymology, a beta-carotene 15,15'-monooxygenase is an enzyme that catalysis the chemical reactionThus, the two substrate of this enzyme are beta-carotene and oxygen, whereas its product is retinal....
 (BCO). Just as carotenoids are the precursors of retinal, retinal is the precursor of the other forms of vitamin A.






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Retinal, also called retinaldehyde or vitamin A aldehyde
Aldehyde

An aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double bond to an oxygen atom , is called the aldehyde group....
, is one of the three forms of vitamin A. Retinal is a polyene
Polyene

Polyenes are poly-Saturation organic compounds that contain one or more sequences of alternating double and single carbon-carbon bonds. These double carbon-carbon bonds interact in a process known as Conjugated system, which results in an overall lower energy state of the molecule....
 chromophore
Chromophore

A chromophore is part of a molecule responsible for its color.When a molecule absorbs certain wavelengths of visible spectrum and transmits or reflects others, the molecule has a color....
, and bound to proteins called opsins, is the chemical basis of animal
Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
 vision. Bound to proteins called type 1 rhodopsins, retinal allows certain microorganisms to convert light into metabolic energy. Animals produce retinal from carotenoids which they must obtain from plants or other photosynthetic organisms. In turn, animals produce the other two forms of vitamin A, retinol
Retinol

Retinol, the animal form of vitamin A, is a fat-soluble vitamin important in visual system and bone growth. It is also a Terpenoid. Retinol is among the most useable forms of vitamin A, which also include Retinal , Retinoic acid and retinyl ester ....
 and retinoic acid
Retinoic acid

Retinoic acid is the oxidized form of Vitamin A. It functions in determining position along embryonic anterior/posterior axis in chordates. It acts through Hox genes, which ultimately controls anterior/posterior patterning in early developmental stages....
, from retinal.

Vitamin A metabolism


Animals produce retinal by irreversible oxidative cleavage of carotenoids. For example

beta-carotene
Beta-carotene

?-Carotene is an organic compound - a terpenoid, a red-orange pigment abundant in plants and fruits. As a carotene with ?-rings at both ends, it is the most common form of carotene....
 + O2 2 retinal


catalyzed by a beta-carotene 15,15'-monooxygenase
Beta-carotene 15,15'-monooxygenase

In enzymology, a beta-carotene 15,15'-monooxygenase is an enzyme that catalysis the chemical reactionThus, the two substrate of this enzyme are beta-carotene and oxygen, whereas its product is retinal....
 (BCO). Just as carotenoids are the precursors of retinal, retinal is the precursor of the other forms of vitamin A. Retinal is interconvertible with retinol
Retinol

Retinol, the animal form of vitamin A, is a fat-soluble vitamin important in visual system and bone growth. It is also a Terpenoid. Retinol is among the most useable forms of vitamin A, which also include Retinal , Retinoic acid and retinyl ester ....
, the transport and storage form of vitamin A

retinal + NADPH
Nicotinamide adenine dinucleotide phosphate

Nicotinamide adenine dinucleotide phosphate is used in anabolic reactions, such as lipid and nucleic acid synthesis, which require NADPH as a reducing agent....
 + H+ retinol + NADP+
retinol + NAD+ retinal + NADH + H+


catalyzed by retinol dehydrogenase
Retinol dehydrogenase

In enzymology, a retinol dehydrogenase is an enzyme that catalysis the chemical reactionThus, the two substrate of this enzyme are retinol and nicotinamide adenine dinucleotide, whereas its 3 product are retinal, nicotinamide adenine dinucleotide, and hydrogen ion....
s (RDHs). Retinol may be called vitamin A alcohol
Alcohol

In chemistry, an alcohol is any organic compound in which a hydroxyl Functional group is bound to a carbon atom of an alkyl or substituted alkyl group....
, or more often, simply vitamin A. Retinal can also be oxidized to retinoic acid
Retinoic acid

Retinoic acid is the oxidized form of Vitamin A. It functions in determining position along embryonic anterior/posterior axis in chordates. It acts through Hox genes, which ultimately controls anterior/posterior patterning in early developmental stages....


retinal + NAD+ + H2O retinoic acid + NADH + H+
retinal + O2 + H2O retinoic acid + H2O2


catalyzed by retinal dehydrogenase
Retinal dehydrogenase

In enzymology, a retinal dehydrogenase is an enzyme that catalysis the chemical reactionThe 3 substrate of this enzyme are retinal, nicotinamide adenine dinucleotide, and water, whereas its 3 product are retinoic acid, nicotinamide adenine dinucleotide, and hydrogen ion....
s and retinal oxidase
Retinal oxidase

In enzymology, a retinal oxidase is an enzyme that catalysis the chemical reactionThe 3 substrate of this enzyme are retinal, oxygen, and water, whereas its two product are retinoic acid and hydrogen peroxide....
s. Retinoic acid, sometimes called vitamin A acid
Carboxylic acid

Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the Chemical formula -COH, usually written -COOH or -CO2H....
, is an important signaling molecule and hormone.

Visual cycle


The visual cycle is a circular enzymatic pathway, which is the first part of the process by which animals see.

Visual cycle (rhodopsin of mammalian rod cells as example)
  1. all-trans-retinyl ester + H2O 11-cis-retinol + fatty acid; RPE65
    RPE65

    Retinal pigment epithelium-specific protein 65kDa, also known as RPE65, is a human gene....
     isomerohydrolases
  2. 11-cis-retinol + NAD(+) 11-cis-retinal + NADH + H+; 11-cis-retinol dehydrogenases
  3. 11-cis-retinal + aporhodopsin rhodopsin + H2O; forms Schiff base
    Schiff base

    A Schiff base , named after Hugo Schiff, is a functional group that contains a carbon-nitrogen double bond with the nitrogen atom connected to an aryl or alkyl group?but not hydrogen ....
     linkage to lysine
    Lysine

    Lysine is an a-amino acid with the chemical formula HO2CCH4NH2. This amino acid is an essential amino acid, which means that humans cannot synthesize it....
    , -CH=N+H-
  4. rhodopsin + h? metarhodopsin II; 11-cis photoisomerizes to all-trans
    rhodopsin + h? bathorhodopsin lumirhodopsin metarhodopsin I metarhodopsin II
  5. metarhodopsin II + H2O aporhodopsin + all-trans-retinal
  6. all-trans-retinal + NADPH + H+ all-trans-retinol + NADP+; all-trans-retinol dehydrogenases
  7. all-trans-retinol + fatty acid all-trans-retinyl ester + H2O; lecithin retinol acyltransferase
    LRAT

    Lecithin retinol acyltransferase , also known as LRAT, is a human gene....
    s (LRATs)


Steps 3,4,5,6 occur in rod cell outer segments
Rod cell

Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells....
; Steps 1, 2, and 7 occur in retinal pigment epithelium (RPE) cells.

Visual opsins


Type 2 rhodopsin is the prototypical member of a very large, and very important family of proteins called GPCRs. GPCR stands for G-protein coupled receptor. G-protein stands for guanine nucleotide-binding protein. The G-protein with which rhodopsin couples is a heterotrimeric G-protein. This makes the full two-carts-before-the-horse name heterotrimeric guanine nucleotide-binding protein coupled receptor. Just wonderful; and rhodopsin is not really a receptor
Receptor (biochemistry)

In biochemistry, a receptor is a protein molecule, embedded in either the plasma membrane or cytoplasm of a cell, to which a mobile signaling molecule may attach....
, although the other members of the family generally are. Both type 1 and type 2 rhodopsins are also retinylidene protein
Retinylidene protein

Retinylidene proteins are a family of proteins that use retinal as chromophore for light reception. Proteins of this family are also called opsins....
s; this just means that they are proteins which have a Schiff base
Schiff base

A Schiff base , named after Hugo Schiff, is a functional group that contains a carbon-nitrogen double bond with the nitrogen atom connected to an aryl or alkyl group?but not hydrogen ....
 linkage to retinal.

Type 1 rhodopsins

All-trans-retinal is also an essential component of type I, or microbial, opsins such as bacteriorhodopsin
Bacteriorhodopsin

Bacteriorhodopsin is a protein used by archaea, most notably halobacteria. It acts as a proton pump, i.e. it captures light energy and uses it to move protons across the membrane out of the cell....
, channelrhodopsin
Channelrhodopsin

Channelrhodopsins are a subfamily of opsin proteins that function as light-gated ion channels. They serve as sensory photoreceptors in unicellular green algae, controlling phototaxis, i.e....
, and halorhodopsin
Halorhodopsin

Halorhodopsin is a light-driven ion pump, specific for chloride ions, and found in phylogenetically ancient archaea, known as halobacteria. It is a seven-transmembrane protein of the retinylidene protein family, homologous to the light-driven proton pump bacteriorhodopsin, and similar in tertiary structure to vertebrate rhodopsins, the pigm...
. In these molecules, light causes the all-trans-retinal to become 13-cis retinal, which then cycles back to all-trans-retinal in the dark state.

History

The American biochemist George Wald
George Wald

George Wald was an United States scientist who is best known for his work with pigments in the retina. He won a share of the 1967 Nobel Prize in Physiology or Medicine with Haldan Keffer Hartline and Ragnar Granit....
 and others had outlined the visual cycle by 1958. For his work, Wald won a share of the 1967 Nobel Prize in Physiology or Medicine
Nobel Prize in Physiology or Medicine

The Nobel Prize in Physiology or Medicine is awarded once a year by the Swedish Karolinska Institutet. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outstanding contributions in Nobel Prize in Physics, Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and Physiology or Medic...
 with Haldan Keffer Hartline
Haldan Keffer Hartline

Haldan Keffer Hartline was an American physiology who was a cowinner of the 1967 Nobel Prize in Physiology or Medicine for his work in analyzing the neurophysiological mechanisms of vision....
 and Ragnar Granit
Ragnar Granit

Ragnar Arthur Granit was a Finnish scientist who won the Nobel Prize in Physiology or Medicine in 1967 along with Haldan Keffer Hartline and George Wald....
.

See also

  • Visual phototransduction
    Visual phototransduction

    Visual phototransduction is a process by which light is converted into electrical signals in the rod cells, cone cells and photosensitive ganglion cells of the retina of the eye....
  • Visual perception
    Visual perception

    Visual perception is the ability to interpret information from visible light reaching the eye. The resulting perception is also known as eyesight, sight or vision....
  • Sensory system
    Sensory system

    A sensory system is a part of the nervous system responsible for processing sense information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception....


Further reading



External links

  • - National Health Museum