Threonine

Threonine

Overview
Threonine is an α-amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 with the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 HO2CCH(NH2)CH(OH)CH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid
Essential amino acid
An essential amino acid or indispensable amino acid is an amino acid that cannot be synthesized de novo by the organism , and therefore must be supplied in the diet.-Essentiality vs. conditional essentiality in humans:...

 is classified as polar
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...

. Together with serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...

, threonine is one of two proteinogenic amino acids bearing an alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

 group (tyrosine
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

 is not an alcohol but a phenol, since its hydroxyl group is bonded directly to an aromatic ring, giving it different acid/base and oxidative properties).
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Encyclopedia
Threonine is an α-amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 with the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 HO2CCH(NH2)CH(OH)CH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid
Essential amino acid
An essential amino acid or indispensable amino acid is an amino acid that cannot be synthesized de novo by the organism , and therefore must be supplied in the diet.-Essentiality vs. conditional essentiality in humans:...

 is classified as polar
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...

. Together with serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...

, threonine is one of two proteinogenic amino acids bearing an alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

 group (tyrosine
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

 is not an alcohol but a phenol, since its hydroxyl group is bonded directly to an aromatic ring, giving it different acid/base and oxidative properties). It is also one of two common amino acids that bear a chiral side chain, along with isoleucine
Isoleucine
Isoleucine is an α-amino acid with the chemical formula HO2CCHCHCH2CH3. It is an essential amino acid, which means that humans cannot synthesize it, so it must be ingested. Its codons are AUU, AUC and AUA....

.

The threonine residue is susceptible to numerous posttranslational modification
Posttranslational modification
Posttranslational modification is the chemical modification of a protein after its translation. It is one of the later steps in protein biosynthesis, and thus gene expression, for many proteins....

s. The hydroxy
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 side-chain can undergo O-linked glycosylation
Glycosylation
Glycosylation is the reaction in which a carbohydrate, i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule . In biology glycosylation refers to the enzymatic process that attaches glycans to proteins, lipids, or other organic molecules...

. In addition, threonine residues undergo phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....

 through the action of a threonine kinase
Kinase
In chemistry and biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation. Kinases are part of the larger family of phosphotransferases...

. In its phosphorylated form, it can be referred to as phosphothreonine.

History


Threonine was discovered as the last of the 20 common proteinogenic amino acids in the 1930s by William Cumming Rose
William Cumming Rose
William Cumming Rose was an American nutritionist whose research in the 1930s discovered the essential amino acid threonine....

.

Stereoisomerism

>
 
L-Threonine (2S,3R) and D-Threonine (2R,3S)
 
L-allo-Threonine (2S,3S) and D-allo-Threonine (2R,3R)


Threonine is one of two amino acids out of the twenty with two chiral
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

 centers. Threonine can exist in four possible stereoisomers with the following configurations: (2S,3R), (2R,3S), (2S,3S) and (2R,3R). However, the name L-threonine is used for one single enantiomer
Enantiomer
In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are non-superposable , much as one's left and right hands are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without...

, (2S,3R)-2-amino-3-hydroxybutanoic acid. The second stereoisomer (2S,3S), which is rarely present in nature, is called L-allo-threonine. The two stereoisomers (2R,3S)- and (2R,3R)-2-amino-3-hydroxybutanoic acid are only of minor importance.

Biosynthesis


As an essential amino acid, threonine is not synthesized in humans, hence we must ingest threonine in the form of threonine-containing proteins. In plants and microorganisms, threonine is synthesized from aspartic acid
Aspartic acid
Aspartic acid is an α-amino acid with the chemical formula HOOCCHCH2COOH. The carboxylate anion, salt, or ester of aspartic acid is known as aspartate. The L-isomer of aspartate is one of the 20 proteinogenic amino acids, i.e., the building blocks of proteins...

 via α-aspartyl-semialdehyde and homoserine
Homoserine
Homoserine is an α-amino acid with the chemical formula HO2CCHCH2CH2OH. L-Homoserine is not one of the common amino acids encoded by DNA. It differs from the proteinogenic amino acid serine by insertion of an additional methylene group...

. Homoserine undergoes O-phosphorylation; this phosphate ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

 undergoes hydrolysis concomitant with relocation of the OH group. Enzymes involved in a typical biosynthesis of threonine include:
  1. aspartokinase
    Aspartokinase
    Aspartokinase is an enzyme that catalyzes the phosphorylation of the amino acid aspartate. This reaction is the first step in the biosynthesis of three essential amino acids: methionine, lysine, and threonine, known as the "aspartate family"...

  2. ß-aspartate semialdehyde dehydrogenase
    Dehydrogenase
    A dehydrogenase is an enzyme that oxidises a substrate by a reduction reaction that transfers one or more hydrides to an electron acceptor, usually NAD+/NADP+ or a flavin coenzyme such as FAD or FMN.-Examples:...

  3. homoserine dehydrogenase
  4. homoserine kinase
    Kinase
    In chemistry and biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation. Kinases are part of the larger family of phosphotransferases...

  5. threonine synthase
    Synthase
    In biochemistry, a synthase is an enzyme that catalyses a synthesis process.Following the EC number classification, they belong to the group of ligases, with lyases catalysing the reverse reaction....

    .



Metabolism


Threonine is metabolized in two ways:
  • It is converted to pyruvate via threonine dehydrogenase. An intermediate in this pathway can undergo thiolysis
    Thiolysis
    Thiolysis is a reaction with a thiol that cleaves one compound into two. This reaction is similar to hydrolysis, which involves water instead of a thiol. This reaction is seen in β-oxidation of fatty acids. The depolymerisation of condensed tannins with the use of benzyl mercaptan as...

     with CoA to produce acetyl-CoA
    Acetyl-CoA
    Acetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...

     and glycine
    Glycine
    Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...

    .
  • In humans, it is converted to alpha-ketobutyrate in a less common pathway via the enzyme serine dehydratase
    Serine dehydratase
    Serine Dehydratase or L-Serine Ammonia Lyase is in the ß-family of Pyridoxal Phosphate-dependent enzymes. SDH is found widely in nature, but its structural and chemical properties vary greatly among species. SDH is found in yeast, bacteria, and the cytoplasm of mammalian hepatocytes...

    , and thereby enters the pathway leading to succinyl-CoA
    Succinyl-CoA
    Succinyl-Coenzyme A, abbreviated as Succinyl-CoA or SucCoA, is a combination of succinic acid and coenzyme A.-Source:It is an important intermediate in the citric acid cycle, where it is synthesized from α-Ketoglutarate by α-ketoglutarate dehydrogenase through decarboxylation...

    .

Sources


Foods high in threonine include cottage cheese
Cottage cheese
Cottage cheese is a cheese curd product with a mild flavor. It is drained, but not pressed, so some whey remains and the individual curds remain loose. The curd is usually washed to remove acidity, giving sweet curd cheese. It is not aged or colored. Different styles of cottage cheese are made from...

, poultry
Poultry
Poultry are domesticated birds kept by humans for the purpose of producing eggs, meat, and/or feathers. These most typically are members of the superorder Galloanserae , especially the order Galliformes and the family Anatidae , commonly known as "waterfowl"...

, fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, meat
Meat
Meat is animal flesh that is used as food. Most often, this means the skeletal muscle and associated fat and other tissues, but it may also describe other edible tissues such as organs and offal...

, lentil
Lentil
The lentil is an edible pulse. It is a bushy annual plant of the legume family, grown for its lens-shaped seeds...

s, and sesame seeds.

Racemic
Racemic
In chemistry, a racemic mixture, or racemate , is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. The first known racemic mixture was "racemic acid", which Louis Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid.- Nomenclature :A...

 threonine can be prepared from crotonic acid
Crotonic acid
Crotonic acid, or trans-2-butenoic acid, is a short-chain unsaturated carboxylic acid, described by the formula CH3CH=CHCO2H. Crotonic acid is so named because it was erroneously thought to be a saponification product of croton oil...

 by alpha-functionalization using mercury(II) acetate
Mercury(II) acetate
Mercury acetate is the chemical compound with the formula Hg2. Commonly abbreviated Hg2, this compound is employed as a reagent to generate organomercury compounds from unsaturated organic precursors.-Structure:...

.

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