Threonine
IUPAC name Threonine
Other names 2-Amino-3-hydroxybutanoic acid
Identifiers
CAS number	80-68-2 Y,  72-19-5 (L-isomer)
PubChem	6288
ChemSpider	6051 Y
EC number	201-300-6
DrugBank	DB00156
ChEBI	CHEBI:57926 Y
ChEMBL	CHEMBL291747 Y
Jmol-3D images	Image 1
SMILES C[C@H]([C@@H](C(=O)O)N)O
InChI InChI=1S/C4H9NO3/c1-2(6)3(5)4(7)8/h2-3,6H,5H2,1H3,(H,7,8)/t2-,3+/m1/s1 Y Key: AYFVYJQAPQTCCC-GBXIJSLDSA-N Y InChI=1S/C4H9NO3/c1-2(6)3(5)4(7)8/h2-3,6H,5H2,1H3,(H,7,8)/t2-,3+/m1/s1
Properties
Molecular formula	C4H9NO3
Molar mass	119.12 g mol−1
Solubility in water	(H2O, g/dl) 10.6(30°),14.1(52°),19.0(61°)
Acidity (pKa)	2.63 (carboxyl), 10.43 (amino)[1]
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Threonine (abbreviated as Thr or T)^[2] is an α-amino acid with the chemical formula HO₂CCH(NH₂)CH(OH)CH₃. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar. Together with serine, threonine is one of two proteinogenic amino acids bearing an alcohol group (tyrosine 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.

The threonine residue is susceptible to numerous posttranslational modifications. The hydroxy side-chain can undergo O-linked glycosylation. In addition, threonine residues undergo phosphorylation through the action of a threonine kinase. In its phosphorylated form, it can be referred to as phosphothreonine.

History[link]

Threonine was discovered as the last of the 20 common proteinogenic amino acids in the 1930s by William Cumming Rose.

Stereoisomerism[link]

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 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 diastereomer, (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[link]

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 via α-aspartyl-semialdehyde and homoserine. Homoserine undergoes O-phosphorylation; this phosphate ester undergoes hydrolysis concomitant with relocation of the OH group.^[3] Enzymes involved in a typical biosynthesis of threonine include:

1. aspartokinase
2. ß-aspartate semialdehyde dehydrogenase
3. homoserine dehydrogenase
4. homoserine kinase
5. threonine synthase.

Threonine biosynthesis

Metabolism[link]

Threonine is metabolized in two ways:

• It is converted to pyruvate via threonine dehydrogenase. An intermediate in this pathway can undergo thiolysis with CoA to produce acetyl-CoA and glycine.
• In humans, it is converted to α-ketobutyrate in a less common pathway via the enzyme serine dehydratase, and thereby enters the pathway leading to succinyl-CoA.

Sources[link]

Foods high in threonine include cottage cheese, poultry, fish, meat, lentils, and sesame seeds.

Racemic threonine can be prepared from crotonic acid by alpha-functionalization using mercury(II) acetate.^[4]

References[link]

External links[link]

• Food Sources of Threonine
• Threonine biosynthesis
• CID 205
• CID 6288