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Threonine (symbol Thr or T[2]) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH+
form under biological conditions), a carboxyl group (which is in the deprotonated −COO form under biological conditions), and a side chain containing a hydroxyl group, making it a polar, uncharged amino acid. It is essential in humans, meaning the body cannot synthesize it: it must be obtained from the diet. Threonine is synthesized from aspartate in bacteria such as E. coli.[3] In the genetic code it is encoded by the codons ACT, ACC, ACA, and ACG.

Skeletal formula
Ball-and-stick model
IUPAC name
Other names
2-Amino-3-hydroxybutanoic acid
3D model (JSmol)
ECHA InfoCard 100.000.704
EC Number 201-300-6
Molar mass 119.12 g·mol−1
(H2O, g/dl) 10.6(30°),14.1(52°),19.0(61°)
Acidity (pKa) 2.63 (carboxyl), 10.43 (amino)[1]
Supplementary data page
Refractive index (n),
Dielectric constantr), etc.
Phase behaviour
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YesYN ?)
Infobox references

Threonine sidechains are often hydrogen bonded; the most common small motifs formed are based on interactions with serine: ST turns, ST motifs (often at the beginning of alpha helices) and ST staples (usually at the middle of alpha helices).



The threonine residue is susceptible to numerous posttranslational modifications. The hydroxyl 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.

It is a precursor of glycine, and can be used as a prodrug to reliably elevate brain glycine levels.


Threonine was the last of the 20 common proteinogenic amino acids to be discovered. It was discovered in 1936 by William Cumming Rose[4], collaborating with Curtis Meyer. The amino acid was named threonine because it was similar in structure to threose, a four-carbon monosaccharide with molecular formula C4H8O4[5]

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 proteinogenic amino acids with two chiral centers, the other being isoleucine. 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.[citation needed]


As an essential amino acid, threonine is not synthesized in humans, and needs to be present in proteins in the diet. Adult humans require about 20 mg/kg body weight/day.[6] 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.[7] Enzymes involved in a typical biosynthesis of threonine include:

  1. aspartokinase
  2. β-aspartate semialdehyde dehydrogenase
  3. homoserine dehydrogenase
  4. homoserine kinase
  5. threonine synthase.
Threonine biosynthesis


Threonine is metabolized in two ways:


Foods high in threonine include cottage cheese, poultry, fish, meat, lentils, Black turtle bean[10] and Sesame seeds.[11]

Racemic threonine can be prepared from crotonic acid by alpha-functionalization using mercury(II) acetate.[12]


  1. ^ Dawson, R.M.C., et al., Data for Biochemical Research, Oxford, Clarendon Press, 1959.
  2. ^ "Nomenclature and Symbolism for Amino Acids and Peptides". IUPAC-IUB Joint Commission on Biochemical Nomenclature. 1983. Archived from the original on 9 October 2008. Retrieved 5 March 2018. 
  3. ^ Raïs, Badr; Chassagnole, Christophe; Lettelier, Thierry; Fell, David; Mazat, Jean-Pierre (2001). "Threonine synthesis from aspartate in Escherichia coli cell-free extracts: pathway dynamics" (PDF). J Biochem. 356: 425–32. PMC 1221853 . PMID 11368769. 
  4. ^ A Dictionary of scientists. Daintith, John., Gjertsen, Derek. Oxford: Oxford University Press. 1999. p. 459. ISBN 9780192800862. OCLC 44963215. 
  5. ^ Meyer, Curtis (20 July 1936). "The Spatial Configuation of Alpha-Amino-Beta-Hydroxy-n-Butyric Acid" (PDF). Journal of Biological Chemistry. 115 (3). 
  6. ^ Institute of Medicine (2002). "Protein and Amino Acids". Dietary Reference Intakes for Energy, Carbohydrates, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: The National Academies Press. pp. 589–768. 
  7. ^ Lehninger, Albert L.; Nelson, David L.; Cox, Michael M. (2000). Principles of Biochemistry (3rd ed.). New York: W. H. Freeman. ISBN 1-57259-153-6. .
  8. ^ Stipanuk, Martha H.; Caudill, Marie A. (2013-08-13). Biochemical, Physiological, and Molecular Aspects of Human Nutrition - E-Book. Elsevier Health Sciences. ISBN 9780323266956. 
  9. ^ Bhardwaj, Uma; Bhardwaj, Ravindra. Biochemistry for Nurses. Pearson Education India. ISBN 9788131795286. 
  10. ^
  11. ^
  12. ^ Carter, Herbert E.; West, Harold D. (1940). "dl-Threonine". Organic Syntheses. 20: 101. ; Collective Volume, 3, p. 813 .

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