threonine, organic compound, one of the 22 α-amino acids commonly found in animal proteins. Only the l-stereoisomer appears in mammalian protein. It is one of several essential amino acids needed in the diet; human beings cannot synthesize it from simpler metabolites. Young adults need about 14 mg of this amino acid per day per kilogram (6 mg per lb) of body weight. Although threonine participates in many reactions in bacteria, including the biosynthesis of vitamin B₁₂ and isoleucine, its metabolic role in higher animals, including man, remains obscure. It is known only as a constituent of proteins, and even in that form it is relatively unreactive. In spite of the fact that its side chain has a hydroxyl group similar to that of serine, there is no indication that it participates in the catalytic functions of any enzyme. Threonine was isolated from the protein fibrin in 1935 and synthesized in the same year.

One of the essential amino acids. It occurs in the proteins of egg, milk, gelatin, and other biological substances and may be synthesized or obtained by hydrolyzing casein. It is used in nutritional and biochemical research and as a dietary supplement.

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Threonine (abbreviated as Thr or T) 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 and tyrosine, threonine is one of three proteinogenic amino acids bearing an alcohol group.

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.

Allo-threonine

With two chiral centers, threonine can exist in four possible stereoisomers, or two possible diastereomers of L-threonine. However, the name L-threonine is used for one single enantiomer, (2S,3R)-2-amino-3-hydroxybutanoic acid. The second diastereomer (2S,3S), which is rarely present in nature, is called L-allo-threonine.

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

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

Metabolism

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 alpha-ketobutyrate in a less common pathway via the enzyme serine dehydratase, and thereby enters the pathway leading to succinyl-CoA.

Sources

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.

References

See also

• Harold Dadford West American Biochemist who first synthetised the Threonine.

External links

• Threonine biosynthesis
• Computational Chemistry Wiki
• CID 205
• CID 6288