Threonine

This essential amino acid is classified as polar and cannot be manufactured by the body and is found in high concentrations in the heart, skeletal muscles and central nervous system.
 * 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.[[image:http://upload.wikimedia.org/wikipedia/commons/a/a0/L-Threonin_-_L-Threonine.svg height="149" align="right"]]



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



** Stereoisomerism **
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: (2//S//,3//R//), (2//R//,3//S//), (2//S//,3//S//) and (2//R//,3//R//). However, the name L -threonine is used for one single diastereomer, (2//S//,3//R//)-2-amino-3-hydroxybutanoic acid. The second stereoisomer (2//S//,3//S//), which is rarely present in nature, is called L -//allo//-threonine. The two stereoisomers (2//R//,3//S//)- and (2//R//,3//R//)-2-amino-3-hydroxybutanoic acid are only of minor importance.
 * [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/a/a0/L-Threonin_-_L-Threonine.svg/120px-L-Threonin_-_L-Threonine.svg.png width="120" height="74" caption="L-Threonin - L-Threonine.svg" link="http://en.wikipedia.org/wiki/File:L-Threonin_-_L-Threonine.svg"]] [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/5/5d/D-Threonine.svg/120px-D-Threonine.svg.png width="120" height="79" caption="D-Threonine.svg" link="http://en.wikipedia.org/wiki/File:D-Threonine.svg"]] ||
 * L -Threonine (2//S//,3//R//) and D -Threonine (2//R//,3//S//) ||
 * [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/b/b9/L-allo-Threonine.svg/120px-L-allo-Threonine.svg.png width="120" height="79" caption="L-allo-Threonine.svg" link="http://en.wikipedia.org/wiki/File:L-allo-Threonine.svg"]] [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/1/1c/D-allo-Threonine.svg/120px-D-allo-Threonine.svg.png width="120" height="79" caption="D-allo-Threonine.svg" link="http://en.wikipedia.org/wiki/File:D-allo-Threonine.svg"]] ||
 * L -//allo//-Threonine (2//S//,3//S//) and D -//allo//-Threonine (2//R//,3//R//) ||

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: Threonine biosynthesis
 * 1) aspartokinase
 * 2) ß-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 produceacetyl-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.