Threonine
Threonine (symbol Thr or T) 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". It is encoded by all the codons starting  AC (ACU, ACC, ACA, and ACG). 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). Modifications. 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. Phosphothreonine has three potential coordination sites (carboxyl, amine and phosphate group) and determination  of the mode of coordination between phosphorylated ligands and metal ions occurring in an organism is important to explain the function of the phosphothreonine in biological processes. History. Threonine was the last of the 20 common proteinogenic amino acids to be discovered. It was discovered in 1936 by William Cumming Rose, collaborating with Curtis Meyer. The amino acid was named threonine because it was similar in structure to threonic acid, a four-carbon monosaccharide with molecular formula CHO Stereoisomers. Threonine is one of two proteinogenic amino acids with two stereogenic centers, the other being isoleucine. 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 -threonine is used for one single stereoisomer, (2"S",3"R")-2-amino-3-hydroxybutanoic acid. The second stereoisomer (2"S",3"S"), which is rarely present in nature, is called -allothreonine. The two stereoisomers (2"R",3"S")- and (2"R",3"R")-2-amino-3-hydroxybutanoic acid are only of minor importance. Biosynthesis. 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. 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: Metabolism. Threonine is metabolized in at least three ways: Sources. Foods high in threonine include cottage cheese, poultry, fish, meat, lentils, black turtle bean and sesame seeds. Racemic threonine can be prepared from crotonic acid by alpha-functionalization using mercury(II) acetate.