Threonine (Thr) or L-threonine is an alpha-amino acid. These are amino acids in which the amino group is attached to the carbon atom immediately adjacent to the carboxylate group (alpha carbon).  Amino acids are organic compounds that contain amino (‚ÄìNH2) and carboxyl (‚ÄìCOOH) functional groups, along with a side chain (R group) specific to each amino acid. L-threonine is one of 20 proteinogenic amino acids, i.e., the amino acids used in the biosynthesis of proteins. Threonine is found in all organisms ranging from bacteria to plants to animals. It is classified as a polar, uncharged (at physiological pH), aliphatic amino acid. Threonine is sometimes considered as a branched chain amino acid. Threonine was actually the last of the 20 amino acids to be discovered (in 1938). It was named threonine because it was similar in structure to threonic acid, a four-carbon monosaccharide. Threonine is an essential amino acid in humans, meaning the body cannot synthesize it and that it must be obtained from the diet. Foods high in threonine include cottage cheese, poultry, fish, meat, lentils, black turtle bean and sesame seeds. Adult humans require about 20 mg/kg body weight/day. In plants and microorganisms, threonine is synthesized from aspartic acid via alpha-aspartyl-semialdehyde and homoserine. In proteins, the threonine residue is susceptible to numerous posttranslational modifications. The hydroxyl side-chain can undergo O-linked glycosylation and phosphorylation through the action of a threonine kinase. Threonine is abundant in human plasma, particularly in newborns. Severe deficiency of threonine causes neurological dysfunction and lameness in experimental animals. Threonine is an immunostimulant which promotes the growth of thymus gland. It also can probably promote cell immune defense function. The threonine content of most of the infant formulas currently on the market is approximately 20% higher than the threonine concentration in human milk. Due to this high threonine content the plasma threonine concentrations are up to twice as high in premature infants fed these formulas than in infants fed human milk. The whey proteins which are used for infant formulas are sweet whey proteins. Sweet whey results from cheese production. Increasing the threonine plasma concentrations leads to accumulation of threonine and glycine in the brain. Such accumulation affects the neurotransmitter balance which may have consequences for the brain development during early postnatal life. Thus, excessive threonine intake during infant feeding should be avoided. (PMID 9853925). Threonine is metabolized in at least two ways. In many animals 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 the gene for threonine dehydrogenase is an inactive pseudogene, so threonine is converted to alpha-ketobutyrate.