Threonine is one of the 20 natural amino acids. Nutritionally, in humans, threonine is an essential amino acid.
Threonine contains two chiral centers, so there are four possible stereoisomers of threonine, 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.
The threonine side chain can undergo O-linked glycosylation.
Threonine can become phosphorylated through the action of a threonine kinase. In its phosphorylated form, it can be referred to as phosphothreonine.
Since threonine cannot be synthesised by humans, it is described as an essential amino acid, referring to the essential requirement for it in the human diet. However, plants and most microorganisms are capable of synthesizing threonine from aspartic acid. The key step is catalyzed by (1) aspartokinase which phosphorylates the β-carboxyl group of aspartic acid. Reduction by (2) β-aspartate semialdehyde dehydrogenase produces β-aspartate-semialdehyde which is an important intermediate in the biosynthesis of threonine, methionine?, and lysine. The rest of the pathway is catalyzed by the enzymes: (3) homoserine dehydrogenase, (4) homoserine kinase, and (5) threonine synthase.
Foods high in threonine are cottage cheese, poultry, fish, meat, lentils, and sesame seeds.