Threonine
Threonine is one of the 20 most common natural amino acids on Earth. Nutritionally, in humans, threonine is also an essential amino acid. Threonine contains two chiral centers, so there are four possible stereoisomers of threonine. It means that there are two possible diastereomers of L-threonine. However, name L-threonine is used for one single enantiomer, (2S,3R)-2-amino-3-hydroxybutanoic acid. The second diastereomer (2S,3S), which is present nature rather rarely, 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. Foods high in threonine are cottage cheese, poultry, fish, meat, lentils, and sesame seeds.
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