threonine

Threonine (abbreviated as Thr or T) is an amino acid. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar. Together with serine, threonine is one of two proteinogenic amino acids bearing an alcohol group. It is also one of two common amino acids that bear a chiral side chain, along with isoleucine.

The threonine residue is susceptible to numerous posttranslational modifications. The hydroxy 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.