Threonine is one of the protein amino acids whose chemical formula is HO2CCH(NH2)CH(OH)CH3. Its three-letter symbol is Thr, and its one-letter symbol is T. As in the case of serine, it has a hydroxyl (−OH) group in its side chain, and it is classified as a polar amino acid. Its molecular weight is 119.12, and its isoelectric point is 6.16. Threonine has two asymmetric carbons (α-carbon and β-carbon) as does isoleucine. Thus, threonine has four stereoisomers: l-threonine, d-threonine, l-allo-threonine, and d-allo-threonine. Only l-threonine is used in biosynthesis of proteins, and it is widely distributed in the biosphere.