Homoserine (also called isothreonine) is an α-amino acid with the chemical formula HO2CCH(NH2)CH2CH2OH. -Homoserine is not one of the common amino acids encoded by DNA. It differs from the proteinogenic amino acid serine by insertion of an additional methylene group. Homoserine, or its lactone form, is the product of a cyanogen bromide cleavage of a peptide by degradation of methionine.
Homoserine is an intermediate in the biosynthesis of three essential amino acids: methionine, threonine (an isomer of homoserine), and isoleucine. It forms by two reductions of aspartic acid via the intermediacy of aspartate semialdehyde.