Homoserine (also called isothreonine) is an α-amino acid with the chemical formula HOCCH(NH)CHCHOH. -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 -CH- unit into the backbone.  Homoserine, or its lactone form, is the product of a cyanogen bromide cleavage of a peptide by degradation of methionine.
-homoserine from -aspartic acid This image shows the wrong enantiomers! -->
Homoserine is an intermediate in the biosynthesis of three essential amino acids: methionine, threonine (an isomer of homoserine), and isoleucine. Its complete biosynthetic pathway includes glycolysis, the tricarboxylic acid (TCA) or citric acid cycle (Krebs cycle), and the aspartate metabolic pathway. It forms by two reductions of aspartic acid via the intermediacy of aspartate semialdehyde. Specifically, the enzyme homoserine dehydrogenase, in association with NADPH, catalyzes a reversible reaction that interconverts -aspartate-4-semialdehyde to -homoserine. Then, two other enzymes, homoserine kinase and homoserine O-succinyltransferase use homoserine as a substrate and produce phosphohomoserine and O-succinyl homoserine respectively.