Homoserine (also known as isothreonine ) is an alpha-amino acid with the chemical formula HO 2 CCH(NH 2 )CH 2 CH 2 OH. l -Homoserine is not one of the common amino acids encoded by DNA. It differs from the protein amino acid serine by the insertion of an additional -CH 2 - unit in the main chain. Homoserine or its lactone form is the product of cyanogen bromide cleavage of the peptide by methionine degradation. 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 or tricarboxylic acid cycle, and the aspartate metabolic pathway. It is formed through the intermediate action of aspartic hemi-aldol via two reductions of aspartic acid. Specifically, the enzyme homoserine dehydrogenase, bound to NADPH, catalyzes the reversible reaction that interconverts L-aspartate-4-half aldehyde to L-homo