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Homoserine is an α-amino acid with the chemical formula HO2CCHCH2CH2OH. l-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 -CH2- 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 is an intermediate in the biosynthesis of three essential amino acids: methionine, threonine , and isoleucine. Its complete biosynthetic pathway includes glycolysis, the tricarboxylic acid or citric acid cycle or the Krebs cycle, and the aspartate metabolic pathway. It forms by two reductions of aspartic acid via the intermediacy of aspartate semialdehyde.I Specifically, the enzyme homoserine dehydrogenase, in association with NADPH, catalyzes a reversible reaction that interconverts L-aspartate-4-semialdehyde to L-homoserine. Then, two other enzymes, homoserine kinase and homoserine o-succinyl transferase use homoserine as a substrate and produce phosphohomoserine and o-succinyl homoserine respectively.