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-homoserine. Two other enzymes, homoserine kinase and homoserine o-succinyltransferase, then use homoserine as a substrate to produce homoserine phosphate and o-succinylhomoserine, respectively.
Applications
Commercially, homoserine can be used as a precursor for the synthesis of isobutanol and 1,4-butanediol. [4] Purified homoserine is used for enzyme structure studies. In addition, homoserine has played an important role in elucidating studies of peptide synthesis and proteoglycan glycopeptide synthesis. Bacterial cell lines can produce large amounts of this amino acid.
Biosynthesis
Homoserine is produced from aspartic acid via aspartic acid-4-semialdehyde, which is produced from β-phosphate aspartate. The hemi-aldehyde is converted to homoserine by the action of homoserine dehydrogenase.
L-homoserine is a substrate for homoserine kinase, which produces homoserine phosphate (homoserine-phosphate), which is converted to L-threonine by threonine synthase.
Homoserine is converted to o-succinylhomoserine by homoserine o-succinyltransferase (a precursor of L-methionine).
Homoserine metathesis inhibits aspartate kinase and glutamate dehydrogenase. Glutamate dehydrogenase reversibly converts glutamate to α-ketoglutarate and converts α-ketoglutarate to oxaloacetate via the citric acid cycle. Threonine acts as another metabotropic inhibitor of aspartate kinase and homoserine dehydrogenase, but it is a competitive inhibitor of homoserine kinase.
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