OphosphoLhomoserine

O-phospho-L-homoserine, often abbreviated OPH, is a phosphorylated derivative of the amino acid homoserine. It is an intermediate in the aspartate family amino acid biosynthesis pathway found in many bacteria and plants, formed by the ATP-dependent phosphorylation of L-homoserine.

The formation is catalyzed by homoserine kinase (ThrB), which transfers a phosphate group from ATP to the hydroxyl side chain of homoserine. As a result, O-phospho-L-homoserine sits at a key branch point in the pathway leading toward threonine production. The primary downstream fate of OPH is conversion to L-threonine by threonine synthase (ThrC), a reaction that typically yields threonine and inorganic phosphate.

Biological context and significance follow from its role as a committed precursor in threonine biosynthesis. Regulation of the enzymes that interconvert homoserine, OPH, and threonine helps balance the flux through the aspartate pathway in response to cellular demands for amino acids. OPH is therefore a useful marker in studies of metabolic regulation and pathway flux in organisms that rely on the aspartate family for essential amino acids.

In summary, O-phospho-L-homoserine is a phosphorylation product of homoserine that serves as a pivotal intermediate directing the flow of carbon from aspartate toward threonine synthesis, with threonine synthase acting as its primary downstream enzyme. It exemplifies the phosphorylation-activated control points common in amino acid biosynthesis. See also homoserine, threonine, and the aspartate family biosynthetic pathway.