<p>Threonine synthase (<db_xref db="EC" dbkey=" 4.2.3.1"/>) is involved in threonine biosynthesis. It catalyses the conversion of O-phospho-L-homoserine and water into L-threonine and orthophosphate, using pyridoxal phosphate as a cofactor. The pyridoxal-phosphate binding site is a Lys (K) residue. The enzyme is distantly related to the serine/threonine dehydratases which are also pyridoxal-phosphate dependent enzymes [<cite idref="PUB00015327"/>].</p>