<p> Serine and threonine dehydratases [<cite idref="PUB00002510"/>, <cite idref="PUB00004637"/>] are functionally and structurally relatedpyridoxal-phosphate dependent enzymes. L-serine dehydratase (<db_xref db="EC" dbkey="4.3.1.17"/>) and D-serine dehydratase (<db_xref db="EC" dbkey=" 4.3.1.18"/>) catalyze the dehydratation of L-serine (respectively D-serine) into ammonia and pyruvate. Threonine dehydratase (<db_xref db="EC" dbkey="4.3.1.19"/>) (TDH) catalyzes the dehydratation of threonine into alpha-ketobutarate and ammonia. In <taxon tax_id="562">Escherichia coli</taxon> and other microorganisms, two classes of TDH are known to exist. One is involvedin the biosynthesis of isoleucine, the other in hydroxamino acid catabolism. Threonine synthase (<db_xref db="EC" dbkey="4.2.3.1"/>) is also a pyridoxal-phosphate enzyme, it catalyzes the transformation of homoserine-phosphate into threonine.It has been shown [<cite idref="PUB00015327"/>] that threonine synthase is distantly related to the serine/threonine dehydratases. In all these enzymes, the pyridoxal-phosphate group is attached to a lysine residue.</p><p> A form of threonine dehydratase with two copies of the C-terminal domain <db_xref db="INTERPRO" dbkey="IPR001721"/> is described by <db_xref db="INTERPRO" dbkey="IPR005787"/>. This model describes a phylogenetically distinct which branches with the catabolic threonine dehydratase of E. coli; many members are designated as catabolic for this reason. However, the catabolic form lacks any <db_xref db="INTERPRO" dbkey="IPR001721"/> domain. Many members of this model are found in species with other isoleucine biosynthetic enzymes.</p> Threonine dehydratase II