<p> Pyridoxal phosphate is the active form of vitamin B6 (pyridoxine or pyridoxal). Pyridoxal 5'-phosphate (PLP) is a versatile catalyst, acting as a coenzyme in a multitude of reactions, including decarboxylation, deamination and transamination [<cite idref="PUB00035505"/>, <cite idref="PUB00006322"/>, <cite idref="PUB00035506"/>]. PLP-dependent enzymes are primarily involved in the biosynthesis of amino acids and amino acid-derived metabolites, but they are also found in the biosynthetic pathways of amino sugars and in the synthesis or catabolism of neurotransmitters; pyridoxal phosphate can also inhibit DNA polymerases and several steroid receptors [<cite idref="PUB00035507"/>]. Inadequate levels of pyridoxal phosphate in the brain can cause neurological dysfunction, particularly epilepsy [<cite idref="PUB00035508"/>].</p><p>PLP enzymes exist in their resting state as a Schiff base, the aldehyde group of PLP forming a linkage with the epsilon-amino group of an active site lysine residue on the enzyme. The alpha-amino group of the substrate displaces the lysine epsilon-amino group, in the process forming a new aldimine with the substrate. This aldimine is the common central intermediate for all PLP-catalysed reactions, enzymatic and non-enzymatic [<cite idref="PUB00035504"/>].</p><p>Serine and threonine dehydratases [<cite idref="PUB00002510"/>, <cite idref="PUB00004637"/>] are functionally and structurally related pyridoxal-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"/>) catalyse the dehydratation of L-serine (respectively D-serine) into ammonia and pyruvate. Threonine dehydratase (<db_xref db="EC" dbkey="4.3.1.19"/>) (TDH) catalyses 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 involved in 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 catalyses 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> Serine/threonine dehydratase, pyridoxal-phosphate-binding site