<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>This entry represents subdomain 2 of the major region of PLP-dependent transferases. This domain has a complex alpha/beta structure. The major region can be found in the following PLP-dependent transferase families:</p><ul><li>Aspartate aminotransferase (AAT)-like enzymes, such as aromatic aminoacid aminotransferase AroAT, glutamine aminotransferase and kynureninase [<cite idref="PUB00035511"/>].</li><li>Beta-eliminating lyases, such as tyrosine phenol lyase and tryptophanase [<cite idref="PUB00035512"/>].</li><li>Pyridoxal-dependent decarboxylases, such as DOPA decarboxylase and glutamate decarboxylase beta (GadB) [<cite idref="PUB00035513"/>].</li><li>Cystathionine synthase-like enzymes, such as cystalysin, methionine gamma-lyase (MGL), and cysteine desulphurase (IscS) [<cite idref="PUB00035514"/>].</li><li>GABA-aminotransferase-like enzymes, such as ornithine aminotransferase and serine hydroxymethyltransferase [<cite idref="PUB00035515"/>].</li><li>Ornithine decarboxylase major domain [<cite idref="PUB00014087"/>].</li></ul> Pyridoxal phosphate-dependent transferase, major region, subdomain 2