Aminotransferases, class-I, pyridoxal-phosphate-binding site <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>Aminotransferases share certain mechanistic features with other pyridoxal-phosphate dependent enzymes, such as the covalent binding of the pyridoxal-phosphate group to a lysine residue. On the basis of sequence similarity, these various enzymes can be grouped [<cite idref="PUB00002679"/>] into subfamilies; these sequences are defined by the aminotransferases class-I pyridoxal-phosphate attachment site signature, which contains the lysine residue involved in pyridoxal-phosphate binding.</p>