<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>Pyridoxal-5'-phosphate-dependent enzymes (B6 enzymes) catalyze manifold reactions in the metabolism of amino acids. Most of these enzymes can be assigned to one of three different families of homologous proteins, the alpha, beta and gamma families. The alpha and gamma family might be distantly related with one another, but are clearly not homologous with the beta family. The beta family includes L- and D-serine dehydratase, threonine dehydratase, the beta subunit of tryptophan synthase, threonine synthase and cysteine synthase. These enzymes catalyze beta-replacement or beta-elimination reactions [<cite idref="PUB00006301"/>].</p><p>Comparison of sequences from eukaryotic, archebacterial, and eubacterial species indicates that the functional specialization of most B6 enzymes has occurred already in the universal ancestor cell. The cofactor pyridoxal-5-phosphate must have emerged very early in biological evolution; conceivably, organic cofactors and metal ions were the first biological catalysts [<cite idref="PUB00006531"/>]. </p><p>The 3D structure of the beta-subunit of tryptophan synthase has been solved. The subunit has two domains that are approximately the same size and similar toeach other in folding pattern. Each has a core containing a four-strandedparallel beta-sheet with three helices on its inner side and one on the outerside. The cofactor is bound at the interface between the domains [<cite idref="PUB00006322"/>].</p> Pyridoxal phosphate-dependent enzyme, beta subunit