<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 pyridoxal kinase (<db_xref db="EC" dbkey="2.7.1.35"/>), which is required for the synthesis of pyridoxal-5-phosphate from vitamin B6, and catalyses the conversion of pyridoxal to pyridoxal 5'-phosphate in the presence of ATP. <taxon tax_id="562">Escherichia coli</taxon> has an enzyme PdxK that acts <i>in vitro</i> as a pyridoxine/pyridoxal/pyridoxamine kinase,but mutants lacking PdxK activity retain a specific pyridoxamine kinase, PdxY [<cite idref="PUB00017544"/>]. PdxY acts in the salvage pathway of pyridoxal 5'-phosphate biosynthesis. Mammalian forms of pyridoxal kinase are more similar to PdxY than to PdxK.</p>