InterPro domain: IPR004625

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 [ 1 , 2 , 3 ]. 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 [ 4 ]. Inadequate levels of pyridoxal phosphate in the brain can cause neurological dysfunction, particularly epilepsy [ 5 ].

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 [ 6 ].

This entry represents pyridoxal kinase ( 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. Escherichia coli has an enzyme PdxK that acts in vitro as a pyridoxine/pyridoxal/pyridoxamine kinase,but mutants lacking PdxK activity retain a specific pyridoxamine kinase, PdxY [ 7 ]. PdxY acts in the salvage pathway of pyridoxal 5'-phosphate biosynthesis. Mammalian forms of pyridoxal kinase are more similar to PdxY than to PdxK.

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2. Pyridoxal phosphate-dependent enzymes. Biochim. Biophys. Acta 1248, 81-96
3. Pyridoxal phosphate enzymes: mechanistic, structural, and evolutionary considerations. Annu. Rev. Biochem. 73, 383-415
4. Exploring the pyridoxal 5'-phosphate-dependent enzymes. J. Inherit. Metab. Dis. 6, 275-87
5. B6-responsive disorders: a model of vitamin dependency. null 29, 317-26
6. Reaction specificity in pyridoxal phosphate enzymes. Arch. Biochem. Biophys. 433, 279-87
7. Identification and function of the pdxY gene, which encodes a novel pyridoxal kinase involved in the salvage pathway of pyridoxal 5'-phosphate biosynthesis in Escherichia coli K-12. J. Bacteriol. 180, 1814-21