InterPro domain: IPR002129

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 ].

A number of pyridoxal-dependent decarboxylases share regions of sequence similarity, particularly in the vicinity of a conserved lysine residue, which provides the attachment site for the pyridoxal-phosphate (PLP) group [ 7 , 8 ]. Among these enzymes are aromatic-L-amino-acid decarboxylase (L-dopa decarboxylase or tryptophan decarboxylase), which catalyses the decarboxylation of tryptophan to tryptamine [ 9 ]; tyrosine decarboxylase, which converts tyrosine into tyramine; histidine decarboxylase, which catalyses the decarboxylation of histidine to histamine [ 10 ]; L-aspartate decarboxylase, which converts aspartate to beta-alanine [ 11 ]; and phenylacetaldehyde synthase that catalyses the decarboxylation of L-phenylalanine to 2-phenylethylamine [ 12 ]. These enzymes belong to the group II decarboxylases [ 13 , 13 ].

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