InterPro domain: IPR015422

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 ].

The monomer of PLP-dependent transferases consists of two domains, a large domain and a small domain. This entry represents small domain, which has a complex alpha/beta structure [ 7 ]. It can be found in the following PLP-dependent transferase families:

• Aspartate aminotransferase (AAT)-like enzymes, such as aromatic aminoacid aminotransferase AroAT, glutamine aminotransferase and kynureninase [ 8 ].
• Beta-eliminating lyases, such as tyrosine phenol lyase and tryptophanase [ 9 ].
• Pyridoxal-dependent decarboxylases, such as DOPA decarboxylase and glutamate decarboxylase beta (GadB) [ 10 ].
• Cystathionine synthase-like enzymes, such as cystalysin, methionine gamma-lyase (MGL), and cysteine desulphurase (IscS) [ 11 ].
• GABA-aminotransferase-like enzymes, such as ornithine aminotransferase and serine hydroxymethyltransferase [ 12 ].
• Ornithine decarboxylase [ 13 ].

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