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 subdomain 1 of the major region of PLP-dependent transferases. This domain has a 3-layer alpha/beta/alpha sandwich topology. The major region 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 [7].
• Beta-eliminating lyases, such as tyrosine phenol lyase and tryptophanase [8].
• Pyridoxal-dependent decarboxylases, such as DOPA decarboxylase and glutamate decarboxylase beta (GadB) [9].
• Cystathionine synthase-like enzymes, such as cystalysin, methionine gamma-lyase (MGL), and cysteine desulphurase (IscS) [10].
• GABA-aminotransferase-like enzymes, such as ornithine aminotransferase and serine hydroxymethyltransferase [11].
• Ornithine decarboxylase major domain [12].

1. Pyridoxal enzymes: mechanistic diversity and uniformity. J. Biochem. 118, 463-73
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. Biochemistry 29, 317-26
6. Reaction specificity in pyridoxal phosphate enzymes. Arch. Biochem. Biophys. 433, 279-87
7. Crystal structure of Homo sapiens kynureninase. Acta Crystallogr. D Biol. Crystallogr. 46, 2735-44
8. Structure of Escherichia coli tryptophanase. Proteins 62, 814-23
9. Structural model of human GAD65: prediction and interpretation of biochemical and immunogenic features. Arch. Biochem. Biophys. 59, 7-14
10. Holo- and apo-cystalysin from Treponema denticola: two different conformations. Biophys. Chem. 455, 31-9
11. A three-dimensional structure of Plasmodium falciparum serine hydroxymethyltransferase in complex with glycine and 5-formyl-tetrahydrofolate. Homology modeling and molecular dynamics. null 115, 1-10
12. Three-dimensional structure of the Gly121Tyr dimeric form of ornithine decarboxylase from Lactobacillus 30a. Acta Crystallogr. D Biol. Crystallogr. 55, 1978-85

Pyridoxal phosphate is the active form of vitamin B6 (pyridoxine or pyridoxal). Pyridoxal 5'-
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