Pyridoxal-phosphate
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Pyridoxal-phosphate (PLP, pyridoxal-5'-phosphate) is a cofactor of many enzymatic reactions. It is the active form of vitamin B6 which comprises three natural organic compounds, pyridoxal, pyridoxamine and pyridoxine.
PLP acts as a coenzyme in all transamination reactions, and in some decarboxylation and deamination reactions of amino acids. The aldehyde group of PLP forms a Schiff-base linkage with the ε-amino group of a specific lysine group of the aminotransferase enzyme. The α-amino group of the amino acid substrate displaces the ε-amino group of the active-site lysine residue. The resulting aldimine becomes deprotonated to become a quinoid intermediate, which in turn accepts a proton at a different position to become a ketimine. The resulting ketimine is hydrolysed so that the amino group remains on the complex. It is also active in the condensation reaction in heme synthesis. It is produced from pyridoxal by the enzyme pyridoxal kinase, and it requires one ATP. It is metabolized in the liver. It is also found on glycogen phosphorylase in the liver, where it is used to break down glycogen when glucagon or epinephrine signals it to do so.
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