Pyridoxal phosphate

Pyridoxal-phosphate (PLP, pyridoxal-5'-phosphate) is a prosthetic group of some enzymes. It is the active form of vitamin B 6  which comprises three natural organic compounds, pyridoxal, pyridoxamine and pyridoxine.

Role as a coenzyme
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 (internal aldimine) 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 external 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.

Additionally, PLP is used by aminotransferases (or transaminases) that act up unusual sugars such as perosamine and desosamine. In these reactions, the PLP reacts with glutamate, which transfers its alpha-amino group to PLP to make pyridoxamine phosphate (PMP). PMP then transfers its nitrogen to the sugar, making an amino-sugar.

PLP is also involved in various beta-elimination reactions such as the reactions carried out by serine dehydratase and GDP-4-keto-6-deoxymannose-3-dehydratase (ColD).

It is also active in the condensation reaction in heme synthesis.

Pyridoxal phosphate is not required in the transaminase reaction of lysine catabolism.

Non-classical examples of PLP
PLP is also found on glycogen phosphorylase in the liver, where it is used to break down glycogen in gluconeogenesis when glucagon or epinephrine signals it to do so. However, this enzyme does not exploit the reactive aldehyde group, but instead utilizes the phosphate group on PLP to perform its reaction.

Although the vast majority of PLP-dependent enzymes form an internal alidmine with PLP via an active site lysine residue, some PLP-dependent enzymes do not have this lysine residue, but instead have an active site histidine. In such a case, the histidine cannot form the internal aldimine and therefore the cofactor never becomes covalently tethered to the enzyme. GDP-4-keto-6-deoxymannose-3-dehydratase (ColD) is an example of such an enzyme.

Production
It is produced from pyridoxal by the enzyme pyridoxal kinase, and it requires one ATP. It is metabolized in the liver.