Transamination is the exchange of the amino group from an amino acid to a keto acid, and provides the most common process for the introduction of nitrogen into amino acids, and for the removal of nitrogen from them. The couple glutamic acid/2-oxoglutaric acid are the usual donor/acceptor molecules for the amino group. Reductive amination of the Krebs cycle intermediate 2-oxoglutaric acid to glutamic acid (Figure 2.14) is responsible for the initial incorporation of nitrogen, a reaction which involves imine formation and subsequent reduction. Transamination then allows the amino group to be transferred from glutamic acid to a suitable keto acid, or in the reverse mode from an amino acid to 2-oxoglutaric acid. This reaction is dependent on the coenzyme pyridoxal phosphate (PLP) and features a Schiff base/imine intermediate (aldimine) with the aldehyde group of PLP (Figure 2.14). The a-hydrogen of the original amino acid is now made considerably more acidic and is removed, leading to the ketimine by a reprotonation process which also restores the aromaticity in the pyridine ring. The keto acid is then liberated by hydrolysis of the Schiff base function, which generates pyridoxam-ine phosphate. The remainder of the sequence is now a reversal of this process, and transfers the amine function from pyridoxamine phosphate to another keto acid.
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