Metabolism
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Transamination and deamination of amino acids
The
first step in the catabolism of most amino acids is removal of the amino group
to form the alpha-keto-acid (correctly an oxo-acid), which is the carbon skeleton
of the amino acid.
A small number of amino acids undergo oxidative or non-oxidative deamination. For example, glutamate is oxidised to alpha-ketoglutarate by glutamate dehydrogenase, glycine is oxidised to glyoxylate by glycine oxidase. There is also a general amino acid oxidase, but this has very low activity, and is not of great importance in amino acid metabolism. Serine undergoes non-oxidative deamination to pyruvate, catalysed by serine deaminase.
For other amino acids there is no direct deamination, but they can undergo transamination. This is a reaction between an amino acid and a keto-acid in which the amino group is transferred from the donor amino acid onto the acceptor keto-acid , leaving the carbon skeleton (keto-acid) of the donor amino acid and forming the amino acid corresponding to the acceptor keto-acid.
In the first half-reaction, the amino group is transferred from the substrate amino acid onto the prosthetic group, pyridoxal phosphate, releasing the keto-acid and forming pyridoxamine phosphate at the active site. In the second half reaction the amino group is transferred onto the acceptor keto-acid, forming the product amino acid, leaving pyridoxal phosphate at the active site, ready to undergo another reaction cycle.
Commonly, the acceptor keto-acid is either alpha-ketoglutarate (forming glutamate) or oxaloacetate, forming aspartate.