Metabolism
on-line - the virtual tutorial room
copyright © 2008 - 2014 David A Bender
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.
How can transamination linked to alpha-ketoglutarate (forming glutamate) account for the overall deamination of most amino acids?
A simple two reaction pathway involving transamination to form glutamate and glutamate dehydrogenase to release the ammonium and reform alpha-ketoglutarate will allow overall deamination of most amino acids for which there is an alpha-ketoglutarate-linked transaminase.
Aspartate transaminase catalyses a reaction between aspartate and alpha-ketoglutarate to form oxaloacetate and glutamate.
