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.
How can transamination linked to oxaloacetate (forming aspartate) account for the overall deamination of most amino acids?
Now we need three reactions:
transamination linked to oxaloacetate, forming aspartate,
aspartate transaminase, forming glutamate from alpha-ketoglutarate and reforming oxaloacetate,
glutamate dehydrogenase to release the ammonium and reform alpha-ketoglutarate.
If the keto-acid corresponding to an amino acid can be synthesised from a common metabolic intermediate, and not only from the amino acid itself, can you explain how the non-essential amino acids are synthesised?
As well as being a mechanism for catabolism of amino acids, transamination also provides a mechanism for synthesis of those amino acids whose carbon skeletons are intermediates in carbohydrate metabolism - the non-essential amino acids.
The treatment of patients in renal failure involves feeding a low protein diet, in order to minimise the burden of nitrogen to be excreted (mainly as urea), while providing just enough protein to meet requirements.
Why do you think that providing the keto-acids of essential amino acids is beneficial in such cases?
If the keto-acids of essential amino acids are provided in the diet then they are substrates for transamination to yield the amino acids, so using nitrogen that would otherwise be metabolised to ammonium and then urea. This permits a greater reduction in total protein intake and further reduces the burden of nitrogen to be excreted.
Experiments
with transaminases
In these experiments you will investigate the activities of two transaminases:
aspartate transaminase, which catalyses the transfer of the amino group from aspartate, forming oxaloacetate, onto alpha-ketoglutarate, forming glutamic acid.
alanine transaminase, which catalyses the transfer of the amino group from alanine, forming pyruvate, onto alpha-ketoglutarate, forming glutamic acid
Experiment: transamination and deamination of glutamate by heart muscle
Ox heart muscle was homogenised in ice cold buffer and centrifuged at 20,000 g for 20 minutes to remove intact cells, cell debris, nuclei and mitochondria. The resultant supernatant was dialysed against three changes of 0.05 mol /L phosphate buffer at pH 7.4.