Metabolism
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Transamination and deamination of amino acids
Key points from this exercise:
Transaminases catalyse the transfer of the amino group of an amino acid onto a keto-acid, forming the corresponding amino acid. The donor amino acid becomes its corresponding keto-acid.
The prosthetic group of transaminases is pyridoxal phosphate, the metabolically active form of vitamin B6. The amino group form the donor amino acid is transferred onto pyridoxal phosphate, forming pyridoxamine phosphate, and releasing the keto-acid corresponding to the donor amino acid. The amino group is then transferred form pyridoxamine phosphate onto the acceptor keto-acid, forming the corresponding amino acid and leaving pyridoxal phosphate at the active site of the enzyme to undergo a further cycle of reaction.
Many transaminases are linked to alpha-ketoglutarate, forming glutamate. This glutamate can be reoxidised to alpha-ketoglutarate by glutamate dehydrogenase, releasing ammonium. This provides a pathway for the overall deamination of a wide variety of amino acids.
Many transaminases are linked to oxaloacetate, forming aspartate. The amino group can then be transferred onto alpha-ketoglutarate, in the reaction catalysed by aspartate transaminase, forming glutamate, which is a substrate for glutamate dehydrogenase, and reforming oxaloacetate. This provides a pathway for the overall deamination of a wide variety of amino acids.
The keto-acids of the non-essential amino acids are relatively common metabolic intermediates, and if they can be synthesised in adequate amounts they are substrates for transamination, permitting synthesis of the non-essential amino acids.
If the keto-acids of essential amino acids are supplied then the corresponding amino acids can be synthesised by transamination. This is useful in the treatment of renal failure, since it permits reduction in the amount of amino nitrogen to be metabolised and excreted.
Red blood cells contain relatively high activities of transaminases, a proportion of which is present as the catalytically inactive apo-enzyme. Vitamin B6 nutritional status can be assessed by measuring the increase in activity of red cell transaminases after pre-incubation with pyridoxal phosphate.