Structural Biochemistry, Proteins, Amino Acid Degradation

Introduction

When there are unneeded amino acids from either protein digestion or turnover, they are broken down into certain compounds. This process usually occurs in the liver.

In amino acid degradation the amino group is removed and turned into an α-ketoacids which is then modified so that the carbon chain could enter the metabolism and eventually become glucose or intermediates of the citric acid cycle.

Amino Acid Degradation

The amino group is transferred to α-ketoglutarate which forms glutamate. Then the glutamate is oxidatively deanimated to form the ammonium ion NH4+

Aminotransferases catalyzes the reaction that turns the α-amino group from an α-amino acid to an α-ketoacid. These enzymes catalyze α-amino groups from a variety of amino acids to α-keto-glutarate for conversion to NH4+

Aspartate aminotransferase, catalyzes the transfer of the amino group of aspartate to α-ketoglutarate.

Alanine aminotransferase catalyzes the transfer of the amino group of alanine to α-ketoglutarate.

The nitrogen from the α-ketoglutarate in the transamination reaction is converted into an ammonium ion by oxidative deamination. This reaction is catalyzed by glutamate dehydrogenase.
This enzyme is special in that it is able to utilize either NAD+ or NADP+. The reaction dehydrogenates the C-N bond, and then hydrolyses of the Schiff base to make a ketoglutarate

The equilibrium for this reaction favors glutamate. But the reaction can be pushed forward by the consumption of ammonia. Glutamate dehydrogenase is found in the mitochondria. This compartmentalization prevents interaction with ammonia. In vertebrates, the activity of glutamate dehydrogenase is allosterically regulated.

NH4+ is converted into urea, which is then excreted as waste.

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