Question

9. The major site of amino acid metabolism is the liver. One of the initial reactions in amino acid metabolism is the transamination. a. What are the two major transaminases in the liver? Write balanced equations for the reactions catalyzed by these two enzymes. b. What is the main physiological function of glutamate dehydrogenase with respect to amino acid metabolism? Write a chemical equation for the reaction catalyzed by this enzyme.c. What is the role of glutamine synthetase in amino acid metabolism? Write the reaction catalyzed by this enzyme. Include all the substrates, products and cofactors.

Answer 1

9.a. The major transaminases in the liver are Aspartate Transaminase (AST) and Alanine Transaminase (ALT). Aspartate Transaminase is responsible for catalyzing the transfer of alpha-amino group from aspartate to an alpha-keto acid, such as alpha ketoglutarate. Similarly, Alanine Transaminase catalyzes the transfer of alpha-amino group from Alanine to an alpha-keto acid. Both of these reactions require the cofactor Pyridoxal 5'-phosphate (PLP).

The reactions catalyzed by both the enzymes has been shown below-

b. Glutamate Dehydrogenase (GDH) carries out oxidative deamination of glutamate. This reaction produces ammonia which enters urea cycle and produces urea. Finally, urea gets eliminated from our body through excretion. This enzyme also produces intermediates that is used to produce glucose (gluconeogenesis), thereby, acting as a link between amino acid metabolism and carbohydrate metabolism. This is especially important during fasting, when blood glucose level is low. The reaction catalysed by this enzyme is shown below-

C. Glutamine synthetase is used to produce glutamine with the help of energy derived from ATP hydrolysis. It helps to detoxify ammonia, by mediating its condensation with glutamate to produce glutamine. Reaction catalyzed by this enzyme is shown below-