S/A AA Oxidation/ Degradation Flashcards
Describe the roles of glutamine synthetase and glutaminase in the metabolism of amino groups in mammals.
s: In tissues that are metabolizing the carbon skeletons of amino acids, the amino groups are transferred by transamination to glutamate, then released as ammonia.
Ammonia, which is toxic, is then combined with glutamate to form glutamine; the reaction is catalyzed by glutamine synthetase and requires ATP.
Glutamine is moved from the extrahepatic tissues to the liver and kidneys, where the amino group is released from glutamine by glutaminase; the products are glutamate and ammonia.
The ammonia delivered in this way to the liver is converted to urea, then excreted.
Describe the reactions and the role of the glucose-alanine cycle.
Toxic ammonia formed by amino acid catabolism in muscle is transported to the liver as alanine, which is nontoxic
Alanine is formed in muscle by transamination of pyruvate (formed by glycolysis); glutamate formed by glutamate dehydrogenase is the amino donor.
In the liver, alanine is reconverted to pyruvate by transamination.
Its amino group is eventually converted to urea, and the pyruvate is converted to glucose by gluconeogenesis in the liver, then exported to muscle
Why does a mammal go to all of the trouble of making urea from ammonia rather than simply excreting ammonia as many bacteria do?
When bacteria release ammonia into the surrounding medium, it is diluted enormously to nontoxic levels. The ammonia produced by amino acid catabolism in mammals cannot be sufficiently diluted in the tissues and the blood to avoid accumulating at toxic levels. Urea is much less toxic than ammonia.
Describe the three general mechanisms for disposing of excess nitrogen obtained in the diet. Which organisms use each mechanism?
(1) Ammonotelic: release into the surrounding medium as NH4+ (bacteria and many marine organisms); (2) Uricotelic: production of uric acid (birds and reptiles); (3) Ureotelic: production and excretion of urea (land-dwelling animals).
Amino acid catabolism involves the breakdown of 20 amino acids all of which contain nitrogen but have different carbon skeletons. What overall strategy is used to deal with this problem? Illustrate the strategy with two examples.
Nitrogen is removed by transamination to glutamate. This converts the amino acid to an α-keto acid that either is an intermediate in carbohydrate catabolism or is converted to one. (
During starvation, more urea production occurs. Explain this observation (
During starvation, cellular proteins are degraded and their carbon skeletons are oxidized for energy. The first step in amino acid catabolism is removal of the amino groups, which are ultimately excreted as urea.
Name four amino acids that can be converted directly (in one step) into pyruvate or a citric acid cycle intermediate, and name the intermediate formed from each.
(1) aspartate; oxaloacetate; (2) glutamate; α-ketoglutarate; (3) alanine; pyruvate; (4) serine; pyruvate.
Degradation of amino acids yields compounds that are common intermediates in the major metabolic pathways. Explain the distinction between glucogenic and ketogenic amino acids in terms of their metabolic fates.
The glucogenic amino acids are those that are catabolized to intermediates that can serve as substrates for gluconeogenesis: pyruvate and any of the four- or five-carbon intermediates of the citric acid cycle. Ketogenic amino acids are catabolized to yield acetyl-CoA or acetoacetyl-CoA, the precursors for ketone body formation.
