Biochemistry : Reactants and Products of Protein Catabolism
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Example Question #11 : Catabolic Pathways And Metabolism
What is produced in one turn of the urea cycle?
1 molecule of ammonia is split into 2 molecules of urea
4 molecules of ammonia and 1 molecule of nitric oxide create 1 molecule of urea and 1 molecule of carbon dioxide
2 molecules of ammonia and 1 molecule of carbon dioxide create 1 molecule of urea and regenerate 1 molecule of ornithine for another turn
4 molecules of ammonia and 1 molecule of carbon dioxide create 2 molecules of urea for excretion and 1 molecule of ornithine
2 molecules of ammonia and 1 molecule of carbon dioxide create 1 molecule of urea and regenerate 1 molecule of ornithine for another turn
2 molecules of ammonia and 1 molecule of carbon dioxide are converted into 1 molecule of urea in every turn of the urea cycle. In addition, each cycle regenerates 1 molecule of ornithine for use in the next turn.
Example Question #11 : Catabolic Pathways And Metabolism
Which process eliminates nitrogen waste generated from protein catabolism in humans?
Krebs cycle
Urea cycle
Purine nucleotide cycle
Citric acid cycle
Gluconeogenesis
Urea cycle
Removal of the amino group is a primary step in amino acid catabolism. Humans (and some other animals) utilize the urea cycle to convert the unnecessary amino groups from amino acids into urea. The carbon skeletons will be fed into the citric acid cycle, and none of the other answers directly relate to human nitrogen metabolism. The Krebs cycle (also known as the citric acid cycle) is an intermediate between glycolysis and oxidative phosphorylation, and involves energy production. Gluconeogenesis is the process by which glucose is synthesized from non-carbohydrate organic substrates. The purine nucleotide cycle involves conversion of nucleotides into Krebs cycle intermediates, and makes ammonia as a byproduct, which is then shunted into the urea cycle to be converted into urea and excreted in the urine.
Example Question #731 : Biochemistry
The first step in the metabolism of most amino acids is the removal of the amino group by aminotransferases. What is the product of this reaction?
Schiff base
Aldimine
Aminotransferases (transaminases) catalyze the conversion of an amino acid to an 
Example Question #1 : Reactants And Products Of Protein Catabolism
Which amino acid cannot be deaminated in protein catabolism?
Cysteine
Glycine
Phenylalanine
Glutamate
Proline
Proline
Due to its amino group's location within the pyrrole ring derivative, proline is unable to be deaminated via the aminotransferase step of protein catabolism. Proline oxidase is the enzyme that catalyzes the first reaction in the catabolism of proline. None of the other amino acid answer choices have this limitation.
Example Question #2 : Reactants And Products Of Protein Catabolism
How is most of the ammonium waste removed from the body?
Excreted in the urine
Conversion to ammonia in the liver
Urea synthesis in the liver
Excreted in the feces
Ammonium remains in the blood until it is needed
Urea synthesis in the liver
When proteins and amino acids are broken down in the body, ammonium is created as a byproduct. Ammonium is dangerous when it remains free in the human body, so something must be done to get rid of it. The major route of removal of ammonium by the body is via urea synthesis in the liver. Urea can then be excreted in urine.
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