Biochemistry : Reactants and Products of Protein Catabolism

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Reactants And Products Of Protein Catabolism

What is produced in one turn of the urea cycle? 

Possible Answers:

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

1 molecule of ammonia is split into 2 molecules of urea

4 molecules of ammonia and 1 molecule of carbon dioxide create 2 molecules of urea for excretion and 1 molecule of ornithine

Correct answer:

2 molecules of ammonia and 1 molecule of carbon dioxide create 1 molecule of urea and regenerate 1 molecule of ornithine for another turn

Explanation:

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 #2 : Reactants And Products Of Protein Catabolism

Which process eliminates nitrogen waste generated from protein catabolism in humans?

Possible Answers:

Krebs cycle

Urea cycle

Purine nucleotide cycle

Citric acid cycle

Gluconeogenesis

Correct answer:

Urea cycle

Explanation:

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 #2 : Reactants And Products Of Protein Catabolism

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?

Possible Answers:

Aldimine

-hydroxy acid

-keto acid

Schiff base

-keto acid

Correct answer:

-keto acid

Explanation:

Aminotransferases (transaminases) catalyze the conversion of an amino acid to an -keto acid. To do so, they utilize a pyridoxyl phosphate coenzyme, which mediates the transfer of the amino group, leaving a ketone functional group next to the carboxyl group. This -keto acid is the product. Since the amino group in an amino acid is attached to the alpha carbon, it will not be a -keto acid. Some of the other answer choices give intermediates of the reaction mechanism, not the product.

Example Question #14 : Catabolic Pathways And Metabolism

Which amino acid cannot be deaminated in protein catabolism?

Possible Answers:

Cysteine

Glutamate

Proline

Phenylalanine

Glycine

Correct answer:

Proline

Explanation:

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 #1 : Reactants And Products Of Protein Catabolism

How is most of the ammonium waste removed from the body?

Possible Answers:

Urea synthesis in the liver

Excreted in the urine

Excreted in the feces

Conversion to ammonia in the liver

Ammonium remains in the blood until it is needed 

Correct answer:

Urea synthesis in the liver

Explanation:

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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