Biochemistry : Catabolic Pathways and Metabolism

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Protein Catabolism Regulation

Which enzyme catalyzes the rate-limiting step of the urea cycle? 

Possible Answers:

Carbamoyl phosphate dehydrogenase

Argininosuccinate lyase

Ornithine transcarbamylase

Carbamoyl phosphate synthetase

Correct answer:

Carbamoyl phosphate synthetase

Explanation:

Carbamoyl phosphate synthetase is the rate-limiting step of the urea cycle. It is the first step of the urea cycle and occurs exclusive in the mitochondria of hepatic and renal cells.

Example Question #11 : Catabolic Pathways And Metabolism

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

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

1 molecule of ammonia is split into 2 molecules of urea

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

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 #11 : Catabolic Pathways And Metabolism

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 #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?

Possible Answers:

-keto acid

-hydroxy acid

Schiff base

Aldimine

-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 #2 : Reactants And Products Of Protein Catabolism

Which amino acid cannot be deaminated in protein catabolism?

Possible Answers:

Phenylalanine

Cysteine

Glutamate

Glycine

Proline

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 #3 : Reactants And Products Of Protein Catabolism

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

Possible Answers:

Urea synthesis in the liver

Conversion to ammonia in the liver

Ammonium remains in the blood until it is needed 

Excreted in the urine

Excreted in the feces

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.

Example Question #11 : Catabolic Pathways And Metabolism

Which of the following statements is false regarding catabolic pathways?

Possible Answers:

Catabolic pathways liberate smaller molecules from larger ones

Catabolic pathways have a net release of energy

The citric acid cycle is a catabolic pathway

Catabolic pathways have a net consumption of ATP

Catabolic reactions are the opposite of anabolic reactions

Correct answer:

Catabolic pathways have a net consumption of ATP

Explanation:

Both catabolic and anabolic reactions are metabolic reactions. The difference is that catabolism is when complex molecules break down into simpler molecules, and anabolism is when simpler molecules are combined to form complex molecules. ATP (adenosine triphosphate) is used to store and transport energy in cells. There is a major difference in how ATP is used in catabolic and anabolic reactions. Anabolic reactions require energy input, and result in a net consumption of ATP. Catabolic reactions produce energy and results in a net synthesis of ATP. Therefore, catabolic pathways do not have a net consumption of ATP.

Example Question #1 : Other Protein Catabolism Concepts

What is the purpose of the urea cycle?

Possible Answers:

Synthesis of urea, a necessary part of renal acid/base balance

Concentration of digestive byproducts in the renal tubules for excretion

Conversion of ammonia to urea so that it can be excreted in the urine

Conversion of urea into ammonia so that it can be excreted in the urine

Correct answer:

Conversion of ammonia to urea so that it can be excreted in the urine

Explanation:

The urea cycle is vital to the excretion of ammonia, a harmful byproduct of amino acid breakdown. Via a series of enzymatic changes, ammonia is converted to urea, which can be excreted into the urine. 

Example Question #11 : Catabolic Pathways And Metabolism

Where does the urea cycle occur?

Possible Answers:

Exclusively in the kidney

Primarily in the liver, secondarily in the kidney

Primarily in the kidney, secondarily in the duodenum

Primarily in the kidney, secondarily in the liver

Correct answer:

Primarily in the liver, secondarily in the kidney

Explanation:

The urea cycle occurs primarily in the liver, and to a lesser extent in renal cells. There is no urea conversion performed by the small intestine. 

Example Question #745 : Biochemistry

In the urea cycle, which of the following amino acids condenses with citrulline, yielding argininosuccinate?

Possible Answers:

Aspartic acid

Arginine

Ornithine

Asparagine

Glutamic acid

Correct answer:

Aspartic acid

Explanation:

Arginine is the amino acid in the cycle that is converted to urea and ornithine via the enzyme arginase, and is one of the products of the lysis of argininosuccinate. Glutamic acid plays a different role in the cycle; it loses its amino group to the synthesis of carbamoyl phosphate, a precursor of citrulline. Asparagine is not present in the urea cycle, but aspartic acid is. It condenses with citrulline, through the action of the enzyme argininosuccinate synthase and ATP, to produce argininosuccinate.

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