Biochemistry : Protein Catabolism Enzymes

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Catabolic Pathways And Metabolism

The urea cycle is vital to the excretion of ammonia, a harmful byproduct of amino acid breakdown. 

What enzyme in the urea cycle converts L-ornithine to L-citrulline?

Possible Answers:

Ornithine transcarbamylase

Argininosuccinate lyase

Ornithine synthetase

 Argininosuccinate synthetase

Correct answer:

Ornithine transcarbamylase

Explanation:

Ornithine transcarbamylase catalyzes the reaction between the substrates ornithine and carbamoyl phosphate to form citrulline and phosphate. This process occurs primarily in the hepatic mitochondria, and to a lesser extent in the mitochondria of the renal cells.

Example Question #2 : Catabolic Pathways And Metabolism

What enzyme is deficient in Lesch-Nyhan syndrome? 

Possible Answers:

Uridine kinase

Uridine phosphorylase

Adenine phosphoribosyltransferase (APRT)

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)

Correct answer:

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)

Explanation:

Lesch-Nyhan syndrome is an x-linked deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT). HGPRT plays a central role in the generation of purine nucleotides through the purine salvage pathway. Cell breakdown products cannot be reused, and are therefore degraded. This gives rise to increased uric acid, a purine breakdown product. This build-up of uric acid is associated with severe gout and kidney problems, poor muscle control and mental retardation, usually in the first year of life. In the second year of life a common sign is self-mutilating behaviors.

Example Question #3 : Catabolic Pathways And Metabolism

Which of the following classes of enzymes could possibly breakdown collagen, the most common protein in the human body?

Possible Answers:

Ligases

Polymerases

Lipases

Isomerases

Proteases

Correct answer:

Proteases

Explanation:

Because collagen is a protein, it must be broken down by a protease. Proteases exert their effects by hydrolyzing peptide bonds. The specific enzyme that breaks down collagen, predictably, is called collagenase.

The other classes of enzymes listed have actions unrelated to protein breakdown. An isomerase rearranges bonds to form an isomer, a polyermase adds nucleotides to DNA, a lipase breaks down fats, and a ligase creates a chemical bond.

Example Question #4 : Catabolic Pathways And Metabolism

Which of these describe(s) the role of aminotransferases in protein catabolism?

I. They are present in the liver and muscle

II. They transfer the amino group from one amino acids to alpha-ketoglutarate

III. In the muscle, these enzymes move the amino group to glutamate, which can be further converted to glutamine which is a excess nitrogen carrier from tissues

IV. They catalyze irreversible reactions and release amino groups as free ammonium

Possible Answers:

II and III

I, II, and III

I and II

I, II, III, and IV

II and IV

Correct answer:

I, II, and III

Explanation:

Aminotransferases do not release amino groups, but rather transfer them to other amino acids. The reactions catalyzed by aminotransferases are reversible. Their blood concentrations can be used as clinical indicators for damage to liver or muscle.

Example Question #5 : Catabolic Pathways And Metabolism

Which of the following describes the role of glutamine synthetase in protein catabolism?

I. Glutamine synthetase converts glutamate and ammonia to glutamine using one molecule of ATP

II. The reaction catalyzed by glutamine synthetase is reversible

III. The reaction catalyzed by glutamine synthetase removes excess nitrogen (in the form of toxic ammonia) from protein degradation

IV. Glutamine synthetase converts glutamine to glutamate

Possible Answers:

III only

I and IV

IV only

I, II, and III

I and III

Correct answer:

I and III

Explanation:

Glutaminase synthetase is present predominately in the brain, liver and kidneys.The importance of the reaction catalyzed by glutamine synthetase is that excess nitrogen (in the form of toxic ammonia) from protein degradation can be removed from tissues, especially the brain. The reaction catalyzed by glutamine synthetase is irreversible. It converts glutamate to glutamine.

Example Question #1 : Protein Catabolism Enzymes

Which of the following regarding mitochondrial carbamoyl phosphate synthetase are true?

I. Mitochondrial carbamoyl phosphate synthetase forms intermediates in the urea cycle

II. Mitochondrial carbamoyl phosphate synthetase utilizes adenosine triphosphate (ATP)

III. A decrease in normal levels of the enzyme leads to a decrease in ammonium ions in the blood

IV. Mitochondrial carbamoyl phosphate synthetase converts ammonium ions to carbamoyl phosphate

Possible Answers:

II and IV

I, II, and III

I and IV

I and III

I, II, and IV

Correct answer:

I, II, and IV

Explanation:

Mitochondrial carbamoyl phosphate synthetase is important in excretion of ammonium ions as urea. Without it there is an increase in blood ammonium levels (which are toxic to the brain), as well as decreased blood urea levels and increased levels of glutamine (the transporter of ammonium in the blood).

Example Question #7 : Catabolic Pathways And Metabolism

Which of these describe the role of ornithine transcarbamoylase in protein degradation?

I. Ornithine transcarbamoylase converts carbamoyl phosphate to citrulline in the presence of ornithine

II. Ornithine transcarbamoylase is present in the mitochondria

III. Deficiency of the enzyme leads to a decrease of ammonium ions in the blood

IV. Deficiency of the enzyme leads to an increase in urea in the blood

Possible Answers:

II and III

I and II

I and III

III and IV

I only

Correct answer:

I and II

Explanation:

Ornithine transcarbamoylase is part of the urea cycle; it converts carbamoyl phosphate to citrulline in the presence of ornithine. It participates in the pathway that transforms toxic ammonium ions released in amino acid degradation to non-toxic urea that can be eliminated in urine. Ornithine transcarbamoylase is a mitochondrial membrane and its deficiency not only affects urea production, but also leads to accumulation of nucleotide bases such as uracil in the blood and urine. The levels of urea in the blood decrease (not increase) in ornithine transcarbamoylase deficiency.

Example Question #8 : Catabolic Pathways And Metabolism

Which of the following is a possible consequence of an arginase deficiency?

Possible Answers:

Hyperammonemia

Arginine deficiency

Excess urea production

No change to the levels of arginine, ornithine, and urea

Excess ornithine production

Correct answer:

Hyperammonemia

Explanation:

Arginase catalyzes the conversion of arginine to ornithine and urea. Therefore, if there is a deficiency in arginase, there will be a buildup of arginine, and a deficiency in ornithine and urea. Without the ability to form urea, the ammonium waste from protein and amino acids degradation will not be able to be converted to urea safely. The buildup of ammonium will cause hyperammonemia, which results in some unfavorable physiological scenarios.

Example Question #9 : Catabolic Pathways And Metabolism

The enzyme responsible for the reaction that produces ammonia during amino acid deamination is ___________.

Possible Answers:

cathepsin

carbamoyl phosphate synthetase

aminotransferase

pyridoxal-5’-phosphate

glutamate dehydrogenase

Correct answer:

glutamate dehydrogenase

Explanation:

The correct answer is "glutamate dehydrogenase." Glutamate is the only amino acid that has an enzyme that removes its amino group as ammonia, rather than just transferring the amino group to a α-keto acid and forming a new amino acid. The transfer of amino groups, which may form glutamate, is performed by aminotransferases that require pyridoxal-5’-phosphate as a cofactor. Cathepsins are a type of proteases contained in lysosomes, and they break down proteins into amino acids rather than metabolize the amino acids themselves. Carbamoyl phosphate synthase is not involved in deamination, but rather condenses ammonia with bicarbonate to enter the urea cycle.

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