Amino Acid Metabolism Practice Questions

Q: Which of the following is a key regulatory enzyme in the urea cycle?

Carbamoyl phosphate synthetase I. ***Carbamoyl phosphate synthetase I*** - This enzyme catalyzes the **first committed step** of the urea cycle, forming **carbamoyl phosphate** from ammonia, bicarbonate, and ATP. - Its activity is tightly regulated, primarily by **N-acetylglutamate**, reflecting changes in protein catabolism and ammonia levels. *Ornithine transcarbamylase* - This enzyme catalyzes the **second step** of the urea cycle, combining **carbamoyl phosphate** with **ornithine** to form citrulline. - While essential for the cycle, it is not considered the primary regulatory point, as its substrate, carbamoyl phosphate, is regulated by CPS I. *Argininosuccinate synthetase* - This enzyme catalyzes the **third step** of the urea cycle, converting **citrulline** and **aspartate** into argininosuccinate. - Its activity is dependent on the availability of citrulline and aspartate, but it does not exert the primary regulatory control over the overall cycle flux. *Arginase* - This enzyme catalyzes the **final step** of the urea cycle, hydrolyzing **arginine** to form urea and regenerating ornithine. - While critical for producing urea and recycling ornithine, its regulation is secondary to the initial rate-limiting step controlled by CPS I.

Q: Which enzyme deficiency is directly responsible for the accumulation of phenylalanine in the blood of individuals with phenylketonuria?

Phenylalanine hydroxylase. ***Correct: Phenylalanine hydroxylase*** - This enzyme is crucial for converting **phenylalanine** into **tyrosine** - Its deficiency leads to the buildup of phenylalanine in the blood, which is characteristic of **phenylketonuria (PKU)** - PKU is an **autosomal recessive disorder** and one of the most common inborn errors of metabolism - Without treatment, elevated phenylalanine causes **intellectual disability, seizures, and behavioral problems** *Incorrect: Hexokinase* - This enzyme is involved in the **first step of glycolysis**, phosphorylating glucose to glucose-6-phosphate - Deficiency of hexokinase does not directly cause an accumulation of phenylalanine - It is unrelated to amino acid metabolism *Incorrect: Glucokinase* - Glucokinase is an enzyme that facilitates the **phosphorylation of glucose to glucose-6-phosphate** in the liver and pancreatic beta cells - Its dysfunction is associated with certain types of **maturity-onset diabetes of the young (MODY)**, but not with phenylalanine metabolism disorders - It serves as the glucose sensor in pancreatic beta cells *Incorrect: Pyruvate kinase* - This enzyme catalyzes the **final step of glycolysis**, converting phosphoenolpyruvate to pyruvate - Deficiencies in pyruvate kinase can lead to **hemolytic anemia** but do not impact phenylalanine levels - It is unrelated to amino acid metabolism

Q: What is the most common inborn error of metabolism that results in phenylketonuria (PKU)?

Phenylalanine hydroxylase deficiency. ***Phenylalanine hydroxylase deficiency*** - This enzyme is crucial for converting **phenylalanine** to **tyrosine**. Its deficiency leads to the accumulation of phenylalanine, causing phenylketonuria (PKU). - More than 95% of PKU cases are attributed to **mutations** in the gene encoding phenylalanine hydroxylase. *Homogentisate oxidase deficiency* - Deficiency of this enzyme causes **alkaptonuria**, a rare genetic disorder characterized by the accumulation of homogentisic acid. - It is distinct from PKU and does not involve the metabolism of phenylalanine to tyrosine. *Branched-chain α-ketoacid dehydrogenase deficiency* - This enzyme deficiency is associated with **maple syrup urine disease (MSUD)**, a disorder of branched-chain amino acid metabolism. - MSUD affects the metabolism of **leucine, isoleucine, and valine**, not phenylalanine. *Ornithine transcarbamylase deficiency* - This deficiency is the most common **urea cycle disorder**, leading to hyperammonemia. - It primarily affects the body's ability to remove **ammonia** and is unrelated to phenylalanine metabolism.

Q: What is the consequence of a defect in the enzyme branched-chain alpha-keto acid dehydrogenase?

Accumulation of branched-chain amino acids. ***Accumulation of branched-chain amino acids*** - **Branched-chain alpha-keto acid dehydrogenase (BCKDH)** is responsible for the oxidative decarboxylation of alpha-keto acids, which are derived from branched-chain amino acids (BCAAs) like **leucine, isoleucine, and valine**. - A defect in this enzyme leads to the **buildup of BCAAs** and their corresponding alpha-keto acids in the blood and urine, a hallmark of **Maple Syrup Urine Disease**. *Reduced synthesis of fatty acids* - While BCAAs can be used for energy production and contribute to lipid metabolism, a direct and primary consequence of BCKDH defect is not a reduction in fatty acid synthesis. - The main issue lies in the **catabolism of BCAAs**, not necessarily downstream fatty acid production. *Increased glucose synthesis* - The elevated levels of BCAAs can actually inhibit several enzymes involved in **gluconeogenesis**. - Therefore, a defect in BCKDH would more likely lead to impaired, rather than increased, glucose synthesis due to the toxic accumulation of metabolites. *Decreased cholesterol levels* - The catabolism of BCAAs, particularly **leucine**, produces metabolites like **acetyl-CoA** which are precursors for cholesterol synthesis. - While a defect in BCKDH would impair this pathway, the primary and most significant consequence is not decreased cholesterol, but rather the toxic effects of accumulated BCAAs and their alpha-keto acids.

Q: A patient has elevated levels of both homocysteine and methionine. Which enzyme deficiency could be the cause?

Cystathionine β-synthase. ***Cystathionine β-synthase*** - A deficiency in **cystathionine β-synthase (CBS)** leads to the accumulation of both **homocysteine** and **methionine** because homocysteine cannot be converted into cystathionine, causing it to build up. - The elevated homocysteine also inhibits methionine metabolism, leading to methionine accumulation as well. - This condition is known as **classical homocystinuria**, characterized by elevated levels of both these amino acids in blood and urine. *Methionine synthase* - **Methionine synthase** (also known as homocysteine methyltransferase) catalyzes the conversion of **homocysteine to methionine**, using methylcobalamin (vitamin B12) and methyltetrahydrofolate. - A deficiency in this enzyme would lead to **elevated homocysteine** but **decreased methionine**, as methionine cannot be regenerated from homocysteine. *Cystathionine γ-lyase* - **Cystathionine γ-lyase (cystathionase)** is the enzyme downstream of CBS that converts **cystathionine to cysteine** in the transsulfuration pathway. - A deficiency causes **cystathioninuria** with elevated cystathionine but typically **normal or mildly elevated homocysteine and methionine**. - This does not match the clinical picture of elevated both homocysteine and methionine. *S-adenosylmethionine synthetase* - **S-adenosylmethionine (SAM) synthetase** converts **methionine into SAM**, which is a crucial methyl donor in methylation reactions. - A deficiency would cause **elevated methionine** but **normal or decreased homocysteine**, as methionine would not be efficiently converted to SAM and subsequently to homocysteine. - This deficiency is rare and presents with hepatic dysfunction and neurological issues.

Q: Which process is primarily responsible for converting ammonia to a less toxic substance in the liver?

The Urea Cycle. ***The Urea Cycle*** - The **urea cycle** is the primary metabolic pathway in the liver for converting highly toxic **ammonia** into less toxic **urea**, which can then be safely excreted by the kidneys. - This cycle is crucial for detoxification of ammonia, a byproduct of amino acid metabolism. - Key enzymes include **carbamoyl phosphate synthetase I**, ornithine transcarbamylase, argininosuccinate synthetase, argininosuccinate lyase, and arginase. *Glycolysis* - Glycolysis is a metabolic pathway that breaks down glucose into pyruvate, producing energy (ATP). - It does not play any direct role in ammonia detoxification. *The Citric Acid Cycle* - The Citric Acid Cycle (Krebs cycle) is a central metabolic pathway for generating ATP through the oxidation of acetyl-CoA. - While it interfaces with amino acid metabolism, it is not directly responsible for converting ammonia to urea. *Beta-oxidation* - Beta-oxidation is the process by which fatty acids are broken down into acetyl-CoA for energy production. - This process is involved in lipid metabolism and has no direct function in ammonia detoxification.

Q: A patient with hyperammonemia is found to have elevated levels of citrulline. Which enzyme deficiency is most likely?

Argininosuccinate synthetase deficiency. ***Argininosuccinate synthetase deficiency*** - A deficiency in **argininosuccinate synthetase** (ASS) prevents the conversion of **citrulline** and **aspartate** to argininosuccinate, leading to the accumulation of **citrulline**. - The accumulated citrulline diffuses out of the hepatocytes and into the bloodstream, resulting in **elevated plasma citrulline** levels, which is a hallmark of this disorder, also known as **Citrullinemia Type I**. *CPS I deficiency* - **Carbamoyl phosphate synthetase I (CPS I)** is the first committed step of the urea cycle; a deficiency results in the inability to synthesize **carbamoyl phosphate**. - This leads to severe **hyperammonemia** but very **low levels of citrulline** and arginine, as the cycle cannot progress beyond the initial stages. *OTC deficiency* - **Ornithine transcarbamylase (OTC)** deficiency prevents the conversion of **carbamoyl phosphate** and **ornithine** to citrulline. - This common X-linked disorder causes **severe hyperammonemia** with **low blood citrulline** levels, contrasting with the elevated citrulline seen in the patient. *Arginase deficiency* - **Arginase** catalyzes the final step of the urea cycle, converting **arginine** to ornithine and urea. - A deficiency leads to an accumulation of **arginine** and **hyperargininemia**, with **normal or only modestly elevated levels of citrulline**, along with hyperammonemia, which is distinct from the primary elevation of citrulline.

Q: A 5-year-old girl with global developmental delay, hypotonia, and failure to thrive presents with severe hyperammonemia. Laboratory results show increased orotic acid, increased glutamine, and decreased BUN. Evaluate and determine the underlying metabolic disorder.

Ornithine transcarbamylase deficiency (OTC deficiency). ***Ornithine transcarbamylase deficiency (OTC deficiency)*** - **Hyperammonemia** with **increased orotic acid** and **decreased BUN** is the hallmark of OTC deficiency, an X-linked urea cycle disorder. - The deficiency in OTC prevents the conversion of carbamoyl phosphate and ornithine to citrulline, leading to the accumulation of carbamoyl phosphate which then enters the **pyrimidine synthesis pathway**, increasing orotic acid. *Maple syrup urine disease (MSUD)* - While it presents with **developmental delay** and **poor feeding**, MSUD is characterized by the accumulation of **branched-chain amino acids** (leucine, isoleucine, valine) and their ketoacids, not hyperammonemia with increased orotic acid. - The urine has a characteristic sweet smell, resembling **maple syrup**. *Phenylketonuria (PKU)* - PKU is caused by a deficiency in **phenylalanine hydroxylase**, leading to the accumulation of phenylalanine. - It does not present with **hyperammonemia** or **orotic aciduria**; instead, it causes severe intellectual disability if untreated and is often detected by newborn screening. *Methylmalonic acidemia (MMA)* - MMA is an organic aciduria characterized by the accumulation of **methylmalonic acid** due to a defect in methylmalonyl-CoA mutase or its cofactor vitamin B12 metabolism. - Patients typically present with metabolic acidosis, hypoglycemia, elevated propionic acid, and ketonuria, but **hyperammonemia and orotic aciduria are not primary features**.

Q: Ochronosis is due to the accumulation of?

Homogentisic acid. ***Homogentisic acid (Correct)*** - **Ochronosis** is a rare genetic disorder characterized by the accumulation of **homogentisic acid** in connective tissues. - This accumulation results from a deficiency of the enzyme **homogentisate 1,2-dioxygenase**, which is crucial in the catabolism of tyrosine and phenylalanine. - Clinically presents with dark pigmentation of cartilage, sclera, and other connective tissues. *Alkapton (Incorrect)* - While **alkaptonuria** is the disease caused by homogentisic acid accumulation, **alkapton** itself is not the substance that accumulates in tissues in ochronosis. - **Alkapton** refers to the dark-colored urine observed in patients with alkaptonuria, which is due to the oxidation of homogentisic acid in the urine. *Xanthurenate (Incorrect)* - **Xanthurenate** is an intermediate in the metabolism of tryptophan, and its accumulation is associated with certain vitamin B6 deficiencies. - It is not involved in the pathogenesis of ochronosis or alkaptonuria. *Glyoxylate (Incorrect)* - **Glyoxylate** is a metabolic intermediate involved in various pathways, including carbohydrate and amino acid metabolism. - Accumulation of glyoxylate is associated with **primary hyperoxaluria type 1**, but not with ochronosis.

Q: Phenylketonuria is due to deficiency of:

Phenylalanine hydroxylase (PAH). ***Phenylalanine hydroxylase (PAH)*** - **Phenylketonuria (PKU)** is an inherited metabolic disorder caused by a deficiency of the enzyme **phenylalanine hydroxylase (PAH)**. - This enzyme is crucial for converting the amino acid **phenylalanine** into **tyrosine**, and its deficiency leads to an accumulation of phenylalanine in the body. - PKU follows an **autosomal recessive** inheritance pattern. *Phenylalanine (amino acid)* - While **phenylalanine** accumulates to toxic levels in PKU, it is the substrate, not the deficient component. - The disease stems from the body's inability to metabolize this amino acid due to the enzyme deficiency. *Tyrosine* - **Tyrosine** is the product of the reaction catalyzed by PAH, and its production is limited in PKU. - However, the deficiency of tyrosine is a secondary effect, not the primary cause of PKU. *None of the options* - This option is incorrect because the deficiency of **phenylalanine hydroxylase (PAH)** is precisely the cause of Phenylketonuria.

Question 1

Which of the following is a key regulatory enzyme in the urea cycle?

Accepted Answer

Carbamoyl phosphate synthetase I

Answer

Ornithine transcarbamylase

Answer

Argininosuccinate synthetase

Answer

Arginase

Question 2

Which enzyme deficiency is directly responsible for the accumulation of phenylalanine in the blood of individuals with phenylketonuria?

Accepted Answer

Phenylalanine hydroxylase

Answer

Hexokinase

Answer

Glucokinase

Answer

Pyruvate kinase

Question 3

What is the most common inborn error of metabolism that results in phenylketonuria (PKU)?

Accepted Answer

Phenylalanine hydroxylase deficiency

Answer

Homogentisate oxidase deficiency

Answer

Branched-chain α-ketoacid dehydrogenase deficiency

Answer

Ornithine transcarbamylase deficiency

Question 4

What is the consequence of a defect in the enzyme branched-chain alpha-keto acid dehydrogenase?

Accepted Answer

Accumulation of branched-chain amino acids

Answer

Reduced synthesis of fatty acids

Answer

Increased glucose synthesis

Answer

Decreased cholesterol levels

Question 5

A patient has elevated levels of both homocysteine and methionine. Which enzyme deficiency could be the cause?

Accepted Answer

Cystathionine β-synthase

Answer

Methionine synthase

Answer

Cystathionine γ-lyase

Answer

S-adenosylmethionine synthetase

Question 6

Which process is primarily responsible for converting ammonia to a less toxic substance in the liver?

Accepted Answer

The Urea Cycle

Answer

Glycolysis

Answer

The Citric Acid Cycle

Answer

Beta-oxidation

Question 7

A patient with hyperammonemia is found to have elevated levels of citrulline. Which enzyme deficiency is most likely?

Accepted Answer

Argininosuccinate synthetase deficiency

Answer

CPS I deficiency

Answer

OTC deficiency

Answer

Arginase deficiency

Question 8

A 5-year-old girl with global developmental delay, hypotonia, and failure to thrive presents with severe hyperammonemia. Laboratory results show increased orotic acid, increased glutamine, and decreased BUN. Evaluate and determine the underlying metabolic disorder.

Accepted Answer

Ornithine transcarbamylase deficiency (OTC deficiency)

Answer

Maple syrup urine disease (MSUD)

Answer

Phenylketonuria (PKU)

Answer

Methylmalonic acidemia (MMA)

Question 9

Ochronosis is due to the accumulation of?

Accepted Answer

Homogentisic acid

Answer

Alkapton

Answer

Xanthurenate

Answer

Glyoxylate

Question 10

Phenylketonuria is due to deficiency of:

Accepted Answer

Phenylalanine hydroxylase (PAH)

Answer

Phenylalanine (amino acid)

Answer

Tyrosine

Answer

None of the options

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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