Amino Acid Metabolism Practice Questions

Q: What is the primary methyl donor in the conversion of norepinephrine to epinephrine?

S-adenosyl methionine. ***S-adenosyl methionine*** (SAM) - **S-adenosyl methionine (SAM)** is the universal methyl donor in many biological methylation reactions, including the conversion of **norepinephrine to epinephrine**. - The enzyme **phenylethanolamine N-methyltransferase (PNMT)** uses SAM to transfer a methyl group to norepinephrine, forming epinephrine. *Arginine* - **Arginine** is an amino acid primarily involved in the synthesis of **nitric oxide**, urea, and creatine, and does not serve as a direct methyl donor. - Its metabolic roles are distinct from methylation processes involving catecholamines. *Phenylalanine* - **Phenylalanine** is an essential amino acid that is a **precursor to tyrosine**, which is then used to synthesize catecholamines like dopamine, norepinephrine, and epinephrine. - It is a building block in the synthesis pathway but does not directly donate methyl groups. *Methionine* - **Methionine** is an essential amino acid that is converted to **S-adenosyl methionine (SAM)** through the action of methionine adenosyltransferase. - While methionine is the precursor to SAM, it is **SAM itself that serves as the active methyl donor**, not free methionine.

Q: In which condition is Vanillylmandelic Acid (VMA) excreted in urine?

Pheochromocytoma. ***Pheochromocytoma*** - **Vanillylmandelic Acid (VMA)** is a breakdown product of catecholamines (epinephrine and norepinephrine), which are overproduced in **pheochromocytoma** [2]. - Elevated VMA levels in urine are a key diagnostic marker for this **adrenal medulla tumor** [3]. *Alkaptonuria* - This is an inherited disorder of **tyrosine metabolism**, leading to the accumulation of **homogentisic acid**. - It causes **ochronosis** (darkening of cartilage and connective tissue) and dark urine upon standing, but not elevated VMA. *Neuroblastoma* - While neuroblastoma also involves **catecholamine overproduction**, it typically leads to elevated levels of **homovanillic acid (HVA)** and **VMA**, but is primarily characterized by HVA in children. - Neuroblastoma is a childhood cancer, distinct from the adrenal tumor seen in adults with pheochromocytoma. *Cushing's syndrome* - This condition is characterized by chronic **excess cortisol** production, leading to symptoms like central obesity, moon facies, and hypertension [1]. - It does not involve alterations in catecholamine metabolism or elevated VMA levels.

Q: Amino acid metabolism is implicated in which disease?

Maple syrup urine disease. ***Maple syrup urine disease*** - This disease is a genetic disorder of **amino acid metabolism**, specifically the breakdown of **branched-chain amino acids** (leucine, isoleucine, and valine). - Its characteristic odor, resembling **maple syrup**, is due to the accumulation of these amino acids and their ketoacids in bodily fluids. *Reye's syndrome* - This is an acute encephalopathy and **liver dysfunction**, typically affecting children, often linked to aspirin use during viral infections. - It primarily involves **mitochondrial damage** and fatty infiltration of the liver, not a direct amino acid metabolism defect. *Von Gierke's disease* - This is a type of **glycogen storage disease (Type I)** caused by a deficiency in **glucose-6-phosphatase**. - It leads to an inability to release glucose from stored glycogen in the liver and kidneys, resulting in **hypoglycemia** and hepatomegaly, not amino acid metabolism issues. *McArdle's disease* - This is another **glycogen storage disease (Type V)**, characterized by a deficiency in **muscle phosphorylase**. - Patients experience exercise intolerance, muscle pain, and cramping due to the inability to break down muscle glycogen for energy, unrelated to amino acid metabolism.

Q: Albinism is due to deficiency of the following enzyme?

Tyrosinase. ***Tyrosinase*** - **Albinism** is characterized by a lack of melanin production, which is primarily due to a defect in the enzyme **tyrosinase**. - **Tyrosinase** is crucial for converting **tyrosine** into **DOPA**, a key step in the biosynthesis of melanin. *Phenylalanine hydroxylase* - Deficiency of **phenylalanine hydroxylase** leads to **phenylketonuria (PKU)**, an inherited metabolic disorder. - This enzyme is responsible for converting **phenylalanine to tyrosine**, not directly involved in melanin synthesis. *Homogentisic acid oxidase* - A deficiency in **homogentisic acid oxidase** causes **alkaptonuria**, a rare metabolic disorder. - This enzyme is involved in the breakdown pathway of **tyrosine and phenylalanine**, causing homogentisic acid accumulation, which results in dark urine and ochronosis. *Decarboxylase* - The term "decarboxylase" refers to a general class of enzymes that remove a **carboxyl group** from a compound. - While decarboxylation reactions occur in many metabolic pathways, no single decarboxylase deficiency is directly responsible for **albinism**.

Q: Which of the following statements about Carbamoyl Phosphate Synthetase I (CPSI) is true?

N-Acetyl Glutamate is an allosteric activator of CPSI.. ***N-Acetyl Glutamate is an allosteric activator of CPSI.*** - **N-acetylglutamate (NAG)** is an essential allosteric activator for **Carbamoyl Phosphate Synthetase I (CPSI)**, signaling a high availability of arginine and a need for urea cycle activity. - Activation by NAG ensures that ammonia is incorporated into the urea cycle only when necessary, preventing its accumulation which can be toxic. *It is present in the cytoplasm.* - **CPSI** is located exclusively in the **mitochondrial matrix** of hepatocytes, where it initiates the urea cycle by converting ammonia to carbamoyl phosphate. - Its cytosolic counterpart, **CPSII**, is involved in pyrimidine synthesis, which differentiates their cellular locations and metabolic roles. *It is involved in pyrimidine synthesis.* - **CPSI** is the rate-limiting enzyme of the **urea cycle**, responsible for detoxifying ammonia by forming carbamoyl phosphate. - **Carbamoyl Phosphate Synthetase II (CPSII)**, located in the cytosol, is the enzyme involved in **pyrimidine synthesis**. *Glutamine is the amino group donor for CPSI.* - **CPSI** uses **free ammonia** (NH3) and **bicarbonate** as substrates to synthesize carbamoyl phosphate, not glutamine. - **Glutamine** is the nitrogen source for **Carbamoyl Phosphate Synthetase II (CPSII)** in pyrimidine synthesis.

Q: Where is Carbamoyl phosphate synthetase I located?

Located in the mitochondria. ***Located in the mitochondria*** - **Carbamoyl phosphate synthetase I (CPS I)** is a key enzyme in the **urea cycle**, which primarily occurs in the **mitochondrial matrix** of liver cells. - CPS I catalyzes the rate-limiting step of the urea cycle, combining ammonia and bicarbonate to form **carbamoyl phosphate**. *Located in the lysosome* - **Lysosomes** are cellular organelles responsible for waste breakdown and degradation, not for the synthesis of molecules like carbamoyl phosphate. - Enzymes involved in the urea cycle, such as CPS I, are not found in lysosomes. *Located in the cytosol* - While some steps of the urea cycle occur in the **cytosol** (e.g., argininosuccinate synthetase and lyase), the initial steps involving CPS I take place exclusively in the **mitochondria**. - CPS I is a mitochondrial enzyme, distinguishing it from **carbamoyl phosphate synthetase II (CPS II)**, which is cytosolic and involved in pyrimidine synthesis. *Located in all of the above* - CPS I is specifically located in the **mitochondria** and is not found in all cellular compartments mentioned. - Its specific mitochondrial location is crucial for its function in the urea cycle and regulation of ammonia detoxification.

Q: Which enzyme catalyzes the transfer of an α-amino group from aspartate to α-ketoglutarate?

Aspartate Transaminase (AST). ***Aspartate Transaminase (AST)*** - **Aspartate transaminase (AST)**, also known as **glutamic-oxaloacetic transaminase (GOT)**, specifically catalyzes the reversible transfer of an **α-amino group** from an L-amino acid (like **aspartate**) to an α-keto acid (like **α-ketoglutarate**). - This reaction forms a new amino acid and a new α-keto acid; in this case, **oxaloacetate** and **glutamate** are formed. *Ornithine transcarbamylase (OTC)* - **OTC** is an enzyme involved in the **urea cycle**, catalyzing the reaction between **ornithine** and **carbamoyl phosphate** to form citrulline. - It does not catalyze the transfer of an α-amino group from aspartate to α-ketoglutarate. *Argininosuccinate lyase (ASL)* - **ASL**, also part of the **urea cycle**, catalyzes the cleavage of **argininosuccinate** into **arginine** and **fumarate**. - Its function is distinct from transamination reactions involving α-amino and α-keto acids. *Alanine Aminotransferase (ALT)* - **ALT**, also known as **glutamic-pyruvic transaminase (GPT)**, specifically catalyzes the transfer of an **α-amino group** from **alanine** to **α-ketoglutarate**. - While it performs a similar type of transamination, it acts on alanine, not aspartate.

Q: Match the enzyme with the disease caused due to its deficiency: **Enzymes:** 1. Fumarylacetoacetate hydrolase 2. Tyrosine transaminase 3. Tyrosinase 4. Homogentisate oxidase **Diseases:** A. Tyrosinemia Type I B. Tyrosinemia Type II C. Albinism D. Alkaptonuria

1 → A, 2 → B, 3 → C, 4 → D. **1 → A, 2 → B, 3 → C, 4 → D** - **Fumarylacetoacetate hydrolase** deficiency causes **Tyrosinemia Type I**, a severe metabolic disorder affecting the liver and kidneys with accumulation of toxic metabolites like succinylacetone. - **Tyrosine transaminase** deficiency leads to **Tyrosinemia Type II** (Richner-Hanhart syndrome), characterized by ocular and cutaneous lesions with elevated plasma tyrosine. - **Tyrosinase** deficiency results in **Albinism** (oculocutaneous albinism type 1), due to lack of melanin production from tyrosine. - **Homogentisate oxidase** deficiency causes **Alkaptonuria**, where homogentisic acid accumulates, turning urine black upon standing and causing ochronosis. *1 → A, 2 → C, 3 → D, 4 → B* - This incorrectly matches tyrosine transaminase with Albinism and tyrosinase with Alkaptonuria, reversing the melanin synthesis pathway enzyme with the tyrosine degradation pathway enzyme. *1 → C, 2 → D, 3 → A, 4 → B* - This incorrectly matches fumarylacetoacetate hydrolase with Albinism (which requires tyrosinase deficiency) and homogentisate oxidase with Tyrosinemia Type II (which requires tyrosine transaminase deficiency). *1 → C, 2 → A, 3 → D, 4 → B* - This incorrectly places the severe hepatorenal disease (Tyrosinemia Type I) with tyrosine transaminase instead of fumarylacetoacetate hydrolase, and mismatches the melanin pathway enzyme with alkaptonuria.

Q: Which of the following amino acids is not converted to alpha-ketoglutarate during catabolism?

Glycine. ***Glycine*** - Glycine catabolism typically produces **serine**, which can then be converted to **pyruvate**, not alpha-ketoglutarate. - Its degradation pathway leads to the formation of **carbon dioxide** and **ammonia** via the glycine cleavage system (GCS), or it can be converted to serine and subsequently pyruvate. *Glutamate* - Glutamate is directly deaminated by **glutamate dehydrogenase** to form **alpha-ketoglutarate**, a key intermediate in the **Krebs cycle**. - This is a reversible reaction that plays a crucial role in nitrogen metabolism. *Histidine* - Histidine catabolism involves several steps, including deamination by **histidase**, eventually leading to the formation of **N-formiminoglutamate (FIGLU)**. - FIGLU is then converted to **glutamate**, which subsequently forms **alpha-ketoglutarate**. *Proline* - Proline is catabolized through a series of oxidation steps to yield **glutamate**. - This glutamate is then converted to **alpha-ketoglutarate** by glutamate dehydrogenase.

Question 1

What is the primary methyl donor in the conversion of norepinephrine to epinephrine?

Accepted Answer

S-adenosyl methionine

Answer

Arginine

Answer

Phenylalanine

Answer

Methionine

Question 2

In which condition is Vanillylmandelic Acid (VMA) excreted in urine?

Accepted Answer

Pheochromocytoma

Answer

Alkaptonuria

Answer

Neuroblastoma

Answer

Cushing's syndrome

Question 3

Amino acid metabolism is implicated in which disease?

Accepted Answer

Maple syrup urine disease

Answer

Reye's syndrome

Answer

Von Gierke's disease

Answer

McArdle's disease

Question 4

Albinism is due to deficiency of the following enzyme?

Accepted Answer

Tyrosinase

Answer

Phenylalanine hydroxylase

Answer

Homogentisic acid oxidase

Answer

Decarboxylase

Question 5

Which of the following statements about Carbamoyl Phosphate Synthetase I (CPSI) is true?

Accepted Answer

N-Acetyl Glutamate is an allosteric activator of CPSI.

Answer

It is present in the cytoplasm.

Answer

It is involved in pyrimidine synthesis.

Answer

Glutamine is the amino group donor for CPSI.

Question 6

Where is Carbamoyl phosphate synthetase I located?

Accepted Answer

Located in the mitochondria

Answer

Located in the lysosome

Answer

Located in the cytosol

Answer

Located in all of the above

Question 7

How does essential amino acid deficiency affect nitrogen balance?

Accepted Answer

Results in negative nitrogen balance due to decreased protein synthesis and increased protein degradation

Answer

Results in positive nitrogen balance due to compensatory mechanisms

Answer

No significant effect on nitrogen balance

Answer

Maintains nitrogen equilibrium through adaptive responses

Question 8

Which enzyme catalyzes the transfer of an α-amino group from aspartate to α-ketoglutarate?

Accepted Answer

Aspartate Transaminase (AST)

Answer

Ornithine transcarbamylase (OTC)

Answer

Argininosuccinate lyase (ASL)

Answer

Alanine Aminotransferase (ALT)

Question 9

Match the enzyme with the disease caused due to its deficiency:

**Enzymes:**
1. Fumarylacetoacetate hydrolase
2. Tyrosine transaminase
3. Tyrosinase
4. Homogentisate oxidase

**Diseases:**
A. Tyrosinemia Type I
B. Tyrosinemia Type II
C. Albinism
D. Alkaptonuria

Accepted Answer

1 → A, 2 → B, 3 → C, 4 → D

Answer

1 → A, 2 → C, 3 → D, 4 → B

Answer

1 → C, 2 → D, 3 → A, 4 → B

Answer

1 → C, 2 → A, 3 → D, 4 → B

Question 10

Which of the following amino acids is not converted to alpha-ketoglutarate during catabolism?

Accepted Answer

Glycine

Answer

Glutamate

Answer

Histidine

Answer

Proline

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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