Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 471: Which of these is a key component of the urea cycle in the liver?

A. Citrulline (Correct Answer)
B. Glycogen
C. Fructose
D. Lactate

Explanation: ***Citrulline*** - **Citrulline** is an amino acid that plays a crucial role as an intermediate in the **urea cycle**, being formed from ornithine and carbamoyl phosphate in the mitochondria. - It is transported out of the mitochondria into the cytoplasm, where it is further converted to argininosuccinate. - This makes it a **key component** of ammonia detoxification. *Glycogen* - **Glycogen** is a polysaccharide that serves as the primary **storage form of glucose** in animals and fungi. - Its metabolism is involved in energy regulation but is **not a component of the urea cycle**. *Fructose* - **Fructose** is a simple sugar, a **monosaccharide**, found in many fruits and honey. - It is primarily metabolized in the liver via fructolysis, but it is **not directly involved in the urea cycle**. *Lactate* - **Lactate** is produced from pyruvate during anaerobic glycolysis and is involved in the **Cori cycle** for gluconeogenesis. - While it is metabolized in the liver, it is **not a component of the urea cycle**.

Question 472: Which of the following is not an amino acid-derived neurotransmitter?

A. Dopamine
B. GABA
C. Serotonin
D. Creatine (Correct Answer)

Explanation: ***Creatine*** - **Creatine** is an organic compound that helps supply energy to cells, primarily muscle cells, but it does **not** function as a neurotransmitter. - It is synthesized from **amino acids** like arginine, glycine, and methionine, but it is not itself an amino acid-derived neurotransmitter. *Dopamine* - **Dopamine** is a **catecholamine neurotransmitter** derived from the amino acid **tyrosine**. - It plays crucial roles in **motivation**, reward, motor control, and various other brain functions. *GABA* - **GABA** (gamma-aminobutyric acid) is the **primary inhibitory neurotransmitter** in the central nervous system, derived from the amino acid **glutamate**. - It works to reduce neuronal excitability throughout the nervous system. *Serotonin* - **Serotonin** (5-hydroxytryptamine or 5-HT) is a **monoamine neurotransmitter** derived from the amino acid **tryptophan**. - It regulates mood, appetite, sleep, and numerous other physiological processes.

Question 473: Which enzyme deficiency leads to hyperhomocysteinemia?

A. Phenylalanine hydroxylase
B. Glucose-6-phosphate dehydrogenase
C. Galactose-1-phosphate uridyltransferase
D. Methionine synthase (Correct Answer)

Explanation: ***Methionine synthase*** - **Methionine synthase** (also known as homocysteine methyltransferase) is critical for converting **homocysteine** back to **methionine** using **methyltetrahydrofolate (MTHF)** as a methyl donor. - A deficiency in this enzyme, or its cofactors like **vitamin B12** or **folate**, leads to the accumulation of homocysteine in the blood, resulting in **hyperhomocysteinemia**. *Phenylalanine hydroxylase* - Deficiency in **phenylalanine hydroxylase** causes **phenylketonuria (PKU)**, characterized by the accumulation of **phenylalanine** and its metabolites, not homocysteine. - This enzyme is responsible for converting **phenylalanine** to **tyrosine**. *Glucose-6-phosphate dehydrogenase* - Deficiency in **glucose-6-phosphate dehydrogenase (G6PD)** leads to **hemolytic anemia** in response to oxidative stress, as it impairs the production of **NADPH** in the **pentose phosphate pathway**. - It is not directly involved in homocysteine metabolism. *Galactose-1-phosphate uridyltransferase* - Deficiency in **galactose-1-phosphate uridyltransferase** is the cause of **classic galactosemia**, leading to the accumulation of **galactose-1-phosphate** and toxicity. - This enzyme is crucial for the metabolism of **galactose**, not homocysteine.

Question 474: Which enzyme deficiency causes Maple Syrup Urine Disease?

A. Glucose-6-phosphatase
B. Branched-chain ketoacid dehydrogenase (Correct Answer)
C. Phenylalanine hydroxylase
D. Pyruvate dehydrogenase

Explanation: ***Branched-chain ketoacid dehydrogenase*** - **Maple Syrup Urine Disease (MSUD)** is caused by deficiency in the branched-chain α-keto acid dehydrogenase complex (BCKDC) - This enzyme is crucial for metabolism of the **branched-chain amino acids (BCAAs)**: leucine, isoleucine, and valine - Accumulation of BCAAs and their keto acids gives urine a characteristic **maple syrup odor** - Presents with **neurological symptoms, ketoacidosis, and intellectual disability** if untreated *Glucose-6-phosphatase* - Deficiency causes Glycogen Storage Disease Type I (von Gierke disease), not MSUD - This enzyme is essential for the final step of gluconeogenesis and glycogenolysis - Leads to hypoglycemia and lactic acidosis *Phenylalanine hydroxylase* - Deficiency causes phenylketonuria (PKU), not MSUD - This enzyme converts phenylalanine to tyrosine - Absence leads to toxic accumulation of phenylalanine causing intellectual disability and musty odor *Pyruvate dehydrogenase* - Deficiency affects conversion of pyruvate to acetyl-CoA, not branched-chain amino acid metabolism - Leads to buildup of pyruvate shunted to lactate - Causes lactic acidosis and neurological symptoms

Question 475: What is the primary source of ammonia in the body?

A. Carbohydrate metabolism
B. Lipid metabolism
C. Amino acid catabolism (Correct Answer)
D. Purine degradation

Explanation: ***Amino acid catabolism*** - The **deamination** of amino acids, particularly in the liver, is the primary metabolic process that removes the amino group and releases **ammonia (NH3)**. - This ammonia is then converted to **urea** via the urea cycle for excretion. *Carbohydrate metabolism* - This process primarily yields **ATP**, carbon dioxide, and water from sugars. - While some intermediates may contain nitrogen, it does not directly produce significant amounts of **ammonia**. *Lipid metabolism* - The breakdown of fats mainly produces **fatty acids** and glycerol, which are then metabolized for energy. - **Ammonia** is not a direct or significant end-product of lipid breakdown. *Purine degradation* - The degradation of **purine nucleotides** (adenine and guanine) produces **uric acid** as the main nitrogenous waste product in humans. - While ammonia can be formed from adenosine deamination, it contributes a relatively small amount compared to **amino acid catabolism**.

Question 476: Which of the following enzymes is not involved in the urea cycle?

A. Ornithine transcarbamylase
B. Carbamoyl phosphate synthetase I
C. Glucose-6-phosphatase (Correct Answer)
D. Arginase

Explanation: ***Glucose-6-phosphatase*** - This enzyme is crucial in **gluconeogenesis** and **glycogenolysis**, catalyzing the final step in the release of glucose from the liver into the bloodstream. It is not part of the **urea cycle**. - Its primary role is to convert **glucose-6-phosphate** to glucose, which is essential for maintaining blood glucose levels. *Ornithine transcarbamylase* - This enzyme is an integral part of the **urea cycle**, catalyzing the reaction of **carbamoyl phosphate** and **ornithine** to form citrulline. - A deficiency in ornithine transcarbamylase leads to **hyperammonemia**, as ammonia cannot be properly converted to urea. *Carbamoyl phosphate synthetase I* - This enzyme is the **rate-limiting step** of the urea cycle, responsible for the synthesis of **carbamoyl phosphate** from ammonia, bicarbonate, and ATP. - It commits ammonia to the urea cycle, making it essential for **detoxification** of nitrogenous waste. *Arginase* - Arginase is the final enzyme in the **urea cycle**, converting **arginine** into **ornithine** and urea. - This reaction releases urea, which is then excreted from the body, and regenerates ornithine to continue the cycle.

Question 477: In the urea cycle, which enzyme catalyzes the formation of argininosuccinate from citrulline and aspartate?

A. Carbamoyl phosphate synthetase I
B. Ornithine transcarbamylase
C. Argininosuccinate synthetase (Correct Answer)
D. Arginase

Explanation: ***Argininosuccinate synthetase*** - This enzyme catalyzes the **ATP-dependent condensation** of **citrulline** and **aspartate** to form **argininosuccinate**. - It is a crucial step in the urea cycle, serving the primary purpose of incorporating the second nitrogen atom (from aspartate) into the urea molecule. *Carbamoyl phosphate synthetase I* - This enzyme catalyzes the **first committed step** of the urea cycle, forming **carbamoyl phosphate** from ammonia, carbon dioxide, and ATP. - It is located in the mitochondria and is the **rate-limiting step** of the urea cycle. *Ornithine transcarbamylase* - This enzyme catalyzes the reaction where **carbamoyl phosphate** condenses with **ornithine** to form **citrulline**. - It acts after carbamoyl phosphate synthetase I in the mitochondrial matrix. *Arginase* - This enzyme catalyzes the **final step** of the urea cycle, hydrolyzing **arginine** into **urea** and **ornithine**. - The ornithine is then regenerated and re-enters the cycle.

Question 478: A patient presents with diarrhea, dermatitis, and dementia. Which amino acid metabolism defect is likely responsible?

A. Phenylketonuria
B. Homocystinuria
C. Hartnup disease (Correct Answer)
D. Maple syrup urine disease

Explanation: ***Hartnup disease*** - This condition is caused by a defect in the **tryptophan absorption** from the diet and reabsorption in the kidneys, leading to **tryptophan deficiency**. - Tryptophan is a precursor for **niacin (vitamin B3)**, and its deficiency results in pellagra-like symptoms: **diarrhea, dermatitis, and dementia**. *Phenylketonuria* - This is an inability to metabolize **phenylalanine**, leading to its accumulation and causing **intellectual disability, seizures, and hypopigmentation**. - It does not present with the "3 D's" (diarrhea, dermatitis, dementia). *Homocystinuria* - This disorder involves the metabolism of **methionine**, leading to the accumulation of **homocysteine**, which affects connective tissue, muscles, the central nervous system, and the cardiovascular system. - Clinical features include **ectopia lentis**, developmental delay, and **thrombosis**, not the symptoms described. *Maple syrup urine disease* - This condition involves a defect in the metabolism of **branched-chain amino acids** (leucine, isoleucine, valine), leading to their accumulation. - It causes **poor feeding, vomiting, lethargy, and a characteristic sweet smell** in urine, but not diarrhea, dermatitis, or dementia.

Question 479: A 2-month-old infant presents with vomiting, lethargy, and metabolic acidosis. Blood tests reveal elevated levels of ammonia. What is the most likely diagnosis?

A. Urea Cycle Disorder (Correct Answer)
B. Phenylketonuria
C. Galactosemia
D. Maple Syrup Urine Disease

Explanation: ***Urea Cycle Disorder*** - The combination of **vomiting**, **lethargy**, metabolic acidosis, and **elevated ammonia** in a 2-month-old infant is highly indicative of a urea cycle disorder due to the impaired detoxification of ammonia. - **Ammonia is neurotoxic**, explaining the lethargy, and its accumulation leads to severe metabolic derangements. *Phenylketonuria* - Characterized by the inability to metabolize **phenylalanine**, leading to its accumulation and neurological damage. - While it can cause developmental delay and seizures, it typically does **not present with acute metabolic acidosis** or hyperammonemia in infancy. *Galactosemia* - An inherited disorder of **galactose metabolism** that can cause vomiting, lethargy, and liver dysfunction, but is typically associated with **jaundice**, hepatomegaly, and **reducing substances in urine**, not primarily hyperammonemia and metabolic acidosis as the presenting features. - The primary defect is in the conversion of galactose to glucose, not ammonia detoxification. *Maple Syrup Urine Disease* - A rare metabolic disorder caused by a defect in the metabolism of **branched-chain amino acids** (leucine, isoleucine, valine). - Presents with feeding difficulties, lethargy, and a characteristic **sweet-smelling urine** ("maple syrup" odor), which is not mentioned in this case.

Question 480: What is the primary function of the urea cycle in humans?

A. To synthesize amino acids
B. To degrade amino acids
C. To convert ammonia to urea (Correct Answer)
D. To break down carbohydrates

Explanation: ***To convert ammonia to urea*** - The primary function of the urea cycle is to safely dispose of **toxic ammonia**, a byproduct of amino acid catabolism, by converting it into **urea**. - Urea is then transported to the kidneys for excretion in the urine. *To synthesize amino acids* - Amino acid synthesis occurs through various metabolic pathways, but it is not the main role of the **urea cycle**. - The urea cycle is specifically designed for the **detoxification of nitrogenous waste**. *To degrade amino acids* - Amino acid degradation, or **catabolism**, occurs through processes like deamination, which generates ammonia. - The urea cycle then processes this ammonia, rather than directly degrading the amino acids themselves. *To break down carbohydrates* - Carbohydrate breakdown, known as **glycolysis** and the **Krebs cycle**, is central to energy production. - This process is distinct from the urea cycle, which focuses on nitrogenous waste management.

Practice by Chapter

Protein Digestion and Absorption

Practice Questions

Transamination and Deamination

Practice Questions

Urea Cycle

Practice Questions

Disorders of Urea Cycle

Practice Questions

Metabolism of Individual Amino Acids

Practice Questions

Inborn Errors of Amino Acid Metabolism

Practice Questions

Phenylketonuria and Alkaptonuria

Practice Questions

Homocystinuria and Methionine Metabolism

Practice Questions

Synthesis of Biologically Important Compounds from Amino Acids

Practice Questions

Nitrogen Balance

Practice Questions

Ammonia Metabolism and Toxicity

Practice Questions

One-Carbon Transfer Reactions

Practice Questions

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