Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 611: Out of the following, which is a substrate for the rate-limiting enzyme of polyamine biosynthesis?

A. Anandamide
B. Cadaverine
C. Ornithine (Correct Answer)
D. Histidine

Explanation: ***Ornithine*** - **Ornithine decarboxylase (ODC)** is the **rate-limiting enzyme** in polyamine biosynthesis, and it uses **ornithine** as its primary substrate. - ODC catalyzes the decarboxylation of **ornithine** to produce **putrescine**, the first polyamine in the pathway. *Anandamide* - **Anandamide** is an **endocannabinoid**, a lipid-based neurotransmitter, and is not involved in polyamine biosynthesis. - Its synthesis involves fatty acid amide hydrolase, a completely different biochemical pathway. *Cadaverine* - **Cadaverine** is a **diamine** produced by the decarboxylation of **lysine**, often associated with putrefaction, and is not a substrate for polyamine biosynthesis's rate-limiting enzyme. - It is distinct from the polyamines produced via the ornithine pathway. *Histidine* - **Histidine** is an **amino acid** that is a precursor for **histamine**, a biogenic amine involved in immune responses. - It is decarboxylated by **histidine decarboxylase**, not ornithine decarboxylase, and is not part of polyamine synthesis.

Question 612: Hydrolysis of which of the following compounds yields urea?

A. Ornithine
B. Argininosuccinate
C. Aspartate
D. Arginine (Correct Answer)

Explanation: ***Arginine*** - **Arginine** is directly hydrolyzed by the enzyme **arginase** to yield **urea** and ornithine. - This reaction is the final step of the **urea cycle**, responsible for detoxifying ammonia in the body. *Ornithine* - **Ornithine** is a product of arginine hydrolysis, not a compound that yields urea upon hydrolysis. - It participates in the urea cycle as a carrier molecule, combining with carbamoyl phosphate to form citrulline. *Argininosuccinate* - **Argininosuccinate** is an intermediate in the urea cycle, formed from citrulline and aspartate. - It is cleaved to form arginine and fumarate, rather than directly yielding urea through hydrolysis. *Aspartate* - **Aspartate** provides one of the nitrogen atoms for urea synthesis by condensing with citrulline to form argininosuccinate. - It does not directly undergo hydrolysis to produce urea.

Question 613: Melatonin is derived from which amino acid?

A. Tyrosine
B. Tryptophan (Correct Answer)
C. Phenylalanine
D. No amino acid is involved.

Explanation: ***Tryptophan*** - **Melatonin** is synthesized from the amino acid **tryptophan** through a series of enzymatic steps involving serotonin as an intermediate. - Tryptophan is an **essential amino acid** obtained from the diet, making its availability crucial for melatonin production. - Synthesis pathway: Tryptophan → 5-Hydroxytryptophan → Serotonin → N-Acetylserotonin → Melatonin *Tyrosine* - **Tyrosine** is a precursor for catecholamines (**dopamine**, **norepinephrine**, **epinephrine**) and thyroid hormones. - It is not involved in melatonin synthesis. *Phenylalanine* - **Phenylalanine** is an essential aromatic amino acid that serves as a precursor to **tyrosine**. - It is not directly involved in the synthesis of melatonin. *No amino acid is involved.* - This statement is incorrect because melatonin is explicitly derived from the amino acid **tryptophan**. - Its biosynthesis pathway clearly demonstrates the involvement of amino acids as building blocks.

Question 614: Which of the following amino acids directly contributes to the formation of Succinyl CoA?

A. Valine (Correct Answer)
B. Histidine
C. Lysine
D. Leucine

Explanation: ***Valine*** - **Valine** is a branched-chain amino acid that is catabolized to **propionyl CoA**, which is then converted to **methylmalonyl CoA** and finally to **succinyl CoA**. - This makes valine a **direct precursor** to succinyl CoA through the propionyl CoA pathway. - **Succinyl CoA** is an important intermediate in the **citric acid cycle** and can also be used for **gluconeogenesis**. *Histidine* - **Histidine** degradation ultimately forms **formiminoglutamate (FIGLU)**, which is then converted to **glutamate**. - Glutamate can enter the citric acid cycle as **α-ketoglutarate**, which is subsequently converted to succinyl CoA in the TCA cycle. - However, histidine does **not directly** form succinyl CoA through its own catabolic pathway. *Lysine* - **Lysine** is a purely **ketogenic amino acid**, meaning its catabolism primarily produces **acetyl CoA** and **acetoacetate**. - It does not directly contribute to the formation of **succinyl CoA**. *Leucine* - **Leucine** is also a purely **ketogenic amino acid**, like lysine. - Its degradation yields **acetyl CoA** and **acetoacetate**, and it does not form **succinyl CoA**.

Question 615: The limiting amino acid in maize is

A. Phenylalanine
B. Tryptophan
C. Lysine (Correct Answer)
D. Cysteine

Explanation: ***Lysine*** - **Lysine** is the **primary limiting amino acid** in maize (corn). - This means that maize contains a relatively low amount of lysine compared to the body's requirements, which limits the overall protein quality and biological value when maize is the sole protein source. - **Quality Protein Maize (QPM)**, developed through breeding programs, has significantly increased levels of **lysine** and **tryptophan** to improve nutritional quality. - Lysine deficiency in maize-based diets can lead to **pellagra** when combined with low niacin intake, as tryptophan (also deficient) is a precursor for niacin synthesis. *Tryptophan* - **Tryptophan** is the **second limiting amino acid** in maize, but not the primary one. - While maize is relatively low in tryptophan, the lysine deficiency is more pronounced and nutritionally significant. - Tryptophan is important as a precursor for **niacin (vitamin B3)**, **serotonin**, and **melatonin**. *Phenylalanine* - **Phenylalanine** is an **essential amino acid** but is generally present in sufficient quantities in maize and is **not considered a limiting factor**. - It serves as a precursor for **tyrosine**, **dopamine**, **epinephrine**, and **norepinephrine**. *Cysteine* - **Cysteine** is a **non-essential (conditionally essential) amino acid** in humans, as the body can synthesize it from methionine. - Therefore, it is **not considered a limiting amino acid** for dietary protein quality assessment.

Question 616: Most common enzyme deficiency in the urea cycle is:

A. Ornithine transcarbamoylase (OTC) deficiency (Correct Answer)
B. Arginase deficiency
C. Carbamoyl phosphate synthase I deficiency
D. Argininosuccinate synthetase deficiency

Explanation: ***Ornithine transcarbamoylase (OTC)*** - **OTC deficiency** is the most common and often the most severe inherited disorder of the **urea cycle**, leading to a buildup of ammonia. - It is an **X-linked recessive** disorder, predominantly affecting males, though carrier females can also exhibit symptoms. *Arginase deficiency* - This deficiency affects the final step of the urea cycle, leading to the accumulation of **arginine** and its precursors. - It is less common than OTC deficiency and typically presents with a later onset and milder symptoms. *Carbamoyl phosphate synthase I deficiency* - **CPS I deficiency** is a severe form of urea cycle disorder but is less common than OTC deficiency. - It results in the inability to synthesize **carbamoyl phosphate**, a crucial substrate for the urea cycle, leading to severe hyperammonemia. *Argininosuccinate synthetase deficiency* - This deficiency, also known as **citrullinemia type I**, leads to the accumulation of **citrulline** in the blood. - While it is a significant urea cycle disorder, it is not as frequently encountered as OTC deficiency.

Question 617: A 32-year-old male is on a weight-maintenance diet, so he does not want to lose or gain any weight. Which amino acid must be present in the diet to prevent the patient from going into a negative nitrogen balance?

A. Alanine
B. Arginine
C. Glycine
D. Threonine (Correct Answer)

Explanation: ***Threonine*** - **Threonine** is an **essential amino acid**, meaning the body cannot synthesize it and it must be obtained from the diet. - To maintain a **neutral nitrogen balance** and prevent a **negative nitrogen balance** (loss of body protein), all essential amino acids, including threonine, must be supplied in adequate amounts. *Alanine* - **Alanine** is a **non-essential amino acid**, which means the body can synthesize it from other compounds; therefore, its absence from the diet would not directly cause a negative nitrogen balance. - It plays a significant role in **gluconeogenesis** and the **glucose-alanine cycle**. *Arginine* - **Arginine** is considered a **conditionally essential amino acid**, meaning it can be synthesized by the body, but sometimes not in sufficient amounts to meet needs (e.g., during rapid growth, illness, or trauma). - Under normal weight-maintenance conditions, the body can typically synthesize enough arginine. *Glycine* - **Glycine** is a **non-essential amino acid** and is the smallest amino acid, often easily synthesized by the body. - Its presence in the diet, while important, is not critical for preventing negative nitrogen balance because the body can produce it.

Question 618: Amino acid carrying ammonia from muscle to liver in the glucose-alanine cycle is?

A. Alanine (Correct Answer)
B. Lysine
C. Glutamine
D. Arginine

Explanation: ***Alanine*** - **Alanine** is a key amino acid involved in the **glucose-alanine cycle**, acting as a non-toxic carrier of **ammonia** and carbon skeletons from muscle to the liver. - In muscle, **pyruvate** is transaminated to alanine using ammonia, which then travels to the liver, where it is converted back to pyruvate for **gluconeogenesis**, and the ammonia is disposed of via the **urea cycle**. *Glutamine* - While **glutamine** is also a major transporter of **ammonia**, it primarily carries ammonia from various tissues (including muscle) to the **kidneys** for excretion and to the **liver** for the urea cycle, but alanine is more prominent in the **glucose-alanine cycle** specific to muscle. - Glutamine synthetase incorporates ammonia into glutamate to form glutamine, which then releases ammonia in the liver or kidneys. *Lysine* - **Lysine** is an essential amino acid with a role in protein synthesis and carnitine synthesis, but it does **not** serve as a primary carrier of **ammonia** between muscle and liver. - It is exclusively **ketogenic**, meaning its breakdown products can form ketone bodies. *Arginine* - **Arginine** is a conditionally essential amino acid and a key component of the **urea cycle** itself (in the liver), but it does not primarily transport **ammonia** from muscle to liver. - It is synthesized in the urea cycle and is a precursor for **nitric oxide** production.

Question 619: For the conversion of aspartate to asparagine, nitrogen comes from which source?

A. Alanine
B. Glutamate
C. Glutamine (Correct Answer)
D. Histidine

Explanation: ***Glutamine*** - The biosynthesis of **asparagine** from **aspartate** is catalyzed by **asparagine synthetase**. - This enzyme utilizes **ATP** and **glutamine** as the amino group donor, with glutamine being hydrolyzed to **glutamate**. *Alanine* - **Alanine** is primarily involved in the **glucose-alanine cycle** for transporting nitrogen from muscle to liver. - It does not directly donate its amino group for the synthesis of asparagine. *Glutamate* - While **glutamate** is a precursor for glutamine, it does not directly donate an amino group in the conversion of aspartate to asparagine. - Glutamate acts as a general amino group donor in many transamination reactions, but not in this specific amidation. *Histidine* - **Histidine** is an essential amino acid involved in various metabolic roles, including acting as a precursor for histamine. - It is not a donor of nitrogen for the biosynthesis of asparagine from aspartate.

Question 620: Which amino acid is derived from arginine and is important for the formation of neutrophilic extracellular traps (NETs)?

A. Leucine
B. Methionine
C. Citrulline (Correct Answer)
D. Valine

Explanation: ***Citrulline*** - **Citrulline** is crucial for the formation of neutrophilic extracellular traps (NETs) [1] as it plays a role in the **post-translational modification** of histones, which is essential for uncoiling DNA. - Its metabolism is directly linked to the production of **neutrophil-derived reactive oxygen species (ROS)**, further supporting NET formation. *Methionine* - While **methionine** is an important amino acid, it is primarily involved in **protein synthesis** and **methylation processes**, not directly in NET formation. - It does not play a significant role in the **nuclear processes** required for the production of NETs. *Valine* - **Valine** is a branched-chain amino acid primarily involved in **energy metabolism** and muscle repair, and does not directly contribute to NET formation. - It lacks a specific role in the **activation of neutrophils** or the structural aspects of NET development. *Leucine* - **Leucine** is another branched-chain amino acid that plays a key role in **protein synthesis**, particularly in muscle tissue, rather than in **immune response mechanisms**. - Its function does not include the direct involvement in the mechanisms of **NET formation** by neutrophils. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, pp. 91-92.

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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