Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 561: Which amino acid in Jowar is responsible for its pellagragenic effect?

A. Leucine (Correct Answer)
B. Lysine
C. Tryptophan
D. Methionine

Explanation: ***Leucine*** - A high intake of **leucine**, an essential amino acid, interferes with the metabolism of **tryptophan** and niacin, leading to **pellagra**. - Jowar (sorghum) contains high levels of leucine, which, when it forms a major part of the diet, can induce **niacin deficiency**. *Lysine* - Lysine is an essential amino acid and is generally considered to be in **limited supply** in many cereal grains, making it a desirable amino acid to increase in diets. - It does not directly contribute to the pellagragenic effect; rather, a deficiency in lysine can be a nutritional concern. *Tryptophan* - Tryptophan is a **precursor to niacin (Vitamin B3)** in the body; a deficiency in tryptophan can lead to pellagra. - The high leucine content in jowar interferes with the conversion of tryptophan to niacin, thus exacerbating niacin deficiency. *Methionine* - Methionine is an **essential sulfur-containing amino acid** important for various metabolic functions and protein synthesis. - It is not directly implicated in the pellagragenic effect associated with high jowar consumption.

Question 562: Beta-alanine is derived from ?

A. Adenosine
B. Uracil (Correct Answer)
C. Guanosine
D. Thymine

Explanation: ***Correct Option: Uracil*** - **Uracil**, a pyrimidine base found in RNA, is the primary source of **β-alanine** through its catabolic pathway. - The degradation sequence: **Uracil** → **Dihydrouracil** → **β-Ureidopropionate** → **β-Alanine** + CO₂ + NH₃ - This pathway is catalyzed by enzymes including dihydropyrimidine dehydrogenase and β-ureidopropionase. - **β-Alanine** is also obtained from dietary sources and is a component of carnosine and pantothenic acid (Vitamin B5). *Incorrect Option: Thymine* - **Thymine** is a pyrimidine base in DNA with a similar catabolic pathway to uracil. - However, thymine produces **β-aminoisobutyrate** (NOT β-alanine) as its final product. - The pathway: **Thymine** → **Dihydrothymine** → **β-Ureidoisobutyrate** → **β-Aminoisobutyrate** - The extra methyl group on thymine (compared to uracil) results in a different end product. *Incorrect Option: Adenosine* - **Adenosine** is a purine nucleoside composed of adenine and ribose. - Purine catabolism leads to **uric acid** formation, not β-alanine. - It follows the pathway: Adenosine → Inosine → Hypoxanthine → Xanthine → Uric acid. *Incorrect Option: Guanosine* - **Guanosine** is a purine nucleoside consisting of guanine and ribose. - Like adenosine, it is catabolized to **uric acid** via xanthine. - It does not participate in β-alanine synthesis.

Question 563: Which of the following is a non-essential amino acid?

A. Tyrosine (Correct Answer)
B. Phenylalanine
C. Lysine
D. Threonine

Explanation: ***Tyrosine*** - **Tyrosine** is considered a **non-essential amino acid** because the human body can synthesize it from the essential amino acid **phenylalanine**. - This synthesis occurs via the enzyme **phenylalanine hydroxylase**, making its dietary intake not strictly necessary if phenylalanine is available. *Phenylalanine* - **Phenylalanine** is an **essential amino acid**, meaning the human body **cannot synthesize it** and it must be obtained through the diet. - It serves as a precursor for various important molecules, including tyrosine, contributing to neurotransmitter synthesis. *Lysine* - **Lysine** is an **essential amino acid** that the human body **cannot synthesize** and must be acquired from dietary sources. - It plays a crucial role in **protein synthesis**, calcium absorption, and the production of hormones and enzymes. *Threonine* - **Threonine** is another example of an **essential amino acid** that the human body is **unable to produce** on its own. - It is important for the formation of **collagen** and elastin, and contributes to immune function.

Question 564: Transamination of Alanine results in formation of ?

A. Oxaloacetate
B. Pyruvate (Correct Answer)
C. Aspartate
D. Arginine

Explanation: **Pyruvate** ✓ - **Transamination** involves the transfer of an amino group from an amino acid to an α-ketoglutarate (catalyzed by aminotransferases). - When **alanine** undergoes transamination via **ALT (alanine aminotransferase)**, its amino group is transferred to α-ketoglutarate, forming glutamate, while alanine is converted to its corresponding α-keto acid, which is **pyruvate**. - Reaction: Alanine + α-Ketoglutarate ⇄ Pyruvate + Glutamate *Oxaloacetate* - **Oxaloacetate** is the α-keto acid formed from the transamination of **aspartate** (via AST/GOT). - It is a key intermediate in the **citric acid cycle** and gluconeogenesis, not a product of alanine transamination. *Aspartate* - **Aspartate** is an amino acid, not an α-keto acid. - It can be formed from oxaloacetate via transamination (reverse reaction), and is involved in the **urea cycle** and nucleotide synthesis. *Arginine* - **Arginine** is a semi-essential amino acid, not an α-keto acid or a product of alanine transamination. - It plays roles in **protein synthesis**, the urea cycle, and nitric oxide production.

Question 565: Which of the following enzymes is not involved in the urea cycle?

A. Arginase
B. Argininosuccinate lyase
C. CPS-II (Correct Answer)
D. CPS-I

Explanation: ***CPS-II*** - Carbamoyl phosphate synthetase II is involved in **pyrimidine synthesis**, not the urea cycle. - It uses **glutamine** as a nitrogen donor and is located in the **cytosol**. *CPS-I* - Carbamoyl phosphate synthetase I is the **rate-limiting enzyme** of the urea cycle. - It catalyzes the formation of **carbamoyl phosphate** from **ammonia**, CO2, and ATP in the mitochondria. *Arginase* - Arginase is the **final enzyme** in the urea cycle, converting **arginine** to **ornithine** and **urea**. - This reaction occurs in the cytosol and releases urea for excretion. *Argininosuccinate lyase* - Argininosuccinate lyase catalyzes the cleavage of **argininosuccinate** into **fumarate** and **arginine**. - This is a key step in regenerating arginine for the final step of the urea cycle.

Question 566: Which enzyme deficiency is responsible for Hyperammonemia type-1?

A. Arginase deficiency
B. Arginosuccinate lyase deficiency
C. Arginosuccinate synthase deficiency
D. Carbamoyl phosphate synthetase I (CPS-1) deficiency (Correct Answer)

Explanation: ***Carbamoyl phosphate synthetase I (CPS-1) deficiency*** - This enzyme deficiency is classified as **Hyperammonemia type-1**, or **CPS1 deficiency**, and results in the inability to initiate the urea cycle. - **CPS-1** catalyzes the first committed step of the urea cycle, combining ammonia and bicarbonate to form carbamoyl phosphate. *Arginase deficiency* - This deficiency causes **Hyperargininemia**, which is a disorder of the urea cycle distinct from Hyperammonemia type-1. - Arginase is involved in the final step of the urea cycle, converting arginine to urea and ornithine. *Arginosuccinate lyase deficiency* - This deficiency leads to **Argininosuccinic aciduria**, another urea cycle disorder. - **Arginosuccinate lyase** is responsible for breaking down argininosuccinate into arginine and fumarate. *Arginosuccinate synthase deficiency* - This deficiency causes **Citrullinemia type 1**, a metabolic disorder characterized by high levels of citrulline and ammonia. - **Arginosuccinate synthase** catalyzes the condensation of citrulline and aspartate to form argininosuccinate.

Question 567: Taurine is biosynthesized from which amino acid?

A. Cysteine (Correct Answer)
B. Valine
C. Arginine
D. Leucine

Explanation: ***Cysteine*** - **Taurine** is primarily synthesized from the amino acid **cysteine** through a pathway involving **cysteine sulfinic acid** and **hypotaurine**. - This pathway utilizes enzymes like **cysteine dioxygenase** and **cysteine sulfinic acid decarboxylase**. - The biosynthetic pathway: Cysteine → Cysteine sulfinic acid → Hypotaurine → Taurine. *Arginine* - **Arginine** is a precursor for **nitric oxide**, **urea**, and **creatine**, not taurine. - It is involved in various metabolic pathways, including the **urea cycle** and protein synthesis. *Valine* - **Valine** is a **branched-chain amino acid (BCAA)** involved in protein synthesis and energy production. - It is not a direct precursor for taurine biosynthesis. *Leucine* - **Leucine** is also a **branched-chain amino acid (BCAA)** crucial for protein synthesis and muscle metabolism. - It does not participate in the synthesis of taurine.

Question 568: In starvation, nitrogen is primarily carried from muscle to liver and kidney by which amino acid?

A. Alanine (Correct Answer)
B. Glycine
C. Aspartic acid
D. Asparagine

Explanation: ***Alanine*** - During starvation, muscles break down proteins, and the amino groups from these proteins are transferred to **pyruvate** to form **alanine** via the **glucose-alanine cycle (Cahill cycle)**. - **Alanine** is then released into the bloodstream and transported primarily to the **liver**, where its carbon skeleton can be used for **gluconeogenesis** and the amino group enters the urea cycle. - Note: While alanine is the primary carrier to the liver, **glutamine** is the main nitrogen carrier to the kidney. However, among the given options, alanine is unequivocally the correct answer. *Aspartic acid* - While aspartate is involved in amino group transfer and is a crucial component of the **urea cycle**, it is not the primary carrier for inter-organ nitrogen transport from muscle to liver during starvation. - Its role is more localized within the liver for the urea cycle rather than as a transport amino acid. *Glycine* - Glycine plays roles in various metabolic pathways, including synthesis of heme, purines, and conjugation reactions, but it is not the primary amino acid for carrying nitrogen from muscle to liver during starvation. - Its small size and simple structure make it less suitable for efficient nitrogen transport compared to alanine. *Asparagine* - Asparagine has a minor role in nitrogen transport but is not the primary carrier during starvation. - It is synthesized from **aspartate** and ammonia and is typically involved in protein synthesis and nitrogen storage in some tissues.

Question 569: Which will activate carbamoyl phosphate synthase I?

A. N-acetyl glutamate (Correct Answer)
B. ATP
C. Acetyl-CoA
D. Ornithine

Explanation: **N-acetyl glutamate** - **N-acetyl glutamate** is an **allosteric activator** of **carbamoyl phosphate synthase I (CPS I)**, which is the mitochondrial enzyme that catalyzes the first committed step of the **urea cycle**. - Its synthesis is stimulated by high levels of **arginine**, linking nitrogen load to urea production. *Acetyl-CoA* - Acetyl-CoA is a common **substrate** and **product** in various metabolic pathways, but it is not a direct activator of CPS I. - It is a precursor for the synthesis of **N-acetyl glutamate**, but does not activate CPS I directly. *Ornithine* - **Ornithine** is a key intermediate of the **urea cycle**, but it does not directly activate CPS I. - It combines with carbamoyl phosphate (the product of CPS I) in the second step of the urea cycle to form citrulline. *ATP* - **ATP** is a **substrate** used by CPS I to provide energy for the synthesis of carbamoyl phosphate. - While essential for the reaction, ATP itself does not act as an allosteric activator of the enzyme.

Question 570: Which amino acid requires ascorbic acid for its formation in the body?

A. Lysine
B. Hydroxyproline (Correct Answer)
C. Cysteine
D. Proline

Explanation: ***Hydroxyproline*** - **Ascorbic acid (Vitamin C)** is an essential cofactor for **prolyl hydroxylase** and **lysyl hydroxylase** enzymes - These enzymes catalyze the **post-translational hydroxylation** of proline and lysine residues within collagen chains to form hydroxyproline and hydroxylysine - This hydroxylation is crucial for **stabilization of the collagen triple helix** structure - Hydroxyproline is formed by **modification of proline after incorporation into collagen**, not as a free amino acid - **Scurvy** (Vitamin C deficiency) results in defective collagen due to inadequate hydroxyproline formation *Lysine* - Lysine is an **essential amino acid** obtained from diet - Does not require ascorbic acid for its synthesis or formation - While lysine residues in collagen can be hydroxylated (forming hydroxylysine), the question asks about the amino acid whose formation requires Vitamin C *Cysteine* - Cysteine is a **sulfur-containing amino acid** synthesized from methionine via transsulfuration pathway - Its synthesis does not involve ascorbic acid *Proline* - Proline is a **non-essential amino acid** synthesized from glutamate - **Proline synthesis does not require ascorbic acid** - Proline serves as the precursor that gets hydroxylated to hydroxyproline within collagen

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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