Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 571: Which enzyme is deficient in Isovaleric acidemia?

A. Isovaleryl CoA dehydrogenase (Correct Answer)
B. Phenylalanine hydroxylase
C. Arginase
D. Methylmalonyl CoA mutase

Explanation: ***Isovaleryl CoA dehydrogenase*** - **Isovaleric acidemia** is an **autosomal recessive** metabolic disorder caused by a deficiency in the enzyme **isovaleryl-CoA dehydrogenase** - This enzyme is crucial for the metabolism of **leucine**, a branched-chain amino acid, leading to the accumulation of toxic byproducts like **isovaleryl-CoA** and **isovaleric acid** - Characteristic **sweaty feet odor** due to isovaleric acid accumulation *Phenylalanine hydroxylase* - A deficiency in **phenylalanine hydroxylase** is responsible for **phenylketonuria (PKU)**, a different metabolic disorder involving the metabolism of **phenylalanine** - This enzyme converts **phenylalanine to tyrosine**, and its deficiency leads to the accumulation of phenylalanine and its metabolites, causing neurological damage if untreated *Arginase* - A deficiency in **arginase** causes **argininemia (hyperargininemia)**, which is a disorder of the **urea cycle** - This enzyme converts **arginine into urea and ornithine**, and its deficiency leads to the buildup of arginine and ammonia in the blood, causing neurological symptoms and developmental delay *Methylmalonyl CoA mutase* - A deficiency in **methylmalonyl CoA mutase** causes **methylmalonic acidemia**, another organic acidemia distinct from isovaleric acidemia - This disorder involves **propionate metabolism** and can present with metabolic acidosis, but affects a different metabolic pathway than leucine catabolism

Question 572: Which of the following substances is not derived from tyrosine?

A. Thyroxine
B. Melanin
C. Nicotinic acid (Correct Answer)
D. Dopamine

Explanation: ***Nicotinic acid*** - **Nicotinic acid** (niacin, vitamin B3) is synthesized from **tryptophan** in the body, not tyrosine. - It plays a crucial role in metabolism as a precursor for NAD+ and NADH, which are involved in various enzymatic reactions. *Thyroxine* - **Thyroxine** (T4), a thyroid hormone, is derived from the amino acid **tyrosine**. - **Iodine** is incorporated into tyrosine residues on thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT), which then couple to form T4 (and T3). *Melanin* - **Melanin**, the pigment responsible for skin, hair, and eye color, is synthesized from **tyrosine** through a pathway involving the enzyme **tyrosinase**. - This process involves the hydroxylation of tyrosine to 3,4-dihydroxyphenylalanine (DOPA) and subsequent oxidation reactions. *Dopamine* - **Dopamine**, an important neurotransmitter, is synthesized from **tyrosine** in a two-step process in the brain and adrenal medulla. - Tyrosine is first hydroxylated to DOPA by **tyrosine hydroxylase**, and then DOPA is decarboxylated to dopamine by DOPA decarboxylase.

Question 573: Methionine can enter the TCA cycle at which level?

A. Fumarate
B. Oxaloacetate
C. Succinyl-CoA (Correct Answer)
D. Citrate

Explanation: ***Succinyl - CoA*** - Methionine is a **glucogenic amino acid** that is catabolized to propionyl-CoA, which is then converted to **methylmalonyl-CoA** and finally to **succinyl-CoA**. - **Succinyl-CoA** is an intermediate of the **TCA cycle**, allowing methionine-derived carbons to enter the cycle. *Fumarate* - Fumarate is an intermediate of the TCA cycle, but methionine catabolism does not directly produce **fumarate**. - Amino acids like **phenylalanine** and **tyrosine** can be catabolized to fumarate. *Oxaloacetate* - **Oxaloacetate** is a TCA cycle intermediate and can be formed from **pyruvate** (via pyruvate carboxylase) or from certain amino acids like **aspartate** and **asparagine**. - Methionine does not directly convert to oxaloacetate. *Citrate* - **Citrate** is the first intermediate formed in the TCA cycle when **acetyl-CoA** combines with **oxaloacetate**. - Methionine catabolism does not lead to the direct formation of citrate.

Question 574: Hydrolysis occurs at which step of urea cycle ?

A. Formation of ornithine
B. Formation of argininosuccinate
C. Formation of citrulline
D. Cleavage of arginine (Correct Answer)

Explanation: ***Cleavage of arginine*** - The final step in the urea cycle, where **arginine** is hydrolyzed by the enzyme **arginase** to form **urea** and **ornithine**. - This reaction involves the addition of a **water molecule** across the guanidino group to release urea. *Formation of argininosuccinate* - This step involves the condensation of **citrulline** and **aspartate**, catalyzed by **argininosuccinate synthetase**. - It is an **ATP-dependent** reaction, not a hydrolysis. *Formation of citrulline* - Occurs when **carbamoyl phosphate** condenses with **ornithine**, catalyzed by **ornithine transcarbamylase**. - This reaction involves the removal of a phosphate group, not the addition of water. *Formation of ornithine* - **Ornithine** is a substrate for the formation of citrulline and is also regenerated at the end of the cycle from arginine. - Its formation from arginine is a **hydrolysis** reaction, but simply stating "formation of ornithine" is less specific than "cleavage of arginine," which directly describes the hydrolytic event.

Question 575: Carnitine is synthesised from -

A. Lysine (Correct Answer)
B. Histidine
C. Choline
D. Arginine

Explanation: ***Lysine*** - **Carnitine** is synthesized in the liver and kidneys from the amino acids **lysine** and methionine. - **Lysine provides the essential carbon backbone** for carnitine synthesis (trimethyllysine is the actual precursor formed from protein-bound lysine residues). - Methionine contributes methyl groups via S-adenosylmethionine (SAM), but lysine is the primary structural precursor. *Arginine* - **Arginine** is a precursor for **nitric oxide**, urea, and creatine, but not a direct precursor for carnitine synthesis. - While arginine is an amino acid, its metabolic pathways are distinct from those involved in carnitine formation. *Histidine* - **Histidine** is a precursor for **histamine** and contributes to protein synthesis, but is not involved in carnitine biosynthesis. - Its metabolic fate differs significantly from the pathway leading to carnitine. *Choline* - **Choline** is a precursor for **acetylcholine** and phospholipids, but not directly for carnitine. - Although both choline and carnitine contain methyl groups, they have different biosynthetic origins.

Question 576: Which amino acid is used by the liver in the urea cycle?

A. Glutamine
B. Glutamate
C. Aspartate (Correct Answer)
D. Ornithine

Explanation: ***Aspartate*** - **Aspartate** provides the second nitrogen atom to the urea cycle, directly contributing to the formation of **argininosuccinate** through condensation with citrulline. - It is crucial for the efficient removal of **ammonia** in the form of urea. *Glutamine* - **Glutamine** transports ammonia from peripheral tissues to the liver and kidneys, but it is typically deamidated to **glutamate** before its nitrogen can enter the urea cycle. - While it's a major ammonia carrier, it's not directly incorporated into urea as an intact amino acid. *Glutamate* - **Glutamate** can donate its amino group to form **aspartate** (via transamination with oxaloacetate) or release ammonia directly (via glutamate dehydrogenase), both of which then enter the urea cycle. - However, glutamate itself is not directly incorporated into the urea molecule in the same way aspartate is. *Ornithine* - **Ornithine** is an amino acid that participates in the urea cycle as a carrier molecule, being regenerated at the end of each cycle. - While essential for the cycle to function, it is not "used" in the sense of being consumed or providing nitrogen for urea formation - rather it acts as a catalytic intermediate that is recycled.

Question 577: Indole ring is present in?

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

Explanation: ***Tryptophan*** - Tryptophan is an **aromatic amino acid** characterized by the presence of an **indole ring** in its side chain. - The indole ring consists of a **benzene ring fused to a pyrrole ring**, which is unique to tryptophan among the standard amino acids. *Tyrosine* - Tyrosine is an **aromatic amino acid** containing a **phenol group** (a benzene ring with a hydroxyl group), not an indole ring. - It is derived from phenylalanine and is a precursor for important molecules like **thyroid hormones** and **catecholamines**. *Phenylalanine* - Phenylalanine is an **aromatic amino acid** with a **benzyl group** (a benzene ring attached to a methylene group) in its side chain. - It lacks the distinct heterocyclic indole structure found in tryptophan. *Threonine* - Threonine is an **aliphatic amino acid** with a **hydroxyl group** on its side chain, classifying it as a **polar, uncharged amino acid**. - It does not contain any ring structures, especially not an indole ring.

Question 578: Where does oxidative deamination primarily occur in the human body?

A. Cytoplasm of all cells
B. Mitochondria of all cells
C. Cytoplasm of liver cells
D. Mitochondria of liver cells (Correct Answer)

Explanation: ***Mitochondria of liver cells*** - **Oxidative deamination**, particularly of glutamate, is a central process in **amino acid catabolism** and occurs predominantly in the **mitochondria of liver cells**. - This process is crucial for removing the **amino group (NH3)** from amino acids, forming ammonia, which is then detoxified into urea. *Cytoplasm of all cells* - While cells have cytoplasmic metabolic pathways, the primary enzyme for oxidative deamination, **glutamate dehydrogenase**, is located in the mitochondria. - The cytoplasm primarily handles glycolysis and various synthetic pathways, but not the bulk of oxidative deamination. *Mitochondria of all cells* - Although mitochondria are the site of oxidative metabolism in most cells, the **liver** is the main organ responsible for processing exogenous amino acids and their subsequent comprehensive deamination. - Other cells perform some amino acid metabolism, but not the large-scale oxidative deamination seen in the liver. *Cytoplasm of liver cells* - The cytoplasm of liver cells is involved in various metabolic processes, including gluconeogenesis and fatty acid synthesis. - However, the key enzymes for oxidative deamination are specifically compartmentalized within the **mitochondria** of these cells, not the cytoplasm.

Question 579: Which cofactor is primarily associated with the activity of glutamate dehydrogenase?

A. NAD+ (Correct Answer)
B. FAD
C. FMN
D. FADH2

Explanation: ***NAD+*** - Glutamate dehydrogenase catalyzes the oxidative deamination of **glutamate** to **α-ketoglutarate** and ammonia, and this reaction primarily uses **NAD+** as an electron acceptor. - In some organisms and contexts, it can also use **NADP+**, but **NAD+** is the more common and significant cofactor for its catabolic role. *FAD* - **FAD (flavin adenine dinucleotide)** is typically associated with **flavoproteins** and enzymes involved in oxidation-reduction reactions, such as those in the **electron transport chain** and the **Krebs cycle**. - Enzymes like **succinate dehydrogenase** use FAD, not glutamate dehydrogenase. *FMN* - **FMN (flavin mononucleotide)** is another flavin coenzyme, similar to FAD, and is found in various **flavoproteins** and enzymes of the **electron transport chain**, such as **NADH dehydrogenase (Complex I)**. - It does not serve as a primary cofactor for **glutamate dehydrogenase** activity. *FADH2* - **FADH2** is the reduced form of **FAD**, carrying high-energy electrons to the **electron transport chain** for ATP synthesis. - It's a product or reactant of various metabolic pathways, but not a direct cofactor for **glutamate dehydrogenase**.

Question 580: 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.

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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