Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 551: Which defect in the urea cycle is an X-linked disease?

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

Explanation: ***Ornithine transcarbamylase*** - **Ornithine transcarbamylase (OTC) deficiency** is the only **X-linked recessive** disorder among the urea cycle defects. - Males are usually more severely affected, while females can be symptomatic carriers. *Carbamoyl phosphate synthetase I* - **Carbamoyl phosphate synthetase I (CPS1) deficiency** is an **autosomal recessive** disorder. - It is one of the more severe urea cycle defects, leading to profound hyperammonemia. *Arginase* - **Arginase deficiency** (hyperargininemia) is an **autosomal recessive** disorder. - It typically presents with a distinct neurological phenotype, including spasticity and developmental delay. *Argininosuccinate synthase* - **Argininosuccinate synthase deficiency**, also known as **Citrullinemia type I**, is an **autosomal recessive** disorder. - It leads to the accumulation of **citrulline** and **ammonia** in the blood.

Question 552: Serotonin is derived from -

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

Explanation: ***Tryptophan*** - **Serotonin**, also known as 5-hydroxytryptamine (5-HT), is synthesized from the essential amino acid **tryptophan** through a two-step enzymatic pathway. - Tryptophan is first hydroxylated by tryptophan hydroxylase to 5-hydroxytryptophan (5-HTP), which is then decarboxylated by L-amino acid decarboxylase to form serotonin. *Tyrosine* - **Tyrosine** is a precursor for the synthesis of **catecholamines** (dopamine, norepinephrine, and epinephrine) and thyroid hormones. - It is not involved in the synthesis pathway for serotonin. *Phenylalanine* - **Phenylalanine** is an essential amino acid that is hydroxylated to form **tyrosine**. - Therefore, it is indirectly involved in catecholamine synthesis but not in serotonin synthesis. *Methionine* - **Methionine** is an essential amino acid primarily known for its role in protein synthesis and as a precursor for **S-adenosylmethionine (SAM)**, a methyl group donor in many biological reactions. - It does not serve as a direct precursor for serotonin.

Question 553: Catecholamines are synthesized from?

A. Tyrosine (Correct Answer)
B. Histidine
C. Methionine
D. Tryptophan

Explanation: ***Tyrosine*** - **Tyrosine** is the direct precursor amino acid for the synthesis of all **catecholamines**, including **dopamine**, **norepinephrine**, and **epinephrine**. - The synthesis pathway begins with the conversion of tyrosine to **L-DOPA** by tyrosine hydroxylase, followed by subsequent enzymatic steps. *Methionine* - **Methionine** is an essential amino acid primarily involved in **protein synthesis** and as a precursor for S-adenosylmethionine (SAM), a key methyl donor in various metabolic reactions. - It is not a direct precursor for the synthesis of **catecholamines**. *Histidine* - **Histidine** is the precursor for the synthesis of **histamine**, a neurotransmitter and inflammatory mediator. - It is not involved in the biosynthesis pathway of **catecholamines**. *Tryptophan* - **Tryptophan** is the precursor for the synthesis of **serotonin** and **melatonin**, important neurotransmitters and hormones. - It does not play a role in the synthesis of **catecholamines**.

Question 554: Which of the following amino acids is not involved in the production of creatine?

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

Explanation: ***Alanine*** - **Alanine** is not directly involved as a precursor for **creatine synthesis**. It can be converted to pyruvate and enter the gluconeogenic pathway. - The primary amino acids involved in **creatine synthesis** are arginine, glycine, and methionine. *Glycine* - **Glycine** is a direct precursor for creatine, reacting with arginine in the first step of its synthesis to form **guanidinoacetate**. - This reaction is catalyzed by **arginine:glycine amidinotransferase (AGAT)**. *Methionine* - **Methionine**, in the form of **S-adenosylmethionine (SAM)**, acts as the methyl donor in the second step of creatine synthesis. - It methylates guanidinoacetate to form **creatine**, a reaction catalyzed by **guanidinoacetate methyltransferase (GAMT)**. *Arginine* - **Arginine** donates its guanidino group to glycine, forming **guanidinoacetate**, the initial intermediate in creatine synthesis. - This is the first committed step in the **creatine biosynthesis pathway**.

Question 555: What is the primary enzymatic source of ammonia production in urine?

A. Glutaminase (Correct Answer)
B. Urease
C. Glutamate dehydrogenase
D. Arginase

Explanation: ***Glutaminase*** - This enzyme catalyzes the **hydrolysis of glutamine** located predominantly in the cells of the **renal tubules**, producing **ammonia** (NH3) and glutamate. - This process is crucial for **acid-base balance**, as the ammonia can bind with excess hydrogen ions to form ammonium (NH4+), which is then excreted in the urine. *Urease* - This enzyme breaks down **urea into ammonia and carbon dioxide**, primarily produced by **bacteria**, not human renal cells, and contributes to ammonia in urine in cases of **urinary tract infections**. - While it produces ammonia, it is not the primary enzymatic source within the healthy human kidney for **acid-base regulation**. *Glutamate dehydrogenase* - This enzyme converts **glutamate into alpha-ketoglutarate and ammonia**, but its contribution to urinary ammonia production is secondary to glutaminase in the kidney. - Its primary role is in **oxidative deamination** and the interconversion of glutamate and alpha-ketoglutarate, acting in both mitochondrial and cytosolic compartments. *Arginase* - This enzyme is involved in the **urea cycle**, converting **arginine into urea and ornithine** in the liver. - While it is important for the detoxification of ammonia by converting it into urea for excretion, it **does not produce ammonia** in the kidney for urinary excretion.

Question 556: Which amino acids accumulate in maple syrup urine disease?

A. Valine
B. Leucine
C. Isoleucine
D. All branched-chain amino acids (Correct Answer)

Explanation: ***All branched-chain amino acids*** - Maple syrup urine disease (MSUD) is characterized by a deficiency in the **branched-chain alpha-keto acid dehydrogenase complex**, which is responsible for the breakdown of branched-chain amino acids (BCAAs). - This deficiency leads to the accumulation of **leucine, isoleucine, and valine**, along with their corresponding alpha-keto acids, in the blood and urine. - The distinctive **maple syrup odor** in the urine is caused by the accumulation of branched-chain keto acids derived from all three BCAAs. *Leucine* - While leucine is one of the BCAAs that accumulates in MSUD, it is not the *only* amino acid involved. - The accumulation of **leucine** is particularly associated with the severe neurological symptoms seen in MSUD, as it is the most neurotoxic of the three BCAAs. *Valine* - Valine is another BCAA that accumulates due to the metabolic block in MSUD. - However, the disease involves the accumulation of all three BCAAs, not just valine in isolation. *Isoleucine* - Isoleucine is the third BCAA that accumulates in MSUD due to the defective enzyme. - Like leucine and valine, isoleucine and its corresponding keto acid accumulate in blood and urine when the branched-chain alpha-keto acid dehydrogenase complex is deficient.

Question 557: Which enzyme catalyzes oxidative deamination?

A. Glutaminase
B. Glutamine synthase
C. Glutamate dehydrogenase (Correct Answer)
D. None of the options

Explanation: ***Glutamate dehydrogenase*** - This enzyme catalyzes the conversion of **glutamate** to **α-ketoglutarate** and ammonia (NH₃), which is an oxidative deamination reaction. - It utilizes **NAD⁺ or NADP⁺** as a coenzyme to remove hydrogen atoms during the oxidation process. - Plays a crucial role in both **amino acid catabolism** and anabolism. *Glutaminase* - This enzyme hydrolyzes **glutamine** to glutamate and ammonia, which is a **hydrolytic deamidation** reaction, not an oxidative deamination. - It does not involve the oxidation of the substrate or require NAD⁺/NADP⁺ as cofactors. *Glutamine synthase* - This enzyme synthesizes **glutamine** from glutamate and ammonia, using ATP, which is a **biosynthetic** reaction, not a catabolic deamination. - It is involved in **ammonia detoxification** and amino acid synthesis, functioning in the opposite direction of deamination. *None of the options* - This option is incorrect because **glutamate dehydrogenase** is a valid correct answer. - Glutamate dehydrogenase is the primary enzyme responsible for oxidative deamination in human metabolism.

Question 558: Sweaty feet odor in urine is seen in which condition?

A. Phenylketonuria
B. Isovaleric acidemia (Correct Answer)
C. Alkaptonuria
D. Maple syrup urine disease

Explanation: ***Isovaleric acidemia*** - This condition is characterized by a distinctive "sweaty feet" odor in body fluids, including urine, due to the accumulation of **isovaleric acid**. - It results from a deficiency in the enzyme **isovaleryl-CoA dehydrogenase**, which is crucial for leucine metabolism. *Phenylketonuria* - Patients with **phenylketonuria (PKU)** typically have a "mousy" or "musty" odor in their urine, not a sweaty feet smell. - This is due to the accumulation of **phenylalanine** and its metabolites. *Maple syrup urine disease* - This metabolic disorder is named for the characteristic sweet, maple syrup-like odor of the urine, which is distinctly different from a sweaty feet odor. - It is caused by a defect in the metabolism of **branched-chain amino acids (leucine, isoleucine, and valine)**. *Alkaptonuria* - This condition is known for urine that turns **dark brown or black** upon standing or when exposed to air, due to the oxidation of **homogentisic acid**. - It does not produce a sweaty feet odor.

Question 559: Which of the following is a neutral amino acid?

A. Aspartate
B. Arginine
C. Glycine (Correct Answer)
D. Histidine

Explanation: ***Glycine*** - **Glycine** has a hydrogen atom as its side chain, making it the **simplest amino acid** and electrically neutral at physiological pH. - Its **nonpolar side chain** contributes to its neutral charge and allows it to fit into various protein structures. *Aspartate* - **Aspartate** is an **acidic amino acid** with a carboxyl group in its side chain. - This **carboxyl group** can lose a proton, giving aspartate a net negative charge at physiological pH. *Arginine* - **Arginine** is a **basic amino acid** characterized by a guanidinium group in its side chain. - The **guanidinium group** contains multiple nitrogen atoms that can accept protons, making arginine positively charged at physiological pH. *Histidine* - **Histidine** is classified as a **basic amino acid** due to the imidazole ring in its side chain. - The **imidazole ring** has a pKa close to physiological pH, allowing it to be protonated and positively charged, but it is not neutral.

Question 560: Transamination of Aspartate forms which compound?

A. Pyruvate
B. Acetyl-CoA
C. Oxaloacetate (Correct Answer)
D. Alanine

Explanation: ***Oxaloacetate*** - **Aspartate** is transaminated by **aspartate aminotransferase (AST)**, transferring its alpha-amino group to **alpha-ketoglutarate**. - This reaction converts aspartate into its corresponding alpha-keto acid, which is **oxaloacetate**. *Pyruvate* - **Pyruvate** is the alpha-keto acid corresponding to the amino acid **alanine**. - Transamination of **alanine** yields **pyruvate**, not aspartate. *Acetyl-CoA* - **Acetyl-CoA** is not a direct product of amino acid transamination; it is formed from **pyruvate** or fatty acid oxidation. - It functions as a key metabolic intermediate in energy production and biosynthesis. *Alanine* - **Alanine** is an amino acid, and therefore a reactant in transamination reactions to form pyruvate, rather than a product of aspartate transamination. - While it can be formed from pyruvate via transamination, it is not formed from aspartate.

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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