Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 551: 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 552: 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.

Question 553: Which of the following amino acids is not classified as polar?

A. Glutamic acid
B. Histidine
C. Glutamine
D. Methionine (Correct Answer)

Explanation: ***Methionine*** - Methionine is classified as a **nonpolar**, **hydrophobic** amino acid due to its side chain containing a **thioether group** (-S-CH3), which has no significant dipole moment. - Its nonpolar nature means it is less likely to interact with water and tends to be found in the interior of proteins. *Glutamic acid* - Glutamic acid is a **polar**, **acidic** amino acid because its side chain contains a **carboxyl group** (-COOH) that is deprotonated (COO-) at physiological pH, carrying a negative charge. - This charged group allows it to participate in **ionic interactions** and hydrogen bonding. *Histidine* - Histidine is a **polar**, **basic** amino acid due to its **imidazole ring** in the side chain, which can be protonated or deprotonated depending on the pH. - Its pKa is close to physiological pH, making it an important **buffer** and active site residue in many enzymes. *Glutamine* - Glutamine is a **polar**, **uncharged** amino acid with an **amide group** (-CONH2) in its side chain. - The carbonyl oxygen and the amine hydrogens in the amide group allow for significant **hydrogen bonding** with water and other polar molecules.

Question 554: In type IA Maple Syrup Urine Disease, which gene mutation is responsible?

A. BCKDHB
B. DBT
C. DLD
D. BCKDHA (Correct Answer)

Explanation: ***BCKDHA*** - **Maple Syrup Urine Disease (MSUD)** type IA is caused by a mutation in the **BCKDHA gene**, which codes for the E1α subunit of the **branched-chain α-keto acid dehydrogenase (BCKD) complex**. - This **enzyme complex** is crucial for the metabolism of **branched-chain amino acids (BCAAs)**: leucine, isoleucine, and valine. *BCKDHB* - The **BCKDHB gene** codes for the E1β subunit of the **BCKD complex**. - Mutations in **BCKDHB** are associated with **type IB MSUD**, not type IA. *DBT* - The **DBT gene** codes for the E2 subunit (dihydrolipoyl transacylase) of the **BCKD complex**. - Mutations in **DBT** are responsible for **type II MSUD**. *DLD* - The **DLD gene** codes for the E3 subunit (dihydrolipoyl dehydrogenase), which is a component shared by several **α-keto acid dehydrogenase complexes**. - Mutations in the **DLD gene** lead to **type III MSUD** and other pyruvate dehydrogenase complex deficiencies, rather than type IA.

Question 555: 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 556: 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 557: 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 558: 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 559: 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 560: 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.

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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