Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 511: Rate limiting enzyme in catecholamine synthesis?

A. Dopa decarboxylase
B. N-methyltransferase
C. Dopamine hydroxylase
D. Tyrosine hydroxylase (Correct Answer)

Explanation: ***Tyrosine hydroxylase*** - **Tyrosine hydroxylase** catalyzes the conversion of **tyrosine to L-DOPA**, which is the first and **rate-limiting step** in the synthesis of **catecholamines** (dopamine, norepinephrine, epinephrine). - Its activity is tightly regulated, making it a key control point for **catecholamine levels**. *Dopa decarboxylase* - **Dopa decarboxylase** converts **L-DOPA to dopamine**, which is a subsequent step in the pathway. - This enzyme is generally **not rate-limiting** and has high activity, quickly processing L-DOPA. *Dopamine hydroxylase* - **Dopamine hydroxylase** converts **dopamine to norepinephrine**. - This enzyme is active after the rate-limiting step and does not control the overall synthesis rate from tyrosine. *N-methyltransferase* - Also known as **phenylethanolamine N-methyltransferase (PNMT)**, this enzyme converts **norepinephrine to epinephrine**. - This is the final step in epinephrine synthesis and occurs after the rate-limiting step, primarily in the adrenal medulla.

Question 512: Which of the following statements about Carbamoyl Phosphate Synthetase I (CPSI) is true?

A. It is present in the cytoplasm.
B. It is involved in pyrimidine synthesis.
C. Glutamine is the amino group donor for CPSI.
D. N-Acetyl Glutamate is an allosteric activator of CPSI. (Correct Answer)

Explanation: ***N-Acetyl Glutamate is an allosteric activator of CPSI.*** - **N-acetylglutamate (NAG)** is an essential allosteric activator for **Carbamoyl Phosphate Synthetase I (CPSI)**, signaling a high availability of arginine and a need for urea cycle activity. - Activation by NAG ensures that ammonia is incorporated into the urea cycle only when necessary, preventing its accumulation which can be toxic. *It is present in the cytoplasm.* - **CPSI** is located exclusively in the **mitochondrial matrix** of hepatocytes, where it initiates the urea cycle by converting ammonia to carbamoyl phosphate. - Its cytosolic counterpart, **CPSII**, is involved in pyrimidine synthesis, which differentiates their cellular locations and metabolic roles. *It is involved in pyrimidine synthesis.* - **CPSI** is the rate-limiting enzyme of the **urea cycle**, responsible for detoxifying ammonia by forming carbamoyl phosphate. - **Carbamoyl Phosphate Synthetase II (CPSII)**, located in the cytosol, is the enzyme involved in **pyrimidine synthesis**. *Glutamine is the amino group donor for CPSI.* - **CPSI** uses **free ammonia** (NH3) and **bicarbonate** as substrates to synthesize carbamoyl phosphate, not glutamine. - **Glutamine** is the nitrogen source for **Carbamoyl Phosphate Synthetase II (CPSII)** in pyrimidine synthesis.

Question 513: Which vitamin is not used in the treatment of homocysteinuria?

A. Vitamin B6
B. Vitamin B12
C. Folate
D. Thiamine (Vitamin B1) (Correct Answer)

Explanation: ***Thiamine (Vitamin B1)*** - **Thiamine** is not directly involved in the metabolic pathways that process homocysteine. - Its primary role is in **carbohydrate metabolism** and nerve function, not homocysteine reduction. *Vitamin B6* - **Vitamin B6 (pyridoxine)** is a cofactor for the enzyme **cystathionine beta-synthase**, which converts homocysteine to cystathionine. - Supplementation with vitamin B6 can help reduce homocysteine levels in some patients with homocystinuria, particularly those with **pyridoxine-responsive forms**. *Vitamin B12* - **Vitamin B12 (cobalamin)** is a cofactor for **methionine synthase**, an enzyme that converts homocysteine back to methionine. - This pathway is crucial for lowering homocysteine levels, making B12 supplementation beneficial in homocystinuria. *Folate* - **Folate (Vitamin B9)**, in the form of 5-methyltetrahydrofolate, provides the methyl group necessary for **methionine synthase** to convert homocysteine to methionine. - Therefore, folate supplementation is essential in the treatment regimen for homocystinuria to support homocysteine metabolism.

Question 514: Which branched-chain amino acid is the PRIMARY contributor to neurotoxicity in maple syrup urine disease?

A. Isoleucine
B. Valine
C. Leucine (Correct Answer)
D. Phenylalanine

Explanation: ***Leucine*** - **Leucine** is the primary branched-chain amino acid (BCAA) responsible for neurotoxicity in **maple syrup urine disease (MSUD)** due to its high concentration in the brain, especially in immature brains. - High levels of leucine and its ketoacid derivative **alpha-ketoisocaproate** interfere with neurotransmitter synthesis, myelin formation, and cerebral energy metabolism, leading to significant neurological damage. *Valine* - While **valine** is one of the BCAAs that accumulates in MSUD, its contribution to direct neurotoxicity is less pronounced compared to leucine. - **Valine** is essential for protein synthesis and tissue repair but does not exert the same level of direct inhibitory effects on cerebral metabolism as leucine. *Isoleucine* - **Isoleucine** is another BCAA whose levels are elevated in MSUD, but it plays a relatively minor role in the acute neurotoxicity compared to leucine. - Its levels must be monitored during dietary treatment of MSUD, as maintaining proper balance of all three BCAAs is essential for preventing metabolic decompensation. *Phenylalanine* - **Phenylalanine** is an aromatic amino acid, not a branched-chain amino acid, and its elevated levels are characteristic of **phenylketonuria (PKU)**, not MSUD. - High phenylalanine causes neurotoxicity in PKU through different mechanisms, distinct from the BCAA-related toxicity seen in MSUD.

Question 515: What is the precursor of proline in the Krebs cycle?

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

Explanation: ***α-ketoglutarate*** - **α-ketoglutarate** is an intermediate of the **Krebs cycle** that can be transaminated to form **glutamate**. - **Glutamate** can then be converted to **glutamate-γ-semialdehyde** (also called glutamate 5-semialdehyde), which cyclizes to form **Δ¹-pyrroline-5-carboxylate**, the immediate precursor for **proline** synthesis. - This pathway links the **Krebs cycle** to amino acid biosynthesis. *Oxaloacetate* - **Oxaloacetate** is a precursor for the synthesis of **aspartate** and **asparagine** in amino acid metabolism. - It is also involved in gluconeogenesis and can be converted to **phosphoenolpyruvate**. *Succinyl CoA* - **Succinyl CoA** is an intermediate in the Krebs cycle that can contribute to the synthesis of **porphyrins** (e.g., heme). - It is derived from the metabolism of certain amino acids like **valine**, **isoleucine**, and **methionine**. *Fumarate* - **Fumarate** is another intermediate of the Krebs cycle that links the cycle to the **urea cycle**. - It is involved in the metabolism of **phenylalanine** and **tyrosine**.

Question 516: Which of the following amino acids contains two amino groups?

A. Asparagine
B. Arginine
C. Lysine (Correct Answer)
D. Glycine

Explanation: ***Lysine*** - Lysine is an **essential amino acid** characterized by a **side chain with a primary amine group** at its epsilon-carbon, in addition to the alpha-amino group present in all amino acids. - This makes lysine one of the **basic amino acids**, as the extra amino group is protonated at physiological pH. *Glycine* - Glycine is the **simplest amino acid**, with a single hydrogen atom as its side chain, thus containing only the standard alpha-amino group. - It is **neither acidic nor basic** due to the absence of additional ionizable groups in its side chain. *Arginine* - Arginine contains a **guanidinium group** in its side chain, which is highly basic and contains three nitrogen atoms, but it is not considered two distinct amino groups. - While it has multiple nitrogen atoms, they are part of a single **complex functional group**, not separate amino groups. *Asparagine* - Asparagine has an **amide group** on its side chain, which contains nitrogen but is not an amino group. - The amide group is **neutral** and does not contribute to the basicity of the amino acid in the same way an amino group would.

Question 517: Glucogenic amino acids give rise to all of the following intermediates of the citric acid cycle except:

A. Succinyl CoA
B. α-ketoglutarate
C. Isocitrate (Correct Answer)
D. Fumarate

Explanation: ***Isocitrate*** - Glucogenic amino acids are metabolized to intermediates that can enter the gluconeogenesis pathway, typically entering the citric acid cycle at points from **alpha-ketoglutarate** onward, excluding early intermediates like isocitrate. - While isocitrate is an intermediate in the citric acid cycle, it is not a direct product of amino acid catabolism that can be used for **net glucose synthesis**. *α-ketoglutarate* - Several amino acids, such as **glutamate**, **glutamine**, **proline**, and **arginine**, are converted to α-ketoglutarate, making it a point of entry for glucogenic amino acids into the citric acid cycle. - This allows the carbon skeletons of these amino acids to be siphoned off for **gluconeogenesis**. *Succinyl CoA* - Amino acids like **valine**, **isoleucine**, **methionine**, and **threonine** are degraded to succinyl CoA, which can then enter the citric acid cycle. - Succinyl CoA is a key intermediate for **glucose synthesis** from these amino acids. *Fumarate* - Amino acids such as **phenylalanine** and **tyrosine** are catabolized to fumarate, which is a citric acid cycle intermediate. - Fumarate's entry into the cycle facilitates the use of these amino acid carbons for **gluconeogenesis**.

Question 518: Which of the following is an acidic amino acid?

A. Alanine
B. Glutamic acid (Correct Answer)
C. Lysine
D. None of the options

Explanation: ***Glutamic acid*** - **Glutamic acid** is an **acidic amino acid** because its side chain (R-group) contains a carboxyl group (-COOH) that can donate a proton, lowering the pH. - At physiological pH, the carboxyl group is deprotonated, giving it a **negative charge**. - The two acidic amino acids are **glutamic acid** and **aspartic acid**. *Alanine* - **Alanine** is a **non-polar, aliphatic amino acid** with a methyl group (-CH₃) as its side chain. - It is **neutral** at physiological pH and does not have acidic or basic properties. *Lysine* - **Lysine** is a **basic amino acid** because its side chain contains an amine group (-NH₂) that can accept a proton. - At physiological pH, the amine group is protonated, giving it a **positive charge**. *None of the options* - This option is incorrect because **glutamic acid** is indeed an acidic amino acid. - Therefore, there is a correct option provided in the list.

Question 519: Creatine is formed from:

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

Explanation: ***Arginine*** - **Creatine** is synthesized from **three amino acids**: arginine, glycine, and methionine. - In the **first step** (kidney/pancreas), arginine donates its **guanidino group** to glycine, forming **guanidinoacetate** via the enzyme arginine:glycine amidinotransferase. - In the **second step** (liver), guanidinoacetate is methylated by **S-adenosylmethionine (SAM)** derived from methionine to form **creatine**. - Among the options given, **arginine** is a direct precursor for creatine synthesis. *Lysine* - **Lysine** is an essential amino acid involved in protein synthesis, collagen formation, and **calcium absorption**. - It is **not involved** in creatine biosynthesis. *Leucine* - **Leucine** is a branched-chain amino acid **(BCAA)** important for muscle protein synthesis and energy production. - It is **not a precursor** for creatine synthesis. *Histidine* - **Histidine** serves as a precursor for **histamine** and participates in various metabolic reactions. - It is **not involved** in creatine biosynthesis.

Question 520: A 3-day-old newborn presents with hyperammonemia in the blood and an unknown inborn error of metabolism. What is the most likely diagnosis?

A. Maple syrup urine disease
B. Organic aciduria
C. Phenylketonuria
D. Urea cycle enzyme deficiency (Correct Answer)

Explanation: ***Urea cycle enzyme deficiency*** - **Hyperammonemia** in a newborn is a hallmark of urea cycle disorders, as the urea cycle is essential for detoxifying ammonia. - The rapid onset of symptoms in a **3-day-old newborn** is consistent with severe forms of urea cycle enzyme deficiencies where ammonia rapidly accumulates after the initiation of protein feeding. *Maple syrup urine disease* - This condition involves impaired metabolism of **branched-chain amino acids** (leucine, isoleucine, valine), leading to their accumulation. - While it can cause neurological symptoms, **hyperammonemia** is not its primary or characteristic metabolic derangement; rather, a distinctive **maple syrup odor** in urine is typical. *Organic aciduria* - These disorders result from defects in pathways of **amino acid or fatty acid metabolism**, leading to the accumulation of various organic acids. - While some severe forms can present with metabolic acidosis and neurological symptoms, **hyperammonemia** is usually secondary and less prominent or central to the pathology compared to urea cycle disorders. *Phenylketonuria* - This is a disorder of **phenylalanine metabolism**, leading to excessive accumulation of phenylalanine in the blood. - It primarily causes developmental delay and neurological issues if untreated, but **hyperammonemia** is not a feature of phenylketonuria.

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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