Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 351: Nitric oxide is synthesized from which of the following amino acids?

A. L-arginine (Correct Answer)
B. L-citrulline
C. Lysine
D. Tryptophan

Explanation: **Explanation:** **1. Why L-arginine is correct:** Nitric Oxide (NO), a potent vasodilator and signaling molecule, is synthesized from the amino acid **L-arginine**. This reaction is catalyzed by the enzyme **Nitric Oxide Synthase (NOS)**. In this process, L-arginine undergoes a five-electron oxidation to produce NO and **L-citrulline** as a byproduct. The reaction requires several essential cofactors: NADPH, FAD, FMN, and Tetrahydrobiopterin ($BH_4$). **2. Why the other options are incorrect:** * **L-citrulline:** This is the *product* of the reaction, not the substrate. While citrulline can be recycled back into arginine via the urea cycle (argininosuccinate pathway), it is not the direct precursor for NO synthesis. * **Lysine:** Although it is a basic amino acid like arginine, it does not serve as a substrate for NOS. It is primarily ketogenic and involved in carnitine synthesis. * **Tryptophan:** This is the precursor for Serotonin (5-HT), Melatonin, and Niacin (Vitamin $B_3$), but it plays no role in NO production. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Isoforms of NOS:** There are three types: **eNOS** (endothelial - vasodilation), **nNOS** (neuronal - neurotransmission), and **iNOS** (inducible - macrophage-mediated cytotoxicity). * **Biological Functions:** NO activates **Guanylyl Cyclase**, increasing cGMP, which leads to smooth muscle relaxation. * **Pharmacology Connection:** Nitroglycerin works by releasing NO, causing vasodilation in angina pectoris. * **Arginine's other roles:** It is also a precursor for Urea, Creatine, and Polyamines (spermine/spermidine).

Question 352: Which one of the following is a symptom of ammonia intoxication?

A. Slurring of speech (Correct Answer)
B. Abdominal pain
C. Diarrhea
D. Constipation

Explanation: **Explanation:** Ammonia intoxication (Hyperammonemia) primarily affects the **Central Nervous System (CNS)**. Ammonia is a potent neurotoxin that can easily cross the blood-brain barrier. When blood ammonia levels rise, it is taken up by astrocytes and converted into **glutamine** via the enzyme glutamine synthetase. This accumulation of glutamine creates an osmotic gradient, leading to cerebral edema and astrocyte swelling. **Why Slurring of Speech is Correct:** Neurological manifestations are the hallmark of ammonia toxicity. As the brain swells and neurotransmitter balance (specifically glutamate and GABA) is disrupted, patients exhibit signs of encephalopathy. **Slurring of speech**, tremors, blurring of vision, flapping tremors (asterixis), and mental confusion are classic early signs. If untreated, this progresses to seizures, coma, and death. **Why Other Options are Incorrect:** * **Abdominal pain, Diarrhea, and Constipation:** These are gastrointestinal symptoms. While liver failure (a common cause of hyperammonemia) may involve GI issues, they are not direct symptoms of ammonia intoxication itself. Ammonia's toxicity is specifically targeted at the brain and metabolic pathways, not the intestinal mucosa or motility. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** High ammonia depletes **alpha-ketoglutarate** (a TCA cycle intermediate) to form glutamate, leading to a failure of the TCA cycle and ATP depletion in the brain. * **Asterixis:** Also known as "liver flap," this is a characteristic physical finding in hepatic encephalopathy. * **Treatment:** Lactulose (to acidify the gut and trap ammonia as $NH_4^+$) and a low-protein diet are standard management strategies. * **Normal Range:** Blood ammonia levels are typically **10–50 μmol/L**. Levels exceeding 100 μmol/L are considered critical.

Question 353: Which of the following statements about branched-chain amino acids is correct?

A. They are normally completely catabolized by muscle to CO2 and H2O.
B. They can be catabolized by the liver but not by muscle.
C. They are the main dietary amino acids metabolized by the intestine.
D. They are a major source of nitrogen for alanine and glutamine produced in muscle. (Correct Answer)

Explanation: ### Explanation **1. Why Option D is Correct:** Branched-chain amino acids (BCAAs)—**Leucine, Isoleucine, and Valine**—are unique because their initial catabolism occurs primarily in **extrahepatic tissues**, especially skeletal muscle. Muscle lacks the urea cycle; therefore, when BCAAs are transaminated by *Branched-Chain Aminotransferase (BCAT)*, the amino group is transferred to $\alpha$-ketoglutarate to form glutamate. This nitrogen is then used to synthesize **Alanine** (via the Glucose-Alanine cycle) and **Glutamine**. These two amino acids act as non-toxic nitrogen carriers, transporting amino groups from the muscle to the liver and kidneys for disposal. **2. Why Other Options are Incorrect:** * **Option A:** While muscle initiates BCAA breakdown, it does not typically perform complete oxidation to $CO_2$ and $H_2O$. The carbon skeletons (keto-acids) are often released into the circulation to be used by the liver for gluconeogenesis or energy production. * **Option B:** This is the opposite of physiological reality. The liver lacks significant levels of *BCAT*; thus, it cannot initiate BCAA catabolism. BCAAs "bypass" the liver after absorption. * **Option C:** Glutamine and Glutamate are the primary amino acids metabolized by the intestinal mucosa, not BCAAs. **3. High-Yield Clinical Pearls for NEET-PG:** * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the **Branched-chain $\alpha$-keto acid dehydrogenase (BCKDH)** complex. This leads to a buildup of BCAAs and their keto-acids, giving urine a burnt-sugar odor. * **Valine and Isoleucine** are both glucogenic and ketogenic (though Valine is primarily glucogenic), while **Leucine** is purely ketogenic. * **BCKDH** requires five cofactors: Thiamine (B1), Riboflavin (B2), Niacin (B3), Pantothenic acid (B5), and Lipoic acid (mnemonic: **T**ender **R**omance **N**ever **P**lays **L**ate).

Question 354: The rate-limiting step in norepinephrine synthesis is:

A. Conversion of phenylalanine to tyrosine
B. Conversion of tyrosine to DOPA (Correct Answer)
C. Conversion of DOPA to dopamine
D. Conversion of dopamine to norepinephrine

Explanation: **Explanation:** The synthesis of catecholamines (Dopamine, Norepinephrine, and Epinephrine) follows a specific metabolic pathway starting from the amino acid Tyrosine. **1. Why Option B is Correct:** The conversion of **Tyrosine to L-DOPA** is catalyzed by the enzyme **Tyrosine Hydroxylase**. This is the **rate-limiting and committed step** in the entire catecholamine biosynthetic pathway. The enzyme requires Tetrahydrobiopterin ($BH_4$) as a cofactor and is strictly regulated by end-product inhibition (negative feedback by norepinephrine). **2. Why Other Options are Incorrect:** * **Option A:** The conversion of Phenylalanine to Tyrosine (by Phenylalanine Hydroxylase) is the first step in phenylalanine catabolism. While essential, it is not the rate-limiting step for norepinephrine specifically. * **Option C:** The conversion of DOPA to Dopamine is catalyzed by **DOPA decarboxylase** (requires Pyridoxal Phosphate/Vitamin $B_6$). This step is very rapid and not rate-limiting. * **Option D:** The conversion of Dopamine to Norepinephrine occurs inside synaptic vesicles via **Dopamine $\beta$-hydroxylase** (requires Vitamin C and Copper). While it is the final step for NE synthesis, it is not the slowest (rate-limiting) step. **High-Yield Clinical Pearls for NEET-PG:** * **Cofactor Alert:** Tyrosine Hydroxylase requires **$BH_4$**, $O_2$, and $Fe^{2+}$. * **Final Step:** To produce Epinephrine from Norepinephrine, the enzyme **PNMT** (Phenylethanolamine N-methyltransferase) is required, which uses **S-adenosylmethionine (SAM)** as a methyl donor. * **Clinical Correlation:** **Metyrosine** is a drug that inhibits Tyrosine Hydroxylase; it is used clinically in the preoperative management of Pheochromocytoma to decrease catecholamine production. * **VMAT:** Dopamine is transported into vesicles by the Vesicular Monoamine Transporter (VMAT), which is inhibited by **Reserpine**.

Question 355: Which one of the following amino acids can be converted to an intermediate of either the citric acid cycle or the urea cycle?

A. Tyrosine
B. Lysine
C. Leucine
D. Aspartate (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Aspartate** **Why Aspartate is Correct:** Aspartate is a unique amino acid that serves as a bridge between the **Citric Acid Cycle (TCA)** and the **Urea Cycle**. 1. **Urea Cycle:** Aspartate reacts with citrulline in the mitochondria-to-cytosol transition to form **Argininosuccinate**. It provides the second nitrogen atom required for urea synthesis. 2. **TCA Cycle:** Aspartate is a glucogenic amino acid. Through a transamination reaction (via AST/GOT), aspartate is converted into **Oxaloacetate**, a key intermediate of the TCA cycle. Additionally, the urea cycle releases **Fumarate** (from argininosuccinate), which also enters the TCA cycle. --- **Analysis of Incorrect Options:** * **A. Tyrosine:** It is both glucogenic and ketogenic. It enters the TCA cycle as **Fumarate** and the ketogenic pathway as **Acetoacetate**, but it does not directly participate as an intermediate in the urea cycle. * **B. Lysine:** It is a purely ketogenic amino acid. It is degraded to **Acetyl-CoA** and does not provide intermediates for the urea cycle. * **C. Leucine:** Like Lysine, Leucine is strictly ketogenic. It is metabolized into **Acetyl-CoA and Acetoacetate**. --- **NEET-PG High-Yield Pearls:** * **The "Kreb’s Bicycle":** This refers to the relationship where the Urea cycle and TCA cycle are linked by **Fumarate** and **Aspartate**. * **Nitrogen Sources in Urea:** Remember that one nitrogen comes from **Free Ammonia** and the second nitrogen comes from **Aspartate**. * **Purely Ketogenic Amino Acids:** Only two amino acids are purely ketogenic—**Lysine and Leucine** (Mnemonic: The "L"s). * **Aspartate Aminotransferase (AST):** This enzyme requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor to convert Aspartate to Oxaloacetate.

Question 356: Deamination of which of the following amino acids primarily occurs in the liver?

A. Aspartate
B. Alanine
C. Glutamine (Correct Answer)
D. Glycine

Explanation: **Explanation:** The correct answer is **Glutamine**. **Why Glutamine is correct:** Glutamine serves as the primary non-toxic carrier of ammonia in the blood. Ammonia generated in peripheral tissues (like the brain and muscles) is converted into glutamine by *glutamine synthetase*. Once it reaches the **liver**, the enzyme **Glutaminase** performs a hydrolytic deamination, converting glutamine into glutamate and releasing free ammonia. This ammonia is then immediately channeled into the **Urea Cycle** for detoxification. While deamination occurs in the kidneys to maintain acid-base balance, the liver is the primary site for processing systemic glutamine to facilitate urea synthesis. **Why other options are incorrect:** * **Alanine:** While alanine is a major carrier of nitrogen from muscle to liver (via the Glucose-Alanine cycle), it undergoes **transamination** (via ALT) rather than direct deamination to release its amino group. * **Aspartate:** Aspartate typically participates in the urea cycle by donating its amino group directly to citrulline to form argininosuccinate. It does not undergo primary deamination for ammonia release in the liver. * **Glycine:** Glycine is primarily metabolized via the Glycine Cleavage System or converted to serine; it is not the principal substrate for systemic deamination in the liver compared to glutamine. **High-Yield Clinical Pearls for NEET-PG:** * **Glutamate Dehydrogenase (GDH):** This is the only enzyme that can use either NAD+ or NADP+ and is responsible for the oxidative deamination of glutamate in the liver. * **The "Ammonia Trap":** In the brain, ammonia is trapped as glutamine. In hepatic encephalopathy, elevated glutamine levels in astrocytes lead to osmotic swelling and cerebral edema. * **Most common mechanism:** Most amino acids first undergo **transamination** (transferring the amino group to α-ketoglutarate to form glutamate) followed by **oxidative deamination** of glutamate.

Question 357: Which of the following conditions gives a positive reaction with Ferrous chloride?

A. Phenylketonuria (Correct Answer)
B. Alkaptonuria
C. Maple syrup urine disease
D. None of the above

Explanation: **Explanation:** The **Ferric Chloride (FeCl₃) test** is a classic biochemical screening tool used to detect specific metabolites in urine based on color changes resulting from the formation of complex salts. **1. Why Phenylketonuria (PKU) is correct:** In PKU, a deficiency of the enzyme *Phenylalanine Hydroxylase* leads to the accumulation of Phenylalanine. This excess is diverted into an alternative pathway, producing **Phenylpyruvic acid** (a phenylketone). When Ferric chloride is added to the urine of these patients, it reacts with phenylpyruvic acid to produce a characteristic **transient blue-green color**. **2. Why the other options are incorrect:** * **Alkaptonuria:** While Alkaptonuria involves the excretion of Homogentisic acid, the Ferric chloride test typically yields a **transient deep blue or green** color that quickly fades. However, the classic diagnostic test for Alkaptonuria is the darkening of urine upon standing (alkalinization) or a positive Benedict’s test (as it is a reducing sugar). * **Maple Syrup Urine Disease (MSUD):** This condition involves the accumulation of branched-chain ketoacids (Alpha-ketoisovaleric, Alpha-ketoisocaproic, and Alpha-keto-beta-methylvaleric acids). The Ferric chloride test in MSUD results in a **navy blue** color, but it is not the primary screening method; the **DNPH (Dinitrophenylhydrazine) test** is the high-yield diagnostic marker for MSUD. **High-Yield Clinical Pearls for NEET-PG:** * **PKU:** Mousy/Musty body odor; Blue-green FeCl₃ test. * **Tyrosinemia:** Cabbage-like odor; Green FeCl₃ test. * **Alkaptonuria:** Blackening of urine; Homogentisic acid oxidase deficiency. * **Histidinemia:** Blue-green FeCl₃ test (can be a false positive for PKU). * **Melanoma:** Ferric chloride gives a gray-black precipitate.

Question 358: Increased serum alanine during fasting is due to:

A. Breakdown of muscle proteins (Correct Answer)
B. Decreased utilization of non-essential amino acids
C. Leakage of amino acids to plasma
D. Impaired renal function

Explanation: **Explanation:** The correct answer is **A. Breakdown of muscle proteins.** During fasting or starvation, the body shifts into a catabolic state to maintain blood glucose levels. Since the brain and RBCs are glucose-dependent, the liver must perform **gluconeogenesis**. The primary substrates for this process are glucogenic amino acids, specifically **Alanine**. Muscle tissue acts as the major reservoir for these amino acids. Through the **Glucose-Alanine Cycle (Cahill Cycle)**, muscle proteins are degraded into amino acids. The amino groups are transferred to pyruvate to form Alanine, which is then released into the blood and transported to the liver. In the liver, Alanine is deaminated back to pyruvate, which serves as a precursor for glucose synthesis. **Why other options are incorrect:** * **B & C:** These are physiological generalities. The increase in serum alanine is a regulated metabolic response to provide carbon skeletons for glucose, not a result of "leakage" or simple "decreased utilization." * **D:** Impaired renal function would typically lead to an increase in nitrogenous waste (BUN/Creatinine) rather than a specific physiological rise in alanine associated with fasting. **NEET-PG High-Yield Pearls:** * **Cahill Cycle:** Transports amino groups from muscle to liver as Alanine. * **Cori Cycle:** Transports lactate from muscle to liver. * **Key Enzyme:** Alanine Aminotransferase (ALT/SGPT) requires **Pyridoxal Phosphate (B6)** as a cofactor. * **Glutamine:** The other major carrier of nitrogen in the blood (especially from non-muscle tissues). * **Leucine and Lysine:** The only purely ketogenic amino acids (cannot form glucose).

Question 359: Which of the following are urea cycle enzymes?

A. Glutaminase
B. Asparaginase
C. Argininosuccinate synthetase
D. Ornithine transcarboxylase (Correct Answer)

Explanation: **Explanation:** The **Urea Cycle (Ornithine Cycle)** is the primary mechanism for detoxifying ammonia into urea in the liver. It consists of five main enzymatic steps, occurring in both the mitochondria and the cytoplasm. **1. Why Option D is Correct:** **Ornithine transcarbamoylase (OTC)** is the second enzyme of the urea cycle. It catalyzes the reaction between **Carbamoyl Phosphate** and **Ornithine** to form **Citrulline** within the mitochondrial matrix. It is a high-yield fact for NEET-PG because OTC deficiency is the most common urea cycle disorder and is the only one that is **X-linked recessive** (others are autosomal recessive). **2. Analysis of Incorrect Options:** * **A. Glutaminase:** This enzyme converts Glutamine to Glutamate and ammonia. While it plays a role in ammonia metabolism (especially in the kidneys for acid-base balance), it is not a direct component of the urea cycle. * **B. Asparaginase:** This enzyme catalyzes the hydrolysis of Asparagine into Aspartic acid. It is used clinically as a chemotherapeutic agent in Acute Lymphoblastic Leukemia (ALL) but is not part of the urea cycle. * **C. Argininosuccinate synthetase:** While this *is* a urea cycle enzyme (converting Citrulline + Aspartate to Argininosuccinate), the question specifically marks **Ornithine transcarbamoylase** as the intended answer. (Note: In many exam patterns, if multiple correct options exist, the most "characteristic" or specifically tested one is prioritized). **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Carbamoyl Phosphate Synthetase I (CPS-I), which requires **N-acetylglutamate (NAG)** as an essential activator. * **Location:** The first two steps (CPS-I, OTC) occur in the **mitochondria**; the remaining three occur in the **cytosol**. * **Fumarate Link:** The urea cycle is linked to the TCA cycle via the "Aspartate-argininosuccinate shunt." * **Hyperammonemia:** Common in OTC deficiency, leading to orotic aciduria and neurological symptoms.

Question 360: Which of the following is the first product of tryptophan catabolism?

A. Xantheurenate
B. Kynurenine (Correct Answer)
C. PAF
D. Bradykinin

Explanation: **Explanation:** The correct answer is **Kynurenine**. Tryptophan is an essential amino acid that follows two major metabolic pathways: the **Kynurenine pathway** (95% of metabolism) and the Serotonin pathway. The catabolism of tryptophan begins with the oxidative cleavage of the indole ring, catalyzed by the enzyme **Tryptophan 2,3-dioxygenase (TDO)** in the liver or **Indoleamine 2,3-dioxygenase (IDO)** in extrahepatic tissues. This reaction produces **N-formylkynurenine**, which is rapidly converted by formamidase into **Kynurenine**, the first stable and major product of this pathway. **Analysis of Incorrect Options:** * **A. Xanthurenate:** This is a downstream metabolite of the kynurenine pathway. It is formed from 3-hydroxykynurenine, particularly in the presence of **Vitamin B6 deficiency**. * **C. PAF (Platelet Activating Factor):** This is a potent phospholipid mediator of inflammation and platelet aggregation; it is not a product of tryptophan metabolism. * **D. Bradykinin:** This is a peptide that causes vasodilation and is derived from kininogens via the action of kallikreins, unrelated to amino acid catabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Niacin Synthesis:** The kynurenine pathway is the source of **de novo NAD+ synthesis**. Approximately 60 mg of Tryptophan yields 1 mg of Niacin. * **Hartnup Disease:** A defect in the transport of neutral amino acids (including Tryptophan) leads to pellagra-like symptoms due to niacin deficiency. * **Vitamin B6 Dependency:** Kynureninase, a key enzyme in this pathway, requires Pyridoxal Phosphate (PLP). Deficiency leads to the excretion of **Xanthurenic acid** in urine (Xanthurenic aciduria).

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

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