Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 191: Pellagra-like symptoms are typically seen in which amino acid deficiency?

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

Explanation: **Explanation:** **Why Tryptophan is Correct:** Pellagra is classically caused by a deficiency of **Niacin (Vitamin B3)**. However, the essential amino acid **Tryptophan** serves as a metabolic precursor for the endogenous synthesis of Niacin. Approximately **60 mg of Tryptophan** is required to produce **1 mg of Niacin**. This conversion requires Vitamin B6 (Pyridoxine) as a cofactor. Therefore, a deficiency in Tryptophan leads to decreased Niacin levels, resulting in the clinical triad of Pellagra: **Dermatitis, Diarrhea, and Dementia** (the 3 Ds). **Why the Other Options are Incorrect:** * **Tyrosine:** A non-essential amino acid (derived from Phenylalanine) used to synthesize catecholamines (Dopamine, Epinephrine), Thyroid hormones, and Melanin. Deficiency does not cause Pellagra. * **Glutamine:** The most abundant free amino acid; it acts as a nitrogen transporter and fuel for rapidly dividing cells. It is not involved in Niacin synthesis. * **Phenylalanine:** An essential amino acid that converts to Tyrosine. Deficiency leads to Phenylketonuria (PKU) symptoms (e.g., musty odor, intellectual disability), not Pellagra. **High-Yield Clinical Pearls for NEET-PG:** * **Hartnup Disease:** An autosomal recessive disorder involving defective transport of neutral amino acids (including Tryptophan) in the gut and kidneys, leading to **Pellagra-like skin rashes**. * **Carcinoid Syndrome:** Can cause Pellagra because tumor cells divert up to 60% of Tryptophan to synthesize **Serotonin**, leaving insufficient amounts for Niacin production. * **Casal’s Necklace:** The characteristic hyperpigmented rash around the neck seen in Pellagra. * **Cofactor Alert:** Iron, Riboflavin (B2), and Pyridoxine (B6) are all essential for the Tryptophan-to-Niacin pathway.

Question 192: Nitric oxide is derived from which amino acid?

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

Explanation: ### Explanation **Correct Answer: D. Arginine** **Mechanism:** 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 is converted into **L-citrulline** and NO in the presence of oxygen and several essential cofactors, including NADPH, FAD, FMN, and Tetrahydrobiopterin ($BH_4$). **Why other options are incorrect:** * **Histidine:** It is the precursor for **Histamine**, a mediator of allergic reactions and gastric acid secretion, via the enzyme histidine decarboxylase. * **Lysine:** It is a purely ketogenic amino acid primarily involved in protein synthesis and the formation of **Carnitine** (required for fatty acid transport). * **Methionine:** It is a sulfur-containing amino acid that serves as the principal methyl donor in the form of **S-adenosylmethionine (SAM)** and is a precursor for cysteine and homocysteine. **High-Yield Clinical Pearls for NEET-PG:** * **Isoforms of NOS:** There are three types: nNOS (Neuronal), eNOS (Endothelial), and iNOS (Inducible/Macrophage). * **Biological Functions:** NO activates **soluble guanylate cyclase**, increasing cGMP levels, which leads to smooth muscle relaxation (vasodilation) and inhibition of platelet aggregation. * **Arginine’s Multi-role:** Besides NO, Arginine is a precursor for **Urea, Creatine, and Polyamines** (spermine, spermidine). * **Pharmacology Link:** Nitroglycerin works by releasing NO, mimicking the endogenous action of Arginine-derived NO to treat angina.

Question 193: Glutathione synthetase requires which of the following ions for its activity?

A. Copper
B. Selenium
C. Magnesium (Correct Answer)
D. Iron

Explanation: **Explanation:** **1. Why Magnesium (Mg²⁺) is Correct:** Glutathione (GSH) is a tripeptide (γ-glutamyl-cysteinyl-glycine) synthesized in two ATP-dependent steps. The enzyme **Glutathione Synthetase** catalyzes the second step: the addition of glycine to γ-glutamylcysteine. Like most enzymes that utilize ATP (kinases and synthetases), Glutathione Synthetase requires **Magnesium (Mg²⁺)** as a cofactor. Magnesium stabilizes the polyphosphate chain of ATP, facilitating the nucleophilic attack required for bond formation. **2. Why Other Options are Incorrect:** * **Copper (A):** Copper is a cofactor for enzymes like Cytochrome c oxidase, Tyrosinase, and **Lysyl oxidase**. In the antioxidant system, it is a component of Cytosolic Superoxide Dismutase (Cu-Zn SOD). * **Selenium (B):** This is a common "distractor" in this topic. Selenium is the essential cofactor for **Glutathione Peroxidase**, the enzyme that uses glutathione to neutralize hydrogen peroxide. It is *not* required for the synthesis of glutathione itself. * **Iron (D):** Iron is vital for Heme-containing enzymes (Catalase, Cytochromes) and Non-heme enzymes like Aconitase, but it does not play a role in glutathione synthesis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Glutathione Components:** Glutamate, Cysteine, and Glycine. Cysteine is the rate-limiting amino acid. * **Linkage:** Glutathione contains an unusual **γ-peptide bond** (between the γ-carboxyl of glutamate and the amino group of cysteine), which protects it from degradation by ordinary peptidases. * **Clinical Correlation:** Glutathione Synthetase deficiency is the most common defect in the γ-glutamyl cycle, leading to 5-oxoprolinuria (pyroglutamic aciduria), metabolic acidosis, and hemolytic anemia. * **G6PD Connection:** Reduced glutathione (GSH) is essential for maintaining erythrocyte membrane integrity; its regeneration requires NADPH from the HMP shunt.

Question 194: Putrescine, a polyamine, is derived from which amino acid?

A. Arginine
B. Ornithine (Correct Answer)
C. Yohimbine
D. Argininosuccinate

Explanation: **Explanation:** The correct answer is **Ornithine**. **1. Why Ornithine is Correct:** Polyamines (Putrescine, Spermidine, and Spermine) are essential for cell growth and proliferation. The synthesis begins with the amino acid **Ornithine**. The rate-limiting step is the decarboxylation of Ornithine by the enzyme **Ornithine Decarboxylase (ODC)**, which requires Pyridoxal Phosphate (PLP) as a cofactor. This reaction directly produces **Putrescine**. Subsequently, Putrescine is converted to Spermidine and then to Spermine through the addition of propylamino groups derived from S-adenosylmethionine (SAM). **2. Why Incorrect Options are Wrong:** * **Arginine:** While Arginine is a precursor to Ornithine (via the enzyme Arginase in the Urea cycle), it is not the *immediate* precursor of Putrescine. * **Yohimbine:** This is an alpha-2 adrenergic antagonist used clinically for erectile dysfunction; it is a plant-derived alkaloid, not an amino acid or a polyamine precursor. * **Argininosuccinate:** This is an intermediate in the Urea cycle formed from Citrulline and Aspartate. It is cleaved into Arginine and Fumarate but does not directly lead to polyamine synthesis. **3. Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Ornithine Decarboxylase (ODC) is the key regulatory enzyme for polyamine synthesis. * **Cancer Link:** ODC activity is often highly elevated in rapidly proliferating cancer cells, making it a target for certain chemotherapeutic research. * **Cofactor:** Like most decarboxylation reactions in biochemistry, ODC requires **Vitamin B6 (PLP)**. * **Function:** Polyamines are highly cationic; they bind to and stabilize DNA/RNA, playing a vital role in cell division.

Question 195: Which amino acid is most stable at physiological pH?

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

Explanation: **Explanation:** The stability and buffering capacity of an amino acid at physiological pH (approximately 7.4) are determined by its **pKa value**. An amino acid is most "stable" in terms of its ionization state and most effective as a buffer when its pKa is closest to the pH of the environment. **1. Why Histidine is Correct:** Histidine is the only amino acid with an ionizable side chain (imidazole ring) that has a **pKa of approximately 6.0**. Because this value is relatively close to the physiological pH of 7.4, histidine exists in a state where it can easily function as both a proton donor and acceptor. In the context of protein structure (like Hemoglobin), histidine residues remain largely uncharged or "stable" in their neutral form compared to more strongly basic amino acids, making it the primary contributor to the buffering capacity of proteins at physiological pH. **2. Why the Other Options are Incorrect:** * **Lysine (pKa ~10.5):** At pH 7.4, lysine is far below its pKa, meaning it is almost entirely protonated and carries a strong positive charge. * **Arginine (pKa ~12.5):** With a very high pKa, arginine is the most basic amino acid and remains fully protonated (positively charged) at physiological pH. * **Glycine:** As the simplest amino acid, it lacks an ionizable side chain. Its α-carboxyl (pKa ~2.3) and α-amino (pKa ~9.6) groups are not near physiological pH, meaning it exists primarily as a zwitterion but lacks the specific side-chain stability/buffering utility of histidine. **High-Yield NEET-PG Pearls:** * **Buffering Capacity:** Histidine is the most important amino acid for the buffering action of **Hemoglobin** (Bohr Effect). * **Essentiality:** Histidine is considered semi-essential (essential during periods of rapid growth/childhood). * **Precursor:** Histidine is the precursor for **Histamine** via the enzyme histidine decarboxylase (requires Vitamin B6/PLP). * **FIGLU Test:** Formiminoglutamate (FIGLU) excretion in urine is a clinical marker for **Folic Acid deficiency**, as histidine metabolism requires THF.

Question 196: What are the sources of nitrogen in the urea cycle?

A. Aspartate and ammonia (Correct Answer)
B. Glutamate and ammonia
C. Arginine and ammonia
D. Uric acid

Explanation: The urea cycle (Ornithine cycle) is the primary mechanism for detoxifying ammonia into urea in the liver. To form one molecule of urea $[NH_2-CO-NH_2]$, two atoms of nitrogen are required. ### **Why Option A is Correct** The two nitrogen atoms in urea originate from two distinct sources: 1. **Free Ammonia ($NH_3$):** The first nitrogen enters the cycle via **Carbamoyl Phosphate Synthetase I (CPS-I)**. This ammonia is primarily derived from the oxidative deamination of glutamate by glutamate dehydrogenase. 2. **Aspartate:** The second nitrogen enters the cycle during the formation of **argininosuccinate**. The enzyme argininosuccinate synthetase condenses citrulline with aspartate, incorporating the amino group of aspartate into the cycle. ### **Analysis of Incorrect Options** * **B. Glutamate and ammonia:** While glutamate is the primary source of free ammonia (via deamination), it does not donate its nitrogen *directly* into the cycle steps; it must first be converted to ammonia or transaminated to aspartate. * **C. Arginine and ammonia:** Arginine is an intermediate *within* the cycle. While its cleavage by arginase releases urea, it is not the original source of the nitrogen atoms. * **D. Uric acid:** Uric acid is the end product of purine metabolism in humans, not a nitrogen donor for the urea cycle. ### **NEET-PG High-Yield Pearls** * **Rate-limiting step:** CPS-I (requires **N-acetylglutamate** as an essential allosteric activator). * **Cellular Localization:** The cycle is "split"—the first two steps occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. * **Link to TCA Cycle:** The "Kreb’s Bicycle" refers to the connection via **Fumarate**, which is released by argininosuccinate lyase and can enter the TCA cycle. * **Most common urea cycle disorder:** Ornithine Transcarbamoylase (OTC) deficiency (X-linked recessive).

Question 197: Cysteine is considered a nonessential amino acid only in the presence of dietary

A. Methionine (Correct Answer)
B. Serine
C. Folate
D. Phenylalanine

Explanation: **Explanation:** The correct answer is **Methionine**. **Why Methionine is correct:** Cysteine is synthesized in the human body through the **transsulfuration pathway**. This process requires two precursors: **Methionine** (an essential amino acid) and **Serine** (a nonessential amino acid). * Methionine provides the **sulfur atom** (via the intermediate Homocysteine). * Serine provides the **carbon skeleton**. Since the sulfur atom must come from Methionine, Cysteine can only be synthesized if there is an adequate dietary supply of Methionine. If Methionine is deficient, Cysteine becomes an "essential" amino acid. Therefore, Cysteine is "spared" by Methionine. **Why other options are incorrect:** * **Serine:** While Serine provides the carbon backbone for Cysteine, it is a nonessential amino acid that the body can synthesize from glucose (via 3-phosphoglycerate). Thus, its dietary presence is not mandatory for Cysteine synthesis. * **Folate:** Folate is a cofactor in the remethylation of Homocysteine back to Methionine, but it does not provide the structural components for Cysteine. * **Phenylalanine:** This is the precursor for Tyrosine, not Cysteine. **High-Yield Clinical Pearls for NEET-PG:** 1. **Cystathionine β-synthase (CBS):** This is the rate-limiting enzyme of the transsulfuration pathway, requiring **Vitamin B6 (Pyridoxine)** as a cofactor. 2. **Homocystinuria:** A deficiency in CBS leads to an accumulation of Homocysteine. In these patients, Cysteine becomes a **truly essential amino acid** because the pathway to synthesize it is blocked. 3. **Sparing Action:** Just as Methionine spares Cysteine, Phenylalanine spares Tyrosine.

Question 198: Tryptophan is:

A. Glucogenic
B. Ketogenic
C. Both glucogenic and ketogenic (Correct Answer)
D. None

Explanation: ### Explanation **1. Why the correct answer is right:** Amino acids are classified as glucogenic, ketogenic, or both based on the metabolic intermediates produced during their catabolism. **Tryptophan** is an aromatic amino acid that undergoes a complex degradative pathway. Its breakdown yields two distinct fragments: * **Alanine:** Which is converted into **Pyruvate**, a precursor for gluconeogenesis (making it **glucogenic**). * **Acetoacetate:** Which is a ketone body precursor (making it **ketogenic**). Because it contributes to both glucose and ketone body synthesis, it is classified as both glucogenic and ketogenic. **2. Why the incorrect options are wrong:** * **Option A (Glucogenic only):** While Tryptophan does produce pyruvate, this option is incomplete as it ignores the production of acetoacetate. Most non-essential amino acids are purely glucogenic. * **Option B (Ketogenic only):** Only two amino acids are "purely" ketogenic: **Leucine and Lysine**. Tryptophan does not belong to this exclusive group. * **Option D:** Incorrect, as Tryptophan has a well-defined metabolic fate in human biochemistry. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Mnemonic for "Both":** Remember **"PITTT"** — **P**henylalanine, **I**soleucine, **T**yrosine, **T**ryptophan, and **T**hreonine are both glucogenic and ketogenic. * **Tryptophan Derivatives:** It is the precursor for **Serotonin** (5-HT), **Melatonin**, **Niacin** (Vitamin B3), and **Indole/Skatole**. * **Hartnup Disease:** A clinical condition caused by the deficiency of a transporter for neutral amino acids (like Tryptophan) in the kidneys and intestine, leading to pellagra-like symptoms. * **Conversion Ratio:** 60 mg of Tryptophan yields 1 mg of Niacin.

Question 199: Which of the following is an acidic amino acid?

A. Aspartic acid (Correct Answer)
B. Arginine
C. Glutamic acid
D. Lysine

Explanation: **Explanation:** Amino acids are classified based on the chemical nature of their side chains (R-groups). **Acidic amino acids** are those that contain a second carboxyl group (-COOH) in their side chain, which carries a negative charge at physiological pH (7.4). **Why Aspartic Acid is Correct:** Aspartic acid (Aspartate) and Glutamic acid (Glutamate) are the only two standard acidic amino acids. They possess a carboxylic acid group in their side chains, making them polar and negatively charged. *Note on the Question:* Both Option A and Option C are technically acidic amino acids. However, in the context of single-best-answer exams like NEET-PG, **Aspartic acid** is the classic representative often tested. (If this were a multiple-choice question, both A and C would be correct). **Analysis of Incorrect Options:** * **Arginine (Option B):** This is a **basic** amino acid. It contains a guanidino group and carries a positive charge at physiological pH. * **Glutamic acid (Option C):** While also an acidic amino acid, if the question requires a single choice and follows standard recall patterns, Aspartic acid is frequently the primary answer provided in keys. * **Lysine (Option D):** This is a **basic** amino acid. It contains an $\epsilon$-amino group in its side chain. **High-Yield Clinical Pearls for NEET-PG:** * **Basic Amino Acids:** Remember the mnemonic **"HAL"** (Histidine, Arginine, Lysine). Arginine is the most basic. * **Charge at pH 7.4:** Acidic amino acids are **anions** (negative), while basic amino acids are **cations** (positive). * **Urea Cycle Connection:** Aspartate provides the second nitrogen atom in the urea cycle, reacting with citrulline to form argininosuccinate. * **Excitatory Neurotransmitters:** Both Glutamate and Aspartate act as excitatory neurotransmitters in the Central Nervous System.

Question 200: Which of the following are acidic amino acids?

A. Asparagine
B. Arginine
C. Glutamine
D. None of the above (Correct Answer)

Explanation: ### Explanation The classification of amino acids based on their side-chain (R-group) charge is a high-yield topic for NEET-PG. **1. Why "None of the above" is correct:** Acidic amino acids are those that contain a **carboxyl group (-COOH)** in their side chain, which carries a negative charge at physiological pH (7.4). There are only two acidic amino acids: * **Aspartic acid (Aspartate)** * **Glutamic acid (Glutamate)** None of the options provided (Asparagine, Arginine, or Glutamine) belong to this category. **2. Analysis of Incorrect Options:** * **Asparagine (A) and Glutamine (C):** These are the **amide derivatives** of aspartic acid and glutamic acid, respectively. While they are polar, their side chains are **uncharged (neutral)** at physiological pH. They are neither acidic nor basic. * **Arginine (B):** This is a **basic amino acid**. Basic amino acids (Arginine, Lysine, and Histidine) contain nitrogenous groups in their side chains that carry a positive charge at physiological pH. **3. High-Yield Clinical Pearls for NEET-PG:** * **Charge at pH 7.4:** Acidic amino acids are **negatively charged** (Anions), while basic amino acids (Arg, Lys) are **positively charged** (Cations). * **Histidine Exception:** Histidine is the only basic amino acid that functions as a significant **buffer** at physiological pH because its pKa (~6.0) is close to the body's pH. * **Urea Cycle Connection:** Aspartate provides the second nitrogen atom in the urea cycle, while Glutamate acts as the primary "collector" of amino groups (via transamination) for ammonia detoxification. * **Excitatory Neurotransmitters:** Both Glutamate and Aspartate serve as major excitatory neurotransmitters in the CNS.

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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