Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 51: Cabbage-like odor in urine is typically found in which condition?

A. Defect in leucine catabolism
B. Defect in valine catabolism
C. Tyrosinemia type 1 (Correct Answer)
D. None of the above

Explanation: **Explanation:** **Tyrosinemia Type 1** (also known as Hepatorenal Tyrosinemia) is caused by a deficiency of the enzyme **Fumarylacetoacetate Hydrolase (FAH)**, the final enzyme in the tyrosine degradation pathway. This deficiency leads to the accumulation of fumarylacetoacetate and its byproduct, **succinylacetone**. The characteristic **"cabbage-like" or "boiled cabbage" odor** in the urine and breath is attributed to these accumulated metabolites and their interaction with sulfur-containing compounds. **Analysis of Options:** * **Option A & B (Leucine and Valine catabolism):** Defects in the catabolism of branched-chain amino acids (Leucine, Isoleucine, and Valine) lead to **Maple Syrup Urine Disease (MSUD)**. As the name suggests, the characteristic odor in MSUD is that of burnt sugar or maple syrup, not cabbage. * **Option C (Tyrosinemia Type 1):** This is the correct association. The condition is clinically significant for causing progressive liver failure, renal tubular dysfunction (Fanconi syndrome), and an increased risk of hepatocellular carcinoma. **NEET-PG High-Yield Pearls (Odor Associations):** * **Mousy/Musty odor:** Phenylketonuria (PKU) * **Sweaty feet odor:** Isovaleric acidemia / Glutaric acidemia type II * **Rotten fish odor:** Trimethylaminuria * **Swimming pool odor:** Hawkinsinuria * **Cat’s urine odor:** β-methylcrotonylglycinuria * **Treatment for Tyrosinemia Type 1:** **Nitisinone (NTBC)**, which prevents the formation of toxic succinylacetone by inhibiting an upstream enzyme (4-hydroxyphenylpyruvate dioxygenase).

Question 52: Which amino acid can cause hypoglycemia?

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

Explanation: **Explanation:** **Leucine** is the correct answer because it is a potent stimulator of insulin secretion from the pancreatic beta cells. It acts via two primary mechanisms: 1. It serves as a substrate for glutamate dehydrogenase (GDH), increasing the production of alpha-ketoglutarate, which enhances ATP production and triggers insulin release. 2. It directly activates the mTOR pathway in beta cells. In clinical conditions like **Hyperinsulinism-Hyperammonemia (HI/HA) syndrome**, mutations in the GDH enzyme make it hypersensitive to Leucine, leading to profound postprandial hypoglycemia (Leucine-induced hypoglycemia). **Analysis of Incorrect Options:** * **Phenylalanine (A):** An aromatic amino acid that is both glucogenic and ketogenic. While it can stimulate some insulin release, it does not clinically cause hypoglycemia; its primary clinical relevance is Phenylketonuria (PKU). * **Lysine (B):** A purely ketogenic amino acid. Like Leucine, it cannot be converted to glucose, but it lacks the specific potent insulinotropic effect required to drive blood glucose levels down to hypoglycemic ranges. * **Valine (D):** A purely glucogenic branched-chain amino acid (BCAA). Since it is converted into Succinyl-CoA to enter the TCA cycle for gluconeogenesis, it would theoretically support blood glucose levels rather than lower them. **NEET-PG High-Yield Pearls:** * **Purely Ketogenic Amino Acids:** Leucine and Lysine (The "L"s). * **Maple Syrup Urine Disease (MSUD):** Involves a defect in the metabolism of BCAAs (Leucine, Isoleucine, Valine). * **Clinical Correlation:** Leucine sensitivity is a classic diagnostic feature in certain pediatric cases of persistent hyperinsulinemic hypoglycemia of infancy (PHHI).

Question 53: The biosynthesis of Epinephrine from Norepinephrine requires which of the following?

A. Pyridoxal phosphate
B. Biotin
C. Cytochrome P450
D. S-adenosyl methionine (Correct Answer)

Explanation: **Explanation:** The conversion of **Norepinephrine to Epinephrine** is the final step in the catecholamine biosynthetic pathway. This reaction is a **methylation** process catalyzed by the enzyme **Phenylethanolamine N-methyltransferase (PNMT)**. 1. **Why S-adenosyl methionine (SAM) is correct:** SAM is the universal methyl group donor in the body. In this reaction, PNMT transfers a methyl group from SAM to the nitrogen atom of norepinephrine to form epinephrine. This step occurs primarily in the adrenal medulla and is induced by cortisol. 2. **Why other options are incorrect:** * **Pyridoxal phosphate (PLP/Vit B6):** Acts as a cofactor for **decarboxylation** reactions (e.g., DOPA to Dopamine). * **Biotin (Vit B7):** Acts as a cofactor for **carboxylation** reactions (e.g., Pyruvate to Oxaloacetate). * **Cytochrome P450:** Involved in hydroxylation and detoxification in the liver, but not in the methylation of catecholamines. (Note: Dopamine $\beta$-hydroxylase, which forms norepinephrine, requires Vitamin C and Copper, not P450). **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Tyrosine hydroxylase (Tyrosine $\rightarrow$ DOPA), which requires **Tetrahydrobiopterin (BH4)**. * **Cortisol Connection:** Cortisol from the adrenal cortex reaches the medulla via the portal system and **upregulates PNMT**, ensuring epinephrine production during stress. * **SAM Metabolism:** After donating a methyl group, SAM is converted to **S-adenosylhomocysteine (SAH)**, which is further metabolized to Homocysteine. * **Sequence:** Tyrosine $\rightarrow$ DOPA $\rightarrow$ Dopamine $\rightarrow$ Norepinephrine $\rightarrow$ Epinephrine.

Question 54: Which of the following is an allosteric activator of hepatic glutamate dehydrogenase?

A. ATP
B. GTP
C. ADP (Correct Answer)
D. NADH

Explanation: **Explanation:** **Glutamate Dehydrogenase (GDH)** is a key mitochondrial enzyme that catalyzes the reversible oxidative deamination of glutamate into $\alpha$-ketoglutarate and ammonia. This reaction serves as a critical bridge between amino acid metabolism and the TCA cycle. **1. Why ADP is the Correct Answer:** The activity of GDH is tightly regulated by the **energy status** of the cell. When cellular energy levels are low, **ADP** (and GDP) concentrations rise. ADP acts as a potent **allosteric activator** of GDH, signaling the need for more fuel. By activating GDH, ADP promotes the conversion of glutamate to $\alpha$-ketoglutarate, which then enters the TCA cycle to generate ATP. **2. Why the Other Options are Incorrect:** * **ATP and GTP (Options A & B):** These are indicators of high cellular energy. They act as **allosteric inhibitors** of GDH. When energy is abundant, the cell does not need to break down amino acids for fuel. * **NADH (Option D):** As a product of the GDH reaction and a carrier of high-energy electrons, high levels of NADH signal that the energy demand is met. Therefore, NADH acts as an **inhibitor** of the enzyme. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Dual Coenzyme Specificity:** GDH is unique because it can use either **NAD+** (primarily for oxidative deamination/catabolism) or **NADP+** (primarily for reductive amination/anabolism). * **Hyperinsulinism-Hyperammonemia Syndrome:** Mutations that abolish the GTP-binding (inhibitory) site of GDH lead to overactive enzyme activity. This results in excessive ammonia production and triggered insulin release (leucine-induced hypoglycemia). * **Directionality:** In the liver, the reaction primarily proceeds toward **ammonia production** for the urea cycle.

Question 55: Which of the following is a glucogenic amino acid?

A. Leucine
B. Lysine
C. Glutamine (Correct Answer)
D. None of the above

Explanation: ### Explanation **1. Why Glutamine is Correct:** Amino acids are classified based on their metabolic end-products. **Glutamine** is a purely **glucogenic** amino acid. In the liver and kidneys, glutamine is converted by the enzyme glutaminase into glutamate, which is then deaminated to form **$\alpha$-ketoglutarate**. Since $\alpha$-ketoglutarate is an intermediate of the TCA cycle, it can be used for gluconeogenesis to produce glucose. Glutamine also serves as the most abundant free amino acid in the body and acts as a major non-toxic carrier of ammonia. **2. Why the Other Options are Incorrect:** * **Leucine (Option A) and Lysine (Option B):** These are the only two **purely ketogenic** amino acids. They are metabolized directly into acetyl-CoA or acetoacetate (precursors for ketone bodies) and cannot be used to synthesize glucose. * **Note on Mixed Amino Acids:** Isoleucine, Phenylalanine, Tryptophan, and Tyrosine are both glucogenic and ketogenic. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **The "L" Rule:** Remember that the two amino acids starting with **L** (Leucine and Lysine) are purely ketogenic. * **Alanine’s Role:** Alanine is the most important glucogenic amino acid in the fasting state (via the Glucose-Alanine cycle). * **Glutamine & Acid-Base Balance:** In the kidneys, the conversion of glutamine to glutamate releases ammonia ($NH_3$), which buffers $H^+$ ions to form $NH_4^+$. This is a crucial renal mechanism for correcting metabolic acidosis. * **Essentiality:** While glutamine is non-essential, it is considered "conditionally essential" during periods of severe metabolic stress or trauma.

Question 56: Which of the following is not an essential amino acid?

A. Tryptophan
B. Threonine
C. Histidine
D. Cysteine (Correct Answer)

Explanation: ### Explanation **Concept Overview:** Amino acids are classified as **essential** (must be obtained from the diet) or **non-essential** (can be synthesized by the body). There are 10 essential amino acids, often remembered by the mnemonic **PVT TIM HALL**. **Why Cysteine is the Correct Answer:** **Cysteine** is a **non-essential** amino acid. Although it contains sulfur, it is synthesized in the body from **Methionine** (an essential amino acid) and **Serine**. Specifically, the sulfur atom is derived from methionine, while the carbon skeleton comes from serine via the cystathionine pathway. Since the body can produce it internally, it is not required in the diet. **Analysis of Incorrect Options:** * **A. Tryptophan:** An essential amino acid. It is a precursor for Serotonin, Melatonin, and Niacin (Vitamin B3). * **B. Threonine:** A strictly essential amino acid. It is one of the few amino acids that does not undergo transamination (along with Lysine). * **C. Histidine:** An essential amino acid. It is often considered "semi-essential" because while adults can synthesize it in small amounts, it is vital for growth in children and during periods of rapid protein synthesis. **NEET-PG High-Yield Pearls:** * **Mnemonic (PVT TIM HALL):** **P**henylalanine, **V**aline, **T**hreonine, **T**ryptophan, **I**soleucine, **M**ethionine, **H**istidine, **A**rginine, **L**eucine, **L**ysine. * **Semi-essential:** Arginine and Histidine (required during growth/pregnancy). * **Purely Ketogenic:** Leucine and Lysine. * **Clinical Correlation:** In **Homocystinuria**, the enzyme cystathionine β-synthase is deficient, making Cysteine a **conditionally essential** amino acid for these patients.

Question 57: Protein is purified using ammonium sulfate by which method?

A. Salting out (Correct Answer)
B. Ion exchange chromatography
C. Mass chromatography
D. Molecular size exclusion

Explanation: **Explanation:** **Salting out** is the correct answer because it is the primary biochemical technique used for protein purification and precipitation using high concentrations of neutral salts like **ammonium sulfate** $((NH_4)_2SO_4)$. * **Mechanism:** At high salt concentrations, the salt ions (cations and anions) compete with protein molecules for water molecules (solvation). As the salt ions become hydrated, the "free" water available to solvate the protein decreases. This exposes the hydrophobic patches on the protein surface, causing them to aggregate and precipitate out of the solution. Ammonium sulfate is preferred because it is highly soluble, inexpensive, and generally does not denature the protein. **Analysis of Incorrect Options:** * **B. Ion exchange chromatography:** This separates proteins based on their **net charge** using a stationary phase (anion or cation exchangers). * **C. Mass chromatography (Mass Spectrometry):** This is used to determine the **molecular weight** and chemical structure of a molecule by measuring the mass-to-charge ratio of ions. * **D. Molecular size exclusion (Gel Filtration):** This separates proteins based on their **size and shape** using porous beads; larger molecules elute first as they cannot enter the pores. **High-Yield Clinical Pearls for NEET-PG:** * **Hofmeister Series:** This ranks ions based on their ability to salt out proteins; Citrate and Sulfate are highly effective. * **Dialysis:** After salting out, dialysis is typically performed to remove the excess ammonium sulfate from the protein sample. * **Salting In:** At low concentrations, salt actually *increases* protein solubility by shielding the ionic charges of the protein.

Question 58: Creatinine is formed from all of the following precursors except?

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

Explanation: **Explanation:** The synthesis of **Creatine** (the precursor to Creatinine) is a high-yield topic in biochemistry, involving three specific amino acids and two primary organs (Kidney and Liver). **Why Asparagine is the Correct Answer:** Asparagine is a non-essential amino acid derived from oxaloacetate. It plays no role in the creatine biosynthetic pathway. Creatinine is formed by the spontaneous, non-enzymatic cyclization of creatine phosphate in the muscles; therefore, any amino acid not involved in making creatine is the correct "except" choice. **Analysis of Incorrect Options (Precursors of Creatine):** 1. **Glycine (Option A):** In the kidney, Glycine combines with Arginine to form Guanidinoacetate (GAA) via the enzyme *AGAT*. It provides the backbone of the creatine molecule. 2. **Arginine (Option B):** Arginine acts as the donor of the amidino group during the first step of synthesis in the kidney. 3. **Methionine (Option C):** In the liver, Methionine (in the form of **S-adenosylmethionine or SAM**) provides the methyl group necessary to convert Guanidinoacetate into Creatine. **NEET-PG Clinical Pearls:** * **Site of Synthesis:** Starts in the **Kidney** (GAA formation) and is completed in the **Liver** (Methylation). * **Storage:** 95% of creatine is stored in skeletal muscle as **Creatine Phosphate**, a high-energy reservoir. * **Excretion:** Creatinine is excreted in the urine. Its excretion rate is relatively constant and proportional to total **muscle mass**, making it a reliable marker for GFR (Glomerular Filtration Rate). * **Mnemonic:** Remember **"GAM"** (Glycine, Arginine, Methionine) for Creatine synthesis.

Question 59: All of the following are essential amino acids except:

A. Methionine
B. Lysine
C. Alanine (Correct Answer)
D. Leucine

Explanation: ### Explanation **Core Concept: Classification of Amino Acids** Amino acids are classified as **essential** or **non-essential** based on whether the body can synthesize them de novo. Essential amino acids cannot be synthesized by the human body and must be obtained through the diet. **1. Why Alanine is the Correct Answer:** **Alanine** is a **non-essential amino acid**. It is synthesized in the body primarily via the transamination of pyruvate (a glycolytic intermediate) by the enzyme Alanine Aminotransferase (ALT). Since the body can produce it internally, it is not a dietary requirement. **2. Why the Other Options are Incorrect:** * **Methionine (A):** An essential, sulfur-containing amino acid. It is the precursor for S-adenosylmethionine (SAM), the body's universal methyl donor. * **Lysine (B):** A strictly ketogenic essential amino acid. It is vital for protein synthesis and collagen cross-linking. * **Leucine (D):** An essential branched-chain amino acid (BCAA). Along with Lysine, it is one of the two purely ketogenic amino acids. --- ### High-Yield Clinical Pearls for NEET-PG * **Mnemonic for Essential Amino Acids:** **"PVT TIM HALL"** * **P**henylalanine, **V**aline, **T**hreonine * **T**ryptophan, **I**soleucine, **M**ethionine * **H**istidine, **A**rginine*, **L**eucine, **L**ysine * **Semi-essential Amino Acids:** Arginine and Histidine are considered semi-essential because they are required in larger quantities during periods of rapid growth (e.g., childhood or pregnancy). * **Purely Ketogenic:** Leucine and Lysine (The "L"s). * **Both Glucogenic and Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, and Isoleucine (Mnemonic: **PITTT**). * **Clinical Link:** Alanine plays a crucial role in the **Cahill Cycle (Glucose-Alanine Cycle)**, transporting nitrogen from the muscles to the liver during fasting.

Question 60: Oxaloacetate is synthesized from which amino acid?

A. Aspartate (Correct Answer)
B. Glycine
C. Serine
D. Valine

Explanation: **Explanation:** The synthesis of **Oxaloacetate (OAA)** from **Aspartate** is a fundamental reaction in amino acid metabolism, occurring via a single-step **transamination** process. **1. Why Aspartate is Correct:** Aspartate is a four-carbon dicarboxylic amino acid. Through the action of the enzyme **Aspartate Aminotransferase (AST/SGOT)** and the cofactor **Pyridoxal Phosphate (B6)**, the amino group of aspartate is transferred to $\alpha$-ketoglutarate. This directly converts the carbon skeleton of aspartate into its corresponding $\alpha$-keto acid, which is **Oxaloacetate**. This reaction is reversible and links amino acid metabolism to the Citric Acid Cycle (TCA cycle) and gluconeogenesis. **2. Why Other Options are Incorrect:** * **Glycine:** A glucogenic amino acid that is primarily converted to **Pyruvate** (via serine) or enters the Glycine Cleavage System. * **Serine:** Converted to **Pyruvate** by the enzyme serine dehydratase. * **Valine:** A branched-chain amino acid (BCAA) that is glucogenic. Its catabolism leads to the formation of **Succinyl-CoA**, not oxaloacetate. **Clinical Pearls & High-Yield Facts:** * **Glucogenic Amino Acids:** Aspartate is purely glucogenic because OAA is a key substrate for gluconeogenesis. * **AST (SGOT):** This enzyme is a critical clinical marker; elevated levels are seen in myocardial infarction and liver cell injury. * **Malate-Aspartate Shuttle:** Aspartate and OAA are vital components of this shuttle, which transports reducing equivalents (NADH) across the inner mitochondrial membrane. * **Asparagine Connection:** Asparagine is first hydrolyzed to Aspartate (via asparaginase), which then forms OAA.

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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