Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 81: How is ammonia removed from the brain predominantly?

A. Asparagine
B. Alanine
C. Glutamine (Correct Answer)
D. Cysteine

Explanation: **Explanation:** Ammonia is highly toxic to the central nervous system. In the brain, the predominant mechanism for ammonia detoxification is the synthesis of **Glutamine**. 1. **Mechanism (Why Glutamine is correct):** Ammonia ($\text{NH}_3$) crosses the blood-brain barrier and is taken up by **astrocytes**. Here, the enzyme **Glutamine Synthetase** catalyzes the ATP-dependent condensation of ammonia with glutamate to form glutamine. Glutamine is a non-toxic, neutral amino acid that can safely be transported out of the brain into the blood and eventually to the kidneys or liver for further processing. 2. **Why other options are incorrect:** * **Alanine:** This is the primary transport form of ammonia from **skeletal muscle** to the liver via the Glucose-Alanine cycle (Cahill cycle). * **Asparagine:** While structurally similar to glutamine, it is not the primary vehicle for ammonia removal in the brain. * **Cysteine:** This is a sulfur-containing amino acid involved in glutathione synthesis but plays no direct role in systemic ammonia transport. **High-Yield NEET-PG Pearls:** * **The "Glutamate Depletion" Theory:** Excessive ammonia in the brain consumes $\alpha$-ketoglutarate (via glutamate dehydrogenase) and glutamate (via glutamine synthetase). This depletes TCA cycle intermediates and the neurotransmitter glutamate, leading to the energy failure seen in **Hepatic Encephalopathy**. * **Astrocyte Swelling:** Accumulation of glutamine exerts an osmotic effect, drawing water into astrocytes, which leads to cerebral edema. * **Rate-limiting enzyme in Brain:** Glutamine Synthetase. * **Rate-limiting enzyme in Liver (Urea Cycle):** Carbamoyl Phosphate Synthetase I (CPS-I).

Question 82: Which amino acid is required for the formation of thyroxine?

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

Explanation: **Explanation:** **Tyrosine** is the correct answer because it serves as the direct structural precursor for thyroid hormones (T3 and T4). The synthesis occurs within the thyroid gland on the protein **thyroglobulin**. Specific tyrosine residues undergo iodination to form Monoiodotyrosine (MIT) and Diiodotyrosine (DIT). The coupling of two DIT molecules results in **Thyroxine (T4)**. **Analysis of Incorrect Options:** * **Tryptophan (A):** This is the precursor for Serotonin, Melatonin, and Niacin (Vitamin B3). It is not involved in thyroid hormone synthesis. * **Glutamine (C):** This acts as a major nitrogen donor in the body and is essential for purine/pyrimidine synthesis and ammonia detoxification, but not for thyroxine. * **Cysteine (D):** A sulfur-containing amino acid involved in glutathione synthesis and disulfide bond formation; it does not contribute to the structure of thyroxine. **High-Yield Clinical Pearls for NEET-PG:** * **Precursor Rule:** Tyrosine is also the precursor for **Catecholamines** (Dopamine, Norepinephrine, Epinephrine) and **Melanin**. * **Enzyme Link:** The conversion of Phenylalanine to Tyrosine is catalyzed by *Phenylalanine hydroxylase*. A deficiency leads to Phenylketonuria (PKU). * **Iodination:** The enzyme **Thyroid Peroxidase (TPO)** is responsible for both the oxidation of iodide and the iodination of tyrosine residues. * **Wolff-Chaikoff Effect:** An autoregulatory phenomenon where high levels of circulating iodide inhibit the organification of iodine (iodination of tyrosine).

Question 83: Which of the following statements correctly describe the transamination reaction?

A. Net deamination with splitting of ammonia
B. α-ketoglutarate is the amino group donor
C. The transaminase enzyme binds covalently to pyridoxal phosphate (Correct Answer)
D. Glutamate is always formed

Explanation: **Explanation:** Transamination is the first step in the catabolism of most amino acids, involving the transfer of an α-amino group to an α-keto acid. **Why Option C is correct:** All transaminases (aminotransferases) require **Pyridoxal Phosphate (PLP)**, a derivative of Vitamin B6, as a mandatory cofactor. During the reaction, the aldehyde group of PLP forms a covalent **Schiff base linkage** (aldimine bond) with the ε-amino group of a specific **lysine residue** at the enzyme's active site. This covalent intermediate is essential for the "ping-pong" mechanism where the amino group is temporarily stored as pyridoxamine phosphate. **Why other options are incorrect:** * **Option A:** Transamination involves the **transfer** of an amino group, not its removal. There is **no net deamination** and no free ammonia is released. Deamination (release of NH₃) primarily occurs later via Glutamate Dehydrogenase. * **Option B:** α-ketoglutarate is typically the amino group **acceptor**, not the donor. It accepts the amino group to become Glutamate. * **Option C:** While Glutamate is the most common product (as α-ketoglutarate is the preferred acceptor), it is not "always" formed. For example, in the AST reaction, oxaloacetate accepts an amino group to form **Aspartate**. **High-Yield NEET-PG Pearls:** * **Exceptions:** Lysine, Threonine, Proline, and Hydroxyproline **do not** undergo transamination. * **Diagnostic Markers:** ALT (SGPT) and AST (SGOT) are clinical markers for liver and heart damage. ALT is more specific for liver disease. * **Cofactor:** Always associate **Vitamin B6 (PLP)** with transamination, decarboxylation, and cystathionine synthesis.

Question 84: Figlu is an intermediate product of the metabolism of which amino acid?

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

Explanation: **Explanation:** **Histidine** is the correct answer because **FIGLU (Formiminoglutamic acid)** is a key intermediate in its catabolic pathway. Histidine is first converted to urocanic acid and then to FIGLU. The enzyme **glutamate formiminotransferase** then transfers the formimino group from FIGLU to Tetrahydrofolate (THF), converting FIGLU into **Glutamate**. **Why the other options are incorrect:** * **Arginine:** It is converted to Ornithine via the urea cycle and subsequently to Glutamate-γ-semialdehyde, bypassing the FIGLU intermediate. * **Cystine:** This is a disulfide of cysteine. Its metabolism primarily involves conversion to pyruvate or taurine, involving sulfur-specific pathways. * **Methionine:** It follows the transmethylation and transsulfuration pathways, leading to the formation of S-adenosylmethionine (SAM), Homocysteine, and eventually Succinyl-CoA. **Clinical Pearls & High-Yield Facts for NEET-PG:** 1. **FIGLU Excretion Test:** In **Folic Acid deficiency**, the conversion of FIGLU to glutamate is blocked because THF (the acceptor) is unavailable. Consequently, FIGLU levels rise and it is excreted in the urine. This is a sensitive functional test for folate status. 2. **Histidinemia:** A deficiency of the enzyme **histidase** leads to elevated histidine levels in blood and urine; it is generally a benign condition but must be differentiated from other metabolic disorders. 3. **Precursor Role:** Histidine is the precursor for **Histamine** (via decarboxylation requiring Vitamin B6). 4. **Glucogenic Status:** Since histidine ultimately forms Glutamate, which enters the TCA cycle as α-ketoglutarate, it is classified as a purely **glucogenic** amino acid.

Question 85: Hartnup disease is due to defective transport of which amino acid?

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

Explanation: **Explanation:** **Hartnup disease** is an autosomal recessive disorder characterized by a defect in the **SLC6A19 transporter** protein. This transporter is responsible for the sodium-dependent neutral amino acid transport in the proximal renal tubules and the brush border of the small intestine. **1. Why Tryptophan is the Correct Answer:** While the defect affects all neutral amino acids (alanine, valine, leucine, isoleucine, phenylalanine, tyrosine, and tryptophan), the clinical manifestations are primarily due to the **malabsorption of Tryptophan**. Tryptophan is the essential precursor for **Niacin (Vitamin B3)** synthesis via the kynurenine pathway. A deficiency in Tryptophan leads to a secondary deficiency of Niacin, resulting in **Pellagra-like symptoms**. **2. Why Incorrect Options are Wrong:** * **Histidine (A):** While it is a neutral amino acid, its deficiency does not drive the clinical pathology of Hartnup disease. Histidine metabolism is primarily linked to FIGLU excretion and histamine production. * **Arginine (B) & Lysine (C):** These are **basic amino acids**. Defects in the transport of basic amino acids (COAL: Cystine, Ornithine, Arginine, Lysine) are seen in **Cystinuria**, not Hartnup disease. **3. NEET-PG High-Yield Clinical Pearls:** * **Clinical Triad:** Dermatitis (photosensitive rash), Cerebellar Ataxia, and Aminoaciduria. * **Diagnosis:** Characterized by **"Neutral Aminoaciduria"** (detected via chromatography). Notably, proline and hydroxyproline levels remain normal (distinguishing it from Fanconi syndrome). * **Treatment:** High-protein diet and **Nicotinamide (Niacin)** supplementation. * **Blue Diaper Syndrome:** A related condition where bacterial breakdown of unabsorbed tryptophan in the gut leads to indicanuria, turning diapers blue.

Question 86: In the liver, ammonia is primarily formed from which amino acid?

A. Glycine
B. Glutamine (Correct Answer)
C. Isoleucine
D. Proline

Explanation: **Explanation:** In the liver, **Glutamine** is the primary source of ammonia. While ammonia is toxic to peripheral tissues, it is safely transported to the liver in the form of Glutamine (the "non-toxic carrier of ammonia"). Once it reaches the liver mitochondria, the enzyme **Glutaminase** hydrolyzes Glutamine into **Glutamate and free ammonia ($NH_3$)**. This released ammonia then enters the Urea Cycle to be detoxified and excreted. **Why other options are incorrect:** * **Glycine:** While glycine can contribute to ammonia via the glycine cleavage system, it is not the *primary* source in the liver. It is more significant in heme and purine synthesis. * **Isoleucine:** This is a branched-chain amino acid (BCAA). BCAAs are primarily metabolized in the **skeletal muscle**, not the liver, due to the absence of BCAA transaminase in hepatic tissue. * **Proline:** Proline is a secondary amino acid (imino acid) that undergoes oxidation to glutamate, but it does not serve as a major direct ammonia donor in the liver compared to glutamine. **NEET-PG High-Yield Pearls:** * **Glutamine** is the most abundant free amino acid in the blood. * **Two-step release:** Ammonia in the liver is released via **Glutaminase** (from Glutamine) and **Glutamate Dehydrogenase** (from Glutamate). * **Brain Protection:** In the brain, ammonia is detoxified by converting Glutamate to Glutamine (via Glutamine Synthetase). High ammonia levels deplete Alpha-ketoglutarate, impairing the TCA cycle—a key mechanism in **Hepatic Encephalopathy**. * **Kidney Role:** Glutamine is also the primary source of ammonia in the kidneys, which is essential for acid-base balance (buffering $H^+$ ions).

Question 87: Which of the following is a sulphur-containing amino acid?

A. Cysteine (Correct Answer)
B. Lysine
C. Arginine
D. Phenylalanine

Explanation: **Explanation:** **Correct Option: A. Cysteine** Amino acids are categorized based on the chemical properties of their side chains (R-groups). **Cysteine** and **Methionine** are the two primary sulfur-containing amino acids found in proteins. Cysteine contains a highly reactive **sulfhydryl (-SH) group**, which is crucial for forming **disulfide bonds** (bridges) between polypeptide chains, providing structural stability to proteins like insulin and keratin. **Incorrect Options:** * **B. Lysine:** This is a basic, positively charged amino acid. It contains an $\epsilon$-amino group in its side chain. * **C. Arginine:** Also a basic amino acid, it contains a **guanidino group**. It is the most basic amino acid and a precursor for Nitric Oxide (NO). * **D. Phenylalanine:** This is an aromatic amino acid containing a benzene ring. It is a precursor for Tyrosine. **High-Yield Clinical Pearls for NEET-PG:** * **Homocystinuria:** A classic metabolic disorder often caused by a deficiency in *Cystathionine $\beta$-synthase*, leading to an accumulation of homocysteine (another sulfur-containing amino acid). * **Glutathione:** Cysteine is the rate-limiting amino acid for the synthesis of Glutathione (GSH), the body's master antioxidant. * **Cystinuria:** A defect in the renal transport of COAL (Cystine, Ornithine, Arginine, Lysine), leading to hexagonal cystine stones in the urine. * **Taurine:** A sulfur-containing derivative of cysteine used in bile acid conjugation. * **Methionine** is an essential amino acid, whereas **Cysteine** is semi-essential (derived from methionine).

Question 88: Carnitine-acylcarnitine translocase acts as an inner membrane exchanger. Carnitine is synthesized from:

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

Explanation: **Explanation:** **Correct Answer: C. Lysine** Carnitine (β-hydroxy-γ-trimethylaminobutyrate) is an essential cofactor required for the transport of long-chain fatty acids across the inner mitochondrial membrane for β-oxidation. It is synthesized primarily in the liver and kidneys from two essential amino acids: **Lysine** and **Methionine**. * **Mechanism:** The carbon skeleton and the nitrogen atom of carnitine are derived from **L-Lysine**. Specifically, lysine residues in certain proteins are methylated using **S-adenosylmethionine (SAM)** as the methyl donor to form trimethyllysine, which is then converted to carnitine through a series of reactions involving Vitamin C, Iron, Vitamin B6, and Niacin. **Analysis of Incorrect Options:** * **A. Alanine:** A non-essential amino acid primarily involved in glucose-alanine cycle and transamination; it does not contribute to carnitine synthesis. * **B. Arginine:** A precursor for Nitric Oxide (NO), urea, and creatine, but not carnitine. * **D. Glycine:** Involved in the synthesis of heme, purines, creatine, and glutathione, but not carnitine. **NEET-PG High-Yield Pearls:** 1. **Carnitine Shuttle:** Essential for fatty acid oxidation. A deficiency leads to non-ketotic hypoglycemia and muscle weakness. 2. **Cofactor Requirement:** The synthesis of carnitine requires **Vitamin C** (as a cofactor for hydroxylases). This explains why muscle weakness is a clinical feature of Scurvy. 3. **Precursors:** Remember the mnemonic **"ML"** (Methionine and Lysine) for Carnitine synthesis. 4. **Rate-limiting step:** The transport of fatty acids into mitochondria via **CPT-1** (inhibited by Malonyl-CoA) is the rate-limiting step of β-oxidation.

Question 89: Blood Urea Nitrogen is decreased in which of the following conditions?

A. Hypo-ammonemia
B. Hyper-ammonemia (Correct Answer)
C. Hyperventilation
D. Hypoventilation

Explanation: **Explanation:** **Understanding the Concept:** Blood Urea Nitrogen (BUN) is the end product of the **Urea Cycle**, which takes place primarily in the liver. The purpose of this cycle is to convert toxic ammonia ($NH_3$) into non-toxic urea for excretion by the kidneys. In **Hyper-ammonemia**, there is a failure in the urea cycle (due to liver failure or enzyme deficiencies like Ornithine Transcarbamylase deficiency). Because the liver cannot process ammonia into urea, ammonia levels rise in the blood while **BUN levels significantly decrease**. Therefore, a low BUN in the presence of neurological symptoms is a classic marker for urea cycle defects or severe hepatic dysfunction. **Analysis of Options:** * **Option A (Hypo-ammonemia):** This is a state of low ammonia. Since ammonia is the substrate for urea, low ammonia would not typically cause a pathological decrease in BUN; rather, it reflects efficient clearance. * **Options C & D (Hyper/Hypoventilation):** These relate to acid-base balance ($CO_2$ levels) and respiratory compensation. While severe alkalosis can theoretically interfere with ion exchange, they do not directly govern the enzymatic rate of the urea cycle. **NEET-PG High-Yield Pearls:** * **Normal BUN:** 7–20 mg/dL. * **Commonest cause of low BUN:** Pregnancy (due to increased GFR), SIADH, and severe liver disease. * **Commonest cause of high BUN:** Renal failure (Azotemia), high protein diet, and GI bleed (blood breakdown increases ammonia load). * **Key Enzyme:** Carbamoyl Phosphate Synthetase I (CPS-I) is the rate-limiting step of the urea cycle, requiring N-acetylglutamate (NAG) as an activator.

Question 90: Which tripeptide is primarily involved in oxidation-reduction reactions?

A. Creatinine
B. Glutathione (Correct Answer)
C. Melanin
D. None of the above

Explanation: **Explanation:** **Glutathione (GSH)** is the correct answer because it is a vital **tripeptide** composed of three amino acids: **Glutamate, Cysteine, and Glycine**. Its primary physiological role is maintaining the redox state of the cell. The thiol (-SH) group of the cysteine residue acts as a reducing agent, neutralizing reactive oxygen species (ROS) and free radicals. During this process, two reduced glutathione molecules (GSH) are oxidized to form a disulfide-linked dimer (GSSG). The enzyme **Glutathione Reductase** then restores GSSG back to GSH using NADPH as a cofactor. **Analysis of Incorrect Options:** * **Creatinine (A):** This is a breakdown product of creatine phosphate in muscles. It is a metabolic waste product excreted by the kidneys and is not a tripeptide involved in redox reactions. * **Melanin (C):** This is a complex polymer derived from the amino acid **Tyrosine**. It is a pigment responsible for skin, hair, and eye color, providing protection against UV radiation, but it is not a tripeptide. **High-Yield Clinical Pearls for NEET-PG:** * **Unique Linkage:** In glutathione, the linkage between Glutamate and Cysteine is a **$\gamma$-glutamyl bond**, which protects the peptide from degradation by ordinary aminopeptidases. * **G6PD Deficiency:** This is clinically linked to glutathione. A deficiency in G6PD leads to decreased NADPH production, meaning glutathione cannot be regenerated. This results in oxidative stress and **hemolysis** (Heinz bodies). * **Drug Detoxification:** Glutathione is essential for the detoxification of **Acetaminophen (Paracetamol)**. In toxicity, glutathione stores are depleted, leading to hepatic necrosis. The antidote, **N-acetylcysteine**, works by replenishing glutathione levels.

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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