Amino Acid Metabolism Practice Questions

Q: Which amino acid, upon decarboxylation, yields a product with potent vasodilator properties?

Histidine. **Explanation:** The correct answer is **Histidine**. This question tests the knowledge of biogenic amines derived from amino acids via decarboxylation reactions. **1. Why Histidine is Correct:** Histidine undergoes decarboxylation catalyzed by the enzyme **histidine decarboxylase** (requiring Pyridoxal Phosphate/PLP as a cofactor) to form **Histamine**. Histamine is a potent vasodilator that increases capillary permeability and plays a central role in allergic responses, gastric acid secretion, and neurotransmission. **2. Analysis of Incorrect Options:** * **Arginine:** While Arginine is the precursor for **Nitric Oxide (NO)**—a potent vasodilator—this conversion occurs via the enzyme *Nitric Oxide Synthase (NOS)*, not simple decarboxylation. Decarboxylation of Arginine yields Agmatine. * **Glutamate:** Decarboxylation of Glutamate yields **GABA** (Gamma-Aminobutyric Acid), which is the primary inhibitory neurotransmitter in the CNS, not a vasodilator. * **Aspartate:** Decarboxylation of Aspartate yields **beta-alanine**, a component of Pantothenic acid (Vitamin B5) and Coenzyme A. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Cofactor Rule:** Almost all amino acid decarboxylation reactions require **Vitamin B6 (Pyridoxal Phosphate)**. * **Histamine Receptors:** H1 (allergy/vasodilation), H2 (gastric acid secretion). * **Other Key Decarboxylations:** * Tyrosine → Dopamine (via DOPA) * Tryptophan → Serotonin (via 5-HTP) * Lysine → Cadaverine (ptomaine) * **Mast Cells:** These are the primary storage sites for histamine in the body.

Q: How is ammonia removed from the brain predominantly?

Glutamine. **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).

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

Glutamine. **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).

Q: Which of the following clinical laboratory observations is suggestive of Hartnup disease?

High fecal levels of tryptophan and indole derivatives. **Explanation:** **Hartnup disease** is an autosomal recessive disorder caused by a mutation in the **SLC6A19 gene**, which encodes a sodium-dependent neutral amino acid transporter. This defect occurs in both the **proximal renal tubules** and the **intestinal mucosa**. 1. **Why Option D is correct:** Because the intestinal transport of neutral amino acids (especially **Tryptophan**) is defective, these amino acids remain in the gut. Colonic bacteria act upon the unabsorbed tryptophan, converting it into **indole derivatives**. Consequently, patients exhibit high fecal levels of tryptophan and indoles. These indoles are absorbed into the portal circulation and excreted in the urine (e.g., indican), sometimes leading to the "Blue Diaper Syndrome." 2. **Why other options are incorrect:** * **Option A:** A "burnt-sugar" or maple syrup smell in urine is characteristic of **Maple Syrup Urine Disease (MSUD)**, caused by a deficiency in the branched-chain alpha-keto acid dehydrogenase complex. * **Option B:** High plasma phenylalanine levels are the hallmark of **Phenylketonuria (PKU)**, typically due to a deficiency of phenylalanine hydroxylase. * **Option C:** Extremely high levels of citrulline (Citrullinemia) indicate a defect in the urea cycle, specifically the enzyme **argininosuccinate synthetase**. **Clinical Pearls for NEET-PG:** * **The 3 D’s:** Hartnup disease presents with Pellagra-like symptoms: **Dermatitis** (photosensitive rash), **Diarrhea**, and **Dementia** (ataxia/neuropsychiatric symptoms). * **Biochemical Basis:** The symptoms arise because Tryptophan is a precursor for **Niacin (Vitamin B3)** synthesis. * **Diagnosis:** Characterized by **neutral aminoaciduria** (alanine, serine, threonine, valine, leucine, isoleucine, phenylalanine, tyrosine, and tryptophan). * **Treatment:** High-protein diet and **Nicotinamide** supplementation.

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

Hyper-ammonemia. **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.

Q: Mousy urine odor in a child is due to a defect in the conversion of phenylalanine to which of the following substances?

Tyrosine. **Explanation:** The clinical presentation of **mousy urine odor** in a child is a hallmark sign of **Phenylketonuria (PKU)**. This autosomal recessive disorder is primarily caused by a deficiency of the enzyme **Phenylphenylalanine hydroxylase (PAH)**. **1. Why Tyrosine is Correct:** Under normal physiological conditions, Phenylalanine hydroxylase converts the essential amino acid **Phenylalanine into Tyrosine**. When this enzyme is defective, phenylalanine cannot be converted to tyrosine, leading to its accumulation in the blood and tissues. Tyrosine becomes a "conditionally essential" amino acid in these patients. **2. Why Incorrect Options are Wrong:** * **Homogentisic acid:** This is an intermediate in the tyrosine degradation pathway. A defect in its breakdown (due to homogentisate oxidase deficiency) leads to **Alkaptonuria**, characterized by urine that turns black upon standing, not a mousy odor. * **Phenyl acetate & Phenyl pyruvate:** These are **alternative metabolites** (phenylketones) produced when phenylalanine levels are high. While these substances are actually responsible for the mousy odor, the question asks which substance phenylalanine *fails* to be converted into due to the primary defect. **3. NEET-PG High-Yield Pearls:** * **Enzyme Defect:** Most commonly Phenylalanine hydroxylase; rarely due to deficiency of **Dihydrobiopterin reductase (BH4)**. * **Clinical Features:** Intellectual disability, "mousy" or "musty" body odor, fair skin/blue eyes (due to decreased melanin synthesis from tyrosine), and seizures. * **Diagnosis:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry. * **Management:** Dietary restriction of phenylalanine and supplementation of tyrosine. Avoid **Aspartame** (contains phenylalanine).

Q: Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is due to deficiency of which enzyme?

Ornithine translocase. **Explanation:** **HHH Syndrome** is an autosomal recessive urea cycle disorder caused by a mutation in the **SLC25A15 gene**, which encodes the **Ornithine Translocase (ORNT1)** protein. 1. **Why Ornithine Translocase is correct:** The urea cycle occurs in both the mitochondria and the cytosol. Ornithine is produced in the cytosol but must be transported into the mitochondria to react with carbamoyl phosphate (via OTC). Ornithine translocase is the specific transporter responsible for this shuttle. When deficient, ornithine cannot enter the mitochondria, leading to: * **Hyperornithinemia:** Accumulation of ornithine in the cytosol/blood. * **Hyperammonemia:** The urea cycle is "stalled" due to lack of mitochondrial ornithine, leading to ammonia buildup. * **Homocitrullinuria:** In the absence of ornithine, the enzyme OTC uses **lysine** as an alternative substrate, producing homocitrulline, which is excreted in the urine. 2. **Why the other options are incorrect:** * **Ornithine permease:** This is a general term for transport proteins in bacteria/lower organisms; it is not the specific human transporter involved in HHH syndrome. * **Ornithine decarboxylase:** This enzyme converts ornithine to putrescine (polyamine synthesis); its deficiency does not cause the HHH triad. * **Ornithine transcarbamoylase (OTC):** Deficiency causes the most common urea cycle disorder (X-linked), characterized by hyperammonemia and orotic aciduria, but *not* hyperornithinemia. **High-Yield Clinical Pearls for NEET-PG:** * **The Triad:** Hyperornithinemia, Hyperammonemia, and Homocitrullinuria. * **Genetics:** Autosomal Recessive (unlike OTC deficiency, which is X-linked). * **Clinical Presentation:** Post-prandial irritability, lethargy, seizures, and characteristic **spastic paraplegia** in chronic cases. * **Diagnosis:** Elevated plasma ornithine and urinary homocitrulline.

Q: All of the following vitamins are involved in sulfur-containing amino acid metabolism except?

Vitamin B3 (Niacin). ### Explanation The metabolism of sulfur-containing amino acids (Methionine and Cysteine) relies heavily on specific B-complex vitamins acting as cofactors. **Vitamin B3 (Niacin)** is the correct answer because it is not directly involved in the primary metabolic pathways of these amino acids (transmethylation and transsulfuration). #### Why Vitamin B3 is the Exception: Niacin (as NAD+/NADP+) is primarily involved in redox reactions and the TCA cycle. While it plays a role in general energy metabolism, it does not serve as a specific cofactor for the enzymes converting methionine to homocysteine or homocysteine to cysteine. #### Role of Other Vitamins: * **Vitamin B6 (Pyridoxine):** Essential for the **Transsulfuration pathway**. It is a cofactor for *Cystathionine β-synthase* (CBS) and *Cystathionase*, which convert Homocysteine to Cysteine. * **Vitamin B9 (Folate) & Vitamin B12 (Cobalamin):** Both are critical for the **Remethylation pathway**. Vitamin B12 is a cofactor for *Methionine Synthase*, while B9 (as N5-methyl THF) provides the methyl group to convert Homocysteine back into Methionine. #### High-Yield Clinical Pearls for NEET-PG: 1. **Hyperhomocysteinemia:** Deficiencies in B6, B9, or B12 lead to elevated homocysteine levels, a significant risk factor for coronary artery disease and thrombosis. 2. **Homocystinuria:** The most common cause is a deficiency of the B6-dependent enzyme **Cystathionine β-synthase**. 3. **Methylfolate Trap:** A B12 deficiency "traps" folate in the N5-methyl THF form, leading to functional folate deficiency and megaloblastic anemia. 4. **Cysteine** becomes an **essential amino acid** if there is a block in the transsulfuration pathway.

Question 1

Which amino acid, upon decarboxylation, yields a product with potent vasodilator properties?

Accepted Answer

Histidine

Answer

Arginine

Answer

Glutamate

Answer

Aspartate

Question 2

How is ammonia removed from the brain predominantly?

Accepted Answer

Glutamine

Answer

Asparagine

Answer

Alanine

Answer

Cysteine

Question 3

Which of the following statements correctly describe the transamination reaction?

Accepted Answer

The transaminase enzyme binds covalently to pyridoxal phosphate

Answer

Net deamination with splitting of ammonia

Answer

α-ketoglutarate is the amino group donor

Answer

Glutamate is always formed

Question 4

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

Accepted Answer

Glutamine

Answer

Glycine

Answer

Isoleucine

Answer

Proline

Question 5

Which of the following clinical laboratory observations is suggestive of Hartnup disease?

Accepted Answer

High fecal levels of tryptophan and indole derivatives

Answer

Burnt-sugar smell in urine

Answer

High plasma phenylalanine levels

Answer

Extremely high levels of citrulline in urine

Question 6

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

Accepted Answer

Hyper-ammonemia

Answer

Hypo-ammonemia

Answer

Hyperventilation

Answer

Hypoventilation

Question 7

Mousy urine odor in a child is due to a defect in the conversion of phenylalanine to which of the following substances?

Accepted Answer

Tyrosine

Answer

Homogentisic acid

Answer

Phenyl acetate

Answer

Phenyl pyruvate

Question 8

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is due to deficiency of which enzyme?

Accepted Answer

Ornithine translocase

Answer

Ornithine permease

Answer

Ornithine decarboxylase

Answer

Ornithine transcarbamoylase

Question 9

All of the following vitamins are involved in sulfur-containing amino acid metabolism except?

Accepted Answer

Vitamin B3 (Niacin)

Answer

Vitamin B6 (Pyridoxine)

Answer

Vitamin B9 (Folate)

Answer

Vitamin B12 (Cobalamin)

Question 10

Mental retardation is not a feature in which of the following aminoacidurias?

Accepted Answer

Albinism

Answer

Phenylketonuria

Answer

Homocystinuria

Answer

Maple syrup urine disease

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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