Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 271: Metabolism of proline takes place in which cellular organelle?

A. Mitochondria (Correct Answer)
B. Cell membrane
C. Ribosome
D. All of the above

Explanation: **Explanation:** The metabolism of proline, specifically its degradation and synthesis, primarily occurs within the **mitochondria**. 1. **Why Mitochondria is correct:** Proline is unique because it is an imino acid. Its catabolism begins with the enzyme **proline oxidase** (also known as proline dehydrogenase), which is located on the inner mitochondrial membrane. This enzyme converts proline to pyrroline-5-carboxylate (P5C), which is then converted to **glutamate** by P5C dehydrogenase. Since these enzymes are mitochondrial, the organelle serves as the central hub for proline flux. 2. **Why other options are incorrect:** * **Cell membrane:** This structure is primarily involved in transport (via amino acid transporters) and signal transduction, not the enzymatic degradation of amino acids. * **Ribosomes:** These are the sites of protein synthesis (translation) where proline is incorporated into polypeptide chains, but they do not host the metabolic pathways for its breakdown or synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Hyperprolinemia Type I:** Caused by a deficiency of **proline oxidase**. * **Hyperprolinemia Type II:** Caused by a deficiency of **P5C dehydrogenase**. A key diagnostic feature is the excretion of P5C in the urine. * **Collagen Connection:** Proline and its derivative, hydroxyproline, constitute about 25% of collagen. Hydroxylation of proline requires **Vitamin C**; deficiency leads to Scurvy due to defective collagen cross-linking. * **Glutamate Link:** Proline is a non-essential, glucogenic amino acid that enters the TCA cycle via glutamate and α-ketoglutarate.

Question 272: Which of the following metabolites is a derivative of tryptophan?

A. Melatonin (Correct Answer)
B. Thyroxine
C. Epinephrine
D. Norepinephrine

Explanation: **Explanation:** **Tryptophan** is an essential aromatic amino acid that serves as a precursor for several biologically active compounds. The correct answer is **Melatonin** because Tryptophan follows the pathway: *Tryptophan → 5-Hydroxytryptophan → Serotonin (5-HT) → N-acetylserotonin → **Melatonin**.* This conversion occurs primarily in the pineal gland, where melatonin regulates the circadian rhythm (sleep-wake cycle). **Analysis of Incorrect Options:** * **Thyroxine (T4), Epinephrine, and Norepinephrine:** These are all derivatives of the amino acid **Tyrosine**. * Tyrosine is converted into **Thyroxine** in the thyroid gland via iodination of thyroglobulin. * In the adrenal medulla and CNS, Tyrosine follows the catecholamine pathway: *Tyrosine → L-DOPA → Dopamine → Norepinephrine → Epinephrine.* **High-Yield Clinical Pearls for NEET-PG:** 1. **Niacin Synthesis:** Tryptophan is also a precursor for **Niacin (Vitamin B3)**. Approximately 60 mg of Tryptophan yields 1 mg of Niacin. 2. **Hartnup Disease:** A defect in the transport of neutral amino acids (like Tryptophan) in the gut and kidneys, leading to pellagra-like symptoms due to Niacin deficiency. 3. **Carcinoid Syndrome:** In patients with carcinoid tumors, up to 60% of Tryptophan is diverted to produce Serotonin, which can lead to secondary Niacin deficiency (Pellagra). 4. **Key Enzyme:** Tryptophan hydroxylase is the rate-limiting enzyme in serotonin synthesis, requiring Tetrahydrobiopterin ($BH_4$) as a cofactor.

Question 273: Hyperphenylalaninemia is due to a defect in which enzyme?

A. Phenylalanine hydroxylase (Correct Answer)
B. Tyrosinase
C. Homogentisic acid oxidase
D. Ornithine transcarbamylase

Explanation: ### Explanation **Correct Option: A. Phenylalanine hydroxylase** Hyperphenylalaninemia, most commonly manifesting as **Phenylketonuria (PKU)**, is primarily caused by a deficiency of the hepatic enzyme **Phenylalanine hydroxylase (PAH)**. This enzyme is responsible for the conversion of the essential amino acid Phenylalanine into Tyrosine. This reaction requires the cofactor **Tetrahydrobiopterin (BH4)**. A defect in PAH leads to the accumulation of Phenylalanine in the blood and tissues, which is then diverted into alternative pathways forming phenylketones (phenylpyruvate, phenyllactate), leading to neurotoxicity and intellectual disability. **Analysis of Incorrect Options:** * **B. Tyrosinase:** A deficiency in this enzyme leads to **Albinism**. Tyrosinase is responsible for converting Tyrosine into Melanin; its defect does not cause Phenylalanine accumulation. * **C. Homogentisic acid oxidase:** A defect here causes **Alkaptonuria**, characterized by the accumulation of homogentisic acid, resulting in ochronosis (darkening of tissues) and dark urine upon standing. * **D. Ornithine transcarbamylase (OTC):** This is an enzyme of the **Urea Cycle**. Its deficiency leads to Hyperammonemia and increased orotic acid levels, not hyperphenylalaninemia. **High-Yield Clinical Pearls for NEET-PG:** * **Mousy/Musty Odor:** A classic clinical sign of PKU due to phenylacetate in sweat and urine. * **Maternal PKU:** If a mother with PKU does not maintain a low-phenylalanine diet during pregnancy, the fetus may suffer from microcephaly and congenital heart defects. * **BH4 Deficiency:** About 2% of hyperphenylalaninemia cases are due to a deficiency in **Dihydropteridine reductase** (which regenerates BH4). These patients require BH4 supplementation in addition to dietary restriction. * **Guthrie Test:** A semi-quantitative bacterial inhibition assay used for neonatal screening of PKU.

Question 274: Conversion of norepinephrine to epinephrine occurs by which biochemical process?

A. Methylation (Correct Answer)
B. Decarboxylation
C. Oxidation
D. Sulphation

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 Methylation is Correct:** In this reaction, a methyl group is transferred from **S-adenosylmethionine (SAM)**, which acts as the universal methyl donor, to the nitrogen atom of norepinephrine. This specific step occurs primarily in the **adrenal medulla**. Interestingly, this enzyme is induced by **glucocorticoids** (cortisol) reaching the medulla via the intra-adrenal portal system. 2. **Why other options are incorrect:** * **Decarboxylation:** This process occurs earlier in the pathway when **L-Dopa** is converted to **Dopamine** by the enzyme Dopa decarboxylase (requiring Vitamin B6). * **Oxidation:** While the pathway involves hydroxylation (a form of oxidation), such as Tyrosine to L-Dopa, the specific conversion of norepinephrine to epinephrine does not involve oxygen addition or electron loss in the functional group. * **Sulphation:** This is a phase II detoxification reaction used for the excretion of catecholamines in urine, not for their synthesis. **NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme** of catecholamine synthesis: **Tyrosine Hydroxylase**. * **Sequence of synthesis:** Tyrosine → L-Dopa → Dopamine → Norepinephrine → Epinephrine. * **Cofactor for PNMT:** S-adenosylmethionine (SAM). * **Vitamin C** is a required cofactor for **Dopamine β-hydroxylase** (Dopamine → Norepinephrine). * **VMA (Vanillylmandellic acid)** is the end-stage urinary metabolite of both norepinephrine and epinephrine, used to diagnose Pheochromocytoma.

Question 275: Which of the following amino acids is involved in the synthesis of thyroxine?

A. Glycine
B. Methionine
C. Threonine
D. Tyrosine (Correct Answer)

Explanation: **Explanation:** **Tyrosine** is the correct answer because it serves as the direct structural precursor for thyroid hormones (**T3 and T4/Thyroxine**). Within the thyroid follicle, tyrosine residues on the protein **thyroglobulin** undergo iodination (forming Monoiodotyrosine and Diiodotyrosine) and subsequent coupling to produce thyroxine. **Analysis of Options:** * **Tyrosine (Correct):** Beyond thyroxine, it is also the precursor for **Catecholamines** (Dopamine, Norepinephrine, Epinephrine) and **Melanin**. * **Glycine (Incorrect):** While a versatile precursor, it is involved in the synthesis of **Heme, Creatine, Purines, and Glutathione**, but not thyroid hormones. * **Methionine (Incorrect):** An essential sulfur-containing amino acid primarily involved in **methylation reactions** (via S-adenosylmethionine/SAM) and the initiation of protein synthesis. * **Threonine (Incorrect):** An essential amino acid primarily involved in the structure of O-linked glycoproteins and mucins; it does not contribute to hormone synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Precursor Rule:** Remember the "T" rule: **Tyrosine** leads to **T**hyroxine and **T**hree catecholamines. * **Enzyme Deficiency:** A defect in the conversion of Phenylalanine to Tyrosine (via Phenylalanine Hydroxylase) leads to **Phenylketonuria (PKU)**. * **Iodination:** The enzyme **Thyroid Peroxidase (TPO)** is responsible for both the iodination of tyrosine residues and the coupling reaction. * **Other Precursors:** * **Tryptophan:** Precursor for Serotonin, Melatonin, and Niacin (B3). * **Histidine:** Precursor for Histamine. * **Arginine:** Precursor for Nitric Oxide (NO) and Urea.

Question 276: Serotonin is a derivative of which amino acid?

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

Explanation: **Explanation:** **1. Why Tryptophan is correct:** Serotonin (5-hydroxytryptamine or 5-HT) is synthesized from the essential amino acid **Tryptophan**. The synthesis occurs in a two-step pathway: * **Step 1:** Tryptophan is hydroxylated to 5-hydroxytryptophan by the enzyme *Tryptophan hydroxylase* (the rate-limiting step), which requires **Tetrahydrobiopterin ($BH_4$)** as a cofactor. * **Step 2:** 5-hydroxytryptophan is decarboxylated to Serotonin by *Aromatic L-amino acid decarboxylase*, requiring **Pyridoxal Phosphate (Vitamin $B_6$)**. **2. Why the other options are incorrect:** * **Tyrosine:** This is the precursor for Catecholamines (Dopamine, Norepinephrine, Epinephrine), Thyroid hormones ($T_3, T_4$), and Melanin. * **Phenylalanine:** This is the precursor for Tyrosine. A deficiency in the enzyme converting Phenylalanine to Tyrosine leads to Phenylketonuria (PKU). * **Melatonin:** This is not an amino acid; it is a hormone synthesized **from** Serotonin in the pineal gland. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Carcinoid Syndrome:** Tumors (usually in the ileum) secrete excessive serotonin. This "diverts" tryptophan from Niacin synthesis, leading to **Pellagra-like symptoms** (Dermatitis, Diarrhea, Dementia). * **Metabolism:** Serotonin is degraded by Monoamine Oxidase (MAO) into **5-HIAA** (5-Hydroxyindoleacetic acid), which is measured in 24-hour urine samples to diagnose Carcinoid syndrome. * **Melatonin Synthesis:** Tryptophan $\rightarrow$ Serotonin $\rightarrow$ Melatonin. * **Niacin Synthesis:** Tryptophan is also a precursor for $NAD^+/NADP^+$ (60 mg of Tryptophan = 1 mg of Niacin).

Question 277: Which amino acid lacks chirality?

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

Explanation: ### Explanation **Correct Answer: D. Glycine** **1. Why Glycine is the Correct Answer:** Chirality in amino acids depends on the presence of an **asymmetric (chiral) alpha-carbon**. A carbon is chiral when it is bonded to four different chemical groups. The general structure of an amino acid includes an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom (-H), and a variable side chain (R-group). In **Glycine**, the R-group is simply another **hydrogen atom**. Because the alpha-carbon is bonded to two identical hydrogen atoms, it lacks asymmetry. Therefore, Glycine is the only **achiral** (optically inactive) proteinogenic amino acid. It does not exist in D- or L- isomeric forms. **2. Why the Other Options are Incorrect:** * **A, B, and C (Lysine, Leucine, Histidine):** These are all standard amino acids with unique side chains (R-groups) that are distinct from the hydrogen, carboxyl, and amino groups. Consequently, their alpha-carbons are bonded to four different groups, making them **chiral** and **optically active**. In human proteins, these exist specifically in the **L-configuration**. **3. NEET-PG High-Yield Clinical Pearls:** * **Smallest Amino Acid:** Due to its tiny side chain (H), Glycine is the smallest amino acid, allowing it to fit into tight spaces, such as the central core of the **Collagen triple helix**. * **Inhibitory Neurotransmitter:** Glycine acts as a major inhibitory neurotransmitter in the **spinal cord** (whereas GABA is primary in the brain). * **Precursor Functions:** Glycine is essential for the synthesis of **Heme, Purines, Creatine, and Glutathione**. * **Proline vs. Glycine:** While Glycine lacks chirality, **Proline** is unique for being an alpha-imino acid (secondary amino group), often acting as a "helix breaker."

Question 278: Melatonin is synthesized from which amino acid?

A. Histidine
B. Methionine
C. Tryptophan (Correct Answer)
D. Phenylalanine

Explanation: **Explanation:** **Correct Answer: C. Tryptophan** Melatonin, the hormone responsible for regulating the circadian rhythm (sleep-wake cycle), is synthesized in the **pineal gland** from the essential amino acid **Tryptophan**. The metabolic pathway follows this sequence: 1. **Tryptophan** is converted to **5-Hydroxytryptophan** (by Tryptophan hydroxylase). 2. It is then decarboxylated to **Serotonin** (5-Hydroxytryptamine). 3. Serotonin undergoes N-acetylation and O-methylation to form **Melatonin**. **Analysis of Incorrect Options:** * **A. Histidine:** This is the precursor for **Histamine**, a mediator of allergic reactions and gastric acid secretion. * **B. Methionine:** While Methionine provides the methyl group (via S-adenosylmethionine) for the final step of melatonin synthesis, it is not the primary structural precursor. It is mainly involved in translation initiation and one-carbon metabolism. * **D. Phenylalanine:** This is the precursor for Tyrosine, which further leads to the synthesis of **Catecholamines** (Dopamine, Norepinephrine, Epinephrine), **Thyroxine**, and **Melanin** (not to be confused with Melatonin). **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The conversion of Serotonin to N-acetylserotonin by **Serotonin N-acetyltransferase** is the rate-limiting step in melatonin synthesis. * **Light Sensitivity:** Melatonin secretion is inhibited by light and stimulated by darkness (via the suprachiasmatic nucleus). * **Other Tryptophan Derivatives:** Besides Melatonin and Serotonin, Tryptophan is a precursor for **Niacin (Vitamin B3)**. * **Hartnup Disease:** A defect in the transport of neutral amino acids (like Tryptophan) leading to pellagra-like symptoms.

Question 279: Increased levels of alanine in serum after fasting suggest what?

A. Increased release of alanine from muscle (Correct Answer)
B. Reduced amino acid utilization for gluconeogenesis
C. Break in continuity of plasma membrane resulting in leakage of amino acids
D. Decreased uptake of alanine by liver

Explanation: ### Explanation **1. Why Option A is Correct: The Glucose-Alanine Cycle (Cahill Cycle)** During fasting, the body shifts to a catabolic state to maintain blood glucose levels. Skeletal muscle begins to break down proteins into amino acids. While many amino acids are oxidized locally, **alanine** and **glutamine** serve as the primary carriers of nitrogen to the liver. In the muscle, pyruvate (from glycolysis or amino acid carbon skeletons) undergoes transamination to form alanine. This alanine is released into the bloodstream and transported to the liver, where it serves as a major substrate for **gluconeogenesis**. Therefore, increased serum alanine during fasting is a direct physiological marker of increased muscle proteolysis and the operation of the Cahill Cycle. **2. Why the Other Options are Incorrect:** * **Option B:** During fasting, amino acid utilization for gluconeogenesis is actually **increased**, not reduced, to prevent hypoglycemia. * **Option C:** While membrane leakage can occur in pathological states (like rhabdomyolysis), the rise of alanine in fasting is a regulated metabolic response, not a result of structural cell damage. * **Option D:** In a fasting state, the liver **increases** its uptake of alanine to convert it into glucose. Decreased uptake would impair glucose homeostasis. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **The Cahill Cycle:** Muscle sends Alanine to the Liver $\rightarrow$ Liver converts Alanine to Glucose $\rightarrow$ Glucose returns to Muscle. * **Key Enzyme:** **ALT (Alanine Transaminase)** requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. * **Nitrogen Transport:** Alanine is the primary transporter of nitrogen from muscle to liver; **Glutamine** is the primary transporter from most other peripheral tissues. * **Glucogenic vs. Ketogenic:** Alanine is a purely **glucogenic** amino acid.

Question 280: Defective branched-chain ketoacid decarboxylation is characteristic of which disorder?

A. Maple syrup urine disease (Correct Answer)
B. Hanup disease
C. Alkaptonuria
D. GM1 gangliosidosis

Explanation: ### Explanation **1. Why Maple Syrup Urine Disease (MSUD) is Correct:** MSUD is caused by a deficiency in the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD) complex**. This multi-enzyme complex is responsible for the oxidative decarboxylation of ketoacids derived from the three branched-chain amino acids (BCAAs): **Leucine, Isoleucine, and Valine**. A defect in this step leads to the toxic accumulation of these amino acids and their corresponding alpha-ketoacids in the blood and urine, giving the urine a characteristic burnt-sugar smell. **2. Analysis of Incorrect Options:** * **B. Hartnup Disease:** This is a transport defect involving **neutral amino acids** (specifically Tryptophan) in the renal tubules and intestine. It presents with pellagra-like symptoms due to niacin deficiency, not a decarboxylation defect. * **C. Alkaptonuria:** This is caused by a deficiency of **Homogentisate oxidase** in the tyrosine catabolic pathway. It leads to the accumulation of homogentisic acid, causing dark urine and ochronosis. * **D. GM1 Gangliosidosis:** This is a **lysosomal storage disorder** caused by a deficiency of $\beta$-galactosidase, leading to the accumulation of GM1 gangliosides in neurons. It is not related to amino acid metabolism. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic (I Love Vermont):** **I**soleucine, **L**eucine, **V**aline are the BCAAs involved. * **Cofactors:** The BCKAD complex requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (**T**ender **R**everence **F**or **N**ew **L**ife). * **Clinical Presentation:** Neonatal onset of encephalopathy, feeding difficulties, and the classic maple syrup odor. * **Diagnosis:** Elevated levels of BCAAs in plasma and **Alloisoleucine** (pathognomonic marker). * **Treatment:** Dietary restriction of BCAAs; some patients respond to high doses of **Thiamine** (Thiamine-responsive MSUD).

Practice by Chapter

Protein Digestion and Absorption

Practice Questions

Transamination and Deamination

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Urea Cycle

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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

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Synthesis of Biologically Important Compounds from Amino Acids

Practice Questions

Nitrogen Balance

Practice Questions

Ammonia Metabolism and Toxicity

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One-Carbon Transfer Reactions

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