Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 381: The amino acid residue having an imino side chain is:

A. Lysine
B. Histidine
C. Tyrosine
D. Proline (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Proline** **Underlying Medical Concept:** Proline is unique among the 20 standard amino acids because its side chain cyclizes back onto the backbone nitrogen atom. This creates a **secondary amine** (where the nitrogen is attached to two carbons) rather than the primary amine group found in other amino acids. Therefore, Proline is technically an **imino acid** rather than an amino acid. This cyclic structure (pyrrolidine ring) imposes significant conformational rigidity, making Proline a "helix breaker" in protein secondary structures. **Analysis of Incorrect Options:** * **A. Lysine:** Contains a primary amino group at the end of its aliphatic side chain (ε-amino group). It is a basic amino acid. * **B. Histidine:** Contains an **imidazole ring** in its side chain. While it contains nitrogen, it does not form an imino group with the peptide backbone. It is vital for its buffering capacity at physiological pH. * **C. Tyrosine:** An aromatic amino acid characterized by a **phenol (hydroxyl) group** attached to a benzene ring. It is a precursor for catecholamines and thyroid hormones. **High-Yield Facts for NEET-PG:** * **Collagen Structure:** Proline and its derivative, **Hydroxyproline**, are essential for the stability of the collagen triple helix. * **Ninhydrin Test:** Most amino acids give a purple/blue color (Ruhemann's purple) with ninhydrin, but Proline gives a characteristic **yellow color** due to its imino structure. * **Helix Disruption:** Due to its rigid ring structure, Proline is rarely found in alpha-helices but is frequently found in **beta-turns**.

Question 382: Cabbage-like odor is found in the body fluids in which of the following metabolic disorders?

A. Phenylketonuria
B. Tyrosinemia (Correct Answer)
C. Maple syrup urine disease
D. Isovaleric academia

Explanation: ### Explanation The correct answer is **Tyrosinemia (specifically Type I)**. **1. Why Tyrosinemia is correct:** Tyrosinemia Type I (Hepatorenal Tyrosinemia) is caused by a deficiency of the enzyme **Fumarylacetoacetate hydrolase (FAH)**. This leads to the accumulation of fumarylacetoacetate and its conversion into **succinylacetone**. The characteristic **cabbage-like or boiled vegetable odor** in body fluids (urine and sweat) is attributed to the accumulation of methionine metabolites and sulfur-containing compounds that occur secondary to liver damage in these patients. **2. Why the other options are incorrect:** * **Phenylketonuria (PKU):** Caused by Phenylalanine hydroxylase deficiency. It is characterized by a **mousy or musty odor** due to the accumulation of phenylacetic acid. * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the Branched-chain alpha-keto acid dehydrogenase complex. It results in a **burnt sugar or maple syrup odor** due to the accumulation of isoleucine. * **Isovaleric Acidemia:** Caused by Isovaleryl-CoA dehydrogenase deficiency. It presents with a characteristic **"sweaty feet" or "cheesy" odor** due to the accumulation of isovaleric acid. **3. Clinical Pearls for NEET-PG:** * **Tyrosinemia Type I:** High risk of early-onset **Hepatocellular Carcinoma (HCC)** and renal rickets. * **Diagnostic Marker:** Succinylacetone in urine is pathognomonic. * **Treatment:** Nitisinone (NTBC), which inhibits 4-hydroxyphenylpyruvate dioxygenase to prevent the formation of toxic metabolites. **Summary of Odors in Metabolic Disorders:** * **Mousy:** PKU * **Cabbage-like:** Tyrosinemia * **Maple Syrup:** MSUD * **Sweaty Feet:** Isovaleric acidemia / Glutaric acidemia Type II * **Fishy:** Trimethylaminuria * **Swimming Pool:** Hawkinsinuria

Question 383: Which of the following amino acids enters the TCA cycle at the succinyl CoA step?

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

Explanation: ### Explanation The entry of amino acids into the TCA cycle depends on their carbon skeleton degradation pathway. **Methionine** is a sulfur-containing amino acid that follows a specific catabolic route to enter the TCA cycle at the **Succinyl CoA** step. **Why Methionine is Correct:** Methionine is converted into **S-adenosylmethionine (SAM)**, which then forms **Homocysteine**. Homocysteine combines with serine to form cystathionine, eventually yielding **Propionyl CoA**. Propionyl CoA undergoes carboxylation (Biotin-dependent) and isomerization (Vitamin B12-dependent) to become **Succinyl CoA**. Other amino acids following this "VOMIT" mnemonic (Valine, Odd-chain fatty acids, Methionine, Isoleucine, Threonine) also enter at this step. **Analysis of Incorrect Options:** * **A. Tryptophan:** This is both glucogenic and ketogenic. It enters the TCA cycle via **Pyruvate** and **Acetoacetyl CoA**. * **B. Tyrosine:** This is also both glucogenic and ketogenic. Its carbon skeleton is degraded to **Fumarate** and **Acetoacetate**. * **C. Glycine:** Glycine is primarily converted to **Pyruvate** (via serine) or undergoes the glycine cleavage system to CO₂ and NH₄⁺. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin B12 Deficiency:** Leads to the accumulation of **Methylmalonic acid (MMA)** because the conversion of Methylmalonyl CoA to Succinyl CoA is blocked. This is a key diagnostic marker to differentiate B12 deficiency from Folate deficiency. * **Homocystinuria:** Most commonly due to a deficiency in **Cystathionine β-synthase**, an enzyme in the methionine catabolic pathway. * **Ketogenic vs. Glucogenic:** Leucine and Lysine are the only *purely* ketogenic amino acids. Methionine is *purely* glucogenic.

Question 384: Phenylbutyrate is used in urea cycle disorders because it:

A. Scavenges nitrogen (Correct Answer)
B. Activates enzymes
C. Maintains renal output
D. Maintains energy production

Explanation: **Explanation:** In Urea Cycle Disorders (UCDs), the body cannot effectively convert toxic ammonia into urea, leading to life-threatening hyperammonemia. **Phenylbutyrate** acts as a **nitrogen scavenger** by providing an alternative pathway for nitrogen excretion that bypasses the urea cycle. **Mechanism of Action:** 1. Phenylbutyrate is a prodrug that undergoes $\beta$-oxidation to form **phenylacetate**. 2. Phenylacetate conjugates with **Glutamine** (which contains two nitrogen atoms) to form **Phenylacetylglutamine**. 3. This compound is water-soluble and is excreted directly by the kidneys. 4. By removing glutamine, the body is forced to synthesize more glutamine from ammonia and glutamate (via Glutamine Synthetase), effectively lowering blood ammonia levels. **Analysis of Incorrect Options:** * **B. Activates enzymes:** Phenylbutyrate does not act as an allosteric activator (like N-acetylglutamate does for CPS-1). Its role is purely stoichiometric in removing nitrogen. * **C. Maintains renal output:** While the metabolite is excreted renally, the drug’s primary therapeutic goal is biochemical detoxification, not the maintenance of kidney hemodynamics. * **D. Maintains energy production:** UCDs cause neurotoxicity due to ammonia; while ammonia interferes with the TCA cycle (by depleting $\alpha$-ketoglutarate), Phenylbutyrate's direct mechanism is nitrogen removal, not metabolic energy support. **NEET-PG High-Yield Pearls:** * **Sodium Benzoate:** Another scavenger used in UCDs; it conjugates with **Glycine** to form **Hippuric acid**. * **Glutamine vs. Glycine:** Remember that Phenylbutyrate removes **two** nitrogens (via Glutamine), whereas Benzoate removes **one** (via Glycine). * **Drug of Choice:** Phenylbutyrate is often preferred for long-term management of UCDs like OTC deficiency.

Question 385: Glycine can be synthesized from all the following precursors, EXCEPT:

A. Choline
B. Serine
C. Alanine
D. None of the above (Correct Answer)

Explanation: **Explanation:** Glycine is a non-essential amino acid, meaning the body can synthesize it from various metabolic precursors. The correct answer is **"None of the above"** because all three listed options (Choline, Serine, and Alanine) can indeed serve as precursors for glycine synthesis. **1. Serine (Option B):** This is the most direct and major pathway for glycine synthesis. The enzyme **Serine Hydroxymethyltransferase (SHMT)** catalyzes the reversible conversion of Serine to Glycine, requiring **Pyridoxal Phosphate (B6)** and **Tetrahydrofolate (THF)** as cofactors. **2. Choline (Option A):** Choline is oxidized to Betaine, which then undergoes demethylation steps to form Dimethylglycine and Sarcosine, eventually yielding Glycine. This pathway is vital for one-carbon metabolism. **3. Alanine (Option C):** While not a direct precursor in a single step, Alanine can be converted to **Pyruvate** via transamination. Pyruvate can enter the TCA cycle or be converted to Glyoxylate. **Glyoxylate** can then be transaminated to Glycine by the enzyme Alanine-Glyoxylate Aminotransferase. **High-Yield Clinical Pearls for NEET-PG:** * **Glycine Cleavage System:** The major catabolic pathway for glycine. A deficiency in this multienzyme complex leads to **Non-ketotic Hyperglycinemia**. * **Primary Hyperoxaluria Type I:** Caused by a deficiency in Alanine-Glyoxylate Aminotransferase, leading to excessive oxalate production and renal stones. * **Heme Synthesis:** Glycine is the starting substrate for heme synthesis (Glycine + Succinyl CoA → ALA). * **Inhibitory Neurotransmitter:** Glycine acts as an inhibitory neurotransmitter in the spinal cord.

Question 386: Which of the following amino acids is NOT both glucogenic and ketogenic?

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

Explanation: ### Explanation Amino acids are classified based on the metabolic intermediates they produce during catabolism. They can be **glucogenic** (converted into glucose via gluconeogenesis), **ketogenic** (converted into ketone bodies or acetyl-CoA), or **both**. **1. Why Leucine is the Correct Answer:** Leucine is one of only two **purely ketogenic** amino acids (the other being Lysine). Its catabolism yields only Acetyl-CoA and Acetoacetate, which cannot be used for the net synthesis of glucose in humans. Therefore, it is NOT both glucogenic and ketogenic. **2. Analysis of Incorrect Options:** The amino acids that are **both glucogenic and ketogenic** can be remembered by the mnemonic **"PITTT"** (Phenylalanine, Isoleucine, Tyrosine, Threonine, Tryptophan). * **A. Phenylalanine:** It is both; it yields Fumarate (glucogenic) and Acetoacetate (ketogenic). * **B. Tyrosine:** It is both; as a derivative of phenylalanine, it also yields Fumarate and Acetoacetate. * **C. Tryptophan:** It is both; it yields Pyruvate (glucogenic) and Acetoacetate (ketogenic). **3. High-Yield Facts for NEET-PG:** * **Purely Ketogenic:** Leucine and Lysine (The "L" amino acids). * **Both Glucogenic & Ketogenic:** Phenylalanine, Isoleucine, Tyrosine, Threonine, Tryptophan. * **Purely Glucogenic:** All other 13 amino acids. * **Clinical Pearl:** In patients with **Maple Syrup Urine Disease (MSUD)**, there is a defect in the branched-chain alpha-keto acid dehydrogenase complex, leading to an accumulation of Leucine, Isoleucine, and Valine. Leucine is the most toxic of these to the CNS.

Question 387: Which of the following is true about glycine?

A. Glycine is an essential amino acid
B. Contains a sulfur atom
C. Has a guanidine group
D. Is optically inactive (Correct Answer)

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** Glycine is the simplest amino acid. Its structure consists of a central alpha-carbon bonded to a carboxyl group, an amino group, and two hydrogen atoms. Because the side chain (R-group) is a **hydrogen atom**, the alpha-carbon is bonded to two identical groups. Therefore, glycine lacks a **chiral center** (asymmetric carbon atom). Since chirality is a prerequisite for optical activity, glycine is the **only amino acid that is optically inactive** (achiral) and does not exist in D- or L- isomeric forms. **2. Why the Other Options are Wrong:** * **Option A:** Glycine is a **non-essential amino acid**. It can be synthesized in the body from serine (via serine hydroxymethyltransferase) or from CO2 and ammonium ions (via the glycine cleavage system). * **Option B:** Glycine does not contain sulfur. The sulfur-containing amino acids are **Methionine and Cysteine**. * **Option C:** Glycine does not have a guanidine group. The guanidine group is the characteristic side chain of **Arginine**. **3. High-Yield Clinical Pearls for NEET-PG:** * **Smallest Amino Acid:** Due to its tiny side chain, glycine is found in restricted spaces of proteins, most notably in **Collagen** (where every third residue is Glycine: *Gly-X-Y*). * **Precursor Functions:** Glycine is essential for the synthesis of **Heme, Purines (C4, C5, N7), Creatine, Glutathione, and Bile Salts** (Glycocholate). * **Inhibitory Neurotransmitter:** It acts as an inhibitory neurotransmitter in the **spinal cord**. * **Metabolic Disorder:** **Non-ketotic hyperglycinemia** is caused by a defect in the glycine cleavage system, leading to severe neurological impairment.

Question 388: Ingestion of l-tryptophan increases the levels of which neurotransmitter in the brain?

A. Serotonin (Correct Answer)
B. Norepinephrine
C. Dopamine
D. Acetylcholine

Explanation: ### Explanation **Correct Option: A. Serotonin** Tryptophan is an essential amino acid that serves as the direct precursor for **Serotonin (5-hydroxytryptamine)**. The synthesis occurs in a two-step pathway: 1. **Tryptophan hydroxylase** (the rate-limiting enzyme) converts L-tryptophan to 5-hydroxytryptophan (5-HTP). This enzyme requires **Tetrahydrobiopterin (BH4)** as a cofactor. 2. **Aromatic L-amino acid decarboxylase** converts 5-HTP to Serotonin (requires Vitamin B6/Pyridoxine). Since the transport of tryptophan across the blood-brain barrier is a saturable process, increasing dietary intake of L-tryptophan directly raises brain serotonin levels. **Why Other Options are Incorrect:** * **B & C (Norepinephrine and Dopamine):** These are catecholamines derived from the amino acid **Tyrosine** (via the L-Dopa pathway). Tryptophan does not contribute to their synthesis. * **D (Acetylcholine):** This neurotransmitter is synthesized from **Choline and Acetyl-CoA** by the enzyme Choline acetyltransferase. It is not derived from an amino acid precursor like tryptophan. **High-Yield Clinical Pearls for NEET-PG:** * **Melatonin Connection:** Serotonin is further converted to Melatonin in the pineal gland; thus, Tryptophan is also the precursor for Melatonin. * **Niacin Synthesis:** About 60 mg of Tryptophan can produce 1 mg of **Niacin (Vitamin B3)**. * **Hartnup Disease:** A defect in the transport of neutral amino acids (like Tryptophan) leads to pellagra-like symptoms due to niacin deficiency. * **Carcinoid Syndrome:** In this condition, tumor cells divert up to 60% of tryptophan metabolism toward serotonin production, leading to secondary niacin deficiency (Pellagra).

Question 389: Which of the following best describes Tryptophan?

A. Non polar, essential, and pure glucogenic
B. Polar, essential, and both glucogenic & ketogenic
C. Polar, non-essential, both glucogenic & ketogenic
D. Non polar, essential, both glucogenic & ketogenic (Correct Answer)

Explanation: ### Explanation **Tryptophan** is a unique and complex amino acid. To arrive at the correct answer, we evaluate it based on three criteria: 1. **Polarity:** Tryptophan contains an **indole ring** (a bicyclic structure). Despite having a nitrogen atom in the ring, the large hydrocarbon structure makes it hydrophobic and **non-polar**. 2. **Nutritional Status:** It is one of the 10 **essential amino acids** (PVT TIM HALL), meaning it cannot be synthesized by the body and must be obtained through diet. 3. **Metabolic Fate:** Tryptophan is **both glucogenic and ketogenic**. Its breakdown yields **Alanine** (which enters gluconeogenesis via pyruvate) and **Acetoacetyl-CoA** (which is ketogenic). #### Why other options are incorrect: * **Option A:** Incorrect because Tryptophan is not purely glucogenic; it is also ketogenic. * **Option B:** Incorrect because Tryptophan is classified as non-polar due to its bulky hydrophobic side chain. * **Option C:** Incorrect because Tryptophan is an essential amino acid, not non-essential. --- ### High-Yield Clinical Pearls for NEET-PG * **Precursor Role:** Tryptophan is the precursor for **Serotonin** (neurotransmitter), **Melatonin** (sleep hormone), and **Niacin** (Vitamin B3). * **Hartnup Disease:** A defect in the transport of neutral amino acids (including Tryptophan) in the gut and kidneys, leading to pellagra-like symptoms. * **Carcinoid Syndrome:** Tumors that divert Tryptophan metabolism toward Serotonin, potentially leading to a secondary Niacin deficiency. * **Largest Amino Acid:** Tryptophan has the highest molecular weight among the 20 standard amino acids. * **Absorbance:** It is responsible for the maximum UV light absorbance of proteins at **280 nm**.

Question 390: Which amino acid spares the use of methionine?

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

Explanation: **Explanation:** **1. Why Cysteine is Correct:** Methionine is an **essential sulfur-containing amino acid**. In the body, methionine is converted to homocysteine, which can then follow two pathways: remethylation back to methionine or the **transsulfuration pathway** to form **Cysteine**. The "sparing effect" occurs because the body utilizes methionine primarily to synthesize cysteine when the latter is deficient in the diet. If adequate Cysteine is provided through the diet, the requirement for methionine decreases because the transsulfuration pathway is downregulated. Thus, Cysteine "spares" Methionine. (Note: A similar relationship exists between Tyrosine and Phenylalanine). **2. Why Other Options are Incorrect:** * **B. Glycine:** A non-essential amino acid involved in heme, purine, and creatine synthesis. It does not contain sulfur and cannot substitute for methionine's functions. * **C. Histidine:** An essential amino acid (semi-essential in adults) primarily involved in histamine production and buffering. It has no metabolic overlap with methionine. * **D. Arginine:** Involved in the urea cycle and nitric oxide production. While it is semi-essential, it does not spare methionine. **3. Clinical Pearls & High-Yield Facts:** * **Cystathionine β-synthase (CBS):** The key enzyme in the transsulfuration pathway. A deficiency leads to **Classical Homocystinuria**, characterized by ectopia lentis, intellectual disability, and thromboembolism. * **Vitamin Cofactors:** The conversion of methionine to cysteine requires **Vitamin B6** (Pyridoxine). The remethylation of homocysteine to methionine requires **Vitamin B12** and **Folate**. * **S-Adenosylmethionine (SAM):** Methionine is the precursor to SAM, the universal methyl donor in the body.

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

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Metabolism of Individual Amino Acids

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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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