Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 371: A newborn infant is diagnosed with phenylketonuria. Which enzyme is deficient in this condition?

A. Phenylamine oxidase
B. Tyrosine hydroxylase
C. Phenylalanine hydroxylase (Correct Answer)
D. Dihydropteridine reductase

Explanation: ### Explanation **1. Why Phenylalanine Hydroxylase (PAH) is Correct:** Phenylketonuria (PKU) is an autosomal recessive disorder caused by a deficiency of the hepatic enzyme **Phenylalanine Hydroxylase (PAH)**. Under normal physiological conditions, PAH converts the essential amino acid Phenylalanine into Tyrosine. This reaction requires **Tetrahydrobiopterin (BH4)** as a mandatory co-factor. When PAH is deficient, phenylalanine accumulates in the blood and tissues, leading to intellectual disability, seizures, and a "mousy" body odor due to the alternative metabolism of phenylalanine into phenylketones (e.g., phenylpyruvate). **2. Analysis of Incorrect Options:** * **Phenylamine oxidase (A):** This is not a recognized enzyme in human amino acid metabolism. * **Tyrosine hydroxylase (B):** This enzyme converts Tyrosine to DOPA in the catecholamine synthesis pathway. While it also requires BH4, its deficiency does not cause classic PKU. * **Dihydropteridine reductase (D):** This enzyme regenerates BH4 from dihydrobiopterin (BH2). While its deficiency causes **Malignant/Atypical PKU** (due to lack of co-factor), "Classic PKU" specifically refers to a deficiency in the PAH enzyme itself. **3. High-Yield Clinical Pearls for NEET-PG:** * **Tyrosine becomes an essential amino acid** in PKU patients because it can no longer be synthesized from phenylalanine. * **Screening:** Done via the **Guthrie Test** (bacterial inhibition assay) or Tandem Mass Spectrometry. * **Clinical Features:** Hypopigmentation (fair skin/blue eyes) occurs because phenylalanine inhibits tyrosinase, reducing melanin synthesis. * **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 (teratogenic effect).

Question 372: Which of the following is a semi-essential amino acid?

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

Explanation: ### Explanation **Correct Answer: A. Arginine** **Why Arginine is the Correct Answer:** Amino acids are classified based on their nutritional requirement into essential, non-essential, and semi-essential. **Arginine** (along with Histidine) is classified as a **semi-essential (or conditionally essential)** amino acid. The underlying medical concept is that while the body can synthesize Arginine via the urea cycle, the rate of synthesis is insufficient to meet the high metabolic demands during periods of **rapid growth** (infancy/childhood), pregnancy, or severe physiological stress (trauma/recovery). In these states, dietary intake becomes mandatory to maintain a positive nitrogen balance. **Analysis of Incorrect Options:** * **B, C, and D (Phenylalanine, Lysine, Tryptophan):** These are **purely essential** amino acids. The human body lacks the biochemical pathways to synthesize their carbon skeletons de novo. Therefore, they must be obtained from the diet regardless of the physiological state. **High-Yield NEET-PG Clinical Pearls:** * **Mnemonic for Essential Amino Acids:** "PVT TIM HALL" (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine). Note that **H**istidine and **A**rginine are the two semi-essential ones. * **Purely Ketogenic Amino Acids:** Leucine and Lysine (The "L"s). * **Both Glucogenic and Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, and Isoleucine (The "Aromatics" + Isoleucine). * **Arginine’s Role:** It is the immediate precursor of **Nitric Oxide (NO)**, mediated by the enzyme Nitric Oxide Synthase (NOS), and is a key intermediate in the **Urea Cycle**.

Question 373: Which group contains non-essential amino acids?

A. Acidic Amino Acids (Correct Answer)
B. Branched Chain Amino Acids
C. Basic Amino Acids
D. Aromatic Amino Acids

Explanation: ### Explanation **1. Why Acidic Amino Acids are Correct:** Non-essential amino acids are those that the human body can synthesize de novo from metabolic intermediates (like those in the TCA cycle) and do not strictly require dietary intake. The **Acidic Amino Acids**—**Aspartic acid (Aspartate)** and **Glutamic acid (Glutamate)**—are classic examples. * **Glutamate** is synthesized via the reductive amination of $\alpha$-ketoglutarate. * **Aspartate** is synthesized via the transamination of oxaloacetate. Since both precursors are readily available in the TCA cycle, these amino acids are non-essential. **2. Analysis of Incorrect Options:** * **Branched Chain Amino Acids (BCAAs):** This group includes **Leucine, Isoleucine, and Valine**. All three are **Essential** amino acids. They must be obtained from the diet and are primarily metabolized in the muscle. * **Basic Amino Acids:** This group includes **Lysine, Arginine, and Histidine**. * **Lysine** is strictly essential. * **Arginine and Histidine** are considered semi-essential (required during periods of rapid growth or stress). * **Aromatic Amino Acids:** This group includes **Phenylalanine, Tyrosine, and Tryptophan**. * **Phenylalanine and Tryptophan** are strictly essential. * **Tyrosine** is non-essential only because it is synthesized from Phenylalanine. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Essential Amino Acids:** "PVT TIM HALL" (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine). * **Purely Ketogenic Amino Acids:** Leucine and Lysine (The "L"s). * **Glucogenic & Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, and Isoleucine (The "Aromatics + Isoleucine"). * **Clinical Correlation:** Defective metabolism of BCAAs leads to **Maple Syrup Urine Disease (MSUD)** due to a deficiency in the Branched-chain $\alpha$-keto acid dehydrogenase complex.

Question 374: What is the most important amino acid transported from muscle to liver for gluconeogenesis?

A. Methionine
B. Tryptophan
C. Alanine (Correct Answer)
D. Arginine

Explanation: ### Explanation The correct answer is **Alanine**. **1. Why Alanine is correct: The Glucose-Alanine Cycle (Cahill Cycle)** During periods of fasting or intense exercise, muscle proteins are broken down into amino acids. The amino groups are transferred to $\alpha$-ketoglutarate to form glutamate. Through the action of **Alanine Aminotransferase (ALT)**, the amino group from glutamate is transferred to pyruvate (a product of glycolysis), forming **Alanine**. Alanine is then released into the blood and transported to the liver. In the liver, ALT converts alanine back into pyruvate, which serves as a key substrate for **gluconeogenesis** to produce glucose. This cycle serves two vital purposes: it provides a carbon skeleton for glucose production and safely transports toxic ammonia from the muscle to the liver for the urea cycle. **2. Why the other options are incorrect:** * **Methionine:** An essential, glucogenic, sulfur-containing amino acid. While it can enter the TCA cycle via succinyl-CoA, it is not a primary inter-organ transporter of nitrogen or carbon for gluconeogenesis. * **Tryptophan:** Both glucogenic and ketogenic. It is a complex aromatic amino acid used primarily for the synthesis of Serotonin, Melatonin, and Niacin, rather than bulk transport for glucose synthesis. * **Arginine:** A semi-essential amino acid involved in the urea cycle and Nitric Oxide (NO) production. It is not the major vehicle for nitrogen transport from muscle. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Alanine and Glutamine** are the two most important amino acids released from muscle. While Alanine is the primary substrate for **gluconeogenesis**, **Glutamine** is the major transporter of ammonia from most other tissues and is the preferred fuel for enterocytes and lymphocytes. * **ALT (SGPT)** requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. * The Glucose-Alanine cycle is analogous to the **Cori Cycle**, but instead of lactate, it uses alanine to transport carbons to the liver.

Question 375: A 7-month-old infant presents with a history of vomiting and failure to thrive. The patient improved with IV glucose. After one month, the infant returns with the same complaints. On evaluation, high glutamine and uracil levels are found. Which is the likely enzyme defect?

A. Carbamoyl phosphate synthetase 1 (CPS1)
B. Ornithine transcarbamylase (Correct Answer)
C. Arginase
D. Argininosuccinate lyase

Explanation: ### Explanation The clinical presentation of vomiting, failure to thrive, and improvement with IV glucose (which reduces protein catabolism) suggests a **Urea Cycle Disorder (UCD)**. **Why Ornithine Transcarbamylase (OTC) Deficiency is correct:** OTC is the most common urea cycle defect and is X-linked recessive. In OTC deficiency, **Carbamoyl Phosphate** cannot be converted to Citrulline. As a result, Carbamoyl Phosphate accumulates in the mitochondria and leaks into the cytosol. Here, it enters the pyrimidine synthesis pathway, leading to the overproduction of **Orotic acid and Uracil**. High ammonia levels are shunted into **Glutamine** synthesis, explaining the elevated glutamine. **Why the other options are incorrect:** * **CPS1 Deficiency:** Like OTC deficiency, it presents with hyperammonemia and high glutamine. However, because CPS1 is upstream of Carbamoyl Phosphate production, there is **no increase** in uracil or orotic acid. * **Argininosuccinate Lyase Deficiency:** This leads to the accumulation of argininosuccinic acid. While it causes hyperammonemia, it does not typically present with the massive uracil elevation seen in OTC deficiency. * **Arginase Deficiency:** This is the least severe UCD. It presents later in childhood with spastic paraplegia rather than acute neonatal/infantile vomiting, and arginine levels would be significantly elevated. **High-Yield Clinical Pearls for NEET-PG:** * **OTC Deficiency** is the *only* X-linked Urea Cycle Disorder; all others are Autosomal Recessive. * **Orotic Aciduria + Hyperammonemia** = OTC Deficiency. * **Orotic Aciduria + Megaloblastic Anemia** (without hyperammonemia) = UMP Synthase deficiency. * **Glutamine** acts as a non-toxic storage form of ammonia; its elevation is a hallmark of most UCDs.

Question 376: Carbamoyl phosphate synthetase I acts on which substrates?

A. Ornithine
B. Carbamoyl phosphate
C. CO2 and ammonia (Correct Answer)
D. Arginine

Explanation: **Explanation:** **Carbamoyl Phosphate Synthetase I (CPS-I)** is the rate-limiting enzyme of the **Urea Cycle**, occurring within the mitochondrial matrix of hepatocytes. Its primary function is to fix nitrogen for excretion. **Why C is correct:** CPS-I catalyzes the condensation of **Ammonia ($NH_3$)** and **Bicarbonate ($HCO_3^-$, derived from $CO_2$)** to form Carbamoyl Phosphate. This reaction requires the consumption of **2 ATP** molecules and is uniquely dependent on **N-acetylglutamate (NAG)** as an essential allosteric activator. **Why the other options are incorrect:** * **A. Ornithine:** This is a substrate for *Ornithine Transcarbamoylase (OTC)*, which combines ornithine with carbamoyl phosphate to form Citrulline. * **B. Carbamoyl phosphate:** This is the **product** of the CPS-I reaction, not the substrate. * **D. Arginine:** This is an intermediate of the urea cycle. It is cleaved by *Arginase* to produce urea and regenerate ornithine. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** CPS-I is **Mitochondrial** (Urea cycle), whereas CPS-II is **Cytosolic** (Pyrimidine synthesis). * **Activator:** NAG is the "obligatory activator" of CPS-I. A deficiency in NAG synthase mimics CPS-I deficiency. * **Clinical Correlation:** CPS-I deficiency is the most severe urea cycle disorder, presenting with **Type I Hyperammonemia**, lethargy, and seizures shortly after birth. Unlike OTC deficiency, there is **no orotic aciduria** in CPS-I deficiency. * **Nitrogen Sources:** The two nitrogens in urea come from **Free Ammonia** (via CPS-I) and **Aspartate**.

Question 377: Maple syrup urine disease can be caused by a problem with the metabolism of which particular compound?

A. Glycogen
B. Collagen
C. Dopamine
D. Valine (Correct Answer)

Explanation: **Explanation:** **Maple Syrup Urine Disease (MSUD)** is an autosomal recessive metabolic disorder caused by a deficiency in the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD) complex**. This enzyme is responsible for the oxidative decarboxylation of the three branched-chain amino acids (BCAAs): **Leucine, Isoleucine, and Valine**. 1. **Why Valine is correct:** Since the BCKAD complex is defective, the body cannot metabolize Valine (along with Leucine and Isoleucine). This leads to a toxic buildup of these amino acids and their corresponding alpha-keto acids in the blood and urine. 2. **Why other options are incorrect:** * **Glycogen:** Disorders of glycogen metabolism are called Glycogen Storage Diseases (e.g., Von Gierke’s), which typically present with hypoglycemia and hepatomegaly. * **Collagen:** Defects in collagen lead to structural disorders like Scurvy (Vitamin C deficiency) or Osteogenesis Imperfecta, not metabolic ketoacidosis. * **Dopamine:** Dopamine is a catecholamine derived from Tyrosine. Abnormalities here are linked to Parkinson’s disease or phenylketonuria (indirectly), but not MSUD. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** Remember **"I Love Vermont"** (Isoleucine, Leucine, Valine) for the BCAAs involved. * **Clinical Presentation:** Infants present with poor feeding, vomiting, and a characteristic **burnt sugar/maple syrup odor** in the urine. * **Diagnosis:** Elevated levels of BCAAs in plasma and **Alloisoleucine** (pathognomonic). * **Cofactor:** The BCKAD complex requires **Thiamine (Vitamin B1)**. Some patients respond to high-dose Thiamine supplementation (Thiamine-responsive MSUD).

Question 378: Which of the following is a sulfur-containing amino acid?

A. Aspartic acid
B. Glutamine
C. Methionine (Correct Answer)
D. Glycine

Explanation: **Explanation:** **Correct Answer: C. Methionine** Amino acids are categorized based on the chemical properties of their side chains. **Methionine** and **Cysteine** are the two primary sulfur-containing amino acids found in proteins. Methionine is an essential amino acid that contains a non-polar thioether group. It serves as the precursor for S-adenosylmethionine (SAM), the body's universal methyl donor, and is the first amino acid incorporated into every polypeptide chain during translation (coded by the start codon AUG). **Analysis of Incorrect Options:** * **A. Aspartic acid:** This is an acidic (dicarboxylic) amino acid. It is negatively charged at physiological pH and plays a key role in the urea cycle and malate-aspartate shuttle. * **B. Glutamine:** This is a polar, uncharged amino acid. It is the most abundant free amino acid in the blood and acts as a major carrier of ammonia from peripheral tissues to the liver. * **C. Glycine:** This is the simplest amino acid, with a hydrogen atom as its R-group. It is optically inactive (achiral) and is a major inhibitory neurotransmitter in the spinal cord. **High-Yield Clinical Pearls for NEET-PG:** * **Homocystinuria:** A deficiency in Cystathionine beta-synthase leads to an accumulation of homocysteine and methionine, presenting with ectopia lentis, intellectual disability, and thromboembolism. * **Cysteine vs. Cystine:** Cysteine contains a sulfhydryl (-SH) group. Two cysteine molecules can form a disulfide bond to create **Cystine**. * **Glutathione:** A tripeptide (Glu-Cys-Gly) that relies on the sulfur group of Cysteine for its antioxidant function.

Question 379: Transamination of aspartate forms which of the following keto acids?

A. Pyruvate
B. Acetyl CoA
C. Oxaloacetate (Correct Answer)
D. Alanine

Explanation: ### Explanation **Core Concept: Transamination** Transamination is the process where an amino group ($\text{-NH}_2$) is transferred from an amino acid to a keto acid, typically catalyzed by aminotransferases (transaminases) using **Pyridoxal Phosphate (Vitamin B6)** as a mandatory cofactor. This reaction converts the amino acid into its corresponding **$\alpha$-keto acid**. **Why Oxaloacetate is Correct:** Aspartate is a four-carbon dicarboxylic amino acid. When it undergoes transamination, catalyzed by **Aspartate Aminotransferase (AST/SGOT)**, it transfers its amino group to $\alpha$-ketoglutarate (forming glutamate). The remaining carbon skeleton of aspartate is **Oxaloacetate (OAA)**. * **Reaction:** $\text{Aspartate} + \alpha\text{-Ketoglutarate} \rightleftharpoons \text{Oxaloacetate} + \text{Glutamate}$ **Analysis of Incorrect Options:** * **A. Pyruvate:** This is the keto acid formed by the transamination of **Alanine** (via ALT). Pyruvate has 3 carbons, while aspartate has 4. * **B. Acetyl CoA:** This is a metabolic intermediate, not a direct product of a simple transamination reaction. It is the entry point for the TCA cycle but does not possess the structure of a primary $\alpha$-keto acid corresponding to a proteinogenic amino acid. * **D. Alanine:** This is an amino acid, not a keto acid. Alanine is the substrate that forms pyruvate. **High-Yield Clinical Pearls for NEET-PG:** 1. **Cofactor:** All transaminases require **PLP (Vitamin B6)**. Deficiency of B6 impairs amino acid metabolism. 2. **Diagnostic Marker:** AST (SGOT) is found in the liver, cardiac muscle, and skeletal muscle. Elevated levels are seen in myocardial infarction and liver cell injury. 3. **TCA Cycle Link:** Aspartate is considered **glucogenic** because its keto acid, oxaloacetate, is a key intermediate in the Citric Acid Cycle and Gluconeogenesis. 4. **Malate-Aspartate Shuttle:** This reaction is crucial for transporting reducing equivalents (NADH) into the mitochondria.

Question 380: Which of the following statements is true regarding catecholamines?

A. Production terminates with dopamine in the brain but epinephrine in the adrenal gland (Correct Answer)
B. Production begins with the action of tyrosine hydroxylase on tyrosine
C. Are metabolized to both glucogenic and ketogenic fragments
D. All contain a methyl group donated by S-adenosylmethionine

Explanation: ### Explanation **1. Why Option A is Correct:** The synthesis of catecholamines follows the pathway: **Tyrosine → L-DOPA → Dopamine → Norepinephrine → Epinephrine**. * In the **adrenal medulla**, the enzyme **Phenylethanolamine N-methyltransferase (PNMT)** is present, which converts norepinephrine to epinephrine. * In specific areas of the **brain** (like the substantia nigra), the pathway terminates at **Dopamine** because the subsequent enzymes (Dopamine $\beta$-hydroxylase and PNMT) are absent in those specific neurons. **2. Analysis of Incorrect Options:** * **Option B:** While Tyrosine hydroxylase is the rate-limiting step, the production technically "begins" with the transport of **Phenylalanine** (which converts to Tyrosine) or Tyrosine itself into the neuron. However, the statement is often considered "partially true" but less accurate than A in the context of tissue-specific termination. * **Option C:** This describes the catabolism of the **amino acid carbon skeletons** (like Phenylalanine/Tyrosine), not the catecholamines themselves. Catecholamines are metabolized by **MAO (Monoamine oxidase)** and **COMT (Catechol-O-methyltransferase)** into inactive metabolites like **VMA (Vanillylmandelic acid)** and **HVA (Homovanillic acid)**. * **Option D:** Only **Epinephrine** contains a methyl group donated by SAM (via the PNMT enzyme). Dopamine and Norepinephrine do not possess this methyl group. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Tyrosine Hydroxylase (requires Tetrahydrobiopterin/BH4). * **PNMT Induction:** Cortisol (from the adrenal cortex) induces PNMT in the medulla, facilitating epinephrine production. * **Metabolites:** * VMA is the end product of Norepinephrine/Epinephrine (elevated in **Pheochromocytoma**). * HVA is the end product of Dopamine (elevated in **Neuroblastoma**). * **Vitamin Cofactors:** Vitamin C is required for Dopamine $\beta$-hydroxylase; Vitamin B6 (PLP) is required for DOPA decarboxylase.

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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