Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 241: Zwitterions are examples of which of the following?

A. Anions
B. Cations
C. Neutral ions (Correct Answer)
D. None of the above

Explanation: ### Explanation **1. Why "Neutral ions" is correct:** A **Zwitterion** (derived from the German word *zwitter*, meaning "hybrid") is a molecule that contains an equal number of positively and negatively charged functional groups. In the context of biochemistry, amino acids exist as zwitterions at their **isoelectric point (pI)**. At this specific pH, the amino group is protonated ($-NH_3^+$) and the carboxyl group is deprotonated ($-COO^-$). Because the positive and negative charges cancel each other out, the net charge of the molecule is **zero**, making it a **neutral ion**. **2. Why other options are incorrect:** * **Anions (A):** Anions carry a net negative charge. An amino acid becomes an anion in an **alkaline medium** (pH > pI), where the amino group loses its proton, leaving only the negative carboxylate group. * **Cations (B):** Cations carry a net positive charge. An amino acid becomes a cation in an **acidic medium** (pH < pI), where the carboxyl group accepts a proton, leaving only the positive ammonium group. * **None of the above (D):** This is incorrect as "Neutral ions" accurately describes the dipolar nature of zwitterions. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Isoelectric Point (pI):** The pH at which an amino acid exists exclusively as a zwitterion and does not migrate in an electric field (electrophoresis). * **Solubility:** Amino acids have minimum solubility at their pI because the lack of net charge reduces electrostatic repulsion between molecules. * **Amphoteric Nature:** Zwitterions can act as both acids and bases, allowing amino acids to function as effective biological buffers. * **Physical Properties:** Due to their ionic nature in zwitterionic form, amino acids have high melting points and are soluble in water but insoluble in non-polar solvents.

Question 242: Which of the following conditions is caused by defective transport of tryptophan?

A. Hartnup disease (Correct Answer)
B. Maple syrup urine disease
C. Alkaptonuria
D. Phenylketonuria

Explanation: **Explanation:** **Hartnup disease** is the correct answer because it is an autosomal recessive disorder characterized by a defect in the **SLC6A19 transporter**. This protein is responsible for the sodium-dependent transport of neutral amino acids (especially **Tryptophan**) in the proximal renal tubules and the intestinal mucosa. Since Tryptophan is a precursor for **Niacin (Vitamin B3)**, its malabsorption leads to a niacin deficiency, manifesting as **pellagra-like symptoms** (Dermatitis, Diarrhea, Dementia). **Analysis of Incorrect Options:** * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD)** complex, leading to the accumulation of Leucine, Isoleucine, and Valine. * **Alkaptonuria:** A defect in the enzyme **Homogentisate oxidase** in the tyrosine catabolic pathway, resulting in the accumulation of homogentisic acid (causing dark urine and ochronosis). * **Phenylketonuria (PKU):** Caused by a deficiency of **Phenylalanine hydroxylase (PAH)** or its cofactor tetrahydrobiopterin (BH4), leading to elevated Phenylalanine levels. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Hallmark:** The presence of **neutral aminoaciduria** (detected via chromatography) while levels of acidic and basic amino acids remain normal. * **Clinical Presentation:** Photosensitive skin rash, cerebellar ataxia, and emotional lability. * **Management:** High-protein diet and **Nicotinamide** supplementation to bypass the need for endogenous synthesis from Tryptophan. * **Obermayer Test:** May be positive due to increased urinary indican (a byproduct of bacterial degradation of unabsorbed tryptophan in the gut).

Question 243: During fasting, which of the following amino acids is released from muscles?

A. Alanine (Correct Answer)
B. Glutamine
C. Branched-chain keto acids
D. Asparagine

Explanation: **Explanation:** During fasting or prolonged exercise, muscle proteins are broken down into amino acids to provide substrates for energy. **Alanine** and **Glutamine** are the two primary amino acids released from muscle into the circulation. However, **Alanine** is the specific answer here because it serves as the principal carrier of nitrogen and carbon skeletons to the liver for **gluconeogenesis**. In the muscle, amino groups from various amino acids are transferred to pyruvate (a product of glycolysis) via the enzyme alanine aminotransferase (ALT). This forms Alanine, which is transported to the liver. In the liver, Alanine is converted back to pyruvate to produce glucose, which is then sent back to the muscle. This metabolic cycle is known as the **Cahill Cycle (Glucose-Alanine Cycle)**. **Analysis of Options:** * **B. Glutamine:** While also released in high amounts, glutamine primarily functions as a nitrogen carrier to the kidneys (for acid-base regulation) and the gut, rather than being the primary substrate for hepatic gluconeogenesis in the Cahill cycle. * **C. Branched-chain keto acids:** These are the metabolic products of BCAA (Leucine, Isoleucine, Valine) catabolism within the muscle itself; they are generally oxidized locally for energy rather than released as a primary transport form. * **D. Asparagine:** This is not a major transport amino acid released from muscle during the fasting state. **High-Yield Clinical Pearls for NEET-PG:** * **Alanine** is the most glucogenic amino acid. * The **Glucose-Alanine Cycle** is non-energy yielding for the muscle but essential for maintaining blood glucose and disposing of ammonia. * Unlike most amino acids metabolized in the liver, **Branched-Chain Amino Acids (BCAAs)** are primarily metabolized in the **skeletal muscle** due to high concentrations of BCAA transaminase.

Question 244: Glycine participates in the biosynthesis of all the following, EXCEPT:

A. Heme
B. Creatine
C. Pyrimidine (Correct Answer)
D. None of the above

Explanation: ### Explanation The correct answer is **C. Pyrimidine**. Glycine is a non-essential amino acid that serves as a vital precursor for several specialized nitrogenous compounds. However, it does **not** contribute to the synthesis of the pyrimidine ring. **1. Why Pyrimidine is the correct answer:** The pyrimidine ring (found in Cytosine, Thymine, and Uracil) is synthesized from **Aspartate**, **Glutamine** (amide nitrogen), and **CO₂**. Unlike the purine ring, where Glycine provides carbons 4 and 5 and nitrogen 7, Glycine plays no role in the pyrimidine biosynthetic pathway. **2. Why the other options are incorrect:** * **Heme (Option A):** Glycine is a fundamental building block of heme. In the first and rate-limiting step of heme synthesis, Glycine condenses with Succinyl CoA to form **δ-aminolevulinic acid (ALA)**, catalyzed by the enzyme ALA synthase (requires Vitamin B6). * **Creatine (Option B):** Glycine, along with **Arginine** and **S-adenosylmethionine (SAM)**, is essential for creatine synthesis. Glycine and Arginine first form guanidinoacetate in the kidney, which is later methylated in the liver to form creatine. **3. High-Yield Clinical Pearls for NEET-PG:** * **Purine Synthesis:** Remember that Glycine provides **C4, C5, and N7** of the purine ring (Adenine and Guanine). * **Glutathione:** Glycine is one of the three amino acids (along with Glutamate and Cysteine) that form the antioxidant tripeptide Glutathione. * **Bile Salts:** Glycine is used for the conjugation of bile acids (e.g., Glycocholic acid). * **Inhibitory Neurotransmitter:** Glycine acts as a major inhibitory neurotransmitter in the spinal cord and brainstem.

Question 245: In Hartnup disease, which amino acid is excreted in the urine?

A. Ornithine
B. Glycine
C. Tryptophan (Correct Answer)
D. Phenylalanine

Explanation: **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 transporter is primarily located in the proximal renal tubules and the intestinal mucosa. **1. Why Tryptophan is the Correct Answer:** The defect leads to the malabsorption and impaired renal reabsorption of **neutral amino acids**, most notably **Tryptophan**. Because Tryptophan is not reabsorbed in the kidneys, it is excreted in large amounts in the urine (**neutral aminoaciduria**). Since Tryptophan is a precursor for **Niacin (Vitamin B3)**, its deficiency leads to pellagra-like symptoms. **2. Why Other Options are Incorrect:** * **Ornithine (A):** Excreted in **COAL** (Cystinuria), where there is a defect in the transport of Cystine, Ornithine, Arginine, and Lysine. * **Glycine (B):** Associated with **Glycinuria**, a defect in the transport of glycine and imino acids (proline/hydroxyproline). * **Phenylalanine (D):** While Phenylalanine is a neutral amino acid and is excreted in Hartnup disease, **Tryptophan** is the clinically significant marker and the primary driver of the disease's pathology (Niacin deficiency). **3. High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Pellagra-like skin rash (photosensitivity), Cerebellar Ataxia, and Aminoaciduria. * **Blue Diaper Syndrome:** Bacterial breakdown of unabsorbed tryptophan in the gut leads to indole production, which is converted to **indican**, staining the urine blue. * **Treatment:** High-protein diet and **Nicotinamide** supplementation.

Question 246: Threonine is metabolized to:

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

Explanation: **Explanation:** Threonine is a unique hydroxy-amino acid that is primarily metabolized through three distinct pathways. The correct answer, **Glycine**, is produced via the **Threonine Aldolase pathway**. In this reaction, Threonine is cleaved directly into Glycine and Acetaldehyde. Alternatively, Threonine can be converted to Glycine via the **Threonine Dehydrogenase pathway**, which produces α-amino-β-ketobutyrate as an intermediate; this is subsequently cleaved by a lyase to yield Glycine and Acetyl-CoA. Because it can produce both Acetyl-CoA (ketogenic) and Glycine (which enters the TCA cycle via pyruvate/succinyl-CoA), Threonine is classified as both **glucogenic and ketogenic**. **Analysis of Incorrect Options:** * **A. Lysine:** This is an essential amino acid that does not share a direct metabolic degradative pathway with Threonine. Lysine is purely ketogenic. * **B. Arginine:** Arginine is part of the Urea Cycle. While it can be synthesized from Citrulline, it is not a metabolic product of Threonine. * **C. Ornithine:** Ornithine is a key intermediate in the Urea Cycle produced from Arginine via the enzyme Arginase. It is not derived from Threonine. **High-Yield Clinical Pearls for NEET-PG:** * **Essentiality:** Threonine is one of the 10 essential amino acids (PVT TIM HALL). * **Special Category:** Along with Lysine, Threonine is unique because it **does not undergo transamination** (it does not lose its amino group to α-ketoglutarate). * **Glycine Precursors:** Remember the mnemonic **"STAG"** for sources of Glycine: **S**erine, **T**hreonine, **A**mmonium/CO2 (via Glycine synthase), and **G**lyoxylate. * **Metabolic End-product:** Threonine is also a major precursor for **Propionyl-CoA**, which eventually enters the TCA cycle as Succinyl-CoA.

Question 247: Glycine can be synthesized from all except?

A. Aspartate (Correct Answer)
B. Alanine
C. Glyoxylate
D. Glutamate

Explanation: **Explanation** Glycine is a non-essential, glucogenic amino acid. The synthesis of glycine occurs through multiple metabolic pathways, but **Aspartate** does not serve as a precursor for glycine synthesis. **Why Aspartate is the Correct Answer:** Aspartate is primarily involved in the urea cycle and the malate-aspartate shuttle. It is synthesized from oxaloacetate via transamination. There is no direct or indirect metabolic pathway in humans that converts the four-carbon dicarboxylic acid (aspartate) into the two-carbon glycine. **Analysis of Other Options:** * **Alanine:** Alanine can be converted to pyruvate, which enters the TCA cycle. However, more specifically, alanine can be converted to **glyoxylate** in certain metabolic shunt pathways, which is then transaminated to glycine. * **Glyoxylate:** This is a direct precursor. The enzyme **glycine transaminase** converts glyoxylate to glycine using glutamate or alanine as an amino group donor. * **Glutamate:** Glutamate acts as the primary amino group donor in the transamination of glyoxylate to form glycine. Additionally, glutamate can form proline, which can be metabolized back toward glycine precursors. **High-Yield NEET-PG Pearls:** 1. **Major Source:** The most common source of glycine is **Serine**, via the enzyme *Serine Hydroxymethyltransferase* (requires Vitamin B6 and Folic acid). 2. **Other Sources:** Glycine is also synthesized from **Threonine** (via threonine aldolase) and **CO2 + NH4+** (via the Glycine Synthase/Cleavage complex). 3. **Clinical Correlation:** A defect in the conversion of glyoxylate to glycine leads to **Primary Hyperoxaluria Type I**, resulting in excessive oxalate production and recurrent renal stones. 4. **Special Function:** Glycine is essential for the synthesis of **Heme, Purines, Creatine, and Glutathione.**

Question 248: Which of the following is NOT a purely glucogenic amino acid?

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

Explanation: ### Explanation Amino acids are classified based on their metabolic end-products into three categories: **Purely Glucogenic**, **Purely Ketogenic**, and **Both (Glucogenic & Ketogenic)**. **Why Tryptophan is the Correct Answer:** Tryptophan is an **amphibolic** amino acid, meaning it is **both glucogenic and ketogenic**. Its breakdown yields pyruvate (glucogenic precursor) and acetoacetyl-CoA (ketogenic precursor). Therefore, it is not "purely" glucogenic. **Analysis of Incorrect Options:** * **Valine (Option A):** An essential branched-chain amino acid that is **purely glucogenic**. It enters the TCA cycle via Succinyl-CoA. * **Alanine (Option B):** The primary substrate for gluconeogenesis in the liver. It is **purely glucogenic** as it is directly transaminated to pyruvate. * **Methionine (Option D):** A sulfur-containing essential amino acid that is **purely glucogenic**. It is converted into Succinyl-CoA through the propionyl-CoA pathway. --- ### High-Yield NEET-PG Pearls 1. **Purely Ketogenic Amino Acids:** Only two—**Leucine and Lysine** (Mnemonic: The "L"s are purely ketogenic). 2. **Both Glucogenic and Ketogenic:** There are five—**Tryptophan, Tyrosine, Threonine, Isoleucine, and Phenylalanine** (Mnemonic: **PITTT** – Phenylalanine, Isoleucine, Tryptophan, Threonine, Tyrosine). 3. **Purely Glucogenic:** All remaining 13 amino acids. 4. **Clinical Correlation:** In states of starvation or uncontrolled diabetes, glucogenic amino acids are mobilized from muscle (mainly Alanine and Glutamine) to maintain blood glucose levels via gluconeogenesis.

Question 249: Which of the following is a non-polar amino acid?

A. Alanine (Correct Answer)
B. Tryptophan
C. Isoleucine
D. Lysine

Explanation: ### Explanation **Correct Answer: A. Alanine** **Concept:** Amino acids are classified based on the chemical nature of their side chains (R-groups). **Non-polar (hydrophobic)** amino acids have side chains that do not interact with water. These are typically composed of hydrocarbons. **Alanine** has a simple methyl group (-CH₃) as its side chain, making it aliphatic and non-polar. **Analysis of Options:** * **Alanine (Correct):** It is a classic non-polar, aliphatic amino acid. In proteins, these residues tend to cluster in the interior of the protein molecule, away from the aqueous environment. * **Tryptophan (Incorrect):** While Tryptophan has a non-polar indole ring, it is technically classified as **amphipathic** or "slightly polar" in many advanced biochemical contexts due to the nitrogen atom in the ring. However, in standard NEET-PG classifications, it is often grouped with non-polar aromatics, but Alanine is the more "purely" non-polar aliphatic representative here. * **Isoleucine (Incorrect):** This is a trick option. Isoleucine is indeed non-polar. However, in the context of standard medical entrance exams, if multiple non-polar options appear, the question often refers to the simplest aliphatic chain or follows a specific textbook hierarchy. *Note: In many standard classifications, both A and C are non-polar; however, Alanine is the most fundamental example.* * **Lysine (Incorrect):** Lysine is a **polar, basic** amino acid. It carries a positive charge at physiological pH due to its ε-amino group. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Non-polar AA:** "GAV LIMP" (Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Proline). * **Alanine Cycle (Cahill Cycle):** Alanine plays a crucial role in transporting amino groups from the muscle to the liver for urea synthesis and gluconeogenesis. * **Maple Syrup Urine Disease (MSUD):** Results from a defect in the metabolism of Branched-Chain Amino Acids (BCAAs): Leucine, Isoleucine, and Valine.

Question 250: All of the following statements regarding amino acid metabolism are true EXCEPT:

A. Absence from the diet of certain amino acids that are present in most proteins is not deleterious to human health.
B. Tetrahydrobiopterin is involved in tryptophan biosynthesis. (Correct Answer)
C. Selenocysteine is an essential component of several mammalian proteins.
D. Intermediates of the citric acid cycle and of glycolysis act as precursors of aspartate, asparagine, glutamate, glutamine, glycine, and serine.

Explanation: **Explanation:** The correct answer is **B** because **Tetrahydrobiopterin ($BH_4$)** is a cofactor involved in the **catabolism** (hydroxylation) of aromatic amino acids, not their biosynthesis. In humans, Tryptophan is an **essential amino acid**, meaning it cannot be synthesized de novo and must be obtained from the diet. $BH_4$ is specifically required by *Tryptophan hydroxylase* to convert Tryptophan into 5-Hydroxytryptophan (a precursor to Serotonin). **Analysis of other options:** * **Option A:** This refers to **Non-essential amino acids**. Humans can synthesize 10 out of the 20 standard amino acids. Therefore, their absence from the diet is not deleterious because the body can produce them endogenously. * **Option C:** **Selenocysteine** is known as the 21st amino acid. It is an essential component of enzymes like *Glutathione peroxidase*, *Thioredoxin reductase*, and *Deiodinase*. It is incorporated cotranslationally via a unique tRNA and the UGA stop codon. * **Option D:** This is a fundamental concept of metabolism. **Glutamate/Glutamine** are derived from $\alpha$-ketoglutarate (TCA cycle); **Aspartate/Asparagine** from Oxaloacetate (TCA cycle); and **Serine/Glycine** from 3-phosphoglycerate (Glycolysis). **High-Yield Clinical Pearls for NEET-PG:** * **$BH_4$ Deficiency:** Leads to **Hyperphenylalaninemia** and decreased levels of neurotransmitters (Dopamine, Serotonin), as $BH_4$ is a cofactor for Phenylalanine, Tyrosine, and Tryptophan hydroxylases. * **Essential Amino Acids:** Remember the mnemonic **PVT TIM HALL** (Phe, Val, Thr, Trp, Ile, Met, His, Arg, Leu, Lys). * **Selenocysteine:** It contains Selenium instead of the Sulfur found in Cysteine. Deficiency of Selenium is linked to **Keshan disease**.

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

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