Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 231: Which of the following is a polar amino acid?

A. Tryptophan
B. Methionine
C. Glutamic acid (Correct Answer)
D. Isoleucine

Explanation: ### Explanation Amino acids are classified based on the chemical nature of their side chains (R-groups) at physiological pH. **Glutamic acid** is a **polar, negatively charged (acidic)** amino acid. It contains a carboxyl group (-COOH) in its side chain which ionizes to form a carboxylate ion (-COO⁻) at pH 7.4. This charge makes it highly hydrophilic and polar, allowing it to interact readily with water and other charged molecules. **Analysis of Incorrect Options:** * **A. Tryptophan:** This is a **non-polar, aromatic** amino acid. Although it contains a nitrogen atom in its indole ring, the large hydrophobic ring structure dominates its properties, making it relatively insoluble in water. * **B. Methionine:** This is a **non-polar, sulfur-containing** amino acid. The sulfur atom is part of a thioether group (C-S-C), which does not carry a charge and is hydrophobic. * **C. Isoleucine:** This is a **non-polar, branched-chain** amino acid (BCAA). Its side chain consists entirely of hydrocarbons, making it strictly hydrophobic. **High-Yield NEET-PG Pearls:** * **Acidic Amino Acids:** Aspartic acid and Glutamic acid (both are polar and negatively charged). * **Basic Amino Acids:** Lysine, Arginine, and Histidine (polar and positively charged). * **Glutamate vs. GABA:** Glutamate is the primary **excitatory** neurotransmitter in the CNS; its decarboxylation (via Vitamin B6) produces GABA, the primary **inhibitory** neurotransmitter. * **BCAAs:** Leucine, Isoleucine, and Valine are metabolized in the muscle (not the liver) and are deficient in **Maple Syrup Urine Disease (MSUD)**.

Question 232: Which co-enzyme is essential for transamination reactions?

A. NAD
B. Biotin
C. Pyridoxal phosphate (Correct Answer)
D. Riboflavin

Explanation: **Explanation:** **1. Why Pyridoxal Phosphate (PLP) is correct:** Transamination is the first step in the catabolism of most amino acids, involving the transfer of an $\alpha$-amino group to an $\alpha$-ketoacid (usually $\alpha$-ketoglutarate). This reaction is catalyzed by **Aminotransferases (Transaminases)**. These enzymes require **Pyridoxal Phosphate (PLP)**, the active form of **Vitamin B6**, as an obligatory co-enzyme. PLP functions as a temporary carrier of the amino group, transitioning between its aldehyde form (PLP) and its aminated form (**Pyridoxamine phosphate**) during the "Ping-Pong" kinetic mechanism of the reaction. **2. Why the other options are incorrect:** * **NAD (Nicotinamide Adenine Dinucleotide):** Derived from Vitamin B3 (Niacin), it is primarily involved in **redox reactions** (e.g., oxidative deamination of glutamate by glutamate dehydrogenase), not transamination. * **Biotin (Vitamin B7):** Acts as a co-enzyme for **carboxylation reactions** (e.g., Pyruvate carboxylase, Acetyl-CoA carboxylase). It carries $CO_2$. * **Riboflavin (Vitamin B2):** The precursor for FMN and FAD, which are involved in **oxidation-reduction reactions** (e.g., Acyl-CoA dehydrogenase in $\beta$-oxidation). **Clinical Pearls for NEET-PG:** * **Diagnostic Markers:** AST (Aspartate Aminotransferase) and ALT (Alanine Aminotransferase) are key transaminases used to assess liver function. * **Exceptions:** All amino acids undergo transamination **except** Lysine, Threonine, Proline, and Hydroxyproline. * **PLP Versatility:** Beyond transamination, PLP is also essential for **decarboxylation** (e.g., GABA synthesis), **deamination**, and **transsulfuration** (homocysteine metabolism). * **Drug Interaction:** Isoniazid (anti-TB drug) can cause Vitamin B6 deficiency, leading to peripheral neuropathy due to impaired PLP-dependent reactions.

Question 233: Creatinine is the breakdown product of:

A. Adenosine triphosphate
B. Purine nucleotides
C. Pyrimidine nucleotides
D. Creatine phosphate (Correct Answer)

Explanation: **Explanation:** **1. Why Creatine Phosphate is Correct:** Creatinine is the cyclic anhydride of **creatine**, formed primarily in skeletal muscle. The process begins with the synthesis of creatine from three amino acids: **Glycine, Arginine, and Methionine** (as S-adenosylmethionine). In the muscle, creatine is phosphorylated to **creatine phosphate** (phosphocreatine) by the enzyme creatine kinase, serving as a high-energy reservoir. Creatinine is formed by the spontaneous, non-enzymatic, and irreversible cyclization of creatine phosphate (and to a lesser extent, creatine). Since this conversion occurs at a constant rate (approx. 1-2% per day), urinary creatinine excretion is directly proportional to the total muscle mass of an individual. **2. Why Other Options are Incorrect:** * **Adenosine triphosphate (ATP):** While ATP provides the phosphate group to form creatine phosphate, its own breakdown products are ADP, AMP, and eventually inorganic phosphate. * **Purine Nucleotides (Adenine/Guanine):** The end product of purine catabolism in humans is **Uric Acid**. * **Pyrimidine Nucleotides (Cytosine/Thymine/Uracil):** These are broken down into highly soluble products like **β-alanine** and **β-aminoisobutyrate**, which are eventually converted to CO₂ and NH₃. **3. Clinical Pearls for NEET-PG:** * **Site of Synthesis:** Creatine synthesis starts in the **Kidney** (Guanidinoacetate formation) and is completed in the **Liver** (Methylation). * **Excretion:** Creatinine is freely filtered by the glomerulus and is not reabsorbed, making it a standard marker for **Glomerular Filtration Rate (GFR)**. * **Diagnostic Marker:** Elevated serum creatinine levels indicate renal impairment. * **Jaffe’s Reaction:** The classic colorimetric method used to estimate creatinine using alkaline picrate.

Question 234: Which of the following is NOT a degradation product of Glycine?

A. Serine
B. Oxalates
C. Pyruvates
D. Ornithine (Correct Answer)

Explanation: **Explanation:** Glycine is a non-essential, glucogenic amino acid with a simple metabolic profile. The correct answer is **Ornithine**, as it is an intermediate of the Urea cycle and a derivative of Arginine/Glutamate, but not a degradation product of Glycine. **Why Ornithine is the correct answer:** Ornithine is synthesized from Glutamate or via the action of Arginase on Arginine. While Glycine contributes to the synthesis of Creatine (alongside Arginine and Methionine), it does not break down into Ornithine. **Analysis of Incorrect Options:** * **Serine:** Glycine can be converted to Serine by the enzyme **Serine Hydroxymethyltransferase (SHMT)** in a reversible reaction requiring Tetrahydrofolate ($N^5, N^{10}$-methylene THF) and Pyridoxal Phosphate (PLP). * **Oxalates:** Glycine can be deaminated by D-amino acid oxidase to form Glyoxylate. Glyoxylate is then oxidized to Oxalate. This is clinically significant in **Primary Hyperoxaluria Type I**, where a defect in glyoxylate metabolism leads to excessive oxalate and renal stones. * **Pyruvates:** Since Glycine can be converted to Serine, and Serine can be converted to Pyruvate by **Serine Dehydratase**, Glycine is considered a glucogenic amino acid. **High-Yield Clinical Pearls for NEET-PG:** * **Glycine Encephalopathy (Non-ketotic Hyperglycinemia):** Caused by a defect in the **Glycine Cleavage System**, leading to severe neurological distress. * **Heme Synthesis:** Glycine + Succinyl CoA are the starting substrates for Heme synthesis (catalyzed by ALA Synthase). * **Glutathione:** Glycine is one of the three constituent amino acids of Glutathione (along with Glutamate and Cysteine). * **Creatine Synthesis:** Remember the mnemonic **GAM**: **G**lycine, **A**rginine, and **M**ethionine are required.

Question 235: Guanidoacetic acid is formed in which organ and from which precursors?

A. Kidney; Arginine + glycine (Correct Answer)
B. Liver; Methionine + glycine
C. Liver; Cysteine + arginine
D. Muscle; Citrulline + aspartate

Explanation: **Explanation:** The synthesis of **Creatine** is a high-yield biochemical pathway involving three amino acids and two organs. The process occurs in two distinct steps: 1. **Step 1 (Kidney):** The enzyme **L-arginine:glycine amidinotransferase (AGAT)** catalyzes the transfer of an amidino group from **Arginine** to **Glycine**. This reaction occurs primarily in the **Kidney** to form **Guanidoacetic acid (GAA)** (also known as glycocyamine). 2. **Step 2 (Liver):** Guanidoacetic acid then travels to the **Liver**, where it undergoes methylation by **S-adenosylmethionine (SAM)** to form Creatine. **Analysis of Options:** * **Option A (Correct):** Correctly identifies the kidney as the site and Arginine + Glycine as the precursors for the first committed step. * **Option B:** Incorrect. While Methionine (as SAM) and Glycine are involved in creatine synthesis, the liver is the site of the *second* step (methylation), not the formation of Guanidoacetic acid. * **Option C:** Incorrect. Cysteine is not involved in this pathway. * **Option D:** Incorrect. Muscle is the site where creatine is stored and converted to **Creatinine** (non-enzymatically), but it does not synthesize Guanidoacetic acid. **High-Yield NEET-PG Pearls:** * **Precursor Amino Acids:** Remember the mnemonic **GAM** (Glycine, Arginine, Methionine). * **Rate-Limiting Step:** The formation of Guanidoacetic acid in the kidney is the rate-limiting step of creatine synthesis. * **Clinical Correlation:** Creatinine is the anhydride form of creatine; its excretion in urine is proportional to total muscle mass and is a marker of GFR. * **Diagnostic Tip:** In GAMT deficiency (a primary creatine deficiency syndrome), Guanidoacetic acid levels are significantly elevated.

Question 236: Phenylalanine is degraded into which of the following metabolic products?

A. Fumarate and succinate
B. Fumarate and acetoacetate (Correct Answer)
C. Fumarate and malate
D. Fumarate and pyruvate

Explanation: **Explanation:** Phenylalanine is classified as both **glucogenic and ketogenic** because its metabolic degradation yields products that can enter both the glucose and ketone body synthesis pathways. 1. **Why Option B is Correct:** The catabolic pathway of Phenylalanine begins with its conversion to **Tyrosine** (via Phenylalanine hydroxylase). Tyrosine is then converted through a series of steps into **Homogentisate**. The aromatic ring is eventually cleaved to produce **4-fumarylacetoacetate**, which is hydrolyzed by the enzyme fumarylacetoacetase into: * **Fumarate:** A TCA cycle intermediate (Glucogenic). * **Acetoacetate:** A ketone body (Ketogenic). 2. **Why Other Options are Incorrect:** * **Option A & C:** Succinate and Malate are TCA cycle intermediates, but they are not the primary end-products of phenylalanine degradation. Succinate is a product of amino acids like Valine, Isoleucine, and Methionine. * **Option D:** Pyruvate is the degradation product of 3-carbon amino acids like Alanine, Glycine, and Serine. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Phenylketonuria (PKU):** Caused by a deficiency of *Phenylalanine hydroxylase*, leading to mental retardation and a "mousy" body odor. * **Alkaptonuria:** Caused by a deficiency of *Homogentisate oxidase*. Characterized by urine that turns black on standing and ochronosis (pigmentation of cartilage). * **Tyrosinemia Type I:** Caused by a deficiency of *Fumarylacetoacetase* (the final enzyme in the pathway), leading to cabbage-like odor and liver failure. * **Mnemonic:** "Ph-T-I-T" (Phenylalanine, Tyrosine, Isoleucine, Tryptophan) are the amino acids that are both glucogenic and ketogenic.

Question 237: All of the following are derivatives of tryptophan EXCEPT:

A. Melatonin
B. Serotonin
C. Niacin
D. Creatinine (Correct Answer)

Explanation: **Explanation:** The correct answer is **Creatinine** because it is a derivative of three specific amino acids: **Glycine, Arginine, and Methionine** (as S-adenosyl methionine). It is formed via the spontaneous, non-enzymatic cyclization of creatine phosphate in skeletal muscle. **Why the other options are incorrect (Tryptophan Derivatives):** Tryptophan is an essential aromatic amino acid that serves as a precursor for several vital biomolecules: * **Serotonin (5-Hydroxytryptamine):** Tryptophan is hydroxylated and decarboxylated to form serotonin, a key neurotransmitter involved in mood and sleep. * **Melatonin:** Produced in the pineal gland, serotonin is further acetylated and methylated to form melatonin, which regulates the circadian rhythm. * **Niacin (Vitamin B3):** Approximately 60 mg of dietary tryptophan can be converted into 1 mg of nicotinic acid via the **Kynurenine pathway**. This pathway requires Vitamin B6 (Pyridoxine) as a cofactor. **Clinical Pearls for NEET-PG:** 1. **Hartnup Disease:** A genetic defect in the transport of neutral amino acids (including tryptophan) in the intestine and kidneys. It clinically presents with **Pellagra-like symptoms** (Dermatitis, Diarrhea, Dementia) due to niacin deficiency. 2. **Carcinoid Syndrome:** In patients with carcinoid tumors, up to 60% of tryptophan is diverted to serotonin production, leading to secondary niacin deficiency and pellagra. 3. **Cofactor Alert:** The conversion of tryptophan to serotonin requires **Tetrahydrobiopterin (BH4)**, similar to Phenylalanine and Tyrosine metabolism.

Question 238: The carbamoyl group of carbamoyl phosphate is derived from which molecule?

A. Ammonia (Correct Answer)
B. Citrulline
C. Ornithine
D. Arginosuccinate

Explanation: **Explanation:** The synthesis of **Carbamoyl Phosphate** is the first and rate-limiting step of the **Urea Cycle**, occurring within the mitochondrial matrix of hepatocytes. This reaction is catalyzed by the enzyme **Carbamoyl Phosphate Synthetase I (CPS-I)**. **Why Ammonia is Correct:** The carbamoyl group ($–CONH_2$) is formed by the condensation of **free ammonia ($NH_3$)** and bicarbonate ($HCO_3^-$). This process requires the consumption of 2 molecules of ATP. The ammonia used here is primarily derived from the oxidative deamination of glutamate by glutamate dehydrogenase. **Why the other options are incorrect:** * **Citrulline:** This is the product formed when carbamoyl phosphate reacts with ornithine. It is transported out of the mitochondria into the cytosol. * **Ornithine:** This acts as a "carrier" molecule that accepts the carbamoyl group to form citrulline. It is regenerated at the end of the cycle. * **Arginosuccinate:** This is an intermediate formed in the cytosol by the condensation of citrulline and aspartate. **High-Yield Clinical Pearls for NEET-PG:** * **Obligatory Activator:** CPS-I requires **N-acetylglutamate (NAG)** as an allosteric activator. Without NAG, the urea cycle cannot initiate. * **CPS-I vs. CPS-II:** Do not confuse them. **CPS-I** is mitochondrial and involved in the urea cycle (source of nitrogen is ammonia). **CPS-II** is cytosolic and involved in pyrimidine synthesis (source of nitrogen is glutamine). * **Hyperammonemia Type I:** Caused by a deficiency of CPS-I, leading to severe elevations in blood ammonia levels shortly after birth.

Question 239: Which of the following dietarily non-essential amino acids becomes dietarily essential in patients with homocystinuria?

A. Cysteine (Correct Answer)
B. Methionine
C. Homocysteine
D. Phenylalanine

Explanation: ### Explanation **1. Why Cysteine is the Correct Answer:** In normal physiology, **Cysteine** is a non-essential amino acid because it is synthesized from the essential amino acid **Methionine**. The pathway involves the conversion of Methionine to **Homocysteine**, which then combines with Serine to form **Cystathionine** (catalyzed by *Cystathionine $\beta$-synthase* or CBS), eventually yielding Cysteine. In **Classical Homocystinuria**, there is a deficiency of the enzyme **CBS**. This creates a metabolic block that prevents the conversion of Homocysteine to Cysteine. Consequently, Cysteine cannot be synthesized endogenously and must be supplied through the diet, making it **conditionally essential** for these patients. **2. Analysis of Incorrect Options:** * **B. Methionine:** This is an essential amino acid. In homocystinuria, Methionine levels are actually **elevated** due to the backup of Homocysteine, which is remethylated back to Methionine. Patients are often put on a Methionine-restricted diet. * **C. Homocysteine:** This is a metabolic intermediate, not a dietary amino acid. Its levels are pathologically high in this condition. * **D. Phenylalanine:** This is an essential amino acid related to PKU, not the sulfur-containing amino acid pathway. **3. Clinical Pearls for NEET-PG:** * **Enzyme Deficiency:** Most common cause is **Cystathionine $\beta$-synthase** deficiency (requires **Vitamin B6** as a cofactor). * **Clinical Triad:** 1. Intellectual disability, 2. Ectopia lentis (downward dislocation of lens), 3. Thromboembolic episodes (premature atherosclerosis). * **Treatment:** High doses of Vitamin B6 (in responders), Methionine-restricted diet, and **Cysteine supplementation**. * **Differentiation:** Unlike Marfan syndrome (upward lens dislocation), Homocystinuria presents with **downward** lens dislocation and autosomal recessive inheritance.

Question 240: Carnitine is synthesized from which amino acids?

A. Leucine
B. Lysine
C. Lysine and methionine (Correct Answer)
D. Arginine

Explanation: **Explanation:** **1. Why Lysine and Methionine are correct:** Carnitine (β-hydroxy-γ-trimethylaminobutyrate) is essential for the transport of long-chain fatty acids into the mitochondrial matrix for β-oxidation. Its synthesis is a multi-step process involving two specific amino acids: * **Lysine:** Provides the **carbon skeleton** of carnitine. Specifically, it is the ε-amino group of lysine (within protein-bound trimethyllysine) that serves as the precursor. * **Methionine:** Acts as the **methyl group donor**. It provides three methyl groups via **S-adenosylmethionine (SAMe)** to convert lysine into trimethyllysine. The synthesis also requires Vitamin C (as a cofactor for hydroxylases), Iron, Vitamin B6, and Niacin. **2. Why other options are incorrect:** * **Leucine (A):** A purely ketogenic amino acid. While it plays a role in muscle metabolism, it does not contribute to the structure or synthesis of carnitine. * **Arginine (D):** Arginine is a precursor for Nitric Oxide (NO), Urea, and Creatine, but it is not involved in the carnitine biosynthetic pathway. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Function:** Carnitine acts as the "shuttle" for the **Carnitine Palmitoyltransferase (CPT) system**. * **Tissue Distribution:** While synthesized primarily in the **liver and kidney**, 97% of carnitine is stored in **skeletal muscle**, which depends on fatty acid oxidation for energy. * **Deficiency:** Carnitine deficiency leads to impaired fatty acid oxidation, presenting as non-ketotic hypoglycemia, muscle weakness, and cardiomyopathy. * **Cofactor Alert:** **Vitamin C** is a crucial cofactor for the enzymes *trimethyllysine hydroxylase* and *gamma-butyrobetaine hydroxylase* in this pathway. This explains why muscle weakness is a symptom of Scurvy.

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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