Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 251: Which amino acid is acidic?

A. Aspartic acid (Correct Answer)
B. Valine
C. Leucine
D. Aspartate

Explanation: **Explanation:** Amino acids are categorized based on the chemical nature of their side chains (R-groups) at physiological pH. **1. Why Aspartic Acid is Correct:** Aspartic acid (and Glutamic acid) are classified as **acidic amino acids**. They possess a carboxylic acid group (-COOH) in their side chain. At physiological pH (~7.4), this group loses a proton, becoming negatively charged (carboxylate ion). Therefore, these are also referred to as "dicarboxylic monoamino acids." *Note on Option D:* While "Aspartate" is the ionized form of Aspartic acid found in the body, in the context of standard biochemical classification questions for NEET-PG, "Aspartic acid" is the traditional nomenclature used to identify the acidic category. **2. Analysis of Incorrect Options:** * **Valine (B):** This is a **branched-chain amino acid (BCAA)** with a non-polar, hydrophobic aliphatic side chain. It is neutral. * **Leucine (C):** Like Valine, Leucine is a **branched-chain amino acid** and is purely ketogenic. It is also neutral and hydrophobic. * **Aspartate (D):** While chemically similar, in MCQ formats, if both "Aspartic acid" and "Aspartate" are provided, the parent name of the acid group is typically preferred unless the question specifically asks for the conjugate base or ionized form. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Acidic AAs:** "As Glue" (**As**partic acid and **Glu**tamic acid). * **Mnemonic for Basic AAs:** "HAL" (**H**istidine, **A**rginine, **L**ysine). * **Isoelectric Point (pI):** Acidic amino acids have a low pI (around 3), meaning they migrate toward the anode in electrophoresis at physiological pH. * **Clinical Correlation:** Aspartate is a key component of the Malate-Aspartate shuttle and a precursor for urea cycle intermediates (combining with citrulline to form argininosuccinate).

Question 252: N-acetylglutamate serves as an allosteric activator of which of the following enzymes in the urea cycle?

A. Argininosuccinate lyase
B. Argininosuccinate synthase
C. Ornithine transcarbamoylase
D. Carbamoyl phosphate synthetase I (Correct Answer)

Explanation: **Explanation:** The urea cycle is the primary pathway for the detoxification of ammonia in the liver. The correct answer is **Carbamoyl Phosphate Synthetase I (CPS-I)** because it is the **rate-limiting and committed step** of the urea cycle. **1. Why CPS-I is correct:** CPS-I, located in the mitochondria, requires **N-acetylglutamate (NAG)** as an absolute allosteric activator. Without NAG, CPS-I is inactive, and the cycle cannot proceed. NAG is synthesized from glutamate and acetyl-CoA by the enzyme *N-acetylglutamate synthase (NAGS)*. High levels of Arginine further stimulate NAGS, signaling that amino acid catabolism is high and the urea cycle must be upregulated to handle the resulting ammonia. **2. Why other options are incorrect:** * **Ornithine transcarbamoylase (OTC):** This is the second step of the cycle. While it is the most common site of urea cycle defects (OTC deficiency), it is regulated by substrate availability (ornithine and carbamoyl phosphate), not NAG. * **Argininosuccinate synthase & Argininosuccinate lyase:** These are the third and fourth steps occurring in the cytosol. They are regulated primarily by the concentration of their respective substrates and do not require NAG for activation. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **CPS-I vs. CPS-II:** Do not confuse them. CPS-I is mitochondrial (Urea cycle); CPS-II is cytosolic (Pyrimidine synthesis). * **NAGS Deficiency:** Mimics CPS-I deficiency clinically (hyperammonemia), but can be treated with **Carglumic acid**, a synthetic analog of NAG. * **Mnemonic:** "NAG activates the first step." * **Location:** The first two steps (CPS-I, OTC) occur in the **mitochondria**, while the remaining steps occur in the **cytosol**.

Question 253: Urea synthesis is mediated by which enzyme?

A. Ornithine decarboxylase
B. Aspartate transaminase
C. Arginase (Correct Answer)
D. Citrulline synthase

Explanation: **Explanation:** The **Urea Cycle (Krebs-Henseleit cycle)** is the primary mechanism for detoxifying ammonia into urea in the liver. **Why Arginase is correct:** Arginase is the final enzyme of the urea cycle. It catalyzes the hydrolytic cleavage of **Arginine** into **Urea** and **Ornithine**. This reaction occurs exclusively in the cytosol of hepatocytes. While Ornithine is recycled back into the mitochondria to continue the cycle, Urea is released into the bloodstream and excreted by the kidneys. **Analysis of Incorrect Options:** * **Ornithine decarboxylase:** This is the rate-limiting enzyme in **polyamine synthesis** (forming putrescine), not the urea cycle. * **Aspartate transaminase (AST):** This enzyme facilitates the transfer of an amino group from glutamate to oxaloacetate to form **aspartate**. While aspartate provides the second nitrogen atom for urea synthesis, AST itself does not mediate the synthesis of urea. * **Citrulline synthase:** This is an alternative name sometimes used for *Ornithine Transcarbamoylase (OTC)*, which produces Citrulline. While it is a key step, it does not result in the formation of urea. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Carbamoyl Phosphate Synthetase I (CPS-I), which requires **N-acetylglutamate (NAG)** as an essential allosteric activator. * **Subcellular location:** The cycle is "split"—the first two steps occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. * **Nitrogen Sources:** One nitrogen comes from free ammonia, and the second comes from **Aspartate**. * **Arginase Deficiency:** Unlike other urea cycle defects, this typically presents with spastic diplegia and developmental delay rather than severe neonatal hyperammonemia.

Question 254: Creatinine is formed from which of the following precursors?

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

Explanation: **Explanation:** Creatinine is the anhydride form of creatine, a molecule essential for energy storage in muscles. The synthesis of creatine involves three specific amino acids: **Arginine, Glycine, and Methionine.** 1. **Why Arginine is correct:** The first step of creatine synthesis occurs in the kidney, where the enzyme *L-arginine:glycine amidinotransferase* transfers an amidino group from **Arginine** to Glycine to form guanidinoacetate. This is the rate-limiting step. Subsequently, in the liver, guanidinoacetate is methylated by S-adenosylmethionine (SAM) to form Creatine. Creatine is then phosphorylated to creatine phosphate in muscles and non-enzymatically cyclized to **Creatinine**, which is excreted in urine. 2. **Why other options are incorrect:** * **Lysine:** Primarily involved in carnitine synthesis (not creatine) and is a purely ketogenic amino acid. * **Leucine:** A branched-chain amino acid (BCAA) that is purely ketogenic; it serves as a precursor for HMG-CoA but not for creatine. * **Histamine:** This is a biogenic amine produced by the decarboxylation of the amino acid **Histidine**, not a precursor for creatinine. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Synthesis:** Creatine synthesis starts in the **Kidney** and is completed in the **Liver**. * **Excretion:** Creatinine is filtered freely by the glomerulus and is not reabsorbed, making it a key clinical marker for **Glomerular Filtration Rate (GFR)**. * **Diagnostic Marker:** Elevated Serum Creatinine indicates impaired renal function. * **Memory Aid:** Remember **"GAM"** for Creatine precursors: **G**lycine, **A**rginine, and **M**ethionine.

Question 255: Which amino acid is used in the synthesis of carnitine?

A. Alanine
B. Lysine (Correct Answer)
C. Arginine
D. Tyrosine

Explanation: **Explanation:** **1. Why Lysine is the Correct Answer:** Carnitine (β-hydroxy-γ-trimethylammonium butyrate) is synthesized primarily in the liver and kidneys. The synthesis begins with the amino acid **Lysine**, which provides the carbon skeleton. Specifically, lysine residues in certain proteins are methylated using **S-adenosylmethionine (SAM)** to form **Trimethyllysine**. Following proteolysis, trimethyllysine undergoes a series of reactions (hydroxylation and cleavage) to eventually form carnitine. **Methionine** is also essential for this process as it acts as the methyl group donor. **2. Why the Other Options are Incorrect:** * **Alanine:** A non-essential amino acid primarily involved in gluconeogenesis (via the Glucose-Alanine cycle) and transamination reactions. It does not contribute to carnitine synthesis. * **Arginine:** A precursor for Nitric Oxide (NO), Urea, and Creatine. While it is involved in nitrogen metabolism, it is not a precursor for carnitine. * **Tyrosine:** A precursor for catecholamines (Dopamine, Epinephrine, Norepinephrine), Thyroid hormones (T3, T4), and Melanin. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Function of Carnitine:** It is essential for the **"Carnitine Shuttle,"** transporting long-chain fatty acids across the inner mitochondrial membrane for **β-oxidation**. * **Cofactors Required:** The synthesis of carnitine requires **Vitamin C (Ascorbic acid)**, Fe²⁺, Vitamin B6, and Niacin. Vitamin C deficiency (Scurvy) leads to decreased carnitine synthesis, contributing to the symptom of fatigue. * **Primary Deficiency:** Caused by defects in the carnitine transporter (OCTN2), leading to hypoketotic hypoglycemia and cardiomyopathy. * **Key Precursors:** Remember the duo **Lysine + Methionine**.

Question 256: Phenylalanine metabolism yields which of the following products, except?

A. Epinephrine
B. Dopamine
C. Melanin
D. Melatonin (Correct Answer)

Explanation: **Explanation:** The core concept tested here is the differentiation between the **Phenylalanine-Tyrosine pathway** and the **Tryptophan pathway**. **1. Why Melatonin is the Correct Answer:** Melatonin is synthesized from the amino acid **Tryptophan**, not Phenylalanine. The pathway involves Tryptophan being converted to 5-Hydroxytryptophan, then to **Serotonin**, and finally to **Melatonin** (primarily in the pineal gland). Since it originates from a different precursor, it is the "except" in this list. **2. Analysis of Incorrect Options (Products of Phenylalanine):** Phenylalanine is first converted to **Tyrosine** by the enzyme *Phenylalanine Hydroxylase*. Tyrosine then serves as the precursor for: * **Dopamine & Epinephrine (Options A & B):** Tyrosine is converted to L-DOPA, which then yields the catecholamines: Dopamine → Norepinephrine → Epinephrine. * **Melanin (Option C):** In melanocytes, Tyrosine is acted upon by the enzyme *Tyrosinase* to produce Melanin pigments. * **Thyroxine:** Tyrosine is also the precursor for thyroid hormones (T3/T4). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **PKU (Phenylketonuria):** Deficiency of *Phenylalanine Hydroxylase* leads to accumulation of Phenylalanine and a deficiency of Tyrosine (making Tyrosine an "essential" amino acid in PKU patients). * **Albinism:** Results from a deficiency of *Tyrosinase*, preventing the conversion of Tyrosine to Melanin. * **Rate-limiting step:** *Tyrosine Hydroxylase* is the rate-limiting enzyme for catecholamine synthesis. * **Vitamin Link:** Both Phenylalanine and Tryptophan hydroxylases require **Tetrahydrobiopterin (BH4)** as a co-factor.

Question 257: The nitrogen atoms of urea are contributed by which two molecules?

A. Ammonium and aspartate (Correct Answer)
B. Ammonium and glutamate
C. Ammonium and glycine
D. Ammonium and asparagine

Explanation: The Urea Cycle (Ornithine cycle) is the primary mechanism for detoxifying ammonia into urea in the liver. Understanding the origin of urea's components is a high-yield topic for NEET-PG. ### **Explanation of the Correct Answer** The urea molecule ($NH_2-CO-NH_2$) contains two nitrogen atoms: 1. **The first nitrogen** enters the cycle as **free ammonium ($NH_4^+$)**. It combines with $CO_2$ to form Carbamoyl Phosphate via the enzyme *Carbamoyl Phosphate Synthetase I (CPS-I)*. 2. **The second nitrogen** is donated by **Aspartate**. This occurs when Citrulline condenses with Aspartate to form Argininosuccinate, catalyzed by *Argininosuccinate synthetase*. ### **Analysis of Incorrect Options** * **B. Glutamate:** While glutamate is the primary source of the free ammonium (via oxidative deamination by *Glutamate Dehydrogenase*), it does not donate the nitrogen atom directly into the urea cycle intermediates. * **C. Glycine:** Glycine is involved in heme and creatine synthesis but does not provide nitrogen for the urea cycle. * **D. Asparagine:** Asparagine must first be converted to aspartate (releasing ammonia) before it can contribute to the cycle, but it is not the direct donor. ### **NEET-PG High-Yield Pearls** * **Rate-limiting step:** CPS-I (requires N-acetylglutamate as an obligate activator). * **Link to TCA Cycle:** The "Kreb’s Bicycle" connects the two cycles via **Fumarate**, which is released when Argininosuccinate is cleaved. * **Cellular Location:** The cycle is "split"—the first two steps occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. * **Carbon Source:** The single carbon atom in urea is derived from **Bicarbonate ($HCO_3^-$)** or $CO_2$.

Question 258: All of the following are tryptophan derivatives except?

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

Explanation: **Explanation:** The correct answer is **Melanin** because it is derived from the amino acid **Tyrosine**, not Tryptophan. In melanocytes, the enzyme tyrosinase converts tyrosine into DOPA and subsequently into melanin. **Analysis of Options:** * **Melanin (Correct):** Derived from Tyrosine. A deficiency in the enzyme tyrosinase leads to Albinism. * **Serotonin (Incorrect):** Tryptophan is hydroxylated by tryptophan hydroxylase (requiring BH4) and then decarboxylated to form Serotonin (5-hydroxytryptamine), a key neurotransmitter. * **Melatonin (Incorrect):** This hormone, which regulates the sleep-wake cycle in the pineal gland, is synthesized via the acetylation and methylation of Serotonin (thus making it a Tryptophan derivative). * **Niacin (Incorrect):** Approximately 60 mg of Tryptophan can be converted into 1 mg of Nicotinic acid (Vitamin B3) via the kynurenine pathway. This process requires Vitamin B6 (Pyridoxine) as a cofactor. **High-Yield Clinical Pearls for NEET-PG:** 1. **Hartnup Disease:** A defect in the transport of neutral amino acids (Tryptophan) in the gut and kidneys. It presents with pellagra-like symptoms because Tryptophan is unavailable for Niacin synthesis. 2. **Carcinoid Syndrome:** Tumor cells divert up to 60% of Tryptophan to produce Serotonin. This "tryptophan steal" leads to a secondary Niacin deficiency (Pellagra). 3. **Cofactor Alert:** Tryptophan metabolism requires **BH4** (for hydroxylation), **B6** (for the kynurenine pathway), and **B2/B3** for subsequent steps.

Question 259: What amino acid is produced by adding a hydroxyl group to the phenyl side chain of phenylalanine?

A. Threonine
B. Histidine
C. Tyrosine (Correct Answer)
D. Serine

Explanation: **Explanation:** The correct answer is **Tyrosine**. **1. Why Tyrosine is Correct:** The conversion of Phenylalanine to Tyrosine is a classic hydroxylation reaction. The enzyme **Phenylalanine Hydroxylase (PAH)** adds a hydroxyl (-OH) group to the para-position of the phenyl ring of Phenylalanine. This reaction requires **Tetrahydrobiopterin (BH4)** as a co-factor and molecular oxygen. Tyrosine is considered a "non-essential" amino acid only because it can be synthesized from the essential amino acid Phenylalanine. **2. Why the Other Options are Incorrect:** * **Threonine:** An essential amino acid containing a hydroxyl group on its aliphatic side chain, not derived from a phenyl ring. * **Histidine:** An essential amino acid characterized by an imidazole ring, not a phenyl ring. * **Serine:** A non-essential amino acid with a hydroxymethyl group. It is primarily synthesized from 3-phosphoglycerate (an intermediate of glycolysis), not from Phenylalanine. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Phenylketonuria (PKU):** A deficiency in Phenylalanine Hydroxylase (Classic PKU) or BH4 leads to the accumulation of Phenylalanine and its metabolites (phenylpyruvate, phenyllactate), causing intellectual disability and a "mousy" body odor. * **Essentiality:** If Phenylalanine Hydroxylase is deficient, Tyrosine becomes a **conditionally essential** amino acid. * **Precursor Role:** Tyrosine is the metabolic precursor for several vital molecules: **Catecholamines** (Dopamine, Epinephrine, Norepinephrine), **Thyroid hormones** (T3, T4), and **Melanin**. * **Alkaptonuria:** A defect in Homogentisate oxidase (part of the Tyrosine degradation pathway) leads to dark urine and ochronosis.

Question 260: Massive aminoaciduria without a corresponding increase in plasma amino acid level is characteristic of which of the following diseases?

A. Homocystinuria
B. Hartnup disease (Correct Answer)
C. Tyrosinemia
D. Maple syrup urine disease

Explanation: ### Explanation The key to this question lies in distinguishing between **Overflow Aminoaciduria** and **Renal Aminoaciduria**. **1. Why Hartnup Disease is Correct:** Hartnup disease is a **Renal Aminoaciduria**. It is caused by a defect in the **SLC6A19 transporter**, which is responsible for the reabsorption of neutral amino acids (especially Tryptophan) in the proximal renal tubules and the intestinal mucosa. Because the defect is in the transport mechanism of the kidney, amino acids are not reabsorbed and are excreted in massive amounts in the urine. Crucially, the **plasma levels remain normal or low** because the primary pathology is a "leak" at the kidney level, not an overproduction in the blood. **2. Why the Other Options are Incorrect:** * **Homocystinuria, Tyrosinemia, and Maple Syrup Urine Disease (MSUD):** These are all examples of **Overflow Aminoaciduria**. In these metabolic disorders, an enzyme deficiency leads to the accumulation of specific amino acids in the **plasma** (e.g., Methionine/Homocysteine, Tyrosine, or Branched-chain amino acids). Once the plasma concentration exceeds the renal threshold for reabsorption, the excess "overflows" into the urine. Therefore, in these conditions, high urinary levels are always accompanied by high plasma levels. **3. High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Hartnup disease mimics **Pellagra** (Dermatitis, Diarrhea, Dementia) because Tryptophan is a precursor for Niacin (Vitamin B3) synthesis. * **Diagnosis:** Look for the "Neutral Aminoaciduria" pattern in urine chromatography. * **Treatment:** High-protein diet and Nicotinamide supplementation. * **Other Renal Aminoacidurias:** Cystinuria (defect in COAL: Cystine, Ornithine, Arginine, Lysine) and Fanconi Syndrome (generalized proximal tubule dysfunction).

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free