Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 261: All of the following regarding the urea cycle are true, except:

A. On consumption of a high amount of proteins, excess urea is formed.
B. Synthesis of argininosuccinate consumes energy.
C. The rate-limiting enzyme is ornithine transcarbamoylase. (Correct Answer)
D. One nitrogen of urea comes from aspartate.

Explanation: ### Explanation The urea cycle (Krebs-Henseleit cycle) is the primary mechanism for detoxifying ammonia into urea in the liver. **Why Option C is the correct answer (The Exception):** The rate-limiting and committed step of the urea cycle is catalyzed by **Carbamoyl Phosphate Synthetase I (CPS-I)**, not Ornithine Transcarbamoylase (OTC). CPS-I requires **N-acetylglutamate (NAG)** as an essential allosteric activator. While OTC deficiency is the most common urea cycle disorder, it does not serve as the rate-limiting step. **Analysis of other options:** * **Option A:** Urea production is directly proportional to protein intake. High protein consumption increases the breakdown of amino acids, leading to increased ammonia production, which induces the enzymes of the urea cycle. * **Option B:** The synthesis of argininosuccinate from citrulline and aspartate is catalyzed by Argininosuccinate Synthetase. This step consumes **one molecule of ATP**, which is hydrolyzed to AMP and PPi (equivalent to 2 high-energy phosphates). * **Option C:** Urea ($NH_2-CO-NH_2$) derives its components from three sources: one nitrogen from **free ammonia**, one nitrogen from **aspartate**, and the carbon atom from **bicarbonate ($CO_2$)**. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** The first two steps occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. * **OTC Deficiency:** It is the only urea cycle disorder that is **X-linked recessive**; all others are autosomal recessive. It presents with orotic aciduria. * **Fumarate Link:** Argininosuccinate lyase cleaves argininosuccinate into arginine and **fumarate**, providing a link to the TCA cycle (the "Urea Bicycle"). * **Hyperammonemia:** Management includes protein restriction and ammonia scavengers like Sodium Benzoate or Phenylbutyrate.

Question 262: Which of the following statements is true regarding the urea cycle?

A. The first two steps occur in the mitochondria.
B. A defect in an enzyme of any step can cause a deficiency disease.
C. Citrulline is formed by the combination of carbamoyl phosphate and L-ornithine.
D. All of the above statements are true. (Correct Answer)

Explanation: The urea cycle (Ornithine cycle) is the primary mechanism for detoxifying ammonia into urea in the liver. **1. Why the correct answer is right:** * **Statement A:** The urea cycle is compartmentalized. The first two reactions—catalyzed by **Carbamoyl Phosphate Synthetase I (CPS-I)** and **Ornithine Transcarbamoylase (OTC)**—occur within the **mitochondrial matrix**. The remaining three steps occur in the cytosol. * **Statement B:** Each enzyme in the cycle is critical. A genetic defect in any of the five enzymes leads to **Urea Cycle Disorders (UCDs)**, characterized by hyperammonemia, encephalopathy, and respiratory alkalosis. * **Statement C:** In the second step, Carbamoyl Phosphate reacts with L-Ornithine (shuttled from the cytosol) to form **Citrulline**. This reaction is catalyzed by OTC. Citrulline then leaves the mitochondria to continue the cycle. Since all individual statements are biochemically accurate, **Option D** is the correct choice. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Rate-Limiting Step:** CPS-I is the rate-limiting enzyme and requires **N-acetylglutamate (NAG)** as an obligatory allosteric activator. * **Most Common Defect:** **OTC deficiency** is the most common urea cycle disorder and is the only one that is **X-linked recessive** (others are autosomal recessive). * **Sources of Nitrogen:** One nitrogen atom in urea comes from free **ammonia**, and the second comes from **Aspartate**. * **Link to TCA Cycle:** The "Krebs Bicycle" refers to the shunt where **Fumarate** (released by Argininosuccinate lyase) enters the TCA cycle.

Question 263: Which of the following is an acidic amino acid?

A. Asparagine
B. Glutamine
C. Aspartate (Correct Answer)
D. Glycine

Explanation: **Explanation:** Amino acids are classified based on the chemical nature of their side chains (R-groups). **Aspartate** (aspartic acid) and **Glutamate** (glutamic acid) are the only two **acidic amino acids**. At physiological pH (7.4), their side-chain carboxyl groups (-COOH) lose a proton, resulting in a negative charge. This makes them polar and hydrophilic. **Analysis of Options:** * **Aspartate (Correct):** It contains a second carboxyl group in its side chain. It is a key intermediate in the urea cycle and acts as an excitatory neurotransmitter in the CNS. * **Asparagine & Glutamine (Incorrect):** These are the **amide derivatives** of aspartate and glutamate, respectively. While they are polar, their side chains are uncharged (neutral) at physiological pH. * **Glycine (Incorrect):** It is the simplest amino acid with only a hydrogen atom as its side chain. It is non-polar, optically inactive (achiral), and serves as a major inhibitory neurotransmitter in the spinal cord. **High-Yield Facts for NEET-PG:** 1. **Basic Amino Acids:** Remember the mnemonic **"HAL"** – **H**istidine, **A**rginine, and **L**ysine. Arginine is the most basic. 2. **Charge at pH 7.4:** Acidic amino acids are negatively charged (anions), while basic amino acids (except Histidine) are positively charged (cations). 3. **Dicarboxylic Acids:** Aspartate and Glutamate are also referred to as dicarboxylic monoamino acids. 4. **Clinical Correlation:** Aspartate is a precursor for the synthesis of nitrogenous bases (purines and pyrimidines) and is involved in the malate-aspartate shuttle for ATP production.

Question 264: A fair-skinned child may be suffering with which of the following diseases?

A. Von Gierke disease
B. Alkaptonuria
C. Albinism (Correct Answer)
D. Tyrosinemia

Explanation: ### Explanation **Correct Option: C. Albinism** Albinism is a group of genetic disorders characterized by a deficit in **melanin production**. The most common cause is a deficiency of the enzyme **Tyrosinase**, which converts Tyrosine to DOPA and subsequently to Melanin. Since melanin is the primary pigment responsible for the color of the skin, hair, and eyes, its absence results in a characteristic **fair-skinned** appearance (hypopigmentation), white hair, and light-colored eyes. **Analysis of Incorrect Options:** * **A. Von Gierke Disease:** This is a Type I Glycogen Storage Disease (GSD) caused by a deficiency of Glucose-6-Phosphatase. Clinical features include hypoglycemia, hepatomegaly, and "doll-like" facies, but it does not typically affect skin pigmentation. * **B. Alkaptonuria:** Caused by a deficiency of **Homogentisate oxidase**. It leads to the accumulation of homogentisic acid, which causes **darkening** of urine upon standing and **ochronosis** (dark pigmentation of connective tissues/cartilage)—the opposite of fair skin. * **C. Tyrosinemia:** This involves defects in the catabolic pathway of tyrosine (e.g., Fumarylacetoacetate hydrolase deficiency in Type I). It primarily presents with liver failure, renal tubular dysfunction (Fanconi syndrome), and a "cabbage-like" odor, rather than generalized hypopigmentation. **High-Yield Clinical Pearls for NEET-PG:** * **Phenylketonuria (PKU):** Also presents with fair skin and blue eyes because high phenylalanine levels competitively inhibit tyrosinase, leading to decreased melanin. * **Ocular Albinism:** Often associated with nystagmus and photophobia due to lack of pigment in the iris and retina. * **Chediak-Higashi Syndrome:** A rare cause of partial albinism (silvery hair) associated with immunodeficiency and giant lysosomal granules.

Question 265: HHH syndrome is due to deficiency of which enzyme?

A. Arginase
B. Arginosuccinase
C. Ornithine transcarbamylase (Correct Answer)
D. None of the above

Explanation: **Explanation** **HHH Syndrome** (Hyperammonemia-Hyperornithinemia-Homocitrullinuria) is a rare autosomal recessive urea cycle disorder. It is caused by a defect in the **SLC25E1 gene**, which encodes the **mitochondrial ornithine transporter (ORNT1)**. This transporter is responsible for moving ornithine from the cytosol into the mitochondrial matrix. **Why the Correct Answer is "None of the above" (Contextual Correction):** While the provided key indicates *Ornithine transcarbamylase* (OTC), this is technically a common distractor. HHH syndrome is specifically due to a **transporter defect**, not a primary enzyme deficiency. However, in the context of NEET-PG questions where the transporter is not listed, it is often grouped with urea cycle defects. If "None of the above" is an option, it is the most accurate choice because the primary pathology is the **ORNT1 transporter deficiency**. **Analysis of Options:** * **A. Arginase:** Deficiency leads to Argininemia. It presents with spastic diplegia and high arginine levels, but not the specific HHH triad. * **B. Arginosuccinase:** Deficiency causes Arginosuccinic Aciduria, characterized by friable hair (Trichorrhexis nodosa). * **C. Ornithine transcarbamylase (OTC):** This is the most common urea cycle disorder (X-linked). While it involves ornithine, it leads to **Orotic aciduria**, not hyperornithinemia. **Clinical Pearls for NEET-PG:** 1. **The Triad:** Hyperammonemia (due to urea cycle failure), Hyperornithinemia (ornithine cannot enter mitochondria and builds up in cytosol), and Homocitrullinuria (excess carbamoyl phosphate reacts with lysine). 2. **Genetics:** Autosomal Recessive (unlike OTC, which is X-linked). 3. **High-Yield Fact:** In HHH syndrome, the urea cycle is blocked because **Ornithine transcarbamylase** lacks its substrate (ornithine) inside the mitochondria, even though the enzyme itself is functional.

Question 266: Cystine is a dimer formed from two molecules of which amino acid?

A. Glycine
B. Cysteine (Correct Answer)
C. Alanine
D. Serine

Explanation: **Explanation:** The correct answer is **Cysteine**. **1. Why Cysteine is Correct:** Cystine is a sulfur-containing amino acid formed by the **oxidative dimerization** of two molecules of Cysteine. This reaction occurs when the thiol (-SH) groups of two cysteine residues react to form a **disulfide bond (S-S)**. This covalent linkage is crucial for stabilizing the tertiary and quaternary structures of proteins (e.g., insulin, keratin, and immunoglobulins). While Cysteine is a single amino acid, Cystine is considered a "derived" amino acid found primarily in extracellular proteins. **2. Why Other Options are Incorrect:** * **Glycine:** The simplest amino acid (achiral) with a hydrogen atom as its R-group. It does not contain sulfur and cannot form disulfide bridges. * **Alanine:** A non-polar amino acid with a methyl group side chain. It lacks the reactive thiol group necessary for dimerization. * **Serine:** Structurally similar to cysteine but contains a hydroxyl (-OH) group instead of a thiol (-SH) group. It forms hydrogen bonds but not disulfide bonds. **3. Clinical Pearls for NEET-PG:** * **Cystinuria:** A defect in the renal tubular reabsorption of COAL (Cystine, Ornithine, Arginine, Lysine). It leads to the formation of **hexagonal cystine stones** in the urine. * **Cyanide-Nitroprusside Test:** Used to screen for cystine in urine (turns purple/magenta). * **Cystinosis:** A lysosomal storage disorder where cystine crystals accumulate in organs due to defective transport out of lysosomes. * **Reducing Agents:** Beta-mercaptoethanol or DTT can break the disulfide bonds of cystine back into two cysteine molecules.

Question 267: Which branched-chain amino acid is both ketogenic and glucogenic?

A. Leucine
B. Isoleucine (Correct Answer)
C. Valine
D. Tryptophan

Explanation: **Explanation:** The classification of amino acids as glucogenic, ketogenic, or both depends on the metabolic intermediates produced during their catabolism. Branched-chain amino acids (BCAAs) follow distinct pathways: 1. **Isoleucine (Correct):** Isoleucine is unique among BCAAs because its carbon skeleton is cleaved into two fragments: **Succinyl-CoA** (which enters the TCA cycle for gluconeogenesis) and **Acetyl-CoA** (which is used for ketogenesis). Therefore, it is both glucogenic and ketogenic. 2. **Leucine (Incorrect):** Leucine is strictly **ketogenic**. Its catabolism yields Acetyl-CoA and Acetoacetate. It cannot contribute to glucose synthesis. 3. **Valine (Incorrect):** Valine is strictly **glucogenic**. It is converted entirely into Succinyl-CoA. 4. **Tryptophan (Incorrect):** While Tryptophan is indeed both glucogenic and ketogenic, it is an **aromatic** amino acid, not a branched-chain amino acid. The question specifically asks for a BCAA. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Dual Nature:** "PhITTT" (Phenylalanine, Isoleucine, Tyrosine, Threonine, Tryptophan) are both glucogenic and ketogenic. * **Purely Ketogenic:** Leucine and Lysine (The only two). * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the **Branched-chain α-keto acid dehydrogenase** complex, leading to the accumulation of Leucine, Isoleucine, and Valine. * **Metabolic Site:** Unlike most amino acids metabolized in the liver, BCAAs are primarily oxidized in **extrahepatic tissues**, specifically skeletal muscle.

Question 268: What is the key enzyme in urea synthesis?

A. Urease
B. Carbamoyl phosphate synthetase I (Correct Answer)
C. Arginase
D. Ornithine transcarbamylase

Explanation: **Explanation:** The urea cycle is the primary mechanism for detoxifying ammonia into urea in the liver. **Carbamoyl Phosphate Synthetase I (CPS-I)** is the **rate-limiting and key regulatory enzyme** of this cycle. It catalyzes the first step: the condensation of ammonia ($NH_3$) and bicarbonate ($HCO_3^-$) to form carbamoyl phosphate. This reaction occurs in the **mitochondria** and requires 2 ATP molecules. Crucially, CPS-I is allosterically activated by **N-acetylglutamate (NAG)**; without NAG, the enzyme is inactive. **Analysis of Incorrect Options:** * **A. Urease:** This enzyme is not found in humans. It is produced by bacteria (e.g., *H. pylori*) to break down urea into ammonia and $CO_2$. * **C. Arginase:** This is the final enzyme of the cycle that cleaves Arginine into Urea and Ornithine. While it produces the final product, it is not the rate-limiting step. * **D. Ornithine transcarbamylase (OTC):** This is the second enzyme of the cycle. OTC deficiency is the most common urea cycle disorder and is unique because it is **X-linked recessive**, whereas others are autosomal recessive. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** The urea cycle occurs in both the **mitochondria** (first two steps) and the **cytosol** (remaining steps)—remembered by the mnemonic: *"**M**any **C**ats"* (Mitochondria/Cytosol). * **NAGS Deficiency:** Mimics CPS-I deficiency, leading to hyperammonemia. It is treated with **Carglumic acid** (a NAG analogue). * **Sources of Nitrogen:** One nitrogen atom in urea comes from free **ammonia**, and the second comes from **Aspartate**.

Question 269: Which of the following glucogenic amino acids enters the citric acid cycle as succinyl CoA?

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

Explanation: **Explanation:** The correct answer is **Isoleucine**. Amino acids are classified as glucogenic, ketogenic, or both, based on the intermediates they produce during catabolism. **1. Why Isoleucine is Correct:** Isoleucine is both glucogenic and ketogenic. Its catabolism follows a pathway shared with other branched-chain amino acids (BCAAs). After transamination and oxidative decarboxylation, the carbon skeleton of Isoleucine is cleaved to yield **Succinyl CoA** (which enters the TCA cycle for gluconeogenesis) and **Acetyl CoA** (which is ketogenic). **2. Analysis of Incorrect Options:** * **Phenylalanine & Tyrosine (Options A & B):** These are both glucogenic and ketogenic. However, their carbon skeletons are degraded into **Fumarate** (glucogenic) and **Acetoacetate** (ketogenic), not Succinyl CoA. * **Tryptophan (Option D):** This is also both glucogenic and ketogenic. Its breakdown yields **Pyruvate** (glucogenic) and **Acetoacetyl CoA** (ketogenic). **3. High-Yield NEET-PG Pearls:** * **VOMIT Pathway:** A useful mnemonic for amino acids that enter the TCA cycle as **Succinyl CoA** is **VOMIT**: **V**aline, **O**dd-chain fatty acids, **M**ethionine, **I**soleucine, and **T**hreonine. * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the *Branched-chain α-keto acid dehydrogenase* complex, leading to the buildup of Leucine, Isoleucine, and Valine. * **Purely Ketogenic Amino Acids:** Remember **Leucine and Lysine** (The "L"s) are the only two amino acids that are exclusively ketogenic.

Question 270: Methionine is synthesized in the human body from which precursor?

A. Cysteine
B. Homocysteine (Correct Answer)
C. Cystine
D. Tryptophan

Explanation: **Explanation:** **Why Homocysteine is Correct:** Methionine is an **essential amino acid**, meaning the carbon skeleton cannot be synthesized de novo in humans. However, it can be **regenerated** from **Homocysteine** via a remethylation pathway. This reaction is catalyzed by the enzyme **Methionine Synthase**, which requires two critical cofactors: **Vitamin B12 (Cobalamin)** and **N5-methyltetrahydrofolate** (a derivative of Folic Acid). In this reaction, homocysteine accepts a methyl group to become methionine, effectively recycling the sulfur-containing amino acid. **Why Other Options are Incorrect:** * **Cysteine & Cystine:** These are synthesized *from* methionine via the transsulfuration pathway (Methionine → Homocysteine → Cystathionine → Cysteine). The process is unidirectional; humans cannot convert cysteine back into methionine. Cystine is simply the oxidized dimer of cysteine. * **Tryptophan:** This is an aromatic essential amino acid and is not involved in the sulfur-containing amino acid metabolic pathway. **High-Yield Clinical Pearls for NEET-PG:** * **The Methyl Trap:** A deficiency in Vitamin B12 leads to folate being "trapped" as N5-methyl-THF, because Methionine Synthase is the only enzyme that can utilize it. This results in functional folate deficiency and megaloblastic anemia. * **Hyperhomocysteinemia:** Deficiencies in B12, Folate, or the enzyme MTHFR lead to elevated homocysteine levels, which is a significant risk factor for coronary artery disease, deep vein thrombosis (DVT), and neural tube defects. * **S-Adenosylmethionine (SAM):** Methionine is the precursor for SAM, the body's universal methyl donor.

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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