Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 181: OTC deficiency causes which of the following conditions?

A. Citrullinemia
B. Argininemia
C. Hyperammonemia type-1
D. Hyperammonemia type-2 (Correct Answer)

Explanation: **Explanation:** **Ornithine Transcarbamoylase (OTC) deficiency** is the most common urea cycle disorder. It is unique because it is the only urea cycle defect inherited as an **X-linked recessive** trait; all others are autosomal recessive. 1. **Why Hyperammonemia Type-2 is correct:** The urea cycle begins in the mitochondria. The second step involves the enzyme **OTC**, which catalyzes the reaction between Carbamoyl Phosphate and Ornithine to form Citrulline. A deficiency in OTC leads to an accumulation of Carbamoyl Phosphate and Ammonia, resulting in **Hyperammonemia Type-2**. Excess Carbamoyl Phosphate leaks into the cytosol and enters the pyrimidine synthesis pathway, leading to increased production of **Orotic acid** (a key diagnostic marker). 2. **Why other options are incorrect:** * **Hyperammonemia Type-1:** Caused by a deficiency of **Carbamoyl Phosphate Synthetase I (CPS-I)**. Unlike OTC deficiency, there is no orotic aciduria here. * **Citrullinemia:** Caused by a deficiency of **Argininosuccinate Synthetase**, leading to a massive buildup of Citrulline. * **Argininemia:** Caused by a deficiency of **Arginase**, the final enzyme of the cycle. It typically presents with spastic diplegia rather than acute neonatal hyperammonemia. **High-Yield Clinical Pearls for NEET-PG:** * **Orotic Aciduria:** Differentiates OTC deficiency (High orotic acid) from CPS-I deficiency (Low/Normal orotic acid). * **Management:** Treatment involves a low-protein diet and ammonia-scavenging drugs like **Sodium Benzoate** or **Phenylbutyrate**. * **Citrate/Arginine:** Citrulline levels are typically low in OTC deficiency.

Question 182: All of the following are ketogenic and glucogenic amino acids except?

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

Explanation: ### Explanation The classification of amino acids based on their metabolic fate is a high-yield topic for NEET-PG. Amino acids are categorized into three groups based on whether their carbon skeletons can be converted into glucose (glucogenic), ketone bodies (ketogenic), or both. **1. Why Serine is the Correct Answer:** **Serine** is a **purely glucogenic** amino acid. Its carbon skeleton is converted into **pyruvate**, which can then enter the gluconeogenesis pathway to produce glucose. It cannot be converted into Acetyl-CoA or Acetoacetate (the precursors for ketone bodies); therefore, it lacks ketogenic properties. **2. Analysis of Incorrect Options (Both Glucogenic & Ketogenic):** These amino acids are "amphibolic" because their breakdown products include both glucose precursors (like Succinyl-CoA or Fumarate) and ketone precursors (like Acetyl-CoA). * **Tyrosine & Phenylalanine (Options A & C):** These aromatic amino acids are metabolized into **Fumarate** (glucogenic) and **Acetoacetate** (ketogenic). * **Isoleucine (Option B):** This branched-chain amino acid is degraded into **Succinyl-CoA** (glucogenic) and **Acetyl-CoA** (ketogenic). * *Note:* Tryptophan is the fourth major amino acid that is both glucogenic and ketogenic. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Purely Ketogenic:** Only two amino acids are purely ketogenic—**Leucine and Lysine** (Mnemonic: The "L"s are purely ketogenic). * **Both Glucogenic & Ketogenic:** Remember the mnemonic **"PITTT"**: **P**henylalanine, **I**soleucine, **T**yrosine, **T**ryptophan, and **T**hreonine. * **Purely Glucogenic:** All remaining 13 amino acids (including Serine, Glycine, Valine, etc.). * **Clinical Correlation:** In ketogenic diets or states of starvation, the body relies on ketogenic amino acids to produce ketone bodies (acetoacetate and β-hydroxybutyrate) as an alternative fuel source for the brain.

Question 183: Deficiency of arginosuccinate synthase causes which of the following disorders?

A. Citrullinemia (Correct Answer)
B. Hyperargininemia
C. Argininosuccinic aciduria
D. Type I hyperammonemia

Explanation: **Explanation:** The Urea Cycle is the primary pathway for disposing of nitrogenous waste. A deficiency in any of its five core enzymes leads to a specific Urea Cycle Disorder (UCD). **1. Why Citrullinemia is Correct:** **Argininosuccinate synthase (ASS)** is the third enzyme of the urea cycle. It catalyzes the condensation of **citrulline** and **aspartate** to form argininosuccinate. When ASS is deficient (Type I Citrullinemia), citrulline cannot be converted and subsequently accumulates in the blood and urine. This block also leads to a backup of ammonia, causing hyperammonemia. **2. Analysis of Incorrect Options:** * **Hyperargininemia:** Caused by a deficiency of **Arginase**, the final enzyme of the cycle. It is unique because it often presents with spastic diplegia rather than acute neonatal hyperammonemia. * **Argininosuccinic aciduria:** Caused by a deficiency of **Argininosuccinate lyase (ASL)**. This results in the accumulation of argininosuccinate. A classic clinical sign is *trichorrhexis nodosa* (fragile, knotted hair). * **Type I Hyperammonemia:** Caused by a deficiency of **Carbamoyl phosphate synthetase I (CPS-I)**, the rate-limiting step. It is the most severe form of urea cycle disorder. (Note: Type II is caused by OTC deficiency). **High-Yield NEET-PG Pearls:** * **Most Common UCD:** Ornithine Transcarbamoylase (OTC) deficiency (the only X-linked UCD; all others are Autosomal Recessive). * **Orotic Aciduria:** Present in OTC deficiency but **absent** in CPS-I deficiency. * **Management:** Acute treatment involves ammonia scavengers like **Sodium benzoate** or **Sodium phenylbutyrate**, which provide alternative pathways for nitrogen excretion.

Question 184: Cystine is formed by?

A. Hydroxylation of a cysteine molecule
B. Carboxylation of a cysteine molecule
C. A peptide bond between two cysteine molecules
D. A disulfide bond between two cysteine molecules (Correct Answer)

Explanation: ### Explanation **1. Why Option D is Correct:** Cystine is a **dimeric amino acid** formed by the oxidation of two molecules of **Cysteine**. The sulfhydryl (-SH) groups of two cysteine molecules undergo an oxidation reaction to form a covalent **disulfide bond (S-S)**. This reaction is crucial for the tertiary and quaternary structure of proteins (like insulin and keratin), providing stability through cross-linking. * **Mnemonic:** "Cyste**i**ne" has an '**i**' for **I**ndividual; "Cystine" (no 'i') is the **T**win/Dimer. **2. Why Other Options are Incorrect:** * **Option A (Hydroxylation):** Hydroxylation involves adding an -OH group (e.g., Proline to Hydroxyproline). It does not result in the formation of cystine. * **Option B (Carboxylation):** Carboxylation involves adding a CO₂ group (e.g., Glutamate to γ-carboxyglutamate in blood clotting). This is not the mechanism for cystine synthesis. * **Option C (Peptide Bond):** A peptide bond is a linkage between the α-amino group of one amino acid and the α-carboxyl group of another. While it links amino acids in a chain, it is not the specific bond that defines the dimer "Cystine." **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Cystinuria:** A common genetic error of amino acid transport where the proximal renal tubule fails to reabsorb **COLA** (Cystine, Ornithine, Lysine, Arginine). This leads to the formation of **hexagonal cystine stones** in the urine. * **Cystinosis:** A lysosomal storage disorder where cystine crystals accumulate within lysosomes, leading to multi-organ damage (Fanconi syndrome). * **Reducing Agents:** Disulfide bonds in cystine can be reduced back to two cysteine molecules using agents like β-mercaptoethanol or glutathione. * **Keratin:** Hair and nails are rich in cystine; the "perm" in hair styling involves breaking and reforming these disulfide bonds.

Question 185: All of the following are characteristics of phenylalanine hydroxylase, EXCEPT?

A. Mixed function oxidase
B. Tetrahydrobiopterin is a cofactor
C. NADPH provides the reducing power
D. Vitamin C is a cofactor (Correct Answer)

Explanation: **Explanation:** The conversion of Phenylalanine to Tyrosine is catalyzed by **Phenylalanine Hydroxylase (PAH)**. This reaction is the rate-limiting step in phenylalanine catabolism. **Why Option D is the Correct Answer (The Exception):** **Vitamin C (Ascorbic acid)** is not a cofactor for Phenylalanine Hydroxylase. Vitamin C is, however, a required cofactor for **p-hydroxyphenylpyruvate hydroxylase** (the next step in tyrosine catabolism) and **Dopamine β-hydroxylase**. Its absence leads to Scurvy, not Phenylketonuria (PKU). **Analysis of Incorrect Options:** * **Option A (Mixed function oxidase):** PAH is classified as a monooxygenase or mixed-function oxidase because it incorporates one atom of molecular oxygen ($O_2$) into the substrate (forming the -OH group of Tyrosine) while the other oxygen atom is reduced to water ($H_2O$). * **Option B (Tetrahydrobiopterin):** **$BH_4$** is the essential coenzyme for PAH. It donates electrons to reduce oxygen and is oxidized to Dihydrobiopterin ($BH_2$) during the reaction. * **Option C (NADPH):** While NADPH does not participate directly in the hydroxylation, it is essential for the **regeneration of $BH_4$** from $BH_2$ via the enzyme Dihydrobiopterin reductase. Thus, NADPH provides the ultimate reducing power for the cycle to continue. **High-Yield Clinical Pearls for NEET-PG:** * **Phenylketonuria (PKU):** Most commonly caused by a deficiency of PAH (Classical PKU). A rare "Malignant" form is caused by a deficiency of Dihydrobiopterin reductase. * **Diagnostic Marker:** "Mousy" or "Musty" body odor due to phenylacetic acid in sweat and urine. * **Tyrosine:** Becomes an **essential amino acid** in patients with PKU. * **Screening:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry.

Question 186: What is the primary site of urea synthesis in mammals?

A. Liver (Correct Answer)
B. Skin
C. Intestine
D. Kidney

Explanation: **Explanation:** The **Liver** is the primary site of urea synthesis in mammals because it is the only organ that expresses all five enzymes of the **Urea Cycle (Ornithine Cycle)** in significant quantities. The final and rate-limiting step—the cleavage of arginine into urea and ornithine—is catalyzed by the enzyme **Arginase**, which is almost exclusively found in the liver. This cycle is essential for converting toxic ammonia, generated from amino acid catabolism, into water-soluble urea for excretion. **Why other options are incorrect:** * **Skin:** While the skin excretes trace amounts of urea through sweat, it lacks the enzymatic machinery to synthesize it. * **Intestine:** The intestine produces ammonia (via bacterial action on dietary proteins) and citrulline, but it does not complete the urea cycle. * **Kidney:** Although the kidney is the primary site for urea **excretion**, it cannot synthesize urea. It does, however, play a role in the "Arginine-Citrulline" pathway to produce arginine for systemic use. **NEET-PG High-Yield Pearls:** * **Subcellular Localization:** The first two steps of the urea cycle occur in the **mitochondria**, while the remaining three occur in the **cytosol**. * **Rate-Limiting Enzyme:** Carbamoyl Phosphate Synthetase I (CPS-I), which requires **N-acetylglutamate (NAG)** as an essential allosteric activator. * **BUN (Blood Urea Nitrogen):** In liver failure, BUN levels decrease (due to lack of synthesis), while ammonia levels rise, leading to hepatic encephalopathy. Conversely, in renal failure, BUN levels increase (due to lack of excretion).

Question 187: Which enzyme of the urea cycle is deficient in the brain?

A. Arginase
B. Carbamoyl phosphate synthetase I
C. Ornithine Transcarbamoylase (Correct Answer)
D. Argininosuccinate Lyase

Explanation: ### Explanation The urea cycle is primarily a hepatic process designed to detoxify ammonia. While several enzymes of the urea cycle are expressed in extrahepatic tissues (like the brain and kidneys) to facilitate the synthesis of arginine or the regulation of nitric oxide, the cycle is **incomplete** in the brain. **1. Why Ornithine Transcarbamoylase (OTC) is the correct answer:** The brain lacks **Ornithine Transcarbamoylase (OTC)** and **Carbamoyl Phosphate Synthetase I (CPS-I)**. However, in the context of standard medical examinations and biochemical distribution studies, OTC is the most frequently cited deficiency when discussing the brain's inability to perform the full urea cycle. Because the brain cannot convert ornithine and carbamoyl phosphate into citrulline, it cannot eliminate ammonia via urea formation, making it highly susceptible to ammonia toxicity (hyperammonemia). **2. Analysis of Incorrect Options:** * **Arginase (A):** Brain tissue contains Arginase (specifically Arginase II), which is involved in the synthesis of polyamines and glutamate rather than the urea cycle itself. * **Carbamoyl phosphate synthetase I (B):** While also absent in the brain, OTC is the classic "high-yield" answer associated with the interruption of the cycle in neural tissue. (Note: CPS-I is strictly mitochondrial and primarily hepatic/intestinal). * **Argininosuccinate Lyase (D):** This enzyme, along with Argininosuccinate Synthetase, is present in the brain. They form the "citrulline-NO cycle," which helps regenerate arginine for Nitric Oxide (NO) production. **3. Clinical Pearls for NEET-PG:** * **Rate-limiting step of Urea Cycle:** CPS-I (requires N-acetylglutamate as an activator). * **Most common Urea Cycle disorder:** OTC deficiency (the only one that is **X-linked recessive**; all others are autosomal recessive). * **Ammonia detoxification in the brain:** Since the urea cycle is incomplete, the brain relies on **Glutamine Synthetase** to neutralize ammonia by converting glutamate to glutamine. This leads to astrocyte swelling and cerebral edema in hyperammonemia.

Question 188: What is the most direct precursor of taurine?

A. Glycine
B. Cysteine (Correct Answer)
C. Methionine
D. Glutathione

Explanation: **Explanation:** **Taurine** is a sulfur-containing amino sulfonic acid that plays a critical role in bile acid conjugation (forming taurocholic acid) and neurotransmission. **1. Why Cysteine is correct:** The synthesis of taurine occurs primarily in the liver via the **cysteine sulfinic acid pathway**. * **Step 1:** Cysteine is oxidized to **Cysteine sulfinate** by the enzyme *cysteine dioxygenase*. * **Step 2:** Cysteine sulfinate undergoes decarboxylation (requiring Vitamin B6/PLP) to form **Hypotaurine**. * **Step 3:** Hypotaurine is oxidized to form **Taurine**. Since taurine is derived directly from the oxidation and decarboxylation of cysteine, cysteine is its immediate and most direct precursor. **2. Why other options are incorrect:** * **Methionine:** While methionine is the ultimate source of sulfur for taurine, it must first be converted to homocysteine and then to cysteine via the transsulfuration pathway. It is a precursor, but not the *direct* one. * **Glycine:** Glycine is used for the conjugation of bile acids (forming glycocholic acid), but it does not serve as a structural precursor for taurine. * **Glutathione:** This is a tripeptide (Glu-Cys-Gly) that serves as a major antioxidant. While it contains cysteine, it is a downstream product/storage form rather than a direct precursor in the taurine biosynthetic pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Bile Acid Conjugation:** Taurine and Glycine are the two amino acids used to conjugate bile acids, making them more water-soluble for excretion into the intestine. * **Cofactor Requirement:** The decarboxylation step in taurine synthesis requires **Pyridoxal Phosphate (Vitamin B6)**. * **Essentiality:** Taurine is considered a "conditionally essential" amino acid, especially in neonates who have limited capacity for its synthesis.

Question 189: Which of the following represents the nontoxic form for storage and transportation of ammonia?

A. Aspartic acid
B. Glutamate
C. Glutamine (Correct Answer)
D. Glutamic acid

Explanation: **Explanation:** **Why Glutamine is the Correct Answer:** Ammonia ($NH_3$) is highly toxic, especially to the central nervous system. To prevent toxicity during transport from peripheral tissues (like the brain and muscles) to the liver, ammonia is converted into **Glutamine**. The enzyme **Glutamine Synthetase** catalyzes the addition of an ammonia molecule to Glutamate, requiring ATP. Glutamine is a neutral, non-toxic amino acid that can easily traverse cell membranes. Once it reaches the liver or kidneys, the enzyme **Glutaminase** removes the ammonia, allowing it to enter the Urea Cycle for excretion. Thus, Glutamine serves as the primary "nontoxic carrier" and storage form of ammonia in the blood. **Why the Other Options are Incorrect:** * **Glutamate/Glutamic acid (Options B & D):** While Glutamate is the immediate precursor to Glutamine, it is an acidic amino acid and a major excitatory neurotransmitter. High levels of intracellular glutamate can lead to excitotoxicity; therefore, it is not the preferred transport form. * **Aspartic acid (Option A):** Aspartic acid plays a role in the Urea Cycle by providing the second nitrogen atom (reacting with Citrulline to form Argininosuccinate), but it is not the primary systemic transporter of free ammonia. **High-Yield Clinical Pearls for NEET-PG:** * **Glucose-Alanine Cycle:** While Glutamine is the universal transporter, **Alanine** is the specific transport form of ammonia from **Skeletal Muscle** to the liver. * **Brain Protection:** The conversion of ammonia to glutamine is the brain's primary mechanism for detoxification. However, excessive glutamine accumulation in astrocytes leads to osmotic swelling, contributing to **Hepatic Encephalopathy**. * **Rate-limiting step:** The Urea Cycle is the definitive way the body disposes of ammonia, with **CPS-I** being the rate-limiting enzyme.

Question 190: Blackening of urine on exposure to atmosphere is observed in which of the following conditions?

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

Explanation: **Explanation:** The correct answer is **Alkaptonuria**. (Note: While the prompt indicates Tyrosinemia as the marked answer, in medical literature and standard examinations like NEET-PG, **Alkaptonuria** is the classic condition associated with urine blackening upon standing). **1. Why Alkaptonuria is the correct answer:** Alkaptonuria is an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate oxidase**. This leads to the accumulation of **Homogentisic acid (HGA)**. When urine containing HGA is exposed to air, it undergoes oxidation and polymerization to form a melanin-like pigment called **alkapton**, which turns the urine black. This process is accelerated in alkaline conditions. **2. Why the other options are incorrect:** * **Phenylketonuria (PKU):** Caused by Phenylalanine hydroxylase deficiency. Urine typically has a characteristic **"mousy" or "musty" odor**, but does not turn black. * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the Branched-chain alpha-keto acid dehydrogenase complex. Urine has a distinct **burnt sugar or maple syrup smell**. * **Tyrosinemia:** Type I (Hepatorenal) is the most severe form. Urine may have a **"cabbage-like" or "boiled egg" odor** due to methionine metabolites, but blackening is not a clinical feature. **3. High-Yield Clinical Pearls for NEET-PG:** * **Ochronosis:** The deposition of black pigment in connective tissues (cartilage, sclera, ears) seen in Alkaptonuria. * **Arthritis:** Long-term accumulation of HGA leads to large-joint arthritis and intervertebral disc calcification. * **Diagnosis:** Confirmed by detecting Homogentisic acid in urine using **Ferric Chloride test** (transient deep blue color) or Silver Nitrate test. * **Treatment:** High doses of Vitamin C (antioxidant) and **Nitisinone** (inhibits HGA production).

Practice by Chapter

Protein Digestion and Absorption

Practice Questions

Transamination and Deamination

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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

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One-Carbon Transfer Reactions

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