Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 291: Arrange the steps of the formation of creatinine from glycine in sequence.

A. Formation of guanidino acetic acid from arginine and glycine -> Formation of creatine by methyl transferase -> Transport of creatine from liver to muscle -> Formation of creatine phosphate -> Conversion of creatine phosphate to creatinine (Correct Answer)
B. Transport of creatine from liver to muscle -> Formation of creatine phosphate -> Formation of creatine by methyl transferase -> Conversion of creatine phosphate to creatinine -> Formation of guanidino acetic acid from arginine and glycine
C. Formation of creatine phosphate -> Transport of creatine from liver to muscle -> Conversion of creatine phosphate to creatinine -> Formation of creatine by methyl transferase -> Formation of guanidino acetic acid from arginine and glycine
D. Formation of creatine by methyl transferase -> Formation of creatine phosphate -> Formation of guanidino acetic acid from arginine and glycine -> Conversion of creatine phosphate to creatinine -> Transport of creatine from liver to muscle

Explanation: Creatine synthesis and its subsequent conversion to creatinine is a multi-organ process involving the kidneys, liver, and skeletal muscle. ### **Step-by-Step Mechanism** 1. **Kidney (Initiation):** The process begins in the kidney where **Arginine** and **Glycine** combine to form **Guanidinoacetate (GAA)** via the enzyme *Amidinotransferase*. 2. **Liver (Methylation):** GAA travels to the liver, where it is methylated by **S-adenosylmethionine (SAM)** via *Methyltransferase* to form **Creatine**. 3. **Transport:** Creatine is released into the blood and transported to high-energy demanding tissues, primarily **Skeletal Muscle**. 4. **Muscle (Storage):** In the muscle, *Creatine Kinase* phosphorylates creatine to form **Creatine Phosphate**, a high-energy reservoir. 5. **Spontaneous Conversion:** Finally, Creatine phosphate undergoes non-enzymatic, irreversible cyclization to form **Creatinine**, which is excreted by the kidneys. ### **Why Other Options are Incorrect** * **Options B, C, and D** are incorrect because they violate the physiological sequence. For instance, methylation (liver) must occur *after* GAA formation (kidney), and transport to the muscle must occur *before* phosphorylation to creatine phosphate. Creatinine is always the final, irreversible waste product. ### **High-Yield NEET-PG Pearls** * **Amino Acid Precursors:** Glycine, Arginine, and Methionine (as SAM). * **Rate-Limiting Step:** The formation of GAA in the kidney. * **Clinical Marker:** Creatinine excretion is proportional to total **muscle mass** and is used to estimate GFR because its production is constant and it is minimally secreted by tubules. * **Diagnostic Tip:** Elevated serum Creatine Kinase (CK) is a marker of muscle damage (e.g., Myocardial Infarction or Rhabdomyolysis).

Question 292: Which of the following is an aromatic amino acid?

A. Serine
B. Histidine
C. Leucine
D. Tryptophan (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Tryptophan** **1. Why Tryptophan is Correct:** Aromatic amino acids are characterized by the presence of a benzene or indole ring in their side chains. There are three primary aromatic amino acids: **Phenylalanine, Tyrosine, and Tryptophan**. Tryptophan contains an **indole ring** system, making it the most complex aromatic amino acid. These molecules are relatively non-polar and can participate in hydrophobic interactions. Notably, they are responsible for the ultraviolet light absorption of proteins at **280 nm**. **2. Why the Other Options are Incorrect:** * **A. Serine:** This is a **hydroxy-containing** (polar, uncharged) amino acid. It plays a crucial role in the catalytic sites of enzymes (serine proteases) and is a site for O-linked glycosylation. * **B. Histidine:** While it contains an **imidazole ring**, it is traditionally classified as a **basic (positively charged)** amino acid. Although it has aromatic properties, in the context of standard NEET-PG classification, it is grouped with Lysine and Arginine. * **C. Leucine:** This is a **branched-chain aliphatic** amino acid. It is purely ketogenic and plays a vital role in muscle protein synthesis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Precursor Functions:** Tryptophan is the precursor for **Serotonin, Melatonin, and Niacin (Vitamin B3)**. * **Hartnup Disease:** A defect in the transport of neutral amino acids (primarily Tryptophan) leads to pellagra-like symptoms due to Niacin deficiency. * **Essentiality:** Phenylalanine and Tryptophan are **Essential**, while Tyrosine is **Non-essential** (derived from Phenylalanine). * **Metabolic Fate:** All three aromatic amino acids are **both glucogenic and ketogenic**.

Question 293: Which of the following is NOT an essential amino acid?

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

Explanation: **Explanation:** Amino acids are categorized as **essential** (must be obtained from the diet) or **non-essential** (can be synthesized by the body). **Why Glycine is the correct answer:** **Glycine** is a **non-essential amino acid**. It is the simplest amino acid (having only a hydrogen atom as its R-group) and is synthesized endogenously from serine by the enzyme *serine hydroxymethyltransferase*. Because the human body can produce sufficient quantities of glycine to meet metabolic demands, it does not need to be supplied through dietary intake. **Why the other options are incorrect:** * **Valine, Isoleucine, and Tryptophan** are all **essential amino acids**. * **Valine and Isoleucine** belong to the Branched-Chain Amino Acids (BCAAs), which are critical for muscle metabolism. * **Tryptophan** is an aromatic essential amino acid and serves as a precursor for Serotonin, Melatonin, and Niacin (Vitamin B3). **High-Yield Facts for NEET-PG:** * **Mnemonic for Essential Amino Acids:** **"PVT TIM HALL"** (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine*, Leucine, Lysine). * **Semi-essential Amino Acids:** Arginine and Histidine (required during periods of rapid growth/pregnancy). * **Purely Ketogenic Amino Acids:** Leucine and Lysine. * **Glycine Clinical Pearls:** It is required for the synthesis of **Heme, Purines, Creatine, and Glutathione**. It also acts as an inhibitory neurotransmitter in the spinal cord.

Question 294: Which of the following is a semi-essential amino acid?

A. Tyrosine
B. Histidine (Correct Answer)
C. Alanine
D. Isoleucine

Explanation: **Explanation:** Amino acids are classified based on their nutritional requirement into essential, non-essential, and semi-essential categories. **Why Histidine is the correct answer:** **Histidine** (along with Arginine) is classified as a **semi-essential (or conditionally essential)** amino acid. While the adult body can synthesize sufficient amounts to maintain nitrogen balance, the rate of synthesis is inadequate during periods of rapid growth, such as infancy, childhood, pregnancy, or recovery from major trauma. Therefore, dietary intake is mandatory during these specific physiological states to meet the increased demand. **Analysis of Incorrect Options:** * **A. Tyrosine:** This is a **non-essential** amino acid because it can be synthesized in the body from the essential amino acid Phenylalanine (via phenylalanine hydroxylase). * **C. Alanine:** This is a **non-essential** amino acid. It is easily synthesized via the transamination of pyruvate, a product of glycolysis. * **D. Isoleucine:** This is a strictly **essential** amino acid. It cannot be synthesized by the human body and must be obtained entirely through the diet. **High-Yield NEET-PG Pearls:** * **Mnemonic for Essential Amino Acids:** "PVT TIM HALL" (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, **Histidine**, **Arginine**, Leucine, Lysine). Note that **H** and **A** are the semi-essential ones. * **Histidine Precursor:** It is the precursor for **Histamine** (via decarboxylation), a key mediator in allergic reactions and gastric acid secretion. * **Clinical Correlation:** Histidine levels are used in the **FIGLU excretion test** to diagnose Folic Acid deficiency.

Question 295: In the biosynthesis of urea, one nitrogen atom is derived from ammonia while the other nitrogen atom is from:

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

Explanation: **Explanation:** The urea cycle (Krebs-Henseleit cycle) is the primary mechanism for detoxifying ammonia into urea in the liver. Urea ($NH_2-CO-NH_2$) contains two nitrogen atoms, which are derived from two distinct sources: 1. **First Nitrogen:** Enters the cycle as **free ammonia ($NH_3$)**, which combines with $CO_2$ to form Carbamoyl Phosphate via the enzyme *Carbamoyl Phosphate Synthetase I (CPS-I)*. 2. **Second Nitrogen:** Enters the cycle during the third step. Citrulline condenses with **Aspartate** to form Argininosuccinate. This reaction is catalyzed by *Argininosuccinate synthetase* and requires ATP. The amino group of Aspartate is what eventually becomes the second nitrogen atom of the urea molecule. **Analysis of Incorrect Options:** * **A. Glycine:** While glycine is involved in the synthesis of heme, purines, and creatine, it does not directly donate a nitrogen atom to the urea cycle. * **B. Alanine:** Alanine is the primary carrier of nitrogen from the muscle to the liver (Cahill cycle). In the liver, it undergoes transamination to form glutamate, but it is not the direct donor in the urea cycle. * **C. Glutamate:** Glutamate acts as a "collection center" for amino groups. Through oxidative deamination (via *Glutamate Dehydrogenase*), it releases the **first** nitrogen as free ammonia. It does not provide the second nitrogen directly. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** CPS-I (requires N-acetylglutamate as an absolute allosteric activator). * **Subcellular location:** The cycle is "split"—the first two steps occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. * **Fumarate Link:** The "Bicycle" link between the Urea and TCA cycles is Fumarate, which is released when Argininosuccinate is cleaved. * **Hyperammonemia:** Deficiency of any urea cycle enzyme leads to ammonia toxicity; *Ornithine Transcarbamoylase (OTC) deficiency* is the most common X-linked urea cycle disorder.

Question 296: Sulfite oxidase is the last enzyme in the pathway for degradation of sulfur-containing amino acids. What is the cofactor for sulfite oxidase?

A. Copper (Cu)
B. Zinc (Zn)
C. Molybdenum (Correct Answer)
D. Iron (Fe)

Explanation: **Explanation:** **Sulfite oxidase** is a mitochondrial enzyme that catalyzes the final step in the degradation of sulfur-containing amino acids (Cysteine and Methionine). It converts toxic **sulfite ($SO_3^{2-}$)** into non-toxic **sulfate ($SO_4^{2-}$)**. 1. **Why Molybdenum is Correct:** Sulfite oxidase requires **Molybdenum (Mo)** as an essential cofactor. Specifically, the molybdenum is held within a complex organic molecule called **Molybdopterin**. This "Molybdenum cofactor" (MoCo) is also required by two other key enzymes: Xanthine oxidase (purine metabolism) and Aldehyde oxidase. 2. **Why Other Options are Incorrect:** * **Copper (Cu):** A cofactor for enzymes like Cytochrome c oxidase, Tyrosinase, and Lysyl oxidase. * **Zinc (Zn):** A cofactor for Carbonic anhydrase, Alcohol dehydrogenase, and Carboxypeptidase. * **Iron (Fe):** Found in Heme-containing enzymes (Catalase, Cytochromes) and Non-heme enzymes (Aconitase). 3. **Clinical Pearls (High-Yield for NEET-PG):** * **Molybdenum Cofactor Deficiency (MoCD):** A rare autosomal recessive disorder. Since sulfite oxidase cannot function, sulfites accumulate, leading to severe neurological damage, seizures, and **ectopia lentis** (dislocation of the lens). * **Biochemical Marker:** Patients with MoCD or Sulfite Oxidase deficiency will show **low plasma sulfate** and **high urinary sulfite** levels. * **Xanthine Connection:** Because Molybdenum is shared with Xanthine oxidase, MoCD also results in **hypouricemia** (low serum uric acid) and increased xanthine excretion.

Question 297: In which organ does the urea cycle primarily occur?

A. Liver (Correct Answer)
B. Gastrointestinal tract
C. Spleen
D. Kidney

Explanation: **Explanation:** The **Urea Cycle (Ornithine Cycle)** is the primary mechanism for detoxifying ammonia, a toxic byproduct of protein catabolism, by converting it into water-soluble urea. **Why Liver is Correct:** The liver is the **exclusive site** for the complete urea cycle. This is because the final enzyme of the pathway, **Arginase**, is expressed almost entirely in the hepatocytes. While the initial steps occur in the mitochondria, the later steps take place in the cytosol. Once urea is formed in the liver, it is released into the bloodstream and transported to the kidneys for excretion. **Why Other Options are Incorrect:** * **Gastrointestinal Tract:** While the gut produces significant amounts of ammonia (via bacterial action on dietary protein), it does not possess the enzymatic machinery to complete the urea cycle. * **Spleen:** The spleen is primarily involved in the destruction of old RBCs and immune surveillance; it plays no role in ammonia detoxification. * **Kidney:** Although the kidneys excrete urea and can perform some initial steps of the cycle (like synthesizing Arginine from Citrulline), they lack **Arginase** and cannot produce urea. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Carbamoyl Phosphate Synthetase I (CPS-I), located in the mitochondria. * **Obligatory Activator:** N-acetylglutamate (NAG) is essential for CPS-I activity. * **Link to TCA Cycle:** The "Krebs' Bicycle" refers to the Shunt where **Fumarate** (produced in the urea cycle) enters the TCA cycle. * **Hyperammonemia:** Liver failure (e.g., Cirrhosis) leads to urea cycle failure, resulting in ammonia toxicity, which manifests clinically as **Hepatic Encephalopathy** (Asterixis, altered sensorium).

Question 298: Which of the following is a non-essential amino acid?

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

Explanation: ### Explanation The classification of amino acids into **essential** and **non-essential** is based on the body's ability to synthesize them de novo. **1. Why Serine is the Correct Answer:** **Serine** is a **non-essential amino acid** because the human body can synthesize it from intermediates of glycolysis, specifically **3-phosphoglycerate**. It plays a crucial role in the synthesis of other compounds like glycine, cysteine, and sphingolipids. Since the body does not rely solely on dietary intake for its supply, it is classified as non-essential. **2. Why the Other Options are Incorrect:** * **Valine (A) & Leucine (B):** These are **Branched-Chain Amino Acids (BCAAs)**. They are strictly essential and must be obtained from the diet. Their metabolism is clinically significant in Maple Syrup Urine Disease (MSUD). * **Tryptophan (C):** This is an **essential aromatic amino acid**. It is a precursor for important molecules like Serotonin, Melatonin, and Niacin (Vitamin B3). **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Essential Amino Acids:** "PVT TIM HALL" (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine*, Leucine, Lysine). * **Semi-essential Amino Acids:** Arginine and Histidine (required during periods of rapid growth/pregnancy). * **Purely Ketogenic Amino Acids:** Leucine and Lysine (the only two that cannot be converted to glucose). * **Both Glucogenic and Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, and Isoleucine (Mnemonic: **Ph**it**T**). * **Serine Deficiency:** Can lead to neurological symptoms and microcephaly due to its role in brain development and sphingolipid synthesis.

Question 299: A 45-year-old male patient complains of blurred vision. The patient has a habit of smoking and is a chronic alcoholic. Investigations reveal that his plasma homocysteine level is 80µmol/L. Which type of thrombosis can occur in this patient?

A. Arterial
B. Venous
C. Both arterial and venous (Correct Answer)
D. None of the above

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The patient presents with **Hyperhomocysteinemia** (plasma level of 80 µmol/L; normal is <15 µmol/L). Elevated homocysteine is a potent pro-thrombotic factor. It induces vascular injury through several mechanisms: * **Endothelial Dysfunction:** It promotes the production of reactive oxygen species (ROS), leading to oxidative stress and endothelial damage. * **Pro-coagulant State:** It activates Factor V, inhibits Protein C activation, and increases platelet aggregation. * **Vascular Remodeling:** It stimulates smooth muscle cell proliferation and reduces nitric oxide bioavailability. Unlike many other risk factors that are specific to one vascular bed, hyperhomocysteinemia is unique because it predisposes to **both arterial thrombosis** (leading to Myocardial Infarction or Stroke) and **venous thrombosis** (leading to Deep Vein Thrombosis or Pulmonary Embolism). **2. Why Other Options are Incorrect:** * **Option A (Arterial only):** While homocysteine accelerates atherosclerosis and arterial clotting, it is equally implicated in venous stasis and clot formation. * **Option B (Venous only):** Although hyperhomocysteinemia is a known cause of "unprovoked" venous thromboembolism, ignoring the arterial risk would be clinically incomplete. **3. Clinical Pearls for NEET-PG:** * **Homocystinuria vs. Hyperhomocysteinemia:** Homocystinuria (levels >100 µmol/L) is often due to **Cystathionine β-synthase (CBS) deficiency**, characterized by the triad of **ectopia lentis** (downward subluxation), intellectual disability, and thromboembolism. * **Acquired Causes:** Chronic alcoholism and smoking (as seen in this patient) deplete **Vitamin B12, B6, and Folate**, which are essential cofactors for homocysteine metabolism (Remethylation and Transsulfuration pathways). * **Ocular Finding:** The "blurred vision" in this patient could be due to **Retinal Vein Occlusion (RVO)** or **Retinal Artery Occlusion**, both associated with high homocysteine.

Question 300: Hyperammonemia type I is due to deficiency of which enzyme?

A. Carbamoyl phosphate synthetase I (CPS-I) (Correct Answer)
B. Carbamoyl phosphate synthetase II (CPS-II)
C. Arginase
D. Argininosuccinase

Explanation: **Explanation:** **Hyperammonemia Type I** is caused by a deficiency of **Carbamoyl Phosphate Synthetase I (CPS-I)**. This is the rate-limiting enzyme of the urea cycle, located in the mitochondria. It converts ammonia and bicarbonate into carbamoyl phosphate. A deficiency leads to a complete block in ammonia detoxification, resulting in severe neonatal hyperammonemia, lethargy, and neurological damage. **Analysis of Options:** * **Option A (Correct):** CPS-I deficiency causes Hyperammonemia Type I. It is unique because it is the only urea cycle disorder (besides NAGS deficiency) that presents with **low or absent orotic acid** levels, as the block occurs before the formation of carbamoyl phosphate. * **Option B (Incorrect):** **CPS-II** is a cytosolic enzyme involved in **Pyrimidine synthesis**, not the urea cycle. It uses glutamine as a nitrogen source. * **Option C (Incorrect):** **Arginase** deficiency causes Hyperargininemia. It is generally the least severe urea cycle disorder and presents with spastic diplegia rather than acute neonatal hyperammonemia. * **Option D (Incorrect):** **Argininosuccinase** (Argininosuccinate Lyase) deficiency causes Argininosuccinic Aciduria. A characteristic clinical finding here is **Trichorrhexis nodosa** (friable, brittle hair). **High-Yield Clinical Pearls for NEET-PG:** * **Hyperammonemia Type II:** Caused by **Ornithine Transcarbamoylase (OTC) deficiency**. This is the **most common** urea cycle disorder and is **X-linked recessive**. It presents with **high orotic acid** levels. * **N-Acetylglutamate (NAG):** This is the essential allosteric activator of CPS-I. Deficiency of NAG Synthetase clinically mimics Hyperammonemia Type I. * **Management:** Acute treatment involves hemodialysis and nitrogen scavengers like **Sodium Benzoate** or **Phenylbutyrate**.

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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