Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 411: Ammonia for the urea cycle is supplied by the action of which of the following enzymes?

A. Aspartate aminotransferase
B. Glutamate dehydrogenase (Correct Answer)
C. Argininosuccinate synthase
D. Argininosuccinate lyase

Explanation: **Explanation:** The urea cycle is the primary mechanism for detoxifying ammonia into urea in the liver. To initiate the cycle, ammonia must be provided for the synthesis of Carbamoyl Phosphate. **Why Glutamate Dehydrogenase (GDH) is correct:** GDH plays a pivotal role in **oxidative deamination**. Most amino acids transfer their $\alpha$-amino group to $\alpha$-ketoglutarate to form **Glutamate** (via transamination). GDH then catalyzes the release of the free ammonium ion ($NH_4^+$) from glutamate in the mitochondrial matrix. This free ammonia then combines with $CO_2$ and ATP, catalyzed by *Carbamoyl Phosphate Synthetase-I (CPS-I)*, to enter the urea cycle. **Analysis of Incorrect Options:** * **Aspartate aminotransferase (AST):** While AST is involved, it provides the **second nitrogen** atom of urea by transferring an amino group from glutamate to oxaloacetate to form **Aspartate**. It does not release free ammonia. * **Argininosuccinate synthase:** This is the third enzyme of the urea cycle that condenses citrulline with aspartate. * **Argininosuccinate lyase:** This enzyme cleaves argininosuccinate into arginine and fumarate. **NEET-PG High-Yield Pearls:** * **Two Sources of Nitrogen:** The first nitrogen of urea comes from **free ammonia** (via GDH); the second nitrogen comes from **Aspartate** (via AST). * **Rate-Limiting Step:** CPS-I is the rate-limiting enzyme of the urea cycle and requires **N-acetylglutamate (NAG)** as an absolute allosteric activator. * **Location:** The first two steps (CPS-I and Ornithine Transcarbamoylase) occur in the **mitochondria**, while the remaining steps occur in the **cytosol**.

Question 412: In which of the following conditions is there an increased level of ammonia in the blood?

A. Ornithine transcarbamoylase deficiency (Correct Answer)
B. Galactosaemia
C. Histidinaemia
D. Phenylketonuria

Explanation: **Explanation:** **1. Why Option A is Correct:** Hyperammonemia (increased blood ammonia) occurs when there is a defect in the **Urea Cycle**, the primary metabolic pathway for detoxifying ammonia into urea in the liver. **Ornithine Transcarbamoylase (OTC) deficiency** is the most common urea cycle disorder. It is an X-linked recessive condition where the enzyme responsible for combining carbamoyl phosphate and ornithine to form citrulline is deficient. This leads to a massive backup of ammonia, resulting in neurotoxicity, cerebral edema, and lethargy. **2. Why Other Options are Incorrect:** * **B. Galactosaemia:** This is a disorder of carbohydrate metabolism (deficiency of GALT enzyme). It presents with jaundice, hepatomegaly, and cataracts, but not primary hyperammonemia. * **C. Histidinaemia:** This is a benign disorder of histidine metabolism (histidase deficiency). While histidine levels rise, the urea cycle remains intact, so ammonia levels are normal. * **D. Phenylketonuria (PKU):** This is a disorder of aromatic amino acid metabolism (Phenylalanine hydroxylase deficiency). It leads to the accumulation of phenylalanine and phenylketones, causing intellectual disability and a "mousy odor," but does not impair ammonia detoxification. **3. NEET-PG High-Yield Pearls:** * **OTC Deficiency Unique Marker:** It is the only urea cycle defect that shows **increased Orotic Acid** in urine (due to the accumulation of carbamoyl phosphate, which enters the pyrimidine synthesis pathway). * **Inheritance:** All urea cycle disorders are Autosomal Recessive **except** OTC deficiency, which is **X-linked Recessive**. * **Management:** Acute hyperammonemia is treated with hemodialysis and ammonia scavengers like **Sodium Benzoate** or **Sodium Phenylbutyrate**.

Question 413: Essential atrophy of the choroid is due to an inborn error of metabolism of which amino acid?

A. Cystine
B. Cysteine
C. Arginine
D. Ornithine (Correct Answer)

Explanation: **Explanation:** **Gyrate Atrophy of the Choroid and Retina** (often referred to as essential atrophy) is an autosomal recessive metabolic disorder caused by a deficiency of the mitochondrial enzyme **Ornithine Aminotransferase (OAT)**. 1. **Why Ornithine is Correct:** The OAT enzyme is responsible for converting **Ornithine** into glutamate-gamma-semialdehyde. A deficiency leads to a significant accumulation of ornithine in the blood (hyperornithinemia), urine, and cerebrospinal fluid. This excess ornithine is toxic to the retinal pigment epithelium and the choroid, leading to progressive circular (gyrate) patches of chorioretinal atrophy, high myopia, and eventual blindness. 2. **Why Incorrect Options are Wrong:** * **Cystine/Cysteine:** Disorders of these sulfur-containing amino acids include **Cystinuria** (renal stones) or **Cystinosis** (lysosomal storage leading to corneal crystals and renal failure), but they do not cause gyrate atrophy. * **Arginine:** While ornithine is a byproduct of the urea cycle involving arginine, a primary deficiency in arginine metabolism (like Arginase deficiency) leads to spasticity and developmental delay, not specific choroidal atrophy. **High-Yield Clinical Pearls for NEET-PG:** * **Key Enzyme:** Ornithine Aminotransferase (OAT). * **Cofactor:** This enzyme requires **Vitamin B6 (Pyridoxine)**. Some patients respond to high doses of B6, which helps lower ornithine levels. * **Dietary Management:** Treatment involves an **Arginine-restricted diet**, as arginine is the metabolic precursor to ornithine. * **Classic Presentation:** Night blindness (nyctalopia) in the first decade of life followed by constricted visual fields.

Question 414: The glucose-alanine cycle is important in which organ?

A. Liver
B. Heart
C. Muscle (Correct Answer)
D. Kidney

Explanation: **Explanation:** The **Glucose-Alanine Cycle (Cahill Cycle)** is a metabolic pathway that facilitates the transport of nitrogen from peripheral tissues to the liver while providing glucose back to the muscles during periods of fasting or intense exercise. **Why Muscle is the Correct Answer:** During muscle contraction, amino acids (primarily branched-chain amino acids) are catabolized, releasing ammonia. To prevent toxic ammonia buildup, the amino group is transferred to α-ketoglutarate to form glutamate, which then transfers the nitrogen to pyruvate (a product of glycolysis) via the enzyme **Alanine Aminotransferase (ALT)**. This forms **Alanine**, which is released into the blood. The muscle is the primary site where this "shuttling" process begins. **Analysis of Incorrect Options:** * **Liver:** While the liver is the destination where alanine is converted back to pyruvate for gluconeogenesis and the nitrogen is processed into urea, the cycle is traditionally defined by its initiation in the **muscle**. * **Heart:** The heart primarily utilizes fatty acids and lactate for energy and does not participate significantly in the glucose-alanine nitrogen shuttle. * **Kidney:** While the kidney is involved in gluconeogenesis and glutamine metabolism (especially during acidosis), it is not the primary site of the glucose-alanine cycle. **NEET-PG High-Yield Pearls:** * **Enzyme:** ALT (Alanine Aminotransferase) requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. * **Purpose:** It serves a dual purpose: (1) Safe transport of ammonia and (2) Maintenance of blood glucose via the glucose-alanine-glucose loop. * **Comparison:** Unlike the **Cori Cycle** (which shuttles Lactate), the Glucose-Alanine cycle shuttles Nitrogen. * **ATP Cost:** The cycle is energy-expensive for the liver (6 ATP used for gluconeogenesis) but provides a net gain of 5-7 ATP for the muscle.

Question 415: In all of the following enzyme deficiencies, hyperammonemia is a common feature, EXCEPT:

A. Argininosuccinate synthetase
B. Carbamoyl Phosphate synthetase I (CPS-I)
C. Ornithine transcarbamoylase (OTC)
D. Ornithine amino transferase (Correct Answer)

Explanation: **Explanation:** The primary mechanism for ammonia detoxification in humans is the **Urea Cycle**, which occurs in the liver. Any enzyme deficiency directly involved in this cycle leads to the accumulation of ammonia, resulting in **hyperammonemia**. **1. Why Ornithine Amino Transferase (OAT) is the correct answer:** OAT is an enzyme involved in the interconversion of ornithine and glutamate/proline. It is **not** a direct component of the urea cycle. A deficiency in OAT leads to **Gyrate Atrophy of the choroid and retina**, characterized by progressive vision loss. While it affects ornithine levels, it does not impair the liver's overall capacity to detoxify ammonia; hence, hyperammonemia is not a feature. **2. Why the other options are incorrect:** * **CPS-I (Option B):** This is the rate-limiting step of the urea cycle. Deficiency causes Type I Hyperammonemia (the most severe form). * **OTC (Option C):** This is the most common urea cycle disorder (X-linked). Deficiency leads to Type II Hyperammonemia and is distinguished by elevated **Orotic acid** levels. * **Argininosuccinate Synthetase (Option A):** Deficiency leads to **Citrullinemia Type I**, characterized by a massive buildup of citrulline and secondary hyperammonemia. **High-Yield Clinical Pearls for NEET-PG:** * **Most common Urea Cycle Disorder:** Ornithine Transcarbamoylase (OTC) deficiency. * **Only X-linked Urea Cycle Disorder:** OTC deficiency (all others are Autosomal Recessive). * **Management of Hyperammonemia:** Low protein diet, Sodium benzoate/phenylbutyrate (ammonia scavengers), and Lactulose. * **Gyrate Atrophy:** Remember the triad of OAT deficiency, high ornithine levels, and circular "punched-out" retinal lesions.

Question 416: Which of the following is NOT a component of the urea cycle?

A. Urease (Correct Answer)
B. Arginase
C. Transcarbamoylase
D. Arginosuccinase

Explanation: The urea cycle (Ornithine cycle) is the primary pathway for the detoxification of ammonia into urea in the liver. **Why Urease is the Correct Answer:** **Urease** is an enzyme that catalyzes the hydrolysis of urea into ammonia and carbon dioxide. It is **not** part of the human urea cycle. Instead, it is produced by certain bacteria (e.g., *H. pylori*, *Proteus mirabilis*) and plants. In humans, bacterial urease in the gut can contribute to hyperammonemia in patients with liver failure by converting urea back into ammonia, which then enters the portal circulation. **Analysis of Incorrect Options:** * **Arginase:** This is the final enzyme of the cycle. It cleaves Arginine into Urea and Ornithine. It is primarily found in the liver. * **Ornithine Transcarbamoylase (OTC):** This mitochondrial enzyme catalyzes the reaction between Carbamoyl Phosphate and Ornithine to form Citrulline. It is the most common site of inherited urea cycle defects. * **Arginosuccinase (Arginosuccinate Lyase):** This enzyme cleaves Arginosuccinate into Arginine and Fumarate. The fumarate produced links the urea cycle to the TCA cycle (the "Krebs Bicycle"). **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Carbamoyl Phosphate Synthetase I (CPS-I), which requires **N-acetylglutamate (NAG)** as an essential activator. * **Subcellular location:** The cycle is "split"—the first two steps occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. * **Most common deficiency:** OTC deficiency (X-linked recessive), characterized by orotic aciduria and hyperammonemia.

Question 417: Amino acid absorption is by which transport mechanism?

A. Facilitated transport
B. Passive transport
C. Active transport (Correct Answer)
D. Pinocytosis

Explanation: **Explanation:** The absorption of amino acids from the intestinal lumen into the enterocytes occurs primarily via **Active Transport**. Specifically, this is a **Secondary Active Transport** mechanism known as the **Sodium-Amino Acid Cotransport system**. 1. **Why Active Transport is Correct:** Amino acids are transported against their concentration gradient. This process is driven by the sodium gradient established by the **Na⁺/K⁺ ATPase pump** on the basolateral membrane. As sodium ions move down their electrochemical gradient into the cell, they "drag" amino acids along with them via specific carrier proteins (Symphorts). A small portion of amino acids (especially neutral ones) are also absorbed via the **γ-Glutamyl cycle (Meister cycle)**, which requires ATP. 2. **Why Other Options are Incorrect:** * **Facilitated Transport:** While some amino acids exit the enterocyte into the portal circulation via facilitated diffusion, the primary uptake from the lumen is active. * **Passive Transport:** Amino acids are polar molecules and cannot freely diffuse across the lipid bilayer; they require specific transporters. * **Pinocytosis:** This mechanism is reserved for the absorption of intact proteins or large peptides, primarily seen in neonates (for immunoglobulin absorption from colostrum), not for standard amino acid absorption. **High-Yield Facts for NEET-PG:** * **Hartnup Disease:** A defect in the transport of neutral amino acids (especially **Tryptophan**), leading to pellagra-like symptoms. * **Cystinuria:** A defect in the **COAL** transporter (Cystine, Ornithine, Arginine, Lysine) in the proximal renal tubule and small intestine, leading to renal stones. * **Site of Absorption:** Occurs mainly in the **duodenum and jejunum**. * **Di- and Tri-peptides:** These are absorbed more rapidly than free amino acids via the **PEPT1** transporter (H⁺-dependent cotransport).

Question 418: Albinism is a genetic disease that results in incomplete metabolism of which amino acid?

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

Explanation: **Explanation:** **Correct Option: C. Tyrosine** Albinism (specifically Oculocutaneous Albinism) is primarily caused by a deficiency of the enzyme **Tyrosinase**. In the melanocytes, Tyrosinase is the rate-limiting enzyme responsible for converting **Tyrosine** into DOPA and subsequently into DOPAquinone, which eventually forms **melanin** (the pigment for skin, hair, and eyes). A genetic defect in this pathway leads to a lack of melanin production, resulting in the characteristic hypopigmentation seen in albinism. **Incorrect Options:** * **A. Histidine:** Metabolism of histidine leads to the formation of Histamine (via decarboxylation) or FIGLU (formiminoglutamate). Defects in this pathway cause Histidinemia, not pigment disorders. * **B. Cystine:** Cystine is involved in the transport disorder Cystinuria (renal stones) or the lysosomal storage disease Cystinosis. While cysteine is a component of pheomelanin, the primary metabolic block in albinism is at the Tyrosine level. * **D. Alanine:** Alanine is a non-essential amino acid primarily involved in glucose-alanine cycle (gluconeogenesis) and transamination reactions. It has no role in melanin synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Most common forms of Albinism are **Autosomal Recessive**. * **Key Enzyme:** Tyrosinase is a **copper-containing enzyme**. * **Clinical Risk:** Patients with albinism have a significantly increased risk of **Squamous Cell Carcinoma** and Basal Cell Carcinoma due to lack of photoprotective melanin. * **Differential:** Do not confuse Albinism (total lack of pigment) with **Vitiligo** (autoimmune destruction of melanocytes) or **Phenylketonuria** (where hypopigmentation occurs because high Phenylalanine inhibits Tyrosinase).

Question 419: Homocysteine is not associated with which of the following?

A. Coronary artery disease
B. Fracture
C. Hearing loss (Correct Answer)
D. Neuropsychiatric manifestations

Explanation: **Explanation:** Hyperhomocysteinemia (elevated levels of homocysteine in the blood) is a significant clinical marker associated with multisystemic pathology. It primarily arises from deficiencies in Vitamin B12, B6, or Folate, or genetic defects in enzymes like MTHFR or Cystathionine beta-synthase. **Why Hearing Loss is the Correct Answer:** There is currently no established clinical or pathophysiological link between elevated homocysteine levels and **hearing loss**. While some studies have explored its role in age-related macular degeneration, it is not a recognized feature of homocystinuria or hyperhomocysteinemia in standard medical curricula (like Harper’s or Harrison’s). **Analysis of Incorrect Options:** * **Coronary Artery Disease (CAD):** Homocysteine is a potent pro-oxidant. It causes endothelial dysfunction, promotes LDL oxidation, and stimulates smooth muscle cell proliferation, making it a well-known independent risk factor for atherosclerosis and myocardial infarction. * **Fracture:** High homocysteine levels interfere with the cross-linking of collagen fibers in the bone matrix. This leads to decreased bone density and increased skeletal fragility, frequently manifesting as osteoporosis and pathological fractures (especially in Homocystinuria). * **Neuropsychiatric Manifestations:** Homocysteine and its metabolites act as NMDA receptor agonists and can be neurotoxic. Clinically, this manifests as cognitive decline, dementia (Alzheimer’s), depression, and in severe cases (Homocystinuria), intellectual disability. **High-Yield Clinical Pearls for NEET-PG:** * **Homocystinuria Triad:** Ectopia lentis (downward subluxation), Intellectual disability, and Thromboembolic episodes. * **Enzyme Deficiency:** The most common cause of homocystinuria is a deficiency of **Cystathionine β-synthase**. * **Treatment:** Pyridoxine (B6) is the first-line cofactor therapy for responsive patients. * **Vascular Risk:** Homocysteine is pro-thrombotic and pro-atherogenic.

Question 420: Negative nitrogen balance is a characteristic feature of which phase of convalescence after surgery?

A. Catabolic phase (Correct Answer)
B. Anabolic phase
C. Fat gain phase
D. Turning point phase

Explanation: **Explanation:** Nitrogen balance is the measure of nitrogen input (protein intake) minus nitrogen output (urea, sweat, feces). A **negative nitrogen balance** occurs when nitrogen excretion exceeds intake, indicating a state of protein breakdown (proteolysis). **1. Why Catabolic Phase is Correct:** Immediately following surgery or major trauma, the body enters the **Catabolic Phase** (also known as the "Ebb" and early "Flow" phases). This is a stress response mediated by hormones like **cortisol, glucagon, and catecholamines**. These hormones promote the breakdown of skeletal muscle protein into amino acids to provide substrates for gluconeogenesis and the synthesis of acute-phase proteins. Since the body is breaking down more protein than it is consuming, nitrogen excretion in the urine increases, leading to a negative nitrogen balance. **2. Why Other Options are Incorrect:** * **Anabolic Phase:** This occurs later in recovery. Here, the body focuses on tissue repair and protein synthesis. Nitrogen intake exceeds excretion, resulting in a **positive nitrogen balance**. * **Fat Gain Phase:** This is the final stage of convalescence where the body restores energy reserves. It is characterized by a neutral or slightly positive nitrogen balance. * **Turning Point Phase:** This is the transition period between catabolism and anabolism. Nitrogen balance begins to shift from negative toward neutral. **High-Yield Clinical Pearls for NEET-PG:** * **Positive Nitrogen Balance:** Seen in growth, pregnancy, and recovery from illness (anabolism). * **Negative Nitrogen Balance:** Seen in starvation, severe burns, major trauma, and uncontrolled diabetes (catabolism). * **Key Marker:** Urinary urea nitrogen is the primary clinical marker used to assess nitrogen balance. * **Hormonal Control:** Insulin promotes positive nitrogen balance; Cortisol promotes negative nitrogen balance.

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

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