Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 171: Ochronosis is found in:

A. Alkaptonuria (Correct Answer)
B. Tyrosinemia
C. Phenylketonuria
D. Homocystinuria

Explanation: **Explanation:** **Alkaptonuria** is the correct answer because it is characterized by a deficiency of the enzyme **Homogentisate 1,2-dioxygenase**, leading to the accumulation of **Homogentisic Acid (HGA)**. When HGA is excreted in urine, it oxidizes upon contact with air, turning the urine black. Within the body, HGA undergoes polymerization into a melanin-like pigment that deposits in connective tissues, cartilages, and joints. This bluish-black pigmentation of tissues is clinically termed **Ochronosis**. **Analysis of Incorrect Options:** * **Tyrosinemia:** Caused by defects in various enzymes of the tyrosine degradation pathway (e.g., Fumarylacetoacetate hydrolase in Type I). It primarily presents with liver failure, renal tubular dysfunction (Fanconi syndrome), and a "cabbage-like" odor, but not ochronosis. * **Phenylketonuria (PKU):** Results from a deficiency of **Phenylalanine hydroxylase**. It presents with intellectual disability, seizures, and a "mousy" odor. Patients often have hypopigmentation (fair skin/blue eyes) due to decreased melanin synthesis, the opposite of ochronosis. * **Homocystinuria:** Due to **Cystathionine beta-synthase** deficiency. It is characterized by ectopia lentis (downward dislocation), marfanoid habitus, and high risk of thromboembolism, but does not involve HGA accumulation. **High-Yield Clinical Pearls for NEET-PG:** * **Triad of Alkaptonuria:** 1. Black urine on standing, 2. Ochronosis (pigmentation of sclera and ear cartilage), 3. Ochronotic arthritis (usually affecting large weight-bearing joints). * **Diagnosis:** Confirmed by detecting Homogentisic acid in urine using **Ferric Chloride test** (transient deep blue color) or Silver Nitrate test. * **Dietary Management:** Restriction of Phenylalanine and Tyrosine; **Nitisinone** is the specific drug used to inhibit HGA production.

Question 172: Creatinine is synthesized from which amino acid?

A. Arginine (Correct Answer)
B. Alanine
C. Histidine
D. Cysteine

Explanation: **Explanation:** Creatinine is a breakdown product of **Creatine phosphate**, which serves as a high-energy reservoir in muscle. The synthesis of creatine is a multi-organ process involving three specific amino acids: **Arginine, Glycine, and Methionine (as S-adenosylmethionine/SAM).** 1. **Step 1 (Kidney):** Arginine and Glycine combine to form Guanidinoacetate (GAA) via the enzyme *Amidinotransferase*. 2. **Step 2 (Liver):** GAA is methylated by SAM to form Creatine. 3. **Step 3 (Muscle):** Creatine is phosphorylated to Creatine Phosphate. It then undergoes non-enzymatic, irreversible cyclization to form **Creatinine**, which is excreted by the kidneys. **Analysis of Incorrect Options:** * **B. Alanine:** A non-essential amino acid primarily involved in the glucose-alanine cycle for transporting nitrogen to the liver; it does not contribute to the guanidino group of creatine. * **C. Histidine:** A precursor for Histamine and a provider of one-carbon units (via FIGLU); it is not involved in creatine synthesis. * **D. Cysteine:** A sulfur-containing amino acid used for synthesizing Taurine and Glutathione; it does not play a role in the formation of the creatine backbone. **Clinical Pearls for NEET-PG:** * **GAMT Deficiency:** A defect in Guanidinoacetate methyltransferase leads to cerebral creatine deficiency, presenting with seizures and intellectual disability. * **Diagnostic Marker:** Serum creatinine is a reliable indicator of GFR (Glomerular Filtration Rate) because its production is constant and proportional to muscle mass. * **Memory Aid:** Remember **"GAM"** (Glycine, Arginine, Methionine) for Creatine synthesis.

Question 173: Which of the following transports nitrogen from muscle to the liver?

A. Lactate
B. Alanine (Correct Answer)
C. Glutamine
D. Aspartate

Explanation: **Explanation:** The correct answer is **Alanine**. This question tests the understanding of the **Glucose-Alanine Cycle (Cahill Cycle)**, a crucial metabolic pathway for nitrogen transport. **1. Why Alanine is Correct:** During periods of fasting or intense exercise, muscle proteins are broken down into amino acids. The amino groups are transferred to $\alpha$-ketoglutarate to form glutamate. In the muscle, the enzyme **ALT (Alanine Aminotransferase)** transfers this amino group from glutamate to pyruvate (a product of glycolysis), forming **Alanine**. Alanine is then released into the blood and transported to the liver. In the liver, it is converted back to pyruvate for gluconeogenesis, while the nitrogen enters the **Urea Cycle** for excretion. **2. Why Other Options are Incorrect:** * **Lactate:** Involved in the **Cori Cycle**. It transports carbon skeletons (energy) from muscle to liver during anaerobic glycolysis but does **not** carry nitrogen. * **Glutamine:** While glutamine is the primary transporter of nitrogen from **most peripheral tissues** (especially the brain) to the liver/kidney, **Alanine** is the specific and preferred transporter for **skeletal muscle**. * **Aspartate:** It is an intermediate in the urea cycle and malate-aspartate shuttle but does not function as a primary inter-organ nitrogen transporter. **Clinical Pearls for NEET-PG:** * **Primary Nitrogen Transporters:** Remember: **Alanine** = Muscle; **Glutamine** = General peripheral tissues/Brain. * **ALT (SGPT):** The key enzyme for the Cahill cycle; it requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. * **Net Result:** The Glucose-Alanine cycle allows muscles to eliminate nitrogen safely while receiving glucose from the liver to sustain activity.

Question 174: At which level does tyrosine enter the citric acid cycle?

A. Succinyl CoA
B. Fumarate (Correct Answer)
C. Pyruvate
D. alpha-ketoglutarate

Explanation: **Explanation:** Tyrosine is a **glucogenic and ketogenic** amino acid. Its catabolism follows a specific pathway where it is first converted to homogentisate and eventually cleaved into two fragments: **Fumarate** and **Acetoacetate**. 1. **Why Fumarate is correct:** The final step of tyrosine (and phenylalanine) degradation involves the enzyme *fumarylacetoacetate hydrolase*, which splits fumarylacetoacetate into fumarate and acetoacetate. Fumarate directly enters the Citric Acid Cycle (TCA), making tyrosine glucogenic, while acetoacetate enters the ketone body pool, making it ketogenic. 2. **Why other options are incorrect:** * **Succinyl CoA:** This is the entry point for "VOMIT" amino acids (Valine, Odd-chain fatty acids, Methionine, Isoleucine, and Threonine). * **Pyruvate:** This is the entry point for 3-carbon amino acids like Alanine, Glycine, Serine, and Cysteine. * **Alpha-ketoglutarate:** This is the entry point for 5-carbon amino acids that form glutamate, such as Glutamine, Proline, Arginine, and Histidine. **Clinical Pearls for NEET-PG:** * **Alkaptonuria:** Caused by a deficiency of *homogentisate oxidase*, leading to dark urine (on standing) and ochronosis. * **Tyrosinemia Type I:** Caused by a deficiency of *fumarylacetoacetate hydrolase* (the final enzyme). It is the most severe form, characterized by a "cabbage-like" odor and liver failure. * **Phenylketonuria (PKU):** Tyrosine becomes an **essential** amino acid in PKU patients because they cannot convert phenylalanine to tyrosine due to *phenylalanine hydroxylase* deficiency.

Question 175: Nitric oxide (NO) is synthesized by which of the following?

A. Uracil
B. Aspartate
C. Guanosine
D. Arginine (Correct Answer)

Explanation: **Explanation:** **Correct Answer: D. Arginine** Nitric Oxide (NO), a potent vasodilator and signaling molecule, is synthesized from the amino acid **L-Arginine**. This reaction is catalyzed by the enzyme **Nitric Oxide Synthase (NOS)**. In this process, Arginine undergoes a five-electron oxidation to form **L-Citrulline** and NO. The reaction requires several essential cofactors: NADPH, FAD, FMN, Heme, and Tetrahydrobiopterin ($BH_4$). **Analysis of Incorrect Options:** * **A. Uracil:** This is a pyrimidine nitrogenous base found in RNA; it is involved in genetic coding, not gasotransmitter synthesis. * **B. Aspartate:** While Aspartate is involved in the Urea cycle (reacting with Citrulline to form Argininosuccinate), it is not the direct precursor of NO. * **C. Guanosine:** This is a purine nucleoside. While NO stimulates **Guanylyl Cyclase** to increase cyclic GMP (cGMP), Guanosine itself is not a substrate for NO production. **High-Yield Clinical Pearls for NEET-PG:** * **Isoforms of NOS:** There are three types: **nNOS** (Neuronal/Type I), **iNOS** (Inducible/Type II - involved in inflammation/macrophages), and **eNOS** (Endothelial/Type III - regulates blood pressure). * **Biological Functions:** NO acts as a potent smooth muscle relaxant, inhibits platelet aggregation, and serves as a neurotransmitter in the CNS. * **Nitroglycerin Mechanism:** It acts as a prodrug to provide NO, causing vasodilation in angina pectoris. * **Precursor Mnemonic:** Remember **"Arg-NO"** (Arginine leads to NO).

Question 176: An infant presents with a history of vomiting and poor feeding. The urine has a burnt sugar odor. Lab examination reveals elevated levels of leucine, isoleucine, and valine. What is the most likely diagnosis?

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

Explanation: ### Explanation **Correct Option: D. Maple Syrup Urine Disease (MSUD)** The clinical presentation of poor feeding, vomiting, and a characteristic **burnt sugar (or maple syrup) odor** in the urine is pathognomonic for MSUD. This autosomal recessive disorder is caused by a deficiency in the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD) complex**. This enzyme is responsible for the oxidative decarboxylation of branched-chain amino acids (BCAAs): **Leucine, Isoleucine, and Valine**. A defect leads to the accumulation of these amino acids and their corresponding alpha-keto acids in the blood and urine. **Why Incorrect Options are Wrong:** * **A. Phenylketonuria (PKU):** Caused by a deficiency of Phenylalanine Hydroxylase. It presents with a "mousy" or "musty" odor and intellectual disability, not a burnt sugar smell. * **B. Alkaptonuria:** Due to Homogentisate Oxidase deficiency. It is characterized by urine that turns black upon standing and ochronosis (pigmentation of connective tissue), typically presenting later in life. * **C. Tyrosinemia:** Type I is caused by Fumarylacetoacetate Hydrolase deficiency. It presents with liver failure, renal tubular dysfunction, and a "cabbage-like" or "boiled egg" odor. **High-Yield Clinical Pearls for NEET-PG:** * **The Odor:** Caused specifically by the accumulation of **S-isoleucine**. * **Cofactors:** The BCKAD complex requires five cofactors: **T**hiamine (B1), **L**ipoic acid, **C**oA (B5), **F**AD (B2), and **N**AD (B3). (*Mnemonic: **T**ender **L**oving **C**are **F**or **N**ancy*). * **Treatment:** Dietary restriction of BCAAs and, in some cases, high-dose **Thiamine** supplementation (Thiamine-responsive MSUD). * **Diagnosis:** Elevated levels of **Alloisoleucine** in the plasma is a highly specific diagnostic marker.

Question 177: Melatonin is synthesized from which amino acid?

A. Tryptophan (Correct Answer)
B. Serotonin
C. Phenylalanine
D. Histidine

Explanation: **Explanation:** **1. Why Tryptophan is Correct:** Melatonin, the hormone responsible for regulating the circadian rhythm (sleep-wake cycle), is synthesized primarily in the **pineal gland** from the essential amino acid **Tryptophan**. The metabolic pathway follows this sequence: * **Tryptophan** → 5-Hydroxytryptophan → **Serotonin** → N-Acetylserotonin → **Melatonin**. The conversion of Serotonin to Melatonin involves two key enzymes: *Serotonin N-acetyltransferase* (the rate-limiting step) and *Hydroxyindole-O-methyltransferase*. **2. Why Incorrect Options are Wrong:** * **Serotonin (Option B):** While Serotonin is the immediate chemical precursor to Melatonin, it is a **biogenic amine**, not an **amino acid**. The question specifically asks for the parent amino acid. * **Phenylalanine (Option C):** This is the precursor for Tyrosine, which leads to the synthesis of Catecholamines (Dopamine, Norepinephrine, Epinephrine), Thyroxine (T4), and Melanin. * **Histidine (Option D):** This amino acid undergoes decarboxylation to form **Histamine**, a mediator of allergic reactions and gastric acid secretion. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Rate-limiting enzyme:** Serotonin N-acetyltransferase (activity increases significantly in the dark). * **Cofactor:** Pyridoxal Phosphate (B6) is required for the decarboxylation step in this pathway. * **Tryptophan Derivatives:** Remember the "3 Ms and 1 S" – **M**elatonin, **M**elanin (via serotonin pathway in some contexts, but primarily Tyrosine), **M**agnesium (not related), and **S**erotonin. Also, Tryptophan is a precursor for **Niacin (Vitamin B3)**; 60 mg of Tryptophan yields 1 mg of Niacin. * **Hartnup Disease:** A defect in Tryptophan transport leading to pellagra-like symptoms due to Niacin deficiency.

Question 178: Glutamate, in the presence of ammonia, is converted into which amino acid within the brain?

A. Glutamine (Correct Answer)
B. Arginine
C. Glyceride
D. Glycine

Explanation: **Explanation:** The correct answer is **Glutamine**. **1. Why Glutamine is Correct:** Ammonia ($NH_3$) is highly neurotoxic and must be rapidly detoxified in the brain. The primary mechanism for this is the conversion of **Glutamate to Glutamine**. This reaction is catalyzed by the enzyme **Glutamine Synthetase**, which adds a molecule of ammonia to glutamate in an ATP-dependent process. Glutamine is a non-toxic, neutral amino acid that can safely cross the blood-brain barrier into the circulation to be transported to the liver or kidneys for further processing. **2. Why Other Options are Incorrect:** * **Arginine:** While Arginine is an intermediate in the Urea Cycle (the body's main ammonia disposal system), this cycle occurs primarily in the **liver**, not the brain. * **Glyceride:** This is a lipid derivative (e.g., triglycerides) and is not an amino acid or involved in nitrogen metabolism. * **Glycine:** Although it is a simple amino acid and an inhibitory neurotransmitter, it is not the primary product of ammonia detoxification from glutamate. **3. Clinical Pearls for NEET-PG:** * **Hepatic Encephalopathy:** In liver failure, elevated blood ammonia levels lead to excessive glutamine accumulation in **astrocytes**. This causes osmotic swelling and cerebral edema, explaining the neurological symptoms. * **The "Glutamate-Glutamine Cycle":** This cycle is crucial for recycling neurotransmitters between neurons and astrocytes. * **Enzyme Marker:** Glutamine synthetase is primarily located in the astrocytes of the brain. * **Alpha-ketoglutarate depletion:** High ammonia levels can also pull $\alpha$-ketoglutarate away from the TCA cycle to form glutamate, leading to an energy deficit in the brain.

Question 179: Albinism is due to deficiency of which enzyme?

A. Homogentisate oxidase
B. Tyrosinase (Correct Answer)
C. Phenylalanine hydroxylase
D. Tyrosine transaminase

Explanation: **Explanation:** **Albinism** is a group of inherited disorders characterized by a lack of melanin pigment in the skin, hair, and eyes. The correct answer is **Tyrosinase** because it is the rate-limiting enzyme in the synthesis of melanin from the amino acid Tyrosine. Tyrosinase catalyzes the conversion of Tyrosine to DOPA (Dihydroxyphenylalanine) and subsequently to Dopaquinone. A deficiency in this enzyme leads to Oculocutaneous Albinism (OCA) Type 1. **Analysis of Incorrect Options:** * **A. Homogentisate oxidase:** Deficiency of this enzyme leads to **Alkaptonuria**, characterized by the accumulation of homogentisic acid, resulting in dark urine (on standing) and ochronosis (pigmentation of connective tissues). * **C. Phenylalanine hydroxylase:** Deficiency causes **Phenylketonuria (PKU)**. While PKU patients may have fair skin due to decreased tyrosine (a melanin precursor), the primary pathology of Albinism specifically involves the melanin synthesis pathway itself. * **D. Tyrosine transaminase:** Deficiency leads to **Tyrosinemia Type II** (Richner-Hanhart syndrome), which presents with painful corneal erosions and palmoplantar keratoderma, not generalized albinism. **Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Tyrosinase is the key regulatory enzyme for melanogenesis. * **Inheritance:** Most forms of Albinism are **Autosomal Recessive**. * **Clinical Risk:** Patients with albinism have a significantly increased risk of **Squamous Cell Carcinoma** and Basal Cell Carcinoma due to the lack of protective melanin against UV radiation. * **Differentiate:** Do not confuse Albinism (absent melanin) with Vitiligo (autoimmune destruction of melanocytes) or Piebaldism (defective migration of melanoblasts).

Question 180: If the tyrosinase gene is defective, what is the resulting condition?

A. Lack of pigmentation (Correct Answer)
B. Maple syrup urine disease
C. Phenylketonuria
D. Kidney stones

Explanation: **Explanation:** The correct answer is **Lack of pigmentation (Albinism)**. **1. Why Option A is correct:** Tyrosinase is a copper-containing enzyme essential for the synthesis of **melanin** from the amino acid **Tyrosine**. It catalyzes the rate-limiting steps: the hydroxylation of Tyrosine to L-DOPA and the subsequent oxidation of L-DOPA to Dopaquinone. A genetic defect in the tyrosinase gene leads to **Oculocutaneous Albinism (Type 1)**, characterized by a complete or partial lack of melanin in the skin, hair, and eyes. **2. Why other options are incorrect:** * **B. Maple syrup urine disease (MSUD):** Caused by a deficiency of the **Branched-chain alpha-keto acid dehydrogenase** complex, leading to the accumulation of Leucine, Isoleucine, and Valine. * **C. Phenylketonuria (PKU):** Results from a deficiency of **Phenylalanine hydroxylase**, preventing the conversion of Phenylalanine to Tyrosine. * **D. Kidney stones:** While various metabolic defects cause stones (e.g., Primary Hyperoxaluria or Cystinuria), they are not linked to tyrosinase deficiency. **3. NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme:** Tyrosinase is the key regulatory enzyme for melanogenesis. * **Cofactor:** Tyrosinase requires **Copper ($Cu^{2+}$)**. Deficiency of copper (as seen in Menkes disease) can also lead to hypopigmentation. * **Differentiate from Alkaptonuria:** Alkaptonuria is due to a deficiency of **Homogentisate oxidase**, leading to dark urine and ochronosis, not albinism. * **Clinical Presentation:** Patients with albinism have increased susceptibility to skin cancers (Squamous cell carcinoma) due to the lack of protective melanin against UV radiation.

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