Amino Acid Metabolism Practice Questions

Q: Which amino acid undergoes the most significant hepatic oxidative deamination, which is used for urea synthesis?

Glutamate. **Explanation:** The correct answer is **Glutamate**. In amino acid metabolism, most peripheral amino acids first undergo **transamination**, where their $\alpha$-amino group is transferred to $\alpha$-ketoglutarate to form **Glutamate**. Glutamate then serves as the "collection center" for amino groups. It undergoes **oxidative deamination** in the mitochondria of hepatocytes, catalyzed by the enzyme **Glutamate Dehydrogenase (GDH)**. This reaction releases free ammonia ($NH_3$), which enters the **Urea Cycle** to be detoxified and excreted. This is the only reaction in humans that can rapidly release nitrogen from amino acids as free ammonia. **Why other options are incorrect:** * **Glutamine:** While it is the primary non-toxic transporter of ammonia in the blood, it undergoes *hydrolysis* by glutaminase to form glutamate, rather than direct oxidative deamination for the bulk of urea synthesis. * **Aspartate:** It provides the second nitrogen atom for the urea cycle by condensing with citrulline, but it does so via *transamination*, not oxidative deamination. * **Arginine:** It is an intermediate of the urea cycle itself. It is cleaved by arginase to produce urea and ornithine, but it does not undergo oxidative deamination to initiate the process. **Clinical Pearls for NEET-PG:** * **Glutamate Dehydrogenase (GDH)** is unique because it can use either **NAD+** (for catabolism/deamination) or **NADP+** (for anabolism/amination) as a coenzyme. * GDH is allosterically inhibited by **GTP/ATP** and activated by **ADP/GDP**, linking nitrogen metabolism to the energy status of the cell. * **Transdeamination:** The combined action of aminotransferases and GDH is the most efficient pathway for nitrogen disposal.

Q: Which enzyme is involved in the regulation of the urea cycle?

Carbamoyl phosphate synthetase. **Explanation:** The urea cycle is the primary mechanism for disposing of nitrogenous waste in humans. The correct answer is **Carbamoyl Phosphate Synthetase I (CPS-I)**, as it serves as the **rate-limiting and committed step** of the cycle. 1. **Why CPS-I is correct:** Located in the mitochondria, CPS-I catalyzes the condensation of $NH_4^+$ and $CO_2$ to form carbamoyl phosphate. It is uniquely regulated by its obligatory allosteric activator, **N-acetylglutamate (NAG)**. Without NAG, CPS-I is inactive. NAG levels rise when arginine levels are high, signaling an abundance of amino acids and the need for increased urea production. 2. **Why other options are incorrect:** * **Ornithine transcarbamoylase (OTC):** While it is the most common site of genetic urea cycle defects (X-linked), it is not the primary regulatory enzyme. * **Argininosuccinase (Argininosuccinate Lyase):** This enzyme cleaves argininosuccinate into arginine and fumarate. It is a reversible step and does not control the flux of the cycle. * **Arginase:** This is the final enzyme that releases urea. While it is essential for completing the cycle, it does not act as the metabolic "pacemaker." **High-Yield Clinical Pearls for NEET-PG:** * **Location:** The urea cycle occurs in both the **mitochondria** (first two steps) and the **cytosol** (remaining steps). * **CPS-I vs. CPS-II:** Do not confuse them. CPS-I is for the **U**rea cycle (**M**itochondrial), while CPS-II is for **P**yrimidine synthesis (**C**ytosolic). * **Hyperammonemia:** Deficiency in any urea cycle enzyme leads to ammonia toxicity. CPS-I deficiency presents with severe hyperammonemia but **no** orotic aciduria (unlike OTC deficiency).

Q: What biochemical substance supplies ammonia to the liver?

Alanine. **Explanation:** The correct answer is **Alanine**. This question tests the understanding of the **Cahill Cycle (Glucose-Alanine Cycle)**, which is the primary mechanism for transporting nitrogen from skeletal muscle to the liver. **Why Alanine is Correct:** During periods of muscle protein catabolism, amino groups are transferred to $\alpha$-ketoglutarate to form glutamate. In the muscle, the enzyme **Alanine Aminotransferase (ALT)** transfers this amino group from glutamate to pyruvate (a product of glycolysis), forming Alanine. Alanine is then released into the blood and travels to the liver. In the liver, ALT reverses the reaction, converting Alanine back into pyruvate and releasing **ammonia**, which enters the **Urea Cycle** for detoxification and excretion. **Why Other Options are Incorrect:** * **Arginine:** While Arginine is an intermediate in the Urea Cycle, it is not a primary transporter of ammonia from peripheral tissues to the liver. Its role is to be cleaved by arginase to produce urea and ornithine. * **Lactate:** Lactate is involved in the **Cori Cycle**, transporting carbon skeletons from muscle to liver for gluconeogenesis during anaerobic metabolism. It does not contain nitrogen and cannot supply ammonia. * **Pyruvate:** Pyruvate is a keto-acid that acts as the carbon skeleton for Alanine. It must be transaminated (receive an amino group) to become Alanine before it can transport nitrogen. **NEET-PG High-Yield Pearls:** * **Glutamine** is the primary transporter of ammonia from **most peripheral tissues** (especially the brain) to the liver/kidneys. * **Alanine** is the specific transporter of ammonia from **skeletal muscle** to the liver. * The Glucose-Alanine cycle serves a dual purpose: it detoxifies ammonia and provides a substrate (pyruvate) for **gluconeogenesis** in the liver. * Key Enzyme: **ALT (Alanine Aminotransferase)** requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor.

Q: What is the first product of tryptophan catabolism?

kynurenine. ### Explanation **Correct Answer: A. Kynurenine** Tryptophan is an essential amino acid that follows two major metabolic pathways: the **Kynurenine pathway** (90-95%) and the Serotonin pathway. The first step of the kynurenine pathway involves the oxidative cleavage of the indole ring of tryptophan. This reaction is catalyzed by the enzyme **Tryptophan 2,3-dioxygenase (TDO)** in the liver or **Indoleamine 2,3-dioxygenase (IDO)** in extrahepatic tissues. The immediate product formed is *N-formylkynurenine*, which is rapidly converted to **Kynurenine** by the enzyme formamidase. In the context of medical exams, Kynurenine is recognized as the first stable major product of this catabolic route. **Analysis of Incorrect Options:** * **B. Bradykinin:** This is a potent vasodilator peptide belonging to the kinin system, derived from high-molecular-weight kininogen (HMWK), not from amino acid catabolism. * **C. PAF (Platelet Activating Factor):** This is a phospholipid mediator of inflammation and platelet aggregation; it is not a product of tryptophan metabolism. * **D. Xanthurenate:** This is a downstream metabolite of the kynurenine pathway. It is clinically significant because it is excreted in excess during **Vitamin B6 deficiency**, as the enzyme kynureninase requires Pyridoxal Phosphate (PLP). **High-Yield Clinical Pearls for NEET-PG:** * **Niacin Synthesis:** The kynurenine pathway is the source of **NAD+/NADP+**. Approximately 60 mg of Tryptophan yields 1 mg of Niacin. * **Hartnup Disease:** A defect in the transport of neutral amino acids (including tryptophan) leads to pellagra-like symptoms due to niacin deficiency. * **Vitamin B6 Dependency:** In B6 deficiency, the conversion of 3-hydroxykynurenine to anthranilic acid is blocked, leading to the side-tracking of metabolites into **Xanthurenic acid** (a diagnostic marker in urine).

Q: 5-HIAA is a metabolite of which of the following?

Serotonin. **Explanation:** **5-HIAA (5-Hydroxyindoleacetic acid)** is the primary end-metabolite of **Serotonin** (5-hydroxytryptamine). Serotonin is synthesized from the amino acid Tryptophan. It undergoes oxidative deamination catalyzed by the enzyme **Monoamine Oxidase (MAO)** and subsequent oxidation by aldehyde dehydrogenase to form 5-HIAA, which is then excreted in the urine. **Analysis of Options:** * **Dopamine (Option B):** The major end-metabolite of dopamine is **Homovanillic Acid (HVA)**. * **Epinephrine (Option C):** Epinephrine and Norepinephrine are metabolized into **Vanillylmandellic Acid (VMA)** and metanephrines. * **Histamine (Option D):** Histamine is primarily metabolized into imidazole acetic acid and methylimidazole acetic acid. **Clinical Pearls for NEET-PG:** 1. **Carcinoid Syndrome:** Urinary 24-hour 5-HIAA levels are the "gold standard" diagnostic marker for Carcinoid tumors (neuroendocrine tumors usually found in the ileum). These tumors secrete excessive serotonin. 2. **Dietary Interference:** Patients must avoid serotonin-rich foods (bananas, walnuts, pineapples, avocados, and tomatoes) for 72 hours before the test to prevent false-positive results. 3. **Hartnup Disease:** This condition involves a defect in tryptophan transport, leading to decreased serotonin and 5-HIAA levels, alongside pellagra-like symptoms. 4. **Pellagra Connection:** In Carcinoid syndrome, secondary Pellagra can occur because up to 60% of dietary tryptophan is diverted to serotonin synthesis, leaving insufficient amounts for Niacin (Vitamin B3) production.

Q: A female neonate presented with lethargy at 24 hours of age, followed by focal seizure activity at 54 hours. A sepsis workup was negative. Her plasma ammonia level was found to be 1,000 mmol/L (normal 5-35 mmol/L). Quantitative plasma amino acid levels revealed a marked elevation of argininosuccinate. Which one of the following enzymic activities is most likely to be deficient in this patient?

Argininosuccinate lyase. ### **Explanation** The clinical presentation of neonatal hyperammonemia (lethargy, seizures, and ammonia >1,000 mmol/L) after a 24-hour symptom-free interval is characteristic of a **Urea Cycle Disorder (UCD)**. **1. Why Argininosuccinate Lyase is Correct:** The definitive clue is the **marked elevation of argininosuccinate** in the plasma. In the urea cycle, **Argininosuccinate Lyase (ASL)** is responsible for cleaving argininosuccinate into **Arginine** and **Fumarate**. A deficiency in this enzyme (Argininosuccinic Aciduria) leads to a massive buildup of argininosuccinate, which is then excreted in the urine. It is the second most common UCD. **2. Why Other Options are Incorrect:** * **Arginase:** Deficiency leads to **Argininemia**. Unlike other UCDs, it typically presents later (infancy/childhood) with progressive spastic diplegia rather than acute neonatal hyperammonemia. * **Argininosuccinate Synthase:** Deficiency causes **Citrullinemia Type I**. It would result in a marked elevation of **Citrulline**, not argininosuccinate. * **Carbamoyl Phosphate Synthetase I (CPS-I):** This is a proximal urea cycle defect. It causes severe hyperammonemia but with **low levels** of citrulline and argininosuccinate, as the cycle is blocked at the very first step. **3. NEET-PG Clinical Pearls:** * **Argininosuccinic Aciduria** is unique because **Trichorrhexis nodosa** (fragile, node-like hair) is a classic physical finding in survivors. * **Treatment Tip:** Unlike other UCDs, supplementation with high-dose **Arginine** can often help "bypass" the cycle in ASL deficiency by promoting the excretion of nitrogen through argininosuccinate. * **Differential Diagnosis:** Always check **Orotic Acid** levels; if elevated with hyperammonemia, suspect **Ornithine Transcarbamylase (OTC) deficiency** (the most common UCD, X-linked).

Q: FIGLU is an intermediate in which metabolic pathway?

Histidine metabolism. **Explanation:** **Histidine metabolism** is the correct answer because FIGLU (**Formiminoglutamate**) is a key intermediate in the catabolism of Histidine to Glutamate. 1. **The Pathway:** Histidine is first converted to urocanate by the enzyme histidase. After a series of steps, it forms FIGLU. The formimino group of FIGLU is then transferred to **Tetrahydrofolate (THF)** by the enzyme *formiminotransferase*, resulting in the formation of Glutamate and $N^5$-formimino-THF. 2. **Clinical Significance:** Since THF is required to clear FIGLU, a deficiency in **Folic Acid** leads to the accumulation and increased urinary excretion of FIGLU. This forms the basis of the **FIGLU Excretion Test** (Histidine loading test) used to diagnose folate deficiency. **Why other options are incorrect:** * **Valine metabolism:** Valine is a branched-chain amino acid (BCAA) that is metabolized into Succinyl-CoA via Methylmalonyl-CoA. * **Methionine metabolism:** Methionine is converted to S-adenosylmethionine (SAM) and eventually Homocysteine. It is linked to the folate cycle but does not produce FIGLU. * **Arginine metabolism:** Arginine is converted to Urea and Ornithine in the urea cycle, or used to produce Nitric Oxide and Creatine. **High-Yield Clinical Pearls for NEET-PG:** * **Histidinemia:** Caused by a deficiency of the enzyme **Histidase**. It is a relatively benign condition but can be associated with mental retardation. * **Folate Trap:** FIGLU levels rise in B12 deficiency as well, because B12 is needed to recycle THF, but FIGLU is specifically used as a functional marker for **Folate status**. * **Mnemonic:** "Histidine goes to Glutamate via FIGLU."

Q: A 16-year-old patient presents with recurrent joint pains, dark spots in the sclera, and blackening of urine on standing. What is the most probable diagnosis?

Alkaptonuria. **Explanation:** The clinical presentation of dark urine on standing, scleral pigmentation, and joint pain is the classic triad of **Alkaptonuria**, an autosomal recessive disorder of phenylalanine and tyrosine metabolism. **1. Why Alkaptonuria is correct:** The condition is caused by a deficiency of the enzyme **Homogentisate Oxidase**, leading to the accumulation of **Homogentisic Acid (HGA)**. * **Dark Urine:** When urine is exposed to air (standing), HGA is oxidized to benzoquinone acetate, which polymerizes into a black pigment (alkapton). * **Ochronosis:** Over time, this pigment deposits in connective tissues, causing dark spots in the sclera and ears. * **Arthritis:** Pigment deposition in large joints leads to ochronotic arthritis, typically manifesting in the teens or early adulthood. **2. Why other options are incorrect:** * **Tyrosinemia:** Presents primarily with liver failure (Type I), cabbage-like odor, or palmoplantar keratosis (Type II). It does not cause blackening of urine. * **Phenylketonuria (PKU):** Characterized by intellectual disability, seizures, and a "mousy" body odor due to phenylacetate. Patients often have fair skin and blue eyes (hypopigmentation), not hyperpigmentation. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Defect:** Homogentisate 1,2-dioxygenase. * **Diagnostic Test:** Ferric chloride test (turns urine deep blue/green) and Silver nitrate test. * **Radiology:** "Bamboo spine" appearance due to calcification of intervertebral discs (mimics Ankylosing Spondylitis). * **Management:** Low protein diet (restriction of Phenylalanine and Tyrosine) and **Nitisinone**, which inhibits the formation of HGA.

Q: Plasma levels of which of the following are likely to be lower than normal in a case of severe liver disease?

Urea. **Explanation:** The liver is the primary site for the **Urea Cycle (Ornithine Cycle)**, the metabolic pathway responsible for converting toxic ammonia (NH₃) into non-toxic urea for excretion by the kidneys. **Why Urea is the Correct Answer:** In severe liver disease (such as cirrhosis or fulminant hepatic failure), the hepatocytes are damaged, leading to a significant impairment of the urea cycle enzymes. Consequently, the liver cannot efficiently convert ammonia into urea. This results in a **decrease in plasma urea levels** (often reflected as a low Blood Urea Nitrogen - BUN) and a reciprocal increase in toxic nitrogenous waste products. **Analysis of Incorrect Options:** * **A & B (NH₃ and NH₄⁺):** Because the liver cannot convert ammonia into urea, ammonia levels **increase** in the blood. This hyperammonemia is a hallmark of liver failure and is the primary driver of hepatic encephalopathy. * **D (Alanine):** Alanine is a major gluconeogenic amino acid that carries nitrogen from muscles to the liver. In liver disease, the liver's ability to uptake and metabolize amino acids for gluconeogenesis is impaired, typically leading to **elevated** or normal plasma levels of aromatic and certain other amino acids, not a decrease. **High-Yield Clinical Pearls for NEET-PG:** * **BUN/Creatinine Ratio:** A very low BUN in the presence of normal creatinine often points toward severe liver dysfunction or malnutrition. * **Hyperammonemia:** Leads to increased brain **Glutamine** levels (via glutamine synthetase in astrocytes), causing osmotic swelling and cerebral edema. * **Deficiency Sign:** The most common urea cycle enzyme deficiency is **Ornithine Transcarbamoylase (OTC) deficiency** (X-linked), which also presents with low urea and high ammonia.

Question 1

Which amino acid undergoes the most significant hepatic oxidative deamination, which is used for urea synthesis?

Accepted Answer

Glutamate

Answer

Glutamine

Answer

Aspartate

Answer

Arginine

Question 2

Which amino acids are absorbed in enterocytes through the Meiser cycle?

Accepted Answer

Neutral amino acids

Answer

Dibasic amino acids

Answer

Basic amino acids

Answer

Dicarboxylic amino acids

Question 3

Which enzyme is involved in the regulation of the urea cycle?

Accepted Answer

Carbamoyl phosphate synthetase

Answer

Ornithine transcarbamoylase

Answer

Argininosuccinase

Answer

Arginase

Question 4

What biochemical substance supplies ammonia to the liver?

Accepted Answer

Alanine

Answer

Arginine

Answer

Lactate

Answer

Pyruvate

Question 5

What is the first product of tryptophan catabolism?

Accepted Answer

kynurenine

Answer

Bradykinin

Answer

PAF

Answer

Xantheurenate

Question 6

5-HIAA is a metabolite of which of the following?

Accepted Answer

Serotonin

Answer

Dopamine

Answer

Epinephrine

Answer

Histamine

Question 7

A female neonate presented with lethargy at 24 hours of age, followed by focal seizure activity at 54 hours. A sepsis workup was negative. Her plasma ammonia level was found to be 1,000 mmol/L (normal 5-35 mmol/L). Quantitative plasma amino acid levels revealed a marked elevation of argininosuccinate. Which one of the following enzymic activities is most likely to be deficient in this patient?

Accepted Answer

Argininosuccinate lyase

Answer

Arginase

Answer

Argininosuccinate synthase

Answer

Carbamoyl phosphate synthetase I

Question 8

FIGLU is an intermediate in which metabolic pathway?

Accepted Answer

Histidine metabolism

Answer

Valine metabolism

Answer

Methionine metabolism

Answer

Arginine metabolism

Question 9

A 16-year-old patient presents with recurrent joint pains, dark spots in the sclera, and blackening of urine on standing. What is the most probable diagnosis?

Accepted Answer

Alkaptonuria

Answer

Tyrosinemia

Answer

Phenylketonuria

Answer

All of the above

Question 10

Plasma levels of which of the following are likely to be lower than normal in a case of severe liver disease?

Accepted Answer

Urea

Answer

NH3

Answer

NH4+

Answer

Alanine

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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