Amino Acid Metabolism Practice Questions

Q: Threonine is metabolized to:

Glycine. **Explanation:** Threonine is a unique hydroxy-amino acid that is primarily metabolized through three distinct pathways. The correct answer, **Glycine**, is produced via the **Threonine Aldolase pathway**. In this reaction, Threonine is cleaved directly into Glycine and Acetaldehyde. Alternatively, Threonine can be converted to Glycine via the **Threonine Dehydrogenase pathway**, which produces α-amino-β-ketobutyrate as an intermediate; this is subsequently cleaved by a lyase to yield Glycine and Acetyl-CoA. Because it can produce both Acetyl-CoA (ketogenic) and Glycine (which enters the TCA cycle via pyruvate/succinyl-CoA), Threonine is classified as both **glucogenic and ketogenic**. **Analysis of Incorrect Options:** * **A. Lysine:** This is an essential amino acid that does not share a direct metabolic degradative pathway with Threonine. Lysine is purely ketogenic. * **B. Arginine:** Arginine is part of the Urea Cycle. While it can be synthesized from Citrulline, it is not a metabolic product of Threonine. * **C. Ornithine:** Ornithine is a key intermediate in the Urea Cycle produced from Arginine via the enzyme Arginase. It is not derived from Threonine. **High-Yield Clinical Pearls for NEET-PG:** * **Essentiality:** Threonine is one of the 10 essential amino acids (PVT TIM HALL). * **Special Category:** Along with Lysine, Threonine is unique because it **does not undergo transamination** (it does not lose its amino group to α-ketoglutarate). * **Glycine Precursors:** Remember the mnemonic **"STAG"** for sources of Glycine: **S**erine, **T**hreonine, **A**mmonium/CO2 (via Glycine synthase), and **G**lyoxylate. * **Metabolic End-product:** Threonine is also a major precursor for **Propionyl-CoA**, which eventually enters the TCA cycle as Succinyl-CoA.

Q: Which of the following is NOT a purely glucogenic amino acid?

Tryptophan. ### Explanation Amino acids are classified based on their metabolic end-products into three categories: **Purely Glucogenic**, **Purely Ketogenic**, and **Both (Glucogenic & Ketogenic)**. **Why Tryptophan is the Correct Answer:** Tryptophan is an **amphibolic** amino acid, meaning it is **both glucogenic and ketogenic**. Its breakdown yields pyruvate (glucogenic precursor) and acetoacetyl-CoA (ketogenic precursor). Therefore, it is not "purely" glucogenic. **Analysis of Incorrect Options:** * **Valine (Option A):** An essential branched-chain amino acid that is **purely glucogenic**. It enters the TCA cycle via Succinyl-CoA. * **Alanine (Option B):** The primary substrate for gluconeogenesis in the liver. It is **purely glucogenic** as it is directly transaminated to pyruvate. * **Methionine (Option D):** A sulfur-containing essential amino acid that is **purely glucogenic**. It is converted into Succinyl-CoA through the propionyl-CoA pathway. --- ### High-Yield NEET-PG Pearls 1. **Purely Ketogenic Amino Acids:** Only two—**Leucine and Lysine** (Mnemonic: The "L"s are purely ketogenic). 2. **Both Glucogenic and Ketogenic:** There are five—**Tryptophan, Tyrosine, Threonine, Isoleucine, and Phenylalanine** (Mnemonic: **PITTT** – Phenylalanine, Isoleucine, Tryptophan, Threonine, Tyrosine). 3. **Purely Glucogenic:** All remaining 13 amino acids. 4. **Clinical Correlation:** In states of starvation or uncontrolled diabetes, glucogenic amino acids are mobilized from muscle (mainly Alanine and Glutamine) to maintain blood glucose levels via gluconeogenesis.

Q: All of the following statements regarding amino acid metabolism are true EXCEPT:

Tetrahydrobiopterin is involved in tryptophan biosynthesis.. **Explanation:** The correct answer is **B** because **Tetrahydrobiopterin ($BH_4$)** is a cofactor involved in the **catabolism** (hydroxylation) of aromatic amino acids, not their biosynthesis. In humans, Tryptophan is an **essential amino acid**, meaning it cannot be synthesized de novo and must be obtained from the diet. $BH_4$ is specifically required by *Tryptophan hydroxylase* to convert Tryptophan into 5-Hydroxytryptophan (a precursor to Serotonin). **Analysis of other options:** * **Option A:** This refers to **Non-essential amino acids**. Humans can synthesize 10 out of the 20 standard amino acids. Therefore, their absence from the diet is not deleterious because the body can produce them endogenously. * **Option C:** **Selenocysteine** is known as the 21st amino acid. It is an essential component of enzymes like *Glutathione peroxidase*, *Thioredoxin reductase*, and *Deiodinase*. It is incorporated cotranslationally via a unique tRNA and the UGA stop codon. * **Option D:** This is a fundamental concept of metabolism. **Glutamate/Glutamine** are derived from $\alpha$-ketoglutarate (TCA cycle); **Aspartate/Asparagine** from Oxaloacetate (TCA cycle); and **Serine/Glycine** from 3-phosphoglycerate (Glycolysis). **High-Yield Clinical Pearls for NEET-PG:** * **$BH_4$ Deficiency:** Leads to **Hyperphenylalaninemia** and decreased levels of neurotransmitters (Dopamine, Serotonin), as $BH_4$ is a cofactor for Phenylalanine, Tyrosine, and Tryptophan hydroxylases. * **Essential Amino Acids:** Remember the mnemonic **PVT TIM HALL** (Phe, Val, Thr, Trp, Ile, Met, His, Arg, Leu, Lys). * **Selenocysteine:** It contains Selenium instead of the Sulfur found in Cysteine. Deficiency of Selenium is linked to **Keshan disease**.

Q: Urea synthesis is mediated by which enzyme?

Arginase. **Explanation:** The **Urea Cycle (Krebs-Henseleit cycle)** is the primary mechanism for detoxifying ammonia into urea in the liver. **Why Arginase is correct:** Arginase is the final enzyme of the urea cycle. It catalyzes the hydrolytic cleavage of **Arginine** into **Urea** and **Ornithine**. This reaction occurs exclusively in the cytosol of hepatocytes. While Ornithine is recycled back into the mitochondria to continue the cycle, Urea is released into the bloodstream and excreted by the kidneys. **Analysis of Incorrect Options:** * **Ornithine decarboxylase:** This is the rate-limiting enzyme in **polyamine synthesis** (forming putrescine), not the urea cycle. * **Aspartate transaminase (AST):** This enzyme facilitates the transfer of an amino group from glutamate to oxaloacetate to form **aspartate**. While aspartate provides the second nitrogen atom for urea synthesis, AST itself does not mediate the synthesis of urea. * **Citrulline synthase:** This is an alternative name sometimes used for *Ornithine Transcarbamoylase (OTC)*, which produces Citrulline. While it is a key step, it does not result in the formation of urea. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Carbamoyl Phosphate Synthetase I (CPS-I), which requires **N-acetylglutamate (NAG)** as an essential allosteric activator. * **Subcellular location:** The cycle is "split"—the first two steps occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. * **Nitrogen Sources:** One nitrogen comes from free ammonia, and the second comes from **Aspartate**. * **Arginase Deficiency:** Unlike other urea cycle defects, this typically presents with spastic diplegia and developmental delay rather than severe neonatal hyperammonemia.

Q: Which amino acid is used in the synthesis of carnitine?

Lysine. **Explanation:** **1. Why Lysine is the Correct Answer:** Carnitine (β-hydroxy-γ-trimethylammonium butyrate) is synthesized primarily in the liver and kidneys. The synthesis begins with the amino acid **Lysine**, which provides the carbon skeleton. Specifically, lysine residues in certain proteins are methylated using **S-adenosylmethionine (SAM)** to form **Trimethyllysine**. Following proteolysis, trimethyllysine undergoes a series of reactions (hydroxylation and cleavage) to eventually form carnitine. **Methionine** is also essential for this process as it acts as the methyl group donor. **2. Why the Other Options are Incorrect:** * **Alanine:** A non-essential amino acid primarily involved in gluconeogenesis (via the Glucose-Alanine cycle) and transamination reactions. It does not contribute to carnitine synthesis. * **Arginine:** A precursor for Nitric Oxide (NO), Urea, and Creatine. While it is involved in nitrogen metabolism, it is not a precursor for carnitine. * **Tyrosine:** A precursor for catecholamines (Dopamine, Epinephrine, Norepinephrine), Thyroid hormones (T3, T4), and Melanin. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Function of Carnitine:** It is essential for the **"Carnitine Shuttle,"** transporting long-chain fatty acids across the inner mitochondrial membrane for **β-oxidation**. * **Cofactors Required:** The synthesis of carnitine requires **Vitamin C (Ascorbic acid)**, Fe²⁺, Vitamin B6, and Niacin. Vitamin C deficiency (Scurvy) leads to decreased carnitine synthesis, contributing to the symptom of fatigue. * **Primary Deficiency:** Caused by defects in the carnitine transporter (OCTN2), leading to hypoketotic hypoglycemia and cardiomyopathy. * **Key Precursors:** Remember the duo **Lysine + Methionine**.

Q: Phenylalanine metabolism yields which of the following products, except?

Melatonin. **Explanation:** The core concept tested here is the differentiation between the **Phenylalanine-Tyrosine pathway** and the **Tryptophan pathway**. **1. Why Melatonin is the Correct Answer:** Melatonin is synthesized from the amino acid **Tryptophan**, not Phenylalanine. The pathway involves Tryptophan being converted to 5-Hydroxytryptophan, then to **Serotonin**, and finally to **Melatonin** (primarily in the pineal gland). Since it originates from a different precursor, it is the "except" in this list. **2. Analysis of Incorrect Options (Products of Phenylalanine):** Phenylalanine is first converted to **Tyrosine** by the enzyme *Phenylalanine Hydroxylase*. Tyrosine then serves as the precursor for: * **Dopamine & Epinephrine (Options A & B):** Tyrosine is converted to L-DOPA, which then yields the catecholamines: Dopamine → Norepinephrine → Epinephrine. * **Melanin (Option C):** In melanocytes, Tyrosine is acted upon by the enzyme *Tyrosinase* to produce Melanin pigments. * **Thyroxine:** Tyrosine is also the precursor for thyroid hormones (T3/T4). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **PKU (Phenylketonuria):** Deficiency of *Phenylalanine Hydroxylase* leads to accumulation of Phenylalanine and a deficiency of Tyrosine (making Tyrosine an "essential" amino acid in PKU patients). * **Albinism:** Results from a deficiency of *Tyrosinase*, preventing the conversion of Tyrosine to Melanin. * **Rate-limiting step:** *Tyrosine Hydroxylase* is the rate-limiting enzyme for catecholamine synthesis. * **Vitamin Link:** Both Phenylalanine and Tryptophan hydroxylases require **Tetrahydrobiopterin (BH4)** as a co-factor.

Q: The nitrogen atoms of urea are contributed by which two molecules?

Ammonium and aspartate. The Urea Cycle (Ornithine cycle) is the primary mechanism for detoxifying ammonia into urea in the liver. Understanding the origin of urea's components is a high-yield topic for NEET-PG. ### **Explanation of the Correct Answer** The urea molecule ($NH_2-CO-NH_2$) contains two nitrogen atoms: 1. **The first nitrogen** enters the cycle as **free ammonium ($NH_4^+$)**. It combines with $CO_2$ to form Carbamoyl Phosphate via the enzyme *Carbamoyl Phosphate Synthetase I (CPS-I)*. 2. **The second nitrogen** is donated by **Aspartate**. This occurs when Citrulline condenses with Aspartate to form Argininosuccinate, catalyzed by *Argininosuccinate synthetase*. ### **Analysis of Incorrect Options** * **B. Glutamate:** While glutamate is the primary source of the free ammonium (via oxidative deamination by *Glutamate Dehydrogenase*), it does not donate the nitrogen atom directly into the urea cycle intermediates. * **C. Glycine:** Glycine is involved in heme and creatine synthesis but does not provide nitrogen for the urea cycle. * **D. Asparagine:** Asparagine must first be converted to aspartate (releasing ammonia) before it can contribute to the cycle, but it is not the direct donor. ### **NEET-PG High-Yield Pearls** * **Rate-limiting step:** CPS-I (requires N-acetylglutamate as an obligate activator). * **Link to TCA Cycle:** The "Kreb’s Bicycle" connects the two cycles via **Fumarate**, which is released when Argininosuccinate is cleaved. * **Cellular Location:** The cycle is "split"—the first two steps occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. * **Carbon Source:** The single carbon atom in urea is derived from **Bicarbonate ($HCO_3^-$)** or $CO_2$.

Q: All of the following are tryptophan derivatives except?

Melanin. **Explanation:** The correct answer is **Melanin** because it is derived from the amino acid **Tyrosine**, not Tryptophan. In melanocytes, the enzyme tyrosinase converts tyrosine into DOPA and subsequently into melanin. **Analysis of Options:** * **Melanin (Correct):** Derived from Tyrosine. A deficiency in the enzyme tyrosinase leads to Albinism. * **Serotonin (Incorrect):** Tryptophan is hydroxylated by tryptophan hydroxylase (requiring BH4) and then decarboxylated to form Serotonin (5-hydroxytryptamine), a key neurotransmitter. * **Melatonin (Incorrect):** This hormone, which regulates the sleep-wake cycle in the pineal gland, is synthesized via the acetylation and methylation of Serotonin (thus making it a Tryptophan derivative). * **Niacin (Incorrect):** Approximately 60 mg of Tryptophan can be converted into 1 mg of Nicotinic acid (Vitamin B3) via the kynurenine pathway. This process requires Vitamin B6 (Pyridoxine) as a cofactor. **High-Yield Clinical Pearls for NEET-PG:** 1. **Hartnup Disease:** A defect in the transport of neutral amino acids (Tryptophan) in the gut and kidneys. It presents with pellagra-like symptoms because Tryptophan is unavailable for Niacin synthesis. 2. **Carcinoid Syndrome:** Tumor cells divert up to 60% of Tryptophan to produce Serotonin. This "tryptophan steal" leads to a secondary Niacin deficiency (Pellagra). 3. **Cofactor Alert:** Tryptophan metabolism requires **BH4** (for hydroxylation), **B6** (for the kynurenine pathway), and **B2/B3** for subsequent steps.

Q: What amino acid is produced by adding a hydroxyl group to the phenyl side chain of phenylalanine?

Tyrosine. **Explanation:** The correct answer is **Tyrosine**. **1. Why Tyrosine is Correct:** The conversion of Phenylalanine to Tyrosine is a classic hydroxylation reaction. The enzyme **Phenylalanine Hydroxylase (PAH)** adds a hydroxyl (-OH) group to the para-position of the phenyl ring of Phenylalanine. This reaction requires **Tetrahydrobiopterin (BH4)** as a co-factor and molecular oxygen. Tyrosine is considered a "non-essential" amino acid only because it can be synthesized from the essential amino acid Phenylalanine. **2. Why the Other Options are Incorrect:** * **Threonine:** An essential amino acid containing a hydroxyl group on its aliphatic side chain, not derived from a phenyl ring. * **Histidine:** An essential amino acid characterized by an imidazole ring, not a phenyl ring. * **Serine:** A non-essential amino acid with a hydroxymethyl group. It is primarily synthesized from 3-phosphoglycerate (an intermediate of glycolysis), not from Phenylalanine. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Phenylketonuria (PKU):** A deficiency in Phenylalanine Hydroxylase (Classic PKU) or BH4 leads to the accumulation of Phenylalanine and its metabolites (phenylpyruvate, phenyllactate), causing intellectual disability and a "mousy" body odor. * **Essentiality:** If Phenylalanine Hydroxylase is deficient, Tyrosine becomes a **conditionally essential** amino acid. * **Precursor Role:** Tyrosine is the metabolic precursor for several vital molecules: **Catecholamines** (Dopamine, Epinephrine, Norepinephrine), **Thyroid hormones** (T3, T4), and **Melanin**. * **Alkaptonuria:** A defect in Homogentisate oxidase (part of the Tyrosine degradation pathway) leads to dark urine and ochronosis.

Q: Massive aminoaciduria without a corresponding increase in plasma amino acid level is characteristic of which of the following diseases?

Hartnup disease. ### Explanation The key to this question lies in distinguishing between **Overflow Aminoaciduria** and **Renal Aminoaciduria**. **1. Why Hartnup Disease is Correct:** Hartnup disease is a **Renal Aminoaciduria**. It is caused by a defect in the **SLC6A19 transporter**, which is responsible for the reabsorption of neutral amino acids (especially Tryptophan) in the proximal renal tubules and the intestinal mucosa. Because the defect is in the transport mechanism of the kidney, amino acids are not reabsorbed and are excreted in massive amounts in the urine. Crucially, the **plasma levels remain normal or low** because the primary pathology is a "leak" at the kidney level, not an overproduction in the blood. **2. Why the Other Options are Incorrect:** * **Homocystinuria, Tyrosinemia, and Maple Syrup Urine Disease (MSUD):** These are all examples of **Overflow Aminoaciduria**. In these metabolic disorders, an enzyme deficiency leads to the accumulation of specific amino acids in the **plasma** (e.g., Methionine/Homocysteine, Tyrosine, or Branched-chain amino acids). Once the plasma concentration exceeds the renal threshold for reabsorption, the excess "overflows" into the urine. Therefore, in these conditions, high urinary levels are always accompanied by high plasma levels. **3. High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Hartnup disease mimics **Pellagra** (Dermatitis, Diarrhea, Dementia) because Tryptophan is a precursor for Niacin (Vitamin B3) synthesis. * **Diagnosis:** Look for the "Neutral Aminoaciduria" pattern in urine chromatography. * **Treatment:** High-protein diet and Nicotinamide supplementation. * **Other Renal Aminoacidurias:** Cystinuria (defect in COAL: Cystine, Ornithine, Arginine, Lysine) and Fanconi Syndrome (generalized proximal tubule dysfunction).

Question 1

Threonine is metabolized to:

Accepted Answer

Glycine

Answer

Lysine

Answer

Arginine

Answer

Ornithine

Question 2

Which of the following is NOT a purely glucogenic amino acid?

Accepted Answer

Tryptophan

Answer

Valine

Answer

Alanine

Answer

Methionine

Question 3

All of the following statements regarding amino acid metabolism are true EXCEPT:

Accepted Answer

Tetrahydrobiopterin is involved in tryptophan biosynthesis.

Answer

Absence from the diet of certain amino acids that are present in most proteins is not deleterious to human health.

Answer

Selenocysteine is an essential component of several mammalian proteins.

Answer

Intermediates of the citric acid cycle and of glycolysis act as precursors of aspartate, asparagine, glutamate, glutamine, glycine, and serine.

Question 4

Urea synthesis is mediated by which enzyme?

Accepted Answer

Arginase

Answer

Ornithine decarboxylase

Answer

Aspartate transaminase

Answer

Citrulline synthase

Question 5

Which amino acid is used in the synthesis of carnitine?

Accepted Answer

Lysine

Answer

Alanine

Answer

Arginine

Answer

Tyrosine

Question 6

Phenylalanine metabolism yields which of the following products, except?

Accepted Answer

Melatonin

Answer

Epinephrine

Answer

Dopamine

Answer

Melanin

Question 7

The nitrogen atoms of urea are contributed by which two molecules?

Accepted Answer

Ammonium and aspartate

Answer

Ammonium and glutamate

Answer

Ammonium and glycine

Answer

Ammonium and asparagine

Question 8

All of the following are tryptophan derivatives except?

Accepted Answer

Melanin

Answer

Serotonin

Answer

Melatonin

Answer

Niacin

Question 9

What amino acid is produced by adding a hydroxyl group to the phenyl side chain of phenylalanine?

Accepted Answer

Tyrosine

Answer

Threonine

Answer

Histidine

Answer

Serine

Question 10

Massive aminoaciduria without a corresponding increase in plasma amino acid level is characteristic of which of the following diseases?

Accepted Answer

Hartnup disease

Answer

Homocystinuria

Answer

Tyrosinemia

Answer

Maple syrup urine disease

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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