Amino Acid Metabolism Practice Questions

Q: Which of the following conditions is caused by defective transport of tryptophan?

Hartnup disease. **Explanation:** **Hartnup disease** is the correct answer because it is an autosomal recessive disorder characterized by a defect in the **SLC6A19 transporter**. This protein is responsible for the sodium-dependent transport of neutral amino acids (especially **Tryptophan**) in the proximal renal tubules and the intestinal mucosa. Since Tryptophan is a precursor for **Niacin (Vitamin B3)**, its malabsorption leads to a niacin deficiency, manifesting as **pellagra-like symptoms** (Dermatitis, Diarrhea, Dementia). **Analysis of Incorrect Options:** * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD)** complex, leading to the accumulation of Leucine, Isoleucine, and Valine. * **Alkaptonuria:** A defect in the enzyme **Homogentisate oxidase** in the tyrosine catabolic pathway, resulting in the accumulation of homogentisic acid (causing dark urine and ochronosis). * **Phenylketonuria (PKU):** Caused by a deficiency of **Phenylalanine hydroxylase (PAH)** or its cofactor tetrahydrobiopterin (BH4), leading to elevated Phenylalanine levels. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Hallmark:** The presence of **neutral aminoaciduria** (detected via chromatography) while levels of acidic and basic amino acids remain normal. * **Clinical Presentation:** Photosensitive skin rash, cerebellar ataxia, and emotional lability. * **Management:** High-protein diet and **Nicotinamide** supplementation to bypass the need for endogenous synthesis from Tryptophan. * **Obermayer Test:** May be positive due to increased urinary indican (a byproduct of bacterial degradation of unabsorbed tryptophan in the gut).

Q: Glycine participates in the biosynthesis of all the following, EXCEPT:

Pyrimidine. ### Explanation The correct answer is **C. Pyrimidine**. Glycine is a non-essential amino acid that serves as a vital precursor for several specialized nitrogenous compounds. However, it does **not** contribute to the synthesis of the pyrimidine ring. **1. Why Pyrimidine is the correct answer:** The pyrimidine ring (found in Cytosine, Thymine, and Uracil) is synthesized from **Aspartate**, **Glutamine** (amide nitrogen), and **CO₂**. Unlike the purine ring, where Glycine provides carbons 4 and 5 and nitrogen 7, Glycine plays no role in the pyrimidine biosynthetic pathway. **2. Why the other options are incorrect:** * **Heme (Option A):** Glycine is a fundamental building block of heme. In the first and rate-limiting step of heme synthesis, Glycine condenses with Succinyl CoA to form **δ-aminolevulinic acid (ALA)**, catalyzed by the enzyme ALA synthase (requires Vitamin B6). * **Creatine (Option B):** Glycine, along with **Arginine** and **S-adenosylmethionine (SAM)**, is essential for creatine synthesis. Glycine and Arginine first form guanidinoacetate in the kidney, which is later methylated in the liver to form creatine. **3. High-Yield Clinical Pearls for NEET-PG:** * **Purine Synthesis:** Remember that Glycine provides **C4, C5, and N7** of the purine ring (Adenine and Guanine). * **Glutathione:** Glycine is one of the three amino acids (along with Glutamate and Cysteine) that form the antioxidant tripeptide Glutathione. * **Bile Salts:** Glycine is used for the conjugation of bile acids (e.g., Glycocholic acid). * **Inhibitory Neurotransmitter:** Glycine acts as a major inhibitory neurotransmitter in the spinal cord and brainstem.

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 $CO_2$.

Question 1

Which of the following conditions is caused by defective transport of tryptophan?

Accepted Answer

Hartnup disease

Answer

Maple syrup urine disease

Answer

Alkaptonuria

Answer

Phenylketonuria

Question 2

A 30-year-old female presents with bluish-black discoloration of the sclera and pinna for the last 4 months. Her urine turns black on standing. What is the most likely diagnosis?

Accepted Answer

Alkaptonuria

Answer

Phenylketonuria

Answer

Tyrosinemia

Answer

Maple syrup urine disease

Question 3

Glycine participates in the biosynthesis of all the following, EXCEPT:

Accepted Answer

Pyrimidine

Answer

Heme

Answer

Creatine

Answer

None of the above

Question 4

Threonine is metabolized to:

Accepted Answer

Glycine

Answer

Lysine

Answer

Arginine

Answer

Ornithine

Question 5

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

Accepted Answer

Tryptophan

Answer

Valine

Answer

Alanine

Answer

Methionine

Question 6

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 7

Urea synthesis is mediated by which enzyme?

Accepted Answer

Arginase

Answer

Ornithine decarboxylase

Answer

Aspartate transaminase

Answer

Citrulline synthase

Question 8

Which amino acid is used in the synthesis of carnitine?

Accepted Answer

Lysine

Answer

Alanine

Answer

Arginine

Answer

Tyrosine

Question 9

Phenylalanine metabolism yields which of the following products, except?

Accepted Answer

Melatonin

Answer

Epinephrine

Answer

Dopamine

Answer

Melanin

Question 10

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

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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