Amino Acid Metabolism Practice Questions

Q: Which amino acid undergoes hepatic deamination?

Glutamate. **Explanation:** The correct answer is **Glutamate**. In amino acid metabolism, the primary mechanism for removing nitrogen is a two-step process: **Transamination** followed by **Oxidative Deamination**. 1. **Why Glutamate is Correct:** Most amino acids transfer their $\alpha$-amino group to $\alpha$-ketoglutarate via transamination, forming **Glutamate**. Glutamate then acts as a "collection center" for amino groups. In the liver mitochondria, Glutamate undergoes rapid **oxidative deamination** catalyzed by the enzyme **Glutamate Dehydrogenase (GDH)**. This reaction releases free ammonia ($NH_3$), which enters the Urea Cycle for detoxification. Glutamate is the only amino acid that undergoes oxidative deamination at a significant rate in mammalian tissues. 2. **Why Other Options are Incorrect:** * **Alanine:** It is the primary carrier of nitrogen from muscles to the liver (Glucose-Alanine cycle). However, it must first be transaminated to Glutamate before the nitrogen can be released. * **Aspartate:** It participates in the Urea Cycle by providing the second nitrogen atom (reacting with Citrulline), but it does not undergo direct deamination to release free ammonia. * **Glycine:** While glycine can be broken down by the Glycine Cleavage System, it is not the primary substrate for hepatic deamination in the context of the general nitrogen pool. **High-Yield NEET-PG Pearls:** * **Glutamate Dehydrogenase (GDH):** It is unique because it can use either $NAD^+$ or $NADP^+$ as a coenzyme. * **Allosteric Regulation:** GDH is inhibited by **ATP/GTP** (high energy) and activated by **ADP/GDP** (low energy). * **Transdeamination:** The combined action of Aminotransferases and GDH is termed "Transdeamination," which is the major pathway for the catabolism of amino acids.

Q: What is the chemical process involved in the conversion of noradrenaline to adrenaline?

Methylation. **Explanation:** The conversion of **Noradrenaline to Adrenaline** is the final step in the catecholamine biosynthetic pathway. This reaction is a **Methylation** process catalyzed by the enzyme **Phenylethanolamine N-methyltransferase (PNMT)**. 1. **Why Methylation is Correct:** In this reaction, a methyl group (–CH₃) is transferred from **S-adenosylmethionine (SAM)**, which acts as the universal methyl donor, to the nitrogen atom of noradrenaline. This N-methylation increases the lipophilicity of the molecule, allowing it to act on different adrenergic receptors (primarily β-receptors). 2. **Why Other Options are Incorrect:** * **Hydroxylation:** This occurs earlier in the pathway (e.g., Phenylalanine to Tyrosine by Phenylalanine hydroxylase, or Tyrosine to DOPA by Tyrosine hydroxylase). * **Carboxylation:** This involves adding CO₂. In catecholamine synthesis, the opposite occurs: **Decarboxylation** (DOPA to Dopamine). * **Dehydrogenation:** This involves the removal of hydrogen (redox reaction), which is not the mechanism for this specific conversion. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Tyrosine hydroxylase is the rate-limiting enzyme for catecholamine synthesis. * **Glucocorticoid Influence:** PNMT is induced by **cortisol**. This is why the adrenal medulla (where noradrenaline is converted to adrenaline) is anatomically surrounded by the adrenal cortex. * **Location:** While noradrenaline is the primary neurotransmitter in sympathetic postganglionic neurons, adrenaline is primarily produced in the **adrenal medulla**. * **Cofactor:** Always remember **SAM** is the donor for this methylation; Vitamin B12 and Folate are indirectly required to regenerate SAM.

Q: In alkaptonuria, what substance is found in the urine?

Homogentisic acid. **Explanation:** **Alkaptonuria** is an autosomal recessive metabolic disorder caused by a deficiency of the enzyme **homogentisate 1,2-dioxygenase**. This enzyme is essential in the catabolic pathway of the aromatic amino acids **phenylalanine and tyrosine**. 1. **Why Homogentisic Acid is Correct:** Due to the enzyme deficiency, the body cannot break down homogentisic acid (HGA). Consequently, HGA accumulates in the blood and is excreted in large quantities in the **urine**. Upon standing or exposure to air, HGA undergoes oxidation and polymerization to form a melanin-like pigment, causing the urine to turn **dark or black**. 2. **Why Incorrect Options are Wrong:** * **Phenylalanine:** Elevated in Phenylketonuria (PKU) due to phenylalanine hydroxylase deficiency, not alkaptonuria. * **Ketones:** Found in the urine (ketonuria) during diabetic ketoacidosis, starvation, or prolonged fasting. * **Acetates:** These are common metabolic intermediates but are not specific diagnostic markers for amino acid metabolism disorders like alkaptonuria. **High-Yield Clinical Pearls for NEET-PG:** * **Ochronosis:** The deposition of black pigment in connective tissues, cartilage (blue-black ears), and sclera. * **Arthritis:** Chronic accumulation of HGA in joints leads to severe, early-onset large-joint arthritis and intervertebral disc calcification. * **Diagnostic Test:** The addition of an alkali (like KOH) to the urine sample will rapidly darken it. It also gives a **false-positive** result with Benedict’s test (reducing property). * **Treatment:** Low protein diet (restricting Phenylalanine/Tyrosine) and **Nitisinone**, which inhibits the enzyme 4-hydroxyphenylpyruvate dioxygenase to reduce HGA production.

Q: Ammonia is toxic to the brain because it leads to depletion of which substrate?

Alpha ketoglutarate. **Explanation:** Ammonia toxicity primarily affects the brain through the depletion of **Alpha-ketoglutarate (α-KG)**, a vital intermediate of the Citric Acid (TCA) cycle. **Why Alpha-ketoglutarate is the correct answer:** When blood ammonia levels rise (hyperammonemia), the brain attempts to detoxify it via two main reactions: 1. **Reductive Amination:** Ammonia reacts with α-KG to form Glutamate, catalyzed by *Glutamate Dehydrogenase*. 2. **Glutamine Synthesis:** Glutamate further reacts with another molecule of ammonia to form Glutamine, catalyzed by *Glutamine Synthetase*. The excessive consumption of α-KG for these reactions shunts it away from the TCA cycle. This depletion inhibits aerobic respiration, leading to a significant decrease in ATP production in neurons, which eventually causes encephalopathy and cerebral edema. **Why other options are incorrect:** * **A, C, and D (Succinate, Isocitrate, Fumarate):** While these are all intermediates of the TCA cycle, they are not the primary substrates consumed to neutralize ammonia. Their levels may decrease secondary to the overall slowing of the TCA cycle, but the initial and direct "pull" occurs at the Alpha-ketoglutarate step. **High-Yield Clinical Pearls for NEET-PG:** * **Glutamine Osmosis:** Accumulation of Glutamine in astrocytes increases osmotic pressure, leading to brain swelling (cerebral edema). * **Neurotransmitter Imbalance:** Depletion of Glutamate (an excitatory neurotransmitter) and accumulation of GABA (inhibitory) further contribute to the CNS depression seen in hepatic encephalopathy. * **Treatment Tip:** Lactulose and Rifaximin are used to reduce intestinal ammonia production/absorption.

Q: Thyroid hormones belong to which class of hormone?

Amino acid derivatives. ### Explanation **Correct Answer: D. Amino acid derivatives** Thyroid hormones (T3 and T4) are synthesized from the amino acid **Tyrosine**. Specifically, they are formed by the iodination of tyrosine residues within the thyroglobulin molecule. Despite being amino acid derivatives, they are unique because they are **lipophilic** (lipid-soluble), allowing them to cross cell membranes and bind to **nuclear receptors**, similar to steroid hormones. **Analysis of Incorrect Options:** * **A. Steroids:** These are derived from **cholesterol**. Examples include cortisol, aldosterone, estrogen, and testosterone. While thyroid hormones share a similar mechanism of action (nuclear binding), their chemical precursor is an amino acid, not a lipid. * **B & C. Proteins and Polypeptides:** These consist of chains of amino acids linked by peptide bonds. Examples include Insulin, PTH, and Anterior Pituitary hormones. Unlike thyroid hormones, these are water-soluble, circulate freely in the blood, and bind to **cell surface receptors**. **High-Yield Clinical Pearls for NEET-PG:** * **The Tyrosine Connection:** Tyrosine is the precursor for three major groups: Thyroid hormones, Catecholamines (Epinephrine, Norepinephrine, Dopamine), and Melanin. * **Storage Exception:** Thyroid hormone is the only endocrine hormone stored extracellularly (within the **colloid** of the thyroid follicle) in large quantities—enough to last for 2–3 months. * **Transport:** Because they are lipophilic, >99% of T3/T4 circulate bound to plasma proteins, primarily **Thyroxine-binding globulin (TBG)**. * **Potency:** T3 is more potent and has a faster onset of action, while T4 is produced in higher quantities and has a longer half-life.

Q: Which of the following amino acids in a protein is involved in the urea cycle and is required for the synthesis of NO?

Arginine. **Explanation:** **1. Why Arginine is Correct:** Arginine is a semi-essential amino acid that plays a dual role in metabolism as highlighted in the question: * **Urea Cycle:** Arginine is a key intermediate in the urea cycle. It is cleaved by the enzyme **Arginase** to produce **Urea** and regenerate **Ornithine**. * **Nitric Oxide (NO) Synthesis:** Arginine is the sole precursor for Nitric Oxide. The enzyme **Nitric Oxide Synthase (NOS)** converts L-Arginine into **L-Citrulline** and NO in a reaction requiring NADPH and Oxygen. NO acts as a potent vasodilator and neurotransmitter. **2. Why Other Options are Incorrect:** * **Histidine:** It is a precursor for **Histamine** (via decarboxylation). While it is basic like arginine, it does not participate in the urea cycle or NO synthesis. * **Tryptophan:** This is an aromatic amino acid used to synthesize **Serotonin, Melatonin, and Niacin (Vitamin B3)**. * **Lysine:** Although it is a basic amino acid, it is purely ketogenic and does not participate in the urea cycle intermediates. **3. NEET-PG High-Yield Pearls:** * **Glucogenic nature:** Arginine is a purely glucogenic amino acid as it enters the TCA cycle via $\alpha$-ketoglutarate. * **Creatine Synthesis:** Arginine, along with Glycine and Methionine (SAM), is required for the synthesis of Creatine. * **Potency:** Arginine is the most basic amino acid due to the presence of the **Guanidinium group**. * **Clinical Link:** In cases of hyperammonemia (Urea cycle disorders), Arginine supplementation is often required because it becomes an "essential" amino acid when the cycle is compromised.

Q: Which amino acid is both glucogenic and ketogenic?

Tryptophan. ### Explanation Amino acids are classified based on their metabolic end-products into three categories: **Glucogenic** (converted to glucose), **Ketogenic** (converted to ketone bodies/Acetyl-CoA), or **Both**. **1. Why Tryptophan is Correct:** Tryptophan is one of the four amino acids that are both glucogenic and ketogenic (the others being **Isoleucine, Phenylalanine, and Tyrosine**). * **Ketogenic component:** Its breakdown produces **Acetyl-CoA** and **Acetoacetyl-CoA**. * **Glucogenic component:** Its breakdown also yields **Alanine**, which can be converted into Pyruvate for gluconeogenesis. **2. Analysis of Incorrect Options:** * **Option A: Leucine:** This is a **purely ketogenic** amino acid. Along with **Lysine**, it is one of only two amino acids that cannot be converted into glucose. (Mnemonic: The "L"s are purely ketogenic). * **Option B: Alanine:** This is a **purely glucogenic** amino acid. It is the primary substrate for gluconeogenesis in the liver via the Glucose-Alanine cycle. * **Option C: Serine:** This is a **purely glucogenic** amino acid. It is easily converted into Pyruvate by the enzyme Serine dehydratase. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Both (Glucogenic & Ketogenic):** "**PITTT**" – **P**henylalanine, **I**soleucine, **T**yrosine, **T**ryptophan, and **T**hreonine (Note: Threonine's status is debated but often included in this group). * **Tryptophan Derivatives:** It is the precursor for **Serotonin, Melatonin, and Niacin (Vitamin B3)**. * **Hartnup Disease:** A clinical condition caused by the deficiency of a transporter for neutral amino acids (primarily Tryptophan), leading to pellagra-like symptoms.

Q: Which of the following is the precursor for adrenaline and thyroxine synthesis?

Tyrosine. **Explanation:** **Tyrosine** is the correct answer because it serves as the primary metabolic precursor for several vital biological molecules, including catecholamines (Dopamine, Norepinephrine, and **Adrenaline**), thyroid hormones (**Thyroxine/T4** and Triiodothyronine/T3), and the pigment Melanin. 1. **Adrenaline Synthesis:** Tyrosine is converted to L-DOPA by *tyrosine hydroxylase* (the rate-limiting step), which eventually leads to the production of adrenaline in the adrenal medulla. 2. **Thyroxine Synthesis:** In the thyroid gland, tyrosine residues on the protein thyroglobulin undergo iodination and coupling to form T3 and T4. **Analysis of Incorrect Options:** * **Phenylalanine (Option A):** While Phenylalanine is the precursor to Tyrosine (via *phenylalanine hydroxylase*), it must first be converted into Tyrosine before it can enter the pathways for adrenaline or thyroxine synthesis. Tyrosine is the immediate precursor. * **Tryptophan (Option C):** This amino acid is the precursor for **Serotonin**, **Melatonin**, and **Niacin** (Vitamin B3). It does not contribute to catecholamine or thyroid hormone synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** *Tyrosine hydroxylase* is the rate-limiting enzyme for catecholamine synthesis. * **Albinism:** Caused by a deficiency in the enzyme *Tyrosinase*, which converts tyrosine to melanin. * **PKU (Phenylketonuria):** Deficiency of *phenylalanine hydroxylase* leads to low tyrosine levels; in these patients, tyrosine becomes an **essential amino acid**. * **Rule of "T":** Remember **T**yrosine for **T**hyroxine, **T**hree catecholamines (Dopamine, Epi, Norepi), and **T**anning (Melanin).

Q: With the help of pyridoxine, serotonin is synthesized from which of the following precursor amino acids?

Tryptophan. **Explanation:** **Tryptophan** is the correct precursor for the synthesis of **Serotonin** (5-hydroxytryptamine). The metabolic pathway occurs in two primary steps: 1. **Hydroxylation:** Tryptophan is converted to 5-hydroxytryptophan by the enzyme *Tryptophan hydroxylase*, which requires Tetrahydrobiopterin ($BH_4$) as a cofactor. 2. **Decarboxylation:** 5-hydroxytryptophan is then converted to Serotonin by *Aromatic L-amino acid decarboxylase*. This enzyme requires **Pyridoxal Phosphate (PLP)**, the active form of **Vitamin $B_6$ (Pyridoxine)**, as a cofactor. **Why other options are incorrect:** * **Tyrosine:** Is the precursor for Catecholamines (Dopamine, Norepinephrine, and Epinephrine), Melanin, and Thyroid hormones ($T_3, T_4$). * **Phenylalanine:** Is an essential amino acid that is converted into Tyrosine by *Phenylalanine hydroxylase*. * **Glycine:** Is used in the synthesis of Heme, Purines, Creatine, and Glutathione, but not serotonin. **High-Yield Clinical Pearls for NEET-PG:** * **Melatonin Connection:** Serotonin is further converted into Melatonin in the Pineal gland (via acetylation and methylation). * **Carcinoid Syndrome:** In patients with carcinoid tumors, up to 60% of dietary tryptophan is diverted to serotonin synthesis, potentially leading to **Pellagra-like symptoms** (due to secondary Niacin deficiency). * **Hartnup Disease:** A defect in the transport of neutral amino acids (like Tryptophan) in the gut and kidneys, also presenting with Pellagra-like symptoms. * **Cofactor Rule:** Almost all decarboxylation reactions in amino acid metabolism require Vitamin $B_6$ (PLP).

Q: A 60-year-old man complains of difficulty arising from a chair and initiating new movements. On examination, a resting hand tremor and cogwheel rigidity are noted. Which of the following amino acids is the precursor for the neurotransmitter that is deficient in the brain of this patient?

Tyrosine. ### Explanation **Clinical Diagnosis: Parkinson’s Disease** The patient presents with the classic triad of **Parkinson’s Disease**: bradykinesia (difficulty initiating movement), resting tremor ("pill-rolling"), and cogwheel rigidity. This condition is caused by the degeneration of dopaminergic neurons in the **substantia nigra pars compacta**, leading to a deficiency of the neurotransmitter **Dopamine**. **Why Tyrosine is Correct:** Dopamine is a catecholamine synthesized from the amino acid **Tyrosine**. The metabolic pathway is as follows: 1. **Tyrosine** → **L-DOPA** (via *Tyrosine Hydroxylase*, the rate-limiting step). 2. **L-DOPA** → **Dopamine** (via *DOPA decarboxylase*). 3. (In other neurons) Dopamine → Norepinephrine → Epinephrine. Since Dopamine is deficient in Parkinson’s, Tyrosine is the essential precursor. **Why Other Options are Incorrect:** * **A. Glutamate:** An excitatory neurotransmitter itself; it is also the precursor for **GABA** (an inhibitory neurotransmitter) via glutamate decarboxylase. * **B. Glycine:** An inhibitory neurotransmitter primarily in the spinal cord; it also contributes to heme and purine synthesis. * **C. Histidine:** The precursor for **Histamine**, involved in allergic reactions and gastric acid secretion. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Tyrosine Hydroxylase requires **Tetrahydrobiopterin (BH4)** as a cofactor. * **Phenylalanine Connection:** Tyrosine itself is derived from Phenylalanine. Therefore, Phenylalanine is the ultimate precursor of catecholamines. * **Treatment Link:** We administer **L-DOPA** (Levodopa) rather than Dopamine because L-DOPA can cross the blood-brain barrier, whereas Dopamine cannot. * **MPTP:** A neurotoxin that can cause permanent Parkinsonian symptoms by destroying dopaminergic neurons.

Question 1

Which amino acid undergoes hepatic deamination?

Accepted Answer

Glutamate

Answer

Alanine

Answer

Aspartate

Answer

Glycine

Question 2

What is the chemical process involved in the conversion of noradrenaline to adrenaline?

Accepted Answer

Methylation

Answer

Hydroxylation

Answer

Carboxylation

Answer

Dehydrogenation

Question 3

In alkaptonuria, what substance is found in the urine?

Accepted Answer

Homogentisic acid

Answer

Phenylalanine

Answer

Ketones

Answer

Acetates

Question 4

Ammonia is toxic to the brain because it leads to depletion of which substrate?

Accepted Answer

Alpha ketoglutarate

Answer

Succinate

Answer

Isocitrate

Answer

Fumarate

Question 5

Thyroid hormones belong to which class of hormone?

Accepted Answer

Amino acid derivatives

Answer

Steroids

Answer

Proteins

Answer

Polypeptides

Question 6

Which of the following amino acids in a protein is involved in the urea cycle and is required for the synthesis of NO?

Accepted Answer

Arginine

Answer

Histidine

Answer

Tryptophan

Answer

Lysine

Question 7

Which amino acid is both glucogenic and ketogenic?

Accepted Answer

Tryptophan

Answer

Leucine

Answer

Alanine

Answer

Serine

Question 8

Which of the following is the precursor for adrenaline and thyroxine synthesis?

Accepted Answer

Tyrosine

Answer

Phenylalanine

Answer

Tryptophan

Answer

None of the above

Question 9

With the help of pyridoxine, serotonin is synthesized from which of the following precursor amino acids?

Accepted Answer

Tryptophan

Answer

Tyrosine

Answer

Phenylalanine

Answer

Glycine

Question 10

A 60-year-old man complains of difficulty arising from a chair and initiating new movements. On examination, a resting hand tremor and cogwheel rigidity are noted. Which of the following amino acids is the precursor for the neurotransmitter that is deficient in the brain of this patient?

Accepted Answer

Tyrosine

Answer

Glutamate

Answer

Glycine

Answer

Histidine

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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