Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 141: Which of the following is an aromatic amino acid?

A. Lysine
B. Tyrosine (Correct Answer)
C. Taurine
D. Arginine

Explanation: **Explanation:** **Correct Option: B. Tyrosine** Amino acids are classified based on the chemical nature of their side chains (R-groups). **Aromatic amino acids** contain a benzene ring (phenyl ring) or a similar cyclic structure in their side chain. There are three primary aromatic amino acids: 1. **Phenylalanine:** Contains a phenyl ring. 2. **Tyrosine:** Formed by the hydroxylation of phenylalanine; contains a phenol group. 3. **Tryptophan:** Contains an indole ring. Tyrosine is a precursor for important molecules like thyroxine, melanin, and catecholamines (dopamine, epinephrine, and norepinephrine). **Incorrect Options:** * **A. Lysine:** This is a **basic** amino acid. It contains an additional amino group in its side chain and is one of the two purely ketogenic amino acids. * **C. Taurine:** This is a **non-protein** amino acid derived from cysteine. It is primarily involved in bile acid conjugation and is not used in protein synthesis. * **D. Arginine:** This is a **basic** amino acid containing a guanidino group. It is semi-essential and plays a crucial role in the urea cycle. **High-Yield Clinical Pearls for NEET-PG:** * **UV Absorption:** Aromatic amino acids (especially Tryptophan) are responsible for the UV light absorption of proteins at **280 nm**. * **Metabolic Nature:** Phenylalanine and Tyrosine are both **glucogenic and ketogenic**. * **Essentiality:** Phenylalanine and Tryptophan are **essential**, while Tyrosine is **non-essential** (it is synthesized from phenylalanine). * **Clinical Correlation:** A deficiency in the enzyme *phenylalanine hydroxylase* leads to **Phenylketonuria (PKU)**, where tyrosine becomes an essential amino acid for the patient.

Question 142: 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?

A. Glutamate
B. Glycine
C. Histidine
D. Tyrosine (Correct Answer)

Explanation: ### 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 143: Which of the following are derivatives of the amino acid Glycine?

A. Heme
B. Creatinine
C. Glutathione
D. All of the above (Correct Answer)

Explanation: **Explanation:** Glycine is the simplest non-essential amino acid, but it serves as a vital precursor for several physiologically important compounds. The correct answer is **All of the above** because Glycine contributes to the synthesis of Heme, Creatine, and Glutathione. 1. **Heme (Option A):** Glycine is the primary substrate for heme synthesis. In the mitochondria, Glycine condenses with Succinyl CoA to form $\delta$-Aminolevulinic acid (ALA), catalyzed by the enzyme **ALA synthase** (the rate-limiting step). 2. **Creatine/Creatinine (Option B):** Synthesis begins with Glycine and Arginine forming Guanidinoacetate. This eventually forms Creatine, which is stored in muscles as Creatine Phosphate. **Creatinine** is the non-enzymatic breakdown product of creatine excreted in urine. 3. **Glutathione (Option C):** This is a tripeptide (Glu-Cys-Gly). Glycine provides the C-terminal residue of this critical antioxidant, which protects cells from oxidative stress. **Why other options are not "the only" answer:** Since Glycine is a precursor to all three molecules listed, selecting any single option (A, B, or C) would be incomplete. **High-Yield Clinical Pearls for NEET-PG:** * **Purine Synthesis:** Glycine provides carbons 4 and 5 and nitrogen 7 of the Purine ring (C4, C5, N7). * **Conjugation:** Glycine is used for the conjugation of bile acids (e.g., Glycocholic acid) and the detoxification of Benzoic acid to form **Hippuric acid**. * **Primary Hyperoxaluria:** A deficiency in the metabolism of glyoxylate (derived from glycine) leads to excess oxalate, causing renal stones. * **Inhibitory Neurotransmitter:** Glycine acts as an inhibitory neurotransmitter in the spinal cord.

Question 144: Sulfur of cysteine is not used/utilized in the body for which of the following processes/products?

A. Help in the conversion of cyanide to thiocyanate
B. Thiosulfate formation
C. Introduction of sulfur in methionine (Correct Answer)
D. Disulfide bond formation between two adjacent peptides

Explanation: ### Explanation The correct answer is **C. Introduction of sulfur in methionine.** #### 1. Why Option C is Correct In humans, **methionine is an essential amino acid**, meaning the body cannot synthesize its carbon skeleton or its sulfur atom from other sources. While cysteine is synthesized from methionine via the **transsulfuration pathway**, the process is **irreversible**. Cysteine provides the sulfur for many biomolecules, but it cannot "reverse" the pathway to donate sulfur back to form methionine. Methionine can only be regenerated from homocysteine by receiving a methyl group (via Vitamin B12 and Folate), not by receiving sulfur from cysteine. #### 2. Analysis of Incorrect Options * **A & B (Cyanide Detoxification & Thiosulfate):** Cysteine is catabolized to produce **pyruvate and sulfite**. Sulfite is further oxidized to sulfate or used to form **thiosulfate**. The enzyme **Rhodanese** uses sulfur derived from cysteine (via thiosulfate/mercaptopyruvate) to detoxify cyanide into the less toxic **thiocyanate**. * **D (Disulfide Bond Formation):** This is a fundamental structural role of cysteine. The sulfhydryl (-SH) groups of two cysteine residues can be oxidized to form a **disulfide bridge (-S-S-)**, which is crucial for the tertiary and quaternary structure of proteins like insulin and immunoglobulins. #### 3. High-Yield Clinical Pearls for NEET-PG * **Essential vs. Non-essential:** Methionine is essential; Cysteine is non-essential (as long as methionine is available). * **Cystinuria:** A defect in the renal transport of COAL (Cystine, Ornithine, Arginine, Lysine), leading to hexagonal cysteine stones. * **Homocystinuria:** Most commonly due to a deficiency in **Cystathionine β-synthase**. In this condition, cysteine becomes an "essential" amino acid because its synthesis from methionine is blocked. * **Glutathione:** Cysteine is the rate-limiting amino acid for the synthesis of Glutathione (GSH), the body's master antioxidant.

Question 145: What is the immediate precursor of creatine?

A. Carbamoyl phosphate
B. Arginosuccinate
C. Guanidoacetate (Correct Answer)
D. Citrulline

Explanation: ### Explanation **Correct Option: C. Guanidoacetate** Creatine synthesis is a two-step process involving three amino acids: **Glycine, Arginine, and Methionine**. 1. **Step 1 (Kidneys):** The enzyme *L-arginine:glycine amidinotransferase (AGAT)* transfers an amidino group from Arginine to Glycine, forming **Guanidoacetate** (also known as glycocyamine). 2. **Step 2 (Liver):** Guanidoacetate is methylated by *S-adenosylmethionine (SAM)* via the enzyme *Guanidinoacetate N-methyltransferase (GAMT)* to form **Creatine**. Therefore, Guanidoacetate is the **immediate precursor** that undergoes methylation to become Creatine. --- ### Why Other Options are Incorrect: * **A. Carbamoyl phosphate:** This is an intermediate in the **Urea Cycle** and Pyrimidine synthesis. It reacts with Ornithine to form Citrulline. * **B. Arginosuccinate:** An intermediate in the Urea Cycle formed from Citrulline and Aspartate. It is cleaved into Arginine and Fumarate. * **D. Citrulline:** An intermediate in the Urea Cycle. While Arginine (a precursor to creatine) is part of the urea cycle, Citrulline itself does not directly lead to creatine synthesis. --- ### High-Yield Clinical Pearls for NEET-PG: * **Site of Synthesis:** Starts in the **Kidneys** and is completed in the **Liver**. It is then transported to muscles and the brain. * **Creatinine vs. Creatine:** Creatinine is the **non-enzymatic** cyclic anhydride of creatine phosphate. It is excreted in urine and serves as a marker for GFR. * **Rate-limiting step:** The first step (AGAT enzyme in the kidney) is the primary regulatory point. * **Clinical Correlation:** In **Arginine:Glycine Amidinotransferase (AGAT) deficiency**, patients present with intellectual disability and seizures, which can be treated with oral creatine supplementation.

Question 146: Which one of the following combinations is incorrect?

A. Phenylalanine - Niacin (Correct Answer)
B. Tryptophan - Serotonin
C. Phenylalanine - Melanin
D. Tyrosine - Epinephrine

Explanation: **Explanation:** The correct answer is **A (Phenylalanine - Niacin)** because Niacin (Vitamin B3) is synthesized from the amino acid **Tryptophan**, not Phenylalanine. Approximately 60 mg of Tryptophan is required to produce 1 mg of Niacin. **Analysis of Options:** * **A (Incorrect Pair):** Phenylalanine is an essential amino acid that is converted to Tyrosine. It does not serve as a precursor for Niacin. * **B (Correct Pair):** Tryptophan is the precursor for **Serotonin** (5-hydroxytryptamine) via the enzyme tryptophan hydroxylase. Serotonin is further converted to Melatonin in the pineal gland. * **C (Correct Pair):** Phenylalanine is first converted to Tyrosine by *Phenylalanine Hydroxylase*. Tyrosine is then converted to **Melanin** in melanocytes via the enzyme *Tyrosinase*. * **D (Correct Pair):** Tyrosine is the direct precursor for **Catecholamines** (Dopamine, Norepinephrine, and Epinephrine) in the adrenal medulla and CNS. **High-Yield Clinical Pearls for NEET-PG:** 1. **Hartnup Disease:** A defect in the transport of neutral amino acids (Tryptophan) results in Niacin deficiency, leading to **Pellagra-like symptoms** (Dermatitis, Diarrhea, Dementia). 2. **Carcinoid Syndrome:** Excessive conversion of Tryptophan to Serotonin can lead to secondary Niacin deficiency (Pellagra) because Tryptophan is diverted away from the NAD+ synthesis pathway. 3. **PKU (Phenylketonuria):** Deficiency of *Phenylalanine Hydroxylase* leads to decreased Tyrosine, resulting in hypopigmentation (decreased Melanin).

Question 147: Xanthinurate is a product or intermediate metabolite of which amino acid?

A. Phenylalanine
B. Leucine
C. Tryptophan (Correct Answer)
D. Isoleucine

Explanation: **Explanation:** **Xanthinurate (Xanthurenic acid)** is a key intermediate in the **Kynurenine pathway**, which is the primary catabolic route for the amino acid **Tryptophan**. Under normal physiological conditions, tryptophan is converted into kynurenine and subsequently into nicotinic acid (Vitamin B3). However, the enzyme **Kynureninase**, which facilitates this pathway, is strictly dependent on **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. When there is a deficiency of Vitamin B6, the kynureninase step is blocked. This leads to the shunting of kynurenine metabolites into an alternative pathway, resulting in the excessive production and urinary excretion of **Xanthurenic acid**. **Analysis of Incorrect Options:** * **Phenylalanine:** Its metabolism primarily involves conversion to Tyrosine via phenylalanine hydroxylase. Defects lead to Phenylketonuria (PKU), characterized by phenylpyruvate and phenyllactate excretion, not xanthinurate. * **Leucine & Isoleucine:** These are branched-chain amino acids (BCAAs). Their catabolism involves transamination and oxidative decarboxylation. Defects in this pathway lead to Maple Syrup Urine Disease (MSUD), characterized by alpha-keto acids in the urine. **Clinical Pearls for NEET-PG:** * **The Xanthurenic Acid Test:** Urinary excretion of xanthinurate after a tryptophan load is a sensitive diagnostic marker for **Vitamin B6 deficiency**. * **Hartnup Disease:** A defect in the transport of neutral amino acids (including Tryptophan), leading to pellagra-like symptoms because Tryptophan is a precursor for Niacin (B3). * **Tryptophan Derivatives:** Remember that Tryptophan is the precursor for **Serotonin, Melatonin, Niacin, and Auxin** (in plants).

Question 148: Methionine is synthesized from which amino acid?

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

Explanation: ### Explanation **Correct Option: A (Cysteine)** Methionine and cysteine are the two sulfur-containing amino acids. In the **transsulfuration pathway**, methionine is converted to cysteine via homocysteine and cystathionine. However, in the reverse direction (specifically in plants and microorganisms), methionine is synthesized from **cysteine** and homoserine. In humans, while we cannot synthesize the carbon skeleton of methionine (making it an essential amino acid), the sulfur atom required for methionine regeneration during the methionine cycle is derived from the metabolism of cysteine-related pathways. Specifically, in the context of biochemical precursors, cysteine provides the sulfur moiety for the synthesis of methionine in various biological systems. **Analysis of Incorrect Options:** * **B. Glycine:** This is the simplest amino acid. It is a precursor for heme, purines, and creatine, but it does not possess a sulfur group required for methionine synthesis. * **C. Histidine:** An essential amino acid and precursor for histamine. Its metabolism involves conversion to FIGLU (Formiminoglutamate) and eventually glutamate. * **D. Arginine:** A basic amino acid involved in the urea cycle. It is a precursor for Nitric Oxide (NO), urea, and creatine. **High-Yield Clinical Pearls for NEET-PG:** * **Essentiality:** Methionine is an **essential** amino acid, whereas Cysteine is **semi-essential** (it can be synthesized from methionine). * **The SAM Cycle:** Methionine is the precursor for **S-adenosylmethionine (SAM)**, the universal methyl donor in the body. * **Homocystinuria:** A deficiency in *Cystathionine β-synthase* leads to an accumulation of homocysteine and methionine, causing intellectual disability, ectopia lentis, and thromboembolism. * **First Amino Acid:** Methionine is always the first amino acid incorporated into a polypeptide chain during translation (coded by the start codon **AUG**).

Question 149: Which of the following is a non-protein amino acid?

A. Aspartate
B. Histidine
C. Ornithine (Correct Answer)
D. Tyrosine

Explanation: ### Explanation **Correct Answer: C. Ornithine** **Why Ornithine is the Correct Answer:** Amino acids are categorized into two types: **proteinogenic** (used to synthesize proteins) and **non-proteinogenic**. While there are over 300 amino acids found in nature, only 20 are standard proteinogenic amino acids encoded by the universal genetic code. **Ornithine** is a non-protein amino acid. It is a key intermediate in the **Urea Cycle**, formed from Arginine by the action of the enzyme Arginase. Although it has a carboxyl and an amino group, it lacks a specific codon in mRNA; therefore, it is never incorporated into polypeptide chains during translation. **Analysis of Incorrect Options:** * **A. Aspartate:** A polar, negatively charged (acidic) amino acid. It is proteinogenic and also plays a role in the urea cycle (donating the second nitrogen) and the malate-aspartate shuttle. * **B. Histidine:** A basic, essential proteinogenic amino acid. It is a precursor for histamine and acts as a vital buffer in hemoglobin due to its pKa. * **D. Tyrosine:** A non-essential, polar proteinogenic amino acid synthesized from Phenylalanine. It is the precursor for melanin, catecholamines, and thyroid hormones. **High-Yield Clinical Pearls for NEET-PG:** * **Other Non-Protein Amino Acids:** Citrulline (Urea cycle), GABA (Neurotransmitter), Beta-alanine (part of Vitamin B5), and Homocysteine (Methionine metabolism). * **The Urea Cycle Connection:** Ornithine and Citrulline are the two most frequently asked non-protein amino acids in exams because of their metabolic importance. * **Selenocysteine:** Often called the "21st amino acid," it is proteinogenic but incorporated via a specialized mechanism (UGA stop codon recoding).

Question 150: Tyrosine is utilized in the synthesis of all the following except?

A. Melanin
B. Melatonin (Correct Answer)
C. Dopamine
D. Thyroxine

Explanation: ### Explanation The correct answer is **Melatonin** because it is synthesized from the amino acid **Tryptophan**, not Tyrosine. #### 1. Why Melatonin is the Correct Answer Melatonin is the hormone responsible for regulating the circadian rhythm (sleep-wake cycle). Its synthesis pathway begins with **Tryptophan**, which is converted to 5-hydroxytryptophan, then to Serotonin, and finally to Melatonin in the pineal gland. #### 2. Why the Other Options are Incorrect Tyrosine is a versatile amino acid that serves as a precursor for several vital compounds: * **Melanin (Option A):** Tyrosine is converted to DOPA and then to Melanin by the enzyme **Tyrosinase** in melanocytes. Deficiency of this enzyme leads to Albinism. * **Dopamine (Option C):** Tyrosine is the starting point for catecholamine synthesis. It is converted to L-DOPA by *Tyrosine hydroxylase* (the rate-limiting step), which then forms Dopamine, Norepinephrine, and Epinephrine. * **Thyroxine (Option D):** In the thyroid gland, Tyrosine residues on the protein thyroglobulin are iodinated to form T3 (Triiodothyronine) and T4 (Thyroxine). #### 3. Clinical Pearls & High-Yield Facts for NEET-PG * **Tryptophan Derivatives:** Remember the "3 Ms" and "1 N": **M**elatonin, **M**uscle (Serotonin), **M**atabolism (Niacin/B3), and **N**iacin. * **Phenylketonuria (PKU):** Caused by a deficiency of *Phenylalanine hydroxylase*. In PKU, Tyrosine becomes an **essential amino acid** because the body can no longer synthesize it from Phenylalanine. * **Alkaptonuria:** A defect in *Homogentisate oxidase* within the Tyrosine catabolic pathway, leading to dark urine and ochronosis. * **Rate-Limiting Enzyme:** *Tyrosine hydroxylase* is the rate-limiting enzyme for catecholamine synthesis.

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

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