Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 131: What is the precursor for melanin synthesis?

A. Tyrosine (Correct Answer)
B. Glycine
C. Phenylalanine
D. Lysine

Explanation: **Explanation:** **1. Why Tyrosine is Correct:** Melanin is the primary pigment responsible for the color of skin, hair, and eyes. It is synthesized within specialized cells called **melanocytes**. The biochemical pathway begins with the amino acid **Tyrosine**. The rate-limiting enzyme in this pathway is **Tyrosinase** (a copper-containing enzyme), which converts Tyrosine into DOPA (3,4-dihydroxyphenylalanine) and subsequently into Dopaquinone, eventually forming Eumelanin or Pheomelanin. **2. Why the Other Options are Incorrect:** * **Phenylalanine:** While Phenylalanine is the precursor to Tyrosine (via Phenylalanine Hydroxylase), it is not the *immediate* precursor for melanin. It must first be converted to Tyrosine in the liver. * **Glycine:** This is the simplest amino acid and serves as a precursor for Heme, Creatine, Purines, and Glutathione, but not melanin. * **Lysine:** This is a purely ketogenic essential amino acid involved in protein synthesis and carnitine production; it has no role in melanogenesis. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Albinism:** A clinical condition caused by a congenital deficiency of the enzyme **Tyrosinase**, leading to a total or partial absence of melanin. * **Phenylketonuria (PKU):** Patients with PKU often have fair skin and blonde hair because high levels of Phenylalanine inhibit Tyrosinase, leading to decreased melanin production. * **Catecholamines:** Tyrosine is also the precursor for Dopamine, Norepinephrine, and Epinephrine. * **Copper Deficiency:** Since Tyrosinase is copper-dependent, severe copper deficiency (e.g., Menkes disease) can lead to hypopigmentation of hair.

Question 132: Which amino acid binds NH4+ covalently and makes it non-toxic for transportation?

A. Histidine
B. Serine
C. Glutamate (Correct Answer)
D. Aspartate

Explanation: **Explanation:** **1. Why Glutamate is Correct:** Ammonia ($NH_4^+$) is highly toxic, especially to the central nervous system. To transport it safely from peripheral tissues to the liver for the urea cycle, it must be converted into a non-toxic form. **Glutamate** plays a central role in this process. In most tissues, the enzyme **Glutamine Synthetase** catalyzes the covalent binding of free ammonia to Glutamate, forming **Glutamine**. Glutamine is a neutral, non-toxic amino acid that serves as the major carrier of ammonia in the blood. In the liver, Glutaminase releases the ammonia for urea synthesis. **2. Why Incorrect Options are Wrong:** * **Histidine:** This is an essential amino acid involved in histamine synthesis and buffering (via hemoglobin), but it does not function as a primary ammonia transporter. * **Serine:** While it can be deaminated to produce ammonia, it does not act as a "sink" or carrier for systemic ammonia transport. * **Aspartate:** Although Aspartate participates in the Urea Cycle (providing the second nitrogen atom), it is not the primary molecule that scavenges free $NH_4^+$ from peripheral tissues for transport. **3. Clinical Pearls & High-Yield Facts:** * **The "Ammonia Traps":** Glutamate and Alanine are the two primary carriers. While **Glutamine** is the transporter for most tissues, **Alanine** is the specific carrier for ammonia from the **Skeletal Muscle** to the liver (via the Glucose-Alanine Cycle). * **Brain Toxicity:** In hyperammonemia, high levels of ammonia force the Glutamine Synthetase reaction in the brain. This depletes Glutamate (a neurotransmitter) and Alpha-ketoglutarate (a TCA cycle intermediate), leading to ATP depletion and cerebral edema. * **Enzyme Key:** Glutamate + $NH_4^+$ $\xrightarrow{ATP}$ Glutamine (Enzyme: Glutamine Synthetase).

Question 133: Which of the following amino acids in a protein is involved in the urea cycle and is required for the synthesis of NO?

A. Arginine (Correct Answer)
B. Histidine
C. Tryptophan
D. Lysine

Explanation: **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.

Question 134: Which enzyme defect causes phenylketonuria?

A. Phenylalanine hydroxylase (Correct Answer)
B. Homogentisate oxidase
C. Pyruvate hydroxylase
D. Fumarylacetoacetate hydroxylase

Explanation: **Explanation:** **Phenylketonuria (PKU)** is an autosomal recessive inborn error of metabolism. The primary defect is a deficiency of the enzyme **Phenylalanine hydroxylase (PAH)**. Under normal physiological conditions, PAH converts the essential amino acid Phenylalanine into Tyrosine using **Tetrahydrobiopterin (BH4)** as a mandatory co-factor. A deficiency in PAH leads to the accumulation of Phenylalanine in the blood and brain, and its diversion into alternative pathways forming phenylketones (phenylpyruvate, phenyllactate, and phenylacetate), which are excreted in the urine. **Analysis of Incorrect Options:** * **B. Homogentisate oxidase:** Deficiency of this enzyme leads to **Alkaptonuria**, characterized by ochronosis (darkening of tissues) and urine that turns black upon standing. * **C. Pyruvate hydroxylase:** This is a distractor; the relevant enzyme in gluconeogenesis is *Pyruvate carboxylase*. * **D. Fumarylacetoacetate hydroxylase:** Deficiency of this enzyme causes **Tyrosinemia Type I**, the most severe form of tyrosinemia, leading to liver and kidney failure. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Features:** Intellectual disability, "mousy" or musty body odor, hypopigmentation (due to decreased melanin), and seizures. * **Diagnosis:** Screened via the **Guthrie Test** (bacterial inhibition assay) or Tandem Mass Spectrometry. * **Maternal PKU:** If a pregnant woman with PKU doesn't maintain a low-phenylalanine diet, the high levels act as a teratogen, causing microcephaly and congenital heart defects in the fetus. * **Variant PKU:** 1-2% of cases are due to **Dihydrobiopterin reductase** deficiency, which affects BH4 regeneration. These cases are more severe as BH4 is also required for neurotransmitter synthesis (Dopamine, Serotonin).

Question 135: Which amino acid is both glucogenic and ketogenic?

A. Leucine
B. Alanine
C. Serine
D. Tryptophan (Correct Answer)

Explanation: ### 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.

Question 136: Which of the following is the precursor for adrenaline and thyroxine synthesis?

A. Phenylalanine
B. Tyrosine (Correct Answer)
C. Tryptophan
D. None of the above

Explanation: **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).

Question 137: What coenzyme is used in transamination reactions?

A. NAD
B. Biotin
C. Pyridoxal phosphate (Correct Answer)
D. Riboflavin

Explanation: **Explanation:** **1. Why Pyridoxal Phosphate (PLP) is correct:** Transamination is the first step in the catabolism of most amino acids, involving the transfer of an $\alpha$-amino group to an $\alpha$-ketoacid (usually $\alpha$-ketoglutarate). This reaction is catalyzed by **Aminotransferases (Transaminases)**. These enzymes strictly require **Pyridoxal Phosphate (PLP)**, which is the active form of **Vitamin $B_6$**, as a mandatory coenzyme. During the reaction, PLP acts as a temporary carrier of the amino group, transforming into Pyridoxamine phosphate (PMP) before transferring the group to the recipient ketoacid. **2. Why other options are incorrect:** * **NAD (Nicotinamide Adenine Dinucleotide):** Derived from Vitamin $B_3$ (Niacin), it is primarily involved in **redox reactions** (e.g., Oxidative deamination by Glutamate Dehydrogenase), not transamination. * **Biotin (Vitamin $B_7$):** This coenzyme is essential for **carboxylation reactions** (e.g., Pyruvate carboxylase, Acetyl-CoA carboxylase). It acts as a carrier of $CO_2$. * **Riboflavin (Vitamin $B_2$):** It is the precursor for FMN and FAD, which serve as prosthetic groups for **oxidoreductases** (e.g., Succinate dehydrogenase). **3. NEET-PG High-Yield Pearls:** * **All amino acids** undergo transamination **EXCEPT** Lysine, Threonine, Proline, and Hydroxyproline. * **Diagnostic Markers:** ALT (SGPT) and AST (SGOT) are transaminases used clinically to assess liver and heart health. * **Mechanism:** Transamination follows a **"Ping-Pong" Bi-Bi mechanism**. * PLP is also a coenzyme for **Decarboxylation** (e.g., Histidine to Histamine) and **Cystathionine synthase** (Heme synthesis).

Question 138: What is the recommended treatment for Alkaptonuria?

A. Vitamin A
B. Vitamin C (Correct Answer)
C. Vitamin D
D. Vitamin K

Explanation: ### Explanation **Alkaptonuria** is an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate Oxidase**. This leads to the accumulation of **Homogentisic Acid (HGA)** in the blood and tissues. #### Why Vitamin C is the Correct Answer: 1. **Inhibition of Oxidation:** Vitamin C (Ascorbic acid) acts as a potent antioxidant. It prevents the non-enzymatic oxidation of Homogentisic acid into **Benzoquinone acetate**, the precursor to the dark pigment **Ochronotic pigment**. 2. **Reduction of Pigmentation:** By inhibiting this oxidation process, Vitamin C reduces the deposition of pigment in connective tissues (Ochronosis), thereby slowing down the progression of joint damage and valvular heart disease. #### Why Other Options are Incorrect: * **Vitamin A:** Primarily involved in vision (rhodopsin formation) and epithelial integrity; it has no role in tyrosine metabolism or HGA oxidation. * **Vitamin D:** Essential for calcium and phosphate homeostasis; while Alkaptonuria affects joints, Vitamin D does not address the underlying metabolic defect. * **Vitamin K:** Necessary for the gamma-carboxylation of clotting factors (II, VII, IX, X); it does not interact with the phenylalanine-tyrosine pathway. #### NEET-PG High-Yield Clinical Pearls: * **Triad of Alkaptonuria:** 1. Homogentisic aciduria (urine turns black on standing/alkalinization), 2. Ochronosis (bluish-black pigmentation of cartilage/sclera), 3. Arthritis (large joints). * **Diagnostic Test:** Ferric Chloride test (yields a transient deep blue color). * **Definitive Treatment:** **Nitisinone** (inhibits 4-hydroxyphenylpyruvate dioxygenase) is the most effective modern treatment as it reduces HGA production. * **Dietary Management:** Restriction of Phenylalanine and Tyrosine.

Question 139: All of the following are tyrosine derivatives, EXCEPT?

A. Serotonin (Correct Answer)
B. Melanin
C. Epinephrine
D. Thyroxine

Explanation: ### Explanation **1. Why Serotonin is the Correct Answer:** Serotonin (5-hydroxytryptamine) is synthesized from the essential amino acid **Tryptophan**, not Tyrosine. The synthesis involves the hydroxylation of Tryptophan by *tryptophan hydroxylase* (requiring BH4) followed by decarboxylation. Therefore, it is the correct "except" option. **2. Analysis of Incorrect Options (Tyrosine Derivatives):** Tyrosine serves as a precursor for several vital biological molecules: * **Melanin (Option B):** Synthesized in melanocytes via the action of the enzyme *tyrosinase*. A deficiency in this pathway leads to Albinism. * **Epinephrine (Option C):** Tyrosine is the precursor for all catecholamines. The pathway follows: Tyrosine → L-DOPA → Dopamine → Norepinephrine → Epinephrine. * **Thyroxine (Option D):** Thyroid hormones (T3 and T4) are synthesized by the iodination of tyrosine residues within the thyroglobulin protein in the thyroid gland. **3. High-Yield Clinical Pearls for NEET-PG:** * **The BH4 Connection:** Both Tyrosine hydroxylase (catecholamine synthesis) and Tryptophan hydroxylase (serotonin synthesis) require **Tetrahydrobiopterin (BH4)** as a cofactor. * **Melatonin vs. Melanin:** Do not confuse the two. **Melanin** comes from Tyrosine, while **Melatonin** (the sleep-regulating hormone) is derived from Tryptophan (via Serotonin). * **Niacin Synthesis:** Tryptophan is also a precursor for **Niacin (Vitamin B3)**; roughly 60 mg of Tryptophan yields 1 mg of Niacin. * **Alkaptonuria:** A deficiency of *homogentisate oxidase* in the Tyrosine catabolic pathway leads to dark urine and ochronosis.

Question 140: With the help of pyridoxine, serotonin is synthesized from which of the following precursor amino acids?

A. Tyrosine
B. Tryptophan (Correct Answer)
C. Phenylalanine
D. Glycine

Explanation: **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).

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

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One-Carbon Transfer Reactions

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