Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 11: Which amino acid contains an amide group?

A. Aspartate
B. Glutamine (Correct Answer)
C. Glutamate
D. Glutamic acid

Explanation: **Explanation:** The correct answer is **Glutamine**. **Why Glutamine is correct:** Amino acids are classified based on the functional groups in their side chains (R-groups). **Glutamine** and **Asparagine** are the two primary amino acids that contain an **amide group** (-CONH₂). Glutamine is formed from Glutamate (Glutamic acid) through the addition of ammonia via the enzyme *Glutamine synthetase*. This amide group is crucial because it allows Glutamine to serve as the primary non-toxic transporter of ammonia in the blood. **Why the other options are incorrect:** * **Aspartate (Option A):** This is the ionized form of Aspartic acid. It contains a **carboxylic acid group** (-COOH) in its side chain, making it an acidic amino acid, not an amide. (Note: Its amide derivative is Asparagine). * **Glutamate / Glutamic acid (Options C & D):** These terms refer to the same molecule (Glutamate is the conjugate base). Like Aspartate, it contains a **carboxylic acid group** in its side chain. It is an acidic amino acid and serves as the precursor to Glutamine. **High-Yield NEET-PG Pearls:** * **Ammonia Detoxification:** Glutamine is the most abundant free amino acid in the body because it carries two nitrogens (one amine, one amide) to the liver and kidneys for urea synthesis and excretion. * **Acid-Base Balance:** In the kidneys, the enzyme **Glutaminase** cleaves the amide group of Glutamine to release ammonia, which buffers H⁺ ions to form ammonium (NH₄⁺), aiding in the excretion of metabolic acids. * **Nitrogen Donor:** The amide nitrogen of Glutamine is a vital donor for the synthesis of **Purines** (N3 and N9) and **Pyrimidines** (C2 and N3 in the cytosol).

Question 12: Tyrosine is involved in the synthesis of which of the following?

A. Melanin
B. Thyroxine
C. Epinephrine and norepinephrine
D. All of the above (Correct Answer)

Explanation: **Explanation:** Tyrosine is a non-essential amino acid (synthesized from phenylalanine) that serves as a precursor for several biologically vital molecules. It is often referred to as a "hub" for the synthesis of hormones and pigments. 1. **Melanin:** In melanocytes, the enzyme **Tyrosinase** converts tyrosine into DOPA and subsequently into melanin. A deficiency in this enzyme leads to **Albinism**. 2. **Thyroid Hormones (T3 & T4):** In the thyroid gland, tyrosine residues on the protein thyroglobulin are iodinated to form Monoiodotyrosine (MIT) and Diiodotyrosine (DIT), which then couple to form Thyroxine. 3. **Catecholamines:** Tyrosine is the starting point for the catecholamine pathway. It is converted to DOPA by **Tyrosine Hydroxylase** (the rate-limiting step), then to **Dopamine**, **Norepinephrine**, and finally **Epinephrine** in the adrenal medulla. **Why "All of the above" is correct:** Since tyrosine is the direct biochemical precursor for the skin pigment (melanin), the metabolic hormones (thyroxine), and the fight-or-flight neurotransmitters (epinephrine/norepinephrine), all options are correct. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Tyrosine hydroxylase is the rate-limiting enzyme for catecholamine synthesis. * **PKU Connection:** In Phenylketonuria (PKU), the enzyme Phenylalanine Hydroxylase is deficient. Consequently, tyrosine becomes an **essential amino acid** for these patients. * **Alkaptonuria:** This is a defect in **Homogentisate oxidase**, an enzyme in the tyrosine degradative pathway, leading to dark urine and ochronosis. * **Tyrosinemia Type I:** Caused by a deficiency of Fumarylacetoacetate hydrolase; it is the most severe form of tyrosinemia.

Question 13: Creatine is synthesized from which of the following amino acids, except?

A. Glycine
B. Alanine (Correct Answer)
C. Methionine
D. Arginine

Explanation: **Explanation:** Creatine synthesis is a high-yield topic in biochemistry, involving three specific amino acids and two primary organs (Kidney and Liver). **1. Why Alanine is the correct answer:** Alanine is a non-essential amino acid primarily involved in gluconeogenesis (via the Cahill cycle) and transamination reactions. It does **not** contribute any atoms or functional groups to the structure of creatine. Therefore, it is the "except" in this list. **2. Analysis of the other options:** Creatine synthesis occurs in a two-step process using the following: * **Arginine (Option D):** In the kidney, the enzyme *Amidinotransferase* transfers the guanidino group from Arginine to Glycine. * **Glycine (Option A):** It acts as the backbone of the molecule. The reaction between Arginine and Glycine forms **Guanidinoacetate (GAA)**. * **Methionine (Option C):** In the liver, GAA is methylated by **S-adenosylmethionine (SAM)**, which is derived from Methionine. This final step produces Creatine. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Site of Synthesis:** Starts in the **Kidney** (formation of Guanidinoacetate) and is completed in the **Liver** (methylation). * **Storage:** 95% of creatine is stored in skeletal muscle as **Creatine Phosphate**, a high-energy compound used to regenerate ATP during the first few seconds of muscle contraction. * **Excretion:** Creatinine (the anhydride form) is excreted in the urine. Its excretion rate is constant and proportional to the total muscle mass of the body. * **Enzyme Marker:** **Creatine Kinase (CK/CPK)** levels are clinically significant markers for muscle damage (CK-MM) and myocardial infarction (CK-MB).

Question 14: Maple syrup urine disease is characterized by all except?

A. Hypotonia
B. Hypertonia
C. Pancreatitis
D. Hypopigmentation (Correct Answer)

Explanation: **Explanation:** Maple Syrup Urine Disease (MSUD) is an autosomal recessive metabolic disorder caused by a deficiency in the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD) complex**. This leads to the accumulation of branched-chain amino acids (BCAAs)—**Leucine, Isoleucine, and Valine**—and their corresponding alpha-keto acids in the blood and urine. **Why Hypopigmentation is the Correct Answer:** Hypopigmentation is **not** a feature of MSUD. It is a classic clinical finding in **Phenylketonuria (PKU)**. In PKU, the deficiency of Phenylalanine Hydroxylase leads to decreased Tyrosine production; since Tyrosine is a precursor for melanin, patients exhibit fair skin and light-colored hair. MSUD does not involve the melanin pathway. **Analysis of Other Options:** * **Hypotonia & Hypertonia:** MSUD typically presents in the first week of life with progressive neurological deterioration. Infants often exhibit alternating bouts of **hypotonia** (floppiness) and **hypertonia** (rigidity/opisthotonus). * **Pancreatitis:** Acute and recurrent pancreatitis is a recognized, though less common, systemic complication of MSUD, likely due to metabolic derangements and toxic accumulation of keto acids. **High-Yield Clinical Pearls for NEET-PG:** * **Odor:** Urine has a characteristic **burnt sugar/maple syrup** smell due to the accumulation of **S-isoleucine**. * **Diagnosis:** Elevated levels of BCAAs in plasma and the presence of **alloisoleucine** (pathognomonic). * **Screening:** Positive **2,4-Dinitrophenylhydrazine (DNPH)** test in urine. * **Treatment:** Dietary restriction of BCAAs and, in some cases, **Thiamine (Vitamin B1)** supplementation (as B1 is a cofactor for the BCKAD complex).

Question 15: HHH syndrome is due to a defect of which pathway?

A. HMP shunt
B. Citric acid cycle
C. Bile acid synthesis
D. Urea cycle (Correct Answer)

Explanation: **Explanation:** **HHH Syndrome** (Hyperammonemia-Hyperornithinemia-Homocitrullinuria) is an autosomal recessive metabolic disorder caused by a mutation in the **SLC25A15 gene**. This gene encodes the **mitochondrial ornithine transporter (ORNT1)**. 1. **Why Urea Cycle is Correct:** The urea cycle occurs partially in the mitochondria and partially in the cytosol. Ornithine, produced in the cytosol, must be transported into the mitochondria to react with carbamoyl phosphate (via Ornithine Transcarbamoylase) to form citrulline. A defect in the ORNT1 transporter leads to: * **Hyperornithinemia:** Ornithine builds up in the cytosol. * **Hyperammonemia:** The urea cycle is stalled due to a lack of mitochondrial ornithine, preventing ammonia detoxification. * **Homocitrullinuria:** Excess carbamoyl phosphate reacts with lysine instead of ornithine, forming homocitrulline. 2. **Why Incorrect Options are Wrong:** * **HMP Shunt:** Involved in NADPH production and pentose sugar synthesis; defects typically lead to hemolytic anemia (e.g., G6PD deficiency). * **Citric Acid Cycle:** The primary aerobic energy pathway; defects usually present with severe lactic acidosis and encephalopathy. * **Bile Acid Synthesis:** Related to cholesterol metabolism; defects cause cholestasis and malabsorption, not ammonia issues. **High-Yield Clinical Pearls for NEET-PG:** * **Triad of HHH:** Hyperammonemia, Hyperornithinemia, and Homocitrullinuria. * **Clinical Presentation:** Episodic confusion, lethargy, seizures, and spastic paraparesis. * **Management:** Protein-restricted diet and supplementation with citrulline or arginine to maintain urea cycle flux. * **Differential:** Unlike OTC deficiency (the most common urea cycle disorder), HHH syndrome specifically features elevated plasma ornithine levels.

Question 16: Urea, creatinine, and nitric oxide are synthesized from which amino acid?

A. Alanine
B. Arginine (Correct Answer)
C. Aspartate
D. Glycine

Explanation: **Explanation:** **Arginine** is a versatile, semi-essential amino acid that serves as a precursor for several biologically vital molecules. It is the correct answer because: 1. **Urea:** In the final step of the Urea Cycle, the enzyme **Arginase** cleaves Arginine into Urea and Ornithine. 2. **Nitric Oxide (NO):** Arginine is the sole precursor of NO, a potent vasodilator. The enzyme **Nitric Oxide Synthase (NOS)** converts L-arginine to L-citrulline and NO. 3. **Creatinine:** Arginine provides the amidino group to Glycine (forming Guanidinoacetate) in the first step of creatine synthesis, which eventually cyclizes to form creatinine. **Why other options are incorrect:** * **Alanine:** Primarily involved in the glucose-alanine cycle for transporting nitrogen to the liver; it is a major gluconeogenic amino acid. * **Aspartate:** While it provides the second nitrogen atom for the Urea Cycle (joining with Citrulline to form Argininosuccinate), it is not the direct precursor for NO or Creatinine. * **Glycine:** Although Glycine is required for **Creatine** and **Heme** synthesis, it does not contribute to the synthesis of Urea or Nitric Oxide. **High-Yield Clinical Pearls for NEET-PG:** * **Arginine** also stimulates the release of Growth Hormone, Insulin, and Glucagon. * **Creatine Synthesis:** Requires three amino acids: **G**lycine, **A**rginine, and **M**ethionine (as S-adenosylmethionine). Remember the mnemonic: **"GAM"**. * **Hyperargininemia:** A deficiency of Arginase leads to elevated arginine levels, causing spastic diplegia and developmental delay (unlike other urea cycle defects which present with severe neonatal hyperammonemia).

Question 17: Which one of the following is not a feature of Phenylketonuria?

A. Severe mental retardation
B. Reduced tendon reflexes (Correct Answer)
C. Enamel hypoplasia
D. Vomiting in early infancy

Explanation: **Explanation:** Phenylketonuria (PKU) is an autosomal recessive disorder caused by a deficiency of the enzyme **phenylalanine hydroxylase (PAH)**, leading to the accumulation of phenylalanine and its metabolites (phenylpyruvate, phenyllactate). **Why "Reduced tendon reflexes" is the correct answer:** In PKU, patients typically exhibit **hyperreflexia** (increased tendon reflexes) and spasticity, rather than reduced reflexes. The accumulation of phenylalanine is neurotoxic and interferes with myelination and neurotransmitter synthesis, leading to upper motor neuron signs and increased muscle tone. **Analysis of other options:** * **Severe mental retardation:** This is a hallmark of untreated PKU. High levels of phenylalanine interfere with the transport of other large neutral amino acids into the brain, disrupting protein synthesis and brain development. * **Enamel hypoplasia:** This is a recognized clinical feature of PKU. Metabolic imbalances during tooth development lead to defects in the dental enamel. * **Vomiting in early infancy:** This is often one of the earliest non-specific symptoms of PKU, appearing before the classic "mousy odor" or skin changes. It can be severe enough to be misdiagnosed as pyloric stenosis. **High-Yield Clinical Pearls for NEET-PG:** * **Mousy/Musty Odor:** Due to phenylacetic acid in sweat and urine. * **Hypopigmentation:** Phenylalanine inhibits **Tyrosinase**, leading to fair skin, blonde hair, and blue eyes (decreased melanin). * **Diagnosis:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry (TMS) for newborn screening. * **Management:** Dietary restriction of phenylalanine and supplementation of **Tyrosine** (which becomes an essential amino acid). Avoid Aspartame.

Question 18: Epinephrine is formed from which amino acid?

A. Methionine
B. Tyrosine (Correct Answer)
C. Valine
D. Glycine

Explanation: **Explanation:** The correct answer is **Tyrosine**. Epinephrine (Adrenaline) is a catecholamine synthesized primarily in the adrenal medulla. The biosynthetic pathway follows a specific sequence: **Phenylalanine → Tyrosine → L-DOPA → Dopamine → Norepinephrine → Epinephrine.** Tyrosine is the direct precursor for all catecholamines. The rate-limiting step is the conversion of Tyrosine to L-DOPA by the enzyme *Tyrosine Hydroxylase*. The final step, conversion of Norepinephrine to Epinephrine, requires the enzyme *Phenylethanolamine N-methyltransferase (PNMT)*, which uses S-adenosylmethionine (SAM) as a methyl donor. **Analysis of Incorrect Options:** * **A. Methionine:** While Methionine (as SAM) provides the methyl group needed to convert Norepinephrine to Epinephrine, it is not the structural amino acid backbone of the hormone. * **C. Valine:** This is a branched-chain amino acid (BCAA) primarily involved in energy metabolism and protein synthesis; it does not contribute to catecholamine synthesis. * **D. Glycine:** This is the simplest amino acid and serves as a precursor for Heme, Purines, Creatine, and Glutathione, but not for Epinephrine. **High-Yield Clinical Pearls for NEET-PG:** 1. **Precursor sequence:** Tyrosine is also the precursor for **Melanin** (via Tyrosinase) and **Thyroid hormones** (T3/T4). 2. **Vitamin Cofactors:** Hydroxylation steps in this pathway require **Tetrahydrobiopterin (BH4)** and **Vitamin C**. 3. **VMA (Vanillylmandelic Acid):** This is the end-stage urinary metabolite of Epinephrine and Norepinephrine; elevated levels are a diagnostic marker for **Pheochromocytoma**.

Question 19: N-methylglycine is also known as:

A. Betaine
B. Sarcosine (Correct Answer)
C. Carnosine
D. Ergothionine

Explanation: ### Explanation **Correct Option: B. Sarcosine** Sarcosine is the chemical name for **N-methylglycine**. It is an intermediate in the metabolism of choline and the conversion of dietary betaine to glycine. The enzyme **sarcosine dehydrogenase** catalyzes the oxidative demethylation of sarcosine into glycine. In the urea cycle and one-carbon metabolism, it serves as a methyl donor. **Analysis of Incorrect Options:** * **A. Betaine (Trimethylglycine):** As the name implies, betaine contains three methyl groups attached to the nitrogen atom of glycine. It acts as an important osmolyte and a methyl donor for the conversion of homocysteine to methionine. * **C. Carnosine (beta-alanyl-L-histidine):** This is a dipeptide composed of beta-alanine and histidine. It is found in high concentrations in muscle and brain tissues, acting as an antioxidant and pH buffer. * **D. Ergothioneine:** This is a naturally occurring sulfur-containing derivative of the amino acid **histidine** (specifically, a thiourea derivative of trimethylhistidine). It is a potent antioxidant obtained primarily through diet (mushrooms). **NEET-PG High-Yield Pearls:** * **Sarcosinemia:** An inborn error of metabolism caused by a deficiency of sarcosine dehydrogenase, leading to elevated levels of sarcosine in blood and urine (usually a benign condition). * **Prostate Cancer Marker:** Recent research highlights sarcosine as a potential metabolic biomarker for prostate cancer progression and invasiveness. * **One-Carbon Metabolism:** Remember that the conversion of Sarcosine → Glycine releases a one-carbon unit that enters the folate pool as 5,10-Methylene THF.

Question 20: A mousy odour in urine is characteristically seen in which of the following conditions?

A. Maple syrup urine disease
B. Phenylketonuria (Correct Answer)
C. Isovaleric acidemia
D. Cystinuria

Explanation: **Explanation:** **Phenylketonuria (PKU)** is the correct answer. This autosomal recessive disorder is caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)** or its cofactor **Tetrahydrobiopterin (BH4)**. This deficiency leads to the accumulation of Phenylalanine, which is alternatively metabolized into phenylketones (such as **phenylacetate**, phenylpyruvate, and phenyllactate). The **mousy or musty odor** characteristic of the urine in PKU patients is specifically attributed to **phenylacetate**. **Analysis of Incorrect Options:** * **A. Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the Branched-Chain Alpha-Keto Acid Dehydrogenase complex. It results in urine that smells like **maple syrup or burnt sugar** due to the accumulation of isoleucine. * **C. Isovaleric Acidemia:** A defect in Isovaleryl-CoA dehydrogenase leads to the accumulation of isovaleric acid, which imparts a characteristic **"sweaty feet" or "cheesy" odor** to the urine. * **D. Cystinuria:** A transport defect of COLA (Cystine, Ornithine, Lysine, Arginine) in the proximal renal tubule. While it leads to hexagonal cystine crystals and stones, it typically presents with a **sulfurous (rotten egg)** smell, if any. **High-Yield Clinical Pearls for NEET-PG:** * **PKU Presentation:** Intellectual disability, hypopigmentation (fair skin/blue eyes due to decreased melanin), and seizures. * **Diagnostic Test:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry. * **Other Odors to Remember:** * **Tyrosinemia:** Boiled cabbage/Rancid butter. * **Trimethylaminuria:** Fishy odor. * **Multiple Carboxylase Deficiency:** Tomcat urine.

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

Practice Questions

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