Amino Acid Metabolism — MCQs
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Which of the following best describes non-essential amino acids?
Catecholamines are synthesized from which amino acid?
The hair abnormality shown in the image is a distinguishing clinical feature of which of the following urea cycle disorders?
Which of the following amino acids is associated with a high risk for coronary heart disease?
All of the following can be used in the treatment of hyperammonemia EXCEPT?
A vitamin B6 deficiency reduces the effectiveness of transaminase enzymes. Which amino acid is formed from the transamination of alpha-ketoglutarate?
Conversion of phenylalanine to tyrosine is hampered in which condition?
Metabolites of tryptophan can give rise to all of the following except?
Richard Hanha syndrome is caused by defective metabolism of which amino acid?
Decarboxylation of amino acids produces all except which of the following?
Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs
Question 221: Which of the following best describes non-essential amino acids?
- A. They are not used by the body.
- B. They are not forming part of proteins.
- C. They are not required in the diet. (Correct Answer)
- D. They are not absorbed in the intestine.
Explanation: ### Explanation **1. Why Option C is Correct:** The classification of amino acids into "essential" and "non-essential" is based strictly on **nutritional requirement**, not biological importance [1]. **Non-essential amino acids (NEAAs)** are those that the body can synthesize de novo from metabolic intermediates [4], such as α-keto acids derived from the TCA cycle or glycolysis [2]. Because the body can produce sufficient quantities endogenously, they are **not required in the diet**. **2. Why Other Options are Incorrect:** * **Option A:** All 20 standard amino acids are vital for life. NEAAs are used extensively for protein synthesis, neurotransmitter production, and gluconeogenesis. * **Option B:** NEAAs (e.g., Glycine, Alanine, Serine) are structural components of almost all human proteins. In fact, Collagen—the most abundant protein in the body—is primarily composed of Glycine and Proline (both NEAAs). * **Option D:** All amino acids, whether essential or non-essential, are absorbed in the small intestine via specific sodium-dependent secondary active transport systems. **3. NEET-PG High-Yield Clinical Pearls:** * **Semi-essential Amino Acids:** Arginine and Histidine are considered semi-essential because they are required during periods of rapid growth (infancy) or recovery from illness, as endogenous synthesis may not meet the high demand [1]. * **Purely Ketogenic Amino Acids:** Leucine and Lysine (both are essential). * **Glucogenic & Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, Isoleucine (Mnemonic: **PITTT**). * **Precursor Fact:** Tyrosine is a non-essential amino acid, but it becomes **conditionally essential** in patients with Phenylketonuria (PKU) because they cannot convert Phenylalanine to Tyrosine [3].
Question 222: Catecholamines are synthesized from which amino acid?
- A. Alanine
- B. Glycine
- C. Cysteine
- D. Tyrosine (Correct Answer)
Explanation: **Explanation:** The correct answer is **Tyrosine**. Catecholamines (Dopamine, Norepinephrine, and Epinephrine) are a group of neurotransmitters and hormones synthesized primarily in the adrenal medulla and the brain from the amino acid **Tyrosine**. **Why Tyrosine is correct:** The synthesis follows a well-defined metabolic pathway: 1. **Tyrosine** is converted to **L-DOPA** by *Tyrosine Hydroxylase* (the rate-limiting step). 2. L-DOPA is decarboxylated to **Dopamine**. 3. Dopamine is hydroxylated to **Norepinephrine**. 4. Norepinephrine is methylated to **Epinephrine** by the enzyme *PNMT* (Phenylethanolamine N-methyltransferase). *Note: Since Tyrosine is derived from Phenylalanine, Phenylalanine is the ultimate precursor.* **Why other options are incorrect:** * **Alanine:** A non-essential amino acid primarily involved in the Cahill cycle (glucose-alanine cycle) for transporting amino groups to the liver. * **Glycine:** Used for the synthesis of Heme, Creatine, Purines, and Glutathione. * **Cysteine:** A sulfur-containing amino acid involved in the synthesis of Glutathione and Taurine. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Tyrosine Hydroxylase requires **Tetrahydrobiopterin (BH4)** as a cofactor. * **VMA (Vanillylmandelic Acid):** The end-product of catecholamine metabolism excreted in urine; elevated levels are a diagnostic marker for **Pheochromocytoma**. * **Parkinson’s Disease:** Characterized by a deficiency of Dopamine in the substantia nigra; treated with L-DOPA (as Dopamine cannot cross the blood-brain barrier). * **Albinism:** Caused by a deficiency of the enzyme *Tyrosinase*, which converts Tyrosine to Melanin.
Question 223: The hair abnormality shown in the image is a distinguishing clinical feature of which of the following urea cycle disorders?
- A. Hyperammonemia type 1
- B. Hyperammonemia type 2
- C. Argininosuccinic aciduria (Correct Answer)
- D. Argininemia
Explanation: ***Argininosuccinic aciduria*** - **Trichorrhexis nodosa** (transverse fractures and fraying of the hair shaft) is a **pathognomonic** clinical feature specific to argininosuccinic aciduria (ASL deficiency). - This hair abnormality results from **argininosuccinic acid accumulation** affecting hair structure, making it a distinguishing diagnostic feature from other urea cycle disorders. *Hyperammonemia type 1* - Caused by **carbamoyl phosphate synthetase I (CPS1) deficiency** and does not present with characteristic hair abnormalities. - Clinical features include **severe hyperammonemia** and neurological symptoms but lack the distinctive hair findings seen in argininosuccinic aciduria. *Hyperammonemia type 2* - Results from **ornithine transcarbamylase (OTC) deficiency** and is **X-linked**, affecting males more severely. - Does not cause **trichorrhexis nodosa** or other specific hair abnormalities, presenting primarily with hyperammonemic episodes. *Argininemia* - Caused by **arginase deficiency** leading to arginine accumulation and presents with **spastic diplegia** and intellectual disability. - Hair abnormalities like **trichorrhexis nodosa** are not associated with this condition, distinguishing it from argininosuccinic aciduria.
Question 224: Which of the following amino acids is associated with a high risk for coronary heart disease?
- A. Homocysteine (Correct Answer)
- B. Methionine
- C. Glycine
- D. None of the above
Explanation: **Explanation:** **Homocysteine** is the correct answer because elevated plasma levels (Hyperhomocysteinemia) are a well-established independent risk factor for coronary heart disease (CHD), peripheral vascular disease, and stroke. **Mechanism:** High levels of homocysteine promote atherosclerosis through several mechanisms: 1. **Endothelial Dysfunction:** It induces oxidative stress, damaging the vascular lining. 2. **Pro-thrombotic state:** It enhances platelet aggregation and interferes with the coagulation cascade. 3. **Smooth Muscle Proliferation:** It stimulates the growth of vascular smooth muscle cells, contributing to plaque formation. **Analysis of Incorrect Options:** * **B. Methionine:** While homocysteine is derived from methionine metabolism (via S-adenosylmethionine), methionine itself is an essential amino acid and is not directly linked to increased cardiovascular risk. * **C. Glycine:** This is a non-essential amino acid involved in heme and purine synthesis; it has no known association with coronary heart disease. **High-Yield Clinical Pearls for NEET-PG:** * **Metabolic Pathways:** Homocysteine is metabolized via two pathways: **Remethylation** to Methionine (requires **Vitamin B12 and Folate**) and **Transsulfuration** to Cysteine (requires **Vitamin B6**). * **Deficiencies:** Deficiencies in B6, B12, or Folate are common causes of hyperhomocysteinemia. * **Enzyme Deficiency:** The most common genetic cause of severe hyperhomocysteinemia (Homocystinuria) is a deficiency of **Cystathionine β-synthase**. * **Classic Triad of Homocystinuria:** Ectopia lentis (downward dislocation), intellectual disability, and premature arterial/venous thrombosis.
Question 225: All of the following can be used in the treatment of hyperammonemia EXCEPT?
- A. Sodium benzoate
- B. L-carnitine (Correct Answer)
- C. L-arginine
- D. Phenylacetate
Explanation: **Explanation:** Hyperammonemia is a medical emergency where the body cannot effectively clear ammonia, typically due to urea cycle enzyme deficiencies or liver failure. The goal of treatment is to provide alternative pathways for nitrogen excretion or replenish cycle intermediates. **Why L-carnitine is the correct answer:** L-carnitine is primarily involved in the transport of long-chain fatty acids into the mitochondria for beta-oxidation. While it is used in specific organic acidemias (to conjugate with toxic organic acids) or Valproate toxicity, it is **not** a standard treatment for primary hyperammonemia or urea cycle disorders. It does not directly facilitate the removal of ammonia or provide an alternative nitrogen disposal pathway. **Analysis of Incorrect Options:** * **Sodium benzoate:** Acts as a "nitrogen scavenger." It conjugates with **Glycine** to form Hippuric acid, which is excreted in the urine. Since glycine contains nitrogen, this bypasses the urea cycle. * **Phenylacetate (or Phenylbutyrate):** Another nitrogen scavenger. It conjugates with **Glutamine** to form Phenylacetylglutamine. Because glutamine contains two nitrogen atoms, this is a highly efficient way to remove excess nitrogen. * **L-arginine:** In many urea cycle defects (except Arginase deficiency), arginine becomes an essential amino acid. Supplementation helps provide intermediates to keep the cycle functioning and promotes the excretion of nitrogen through citrulline or argininosuccinate. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice for acute hyperammonemia:** Hemodialysis (fastest way to reduce levels). * **Nitrogen Scavenger Mechanism:** Benzoate → Glycine; Phenylacetate → Glutamine. * **Lactulose:** Used in hepatic encephalopathy to "trap" ammonia ($NH_3$) as ammonium ($NH_4^+$) in the gut by lowering luminal pH. * **N-acetylglutamate (NAG) analog:** Carglumic acid is used specifically for NAGS deficiency.
Question 226: A vitamin B6 deficiency reduces the effectiveness of transaminase enzymes. Which amino acid is formed from the transamination of alpha-ketoglutarate?
- A. Glycine
- B. Glutamine
- C. Asparagine
- D. Glutamate (Correct Answer)
Explanation: ### Explanation **Correct Answer: D. Glutamate** **The Concept of Transamination** Transamination is the process where an amino group ($\text{-NH}_2$) is transferred from an amino acid to a keto acid, resulting in the formation of a new amino acid and a new keto acid. This reaction is catalyzed by **aminotransferases (transaminases)**, which require **Pyridoxal Phosphate (PLP)**, the active form of **Vitamin B6**, as an essential cofactor. In this specific reaction, **$\alpha$-ketoglutarate** (a 5-carbon dicarboxylic keto acid) acts as the universal amino group acceptor. When it accepts an amino group, it is converted into its corresponding amino acid, **Glutamate**. **Analysis of Incorrect Options:** * **A. Glycine:** This is the simplest amino acid. Its corresponding keto acid is glyoxylate. It is not derived from $\alpha$-ketoglutarate. * **B. Glutamine:** While related to glutamate, glutamine is formed via the action of **glutamine synthetase**, which adds a second amino group (as free ammonia) to glutamate. This is an ATP-dependent amidation, not a transamination. * **C. Asparagine:** This is the amide form of aspartate. It is synthesized from aspartate and glutamine via asparagine synthetase. The keto acid precursor for aspartate (via transamination) is **oxaloacetate**, not $\alpha$-ketoglutarate. **NEET-PG High-Yield Pearls:** * **Cofactor:** All transaminases require **Vitamin B6 (PLP)**. Deficiency leads to impaired amino acid metabolism and neurological symptoms (due to decreased GABA synthesis). * **Key Pairs to Remember:** * Pyruvate $\leftrightarrow$ Alanine (via ALT) * Oxaloacetate $\leftrightarrow$ Aspartate (via AST) * $\alpha$-ketoglutarate $\leftrightarrow$ Glutamate * **Exception:** Lysine, Threonine, Proline, and Hydroxyproline **do not** undergo transamination. * **Clinical Marker:** AST and ALT are sensitive markers for liver injury; AST is also found in cardiac and skeletal muscle.
Question 227: Conversion of phenylalanine to tyrosine is hampered in which condition?
- A. Phenylketonuria (Correct Answer)
- B. Alkaptonuria
- C. Maple syrup disease
- D. Tyrosinemia
Explanation: ### Explanation **1. Why Phenylketonuria (PKU) is the Correct Answer:** Phenylketonuria is an autosomal recessive disorder caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)**. Under normal physiological conditions, PAH converts the essential amino acid Phenylalanine into **Tyrosine** using the cofactor Tetrahydrobiopterin ($BH_4$). When this enzyme is deficient or absent, the conversion is blocked, leading to the accumulation of Phenylalanine in the blood and tissues, and its diversion into alternative pathways (forming phenylpyruvate, phenyllactate, and phenylacetate). **2. Why the Other Options are Incorrect:** * **Alkaptonuria:** This is a defect in the enzyme **Homogentisate Oxidase** in the tyrosine degradation pathway. It leads to the accumulation of homogentisic acid, causing dark urine and ochronosis, but does not affect the initial conversion of phenylalanine to tyrosine. * **Maple Syrup Urine Disease (MSUD):** This involves a deficiency of the **Branched-Chain Alpha-Keto Acid Dehydrogenase** complex. it affects the metabolism of branched-chain amino acids (**Leucine, Isoleucine, and Valine**), not phenylalanine. * **Tyrosinemia:** This refers to a group of disorders (Type I, II, and III) caused by defects in enzymes *downstream* of tyrosine (e.g., Fumarylacetoacetate hydrolase). While tyrosine levels are elevated, the conversion of phenylalanine to tyrosine itself is not hampered. **3. NEET-PG High-Yield Clinical Pearls:** * **Mousy/Musty Odor:** A classic clinical sign of PKU due to phenylacetate in sweat and urine. * **Tyrosine becomes "Essential":** In PKU patients, tyrosine cannot be synthesized and must be supplied in the diet. * **Maternal PKU:** If a pregnant woman with PKU doesn't control her phenylalanine levels, the infant may suffer from microcephaly, mental retardation, and congenital heart defects (teratogenic effect). * **Cofactor:** A small percentage of PKU cases are due to a deficiency in **Dihydrobiopterin reductase**, which regenerates $BH_4$.
Question 228: Metabolites of tryptophan can give rise to all of the following except?
- A. Diarrhoea
- B. Vasoconstriction
- C. Flushing
- D. Phenylketonuria (Correct Answer)
Explanation: **Explanation:** Tryptophan is an essential amino acid that serves as a precursor for several vital molecules, including **Serotonin (5-HT)**, **Melatonin**, and **Niacin (Vitamin B3)**. **Why Phenylketonuria (PKU) is the correct answer:** Phenylketonuria is an autosomal recessive disorder caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)** or its cofactor, Tetrahydrobiopterin (BH4). This leads to the accumulation of **Phenylalanine**, not Tryptophan. Therefore, Tryptophan metabolites are not involved in the pathogenesis of PKU. **Analysis of Incorrect Options:** * **Vasoconstriction & Flushing:** Tryptophan is converted into Serotonin. In **Carcinoid Syndrome** (a tumor of enterochromaffin cells), there is massive overproduction of Serotonin. Serotonin acts as a potent **vasoconstrictor** in certain vascular beds, while its metabolites and associated kinins cause episodic **flushing**. * **Diarrhoea:** Serotonin increases gastrointestinal motility. In Carcinoid Syndrome, the excess production of Serotonin leads to secretory diarrhoea. **High-Yield Clinical Pearls for NEET-PG:** * **Carcinoid Triad:** Flushing, Diarrhoea, and Right-sided heart failure (Valvular lesions). * **Diagnostic Marker:** Elevated urinary **5-HIAA** (5-Hydroxyindoleacetic acid), which is the end-metabolite of Serotonin. * **Hartnup Disease:** A defect in the transport of neutral amino acids (Tryptophan) in the gut and kidneys, leading to **Pellagra-like symptoms** (Dermatitis, Diarrhoea, Dementia) due to Niacin deficiency. * **Conversion Ratio:** Approximately 60 mg of Tryptophan is required to synthesize 1 mg of Niacin.
Question 229: Richard Hanha syndrome is caused by defective metabolism of which amino acid?
- A. Lysine
- B. Arginine
- C. Tyrosine (Correct Answer)
- D. Glycine
Explanation: **Explanation:** **Richard-Hanhart Syndrome** (also known as Tyrosinemia Type II or Oculocutaneous Tyrosinemia) is an autosomal recessive disorder caused by a deficiency of the enzyme **Tyrosine Aminotransferase (TAT)**. This enzyme is responsible for the first step of tyrosine catabolism, converting tyrosine into p-hydroxyphenylpyruvate. Its deficiency leads to significant hypertyrosinemia, resulting in the characteristic clinical triad: 1. **Ocular:** Herpetiform corneal ulcers and photophobia. 2. **Cutaneous:** Painful palmoplantar keratoderma (hyperkeratosis). 3. **Neurological:** Variable degrees of intellectual disability. **Analysis of Options:** * **Option C (Tyrosine):** Correct. The defect lies in the cytosolic tyrosine aminotransferase enzyme, leading to the accumulation of tyrosine in tissues. * **Option A (Lysine):** Incorrect. Defects in lysine metabolism are associated with conditions like Hyperlysinemia or Glutaric Aciduria Type I, which present with neurological deterioration but not the oculocutaneous features of Hanhart syndrome. * **Option B (Arginine):** Incorrect. Arginase deficiency leads to Hyperargininemia, characterized by spastic diplegia and episodic hyperammonemia. * **Option D (Glycine):** Incorrect. Glycine metabolism defects lead to Non-ketotic Hyperglycinemia, presenting with severe neonatal seizures and hiccups. **High-Yield Clinical Pearls for NEET-PG:** * **Tyrosinemia Type I (Hepatorenal):** Defect in **Fumarylacetoacetate hydrolase**. Features liver failure, renal tubular dysfunction (Fanconi syndrome), and a "cabbage-like" odor. * **Tyrosinemia Type III:** Defect in **p-hydroxyphenylpyruvate dioxygenase** (extremely rare). * **Treatment for Type II:** Dietary restriction of Tyrosine and Phenylalanine. Unlike Type I, Nitisinone (NTBC) is not typically used.
Question 230: Decarboxylation of amino acids produces all except which of the following?
- A. Histamine
- B. Proline (Correct Answer)
- C. Tryptamine
- D. Tyramine
Explanation: **Explanation:** Decarboxylation is a chemical reaction that removes a carboxyl group (-COOH) from an amino acid, typically catalyzed by pyridoxal phosphate (PLP)-dependent enzymes, to produce a corresponding **biogenic amine**. * **Why Proline is the correct answer:** Proline is a cyclic secondary amino acid (imino acid). It is synthesized from glutamate and does not undergo a simple decarboxylation reaction to form a functional amine in the human body. Unlike the other options, Proline is a primary amino acid itself, not a product of decarboxylation. * **Why other options are incorrect:** * **Histamine:** Produced by the decarboxylation of **Histidine** (via histidine decarboxylase). It is a key mediator in allergic reactions and gastric acid secretion. * **Tryptamine:** Produced by the decarboxylation of **Tryptophan**. While serotonin is the more famous derivative, tryptamine is a direct decarboxylation product. * **Tyramine:** Produced by the decarboxylation of **Tyrosine**. It is clinically significant due to the "Cheese Reaction" in patients taking MAO inhibitors. **High-Yield NEET-PG Clinical Pearls:** 1. **PLP Dependency:** Almost all amino acid decarboxylases require **Vitamin B6 (Pyridoxine)** as a cofactor. 2. **GABA:** Produced by the decarboxylation of **Glutamate** (via GAD enzyme); it is the major inhibitory neurotransmitter in the CNS. 3. **Dopamine:** Produced by the decarboxylation of **L-Dopa** (derived from Tyrosine). 4. **Cadaverine and Putrescine:** These are polyamines produced by the decarboxylation of Lysine and Ornithine, respectively, often associated with the smell of decaying tissue.
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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