Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 121: Which of the following contains glycine?

A. Hemoglobin
B. Glutathione
C. Purine
D. All of the above (Correct Answer)

Explanation: ### Explanation Glycine is the simplest non-essential amino acid and serves as a vital precursor for several physiologically important compounds. The correct answer is **"All of the above"** because glycine is a fundamental building block for heme, glutathione, and purine rings. **1. Hemoglobin (Heme Synthesis):** The first and rate-limiting step of heme synthesis involves the condensation of **Succinyl CoA and Glycine** to form $\delta$-aminolevulinic acid (ALA), catalyzed by the enzyme ALA synthase (with Vitamin B6 as a cofactor). Since heme is the prosthetic group of hemoglobin, glycine is essential for its formation. **2. Glutathione (GSH):** Glutathione is a potent intracellular antioxidant and a tripeptide composed of three amino acids: **Glutamate, Cysteine, and Glycine**. It plays a critical role in neutralizing free radicals and maintaining red blood cell integrity. **3. Purine Synthesis:** Glycine contributes to the formation of the purine ring (Adenine and Guanine). Specifically, the **entire glycine molecule** is incorporated to provide carbons C4, C5, and nitrogen N7 of the purine skeleton. ### High-Yield NEET-PG Pearls: * **Creatine Synthesis:** Glycine combines with Arginine and S-adenosylmethionine (SAM) to form creatine. * **Bile Acid Conjugation:** Glycine conjugates with cholic acid to form glycocholic acid (a primary bile salt). * **Collagen Structure:** Glycine is the most abundant amino acid in collagen, occurring at every third position (Gly-X-Y) to allow for tight triple-helix packing. * **Inhibitory Neurotransmitter:** Glycine acts as an inhibitory neurotransmitter in the spinal cord and brainstem.

Question 122: In mammalian tissue, glycine is synthesized from which of the following precursors?

A. Choline
B. Serine
C. Alanine
D. All of the above (Correct Answer)

Explanation: **Explanation:** Glycine is a non-essential amino acid, meaning it can be synthesized endogenously in mammalian tissues through multiple metabolic pathways. 1. **From Serine (Major Pathway):** The most significant source of glycine is serine. The enzyme **Serine Hydroxymethyltransferase (SHMT)** catalyzes the reversible conversion of serine to glycine. This reaction requires **Pyridoxal Phosphate (B6)** and **Tetrahydrofolate (THF)**, which acts as the one-carbon acceptor. 2. **From Choline:** Choline is oxidized to betaine, which subsequently undergoes demethylation to form dimethylglycine and then sarcosine (N-methylglycine). Sarcosine is finally converted into glycine by sarcosine dehydrogenase. 3. **From Alanine:** While not the primary pathway, glycine can be synthesized from **Threonine** via the threonine aldolase pathway, which produces acetaldehyde and glycine. Additionally, glyoxylate (derived from hydroxyproline or via transamination of alanine) can be converted to glycine by the enzyme **Alanine-Glyoxylate Aminotransferase**. **Why "All of the Above" is correct:** Glycine synthesis is highly versatile. While Serine is the immediate precursor, both Choline and Alanine (via glyoxylate) contribute to the glycine pool through distinct metabolic intermediates. **High-Yield Clinical Pearls for NEET-PG:** * **Glycine Encephalopathy (Non-ketotic Hyperglycinemia):** Caused by a defect in the **Glycine Cleavage System**, leading to massive accumulation of glycine in the CNS. * **Primary Hyperoxaluria Type 1:** Deficiency of **Alanine-Glyoxylate Aminotransferase** leads to the accumulation of glyoxylate, which is oxidized to oxalate, causing renal stones and nephrocalcinosis. * **Heme Synthesis:** Glycine is the starting substrate for heme synthesis (Glycine + Succinyl CoA → ALA). * **Inhibitory Neurotransmitter:** Glycine acts as a major inhibitory neurotransmitter in the spinal cord.

Question 123: Hyperammonemia inhibits the TCA cycle by depleting which intermediate?

A. Oxaloacetate
B. α-ketoglutarate (Correct Answer)
C. Citrate
D. Succinyl CoA

Explanation: **Explanation:** The primary mechanism by which hyperammonemia inhibits the TCA cycle is through the **depletion of α-ketoglutarate**. When blood ammonia ($NH_3$) levels rise, the body attempts to detoxify it, particularly in the brain. This occurs via two sequential reactions: 1. **Reductive Amination:** Ammonia reacts with **α-ketoglutarate** 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*. Because α-ketoglutarate is a vital intermediate of the TCA cycle, its excessive consumption to "mop up" ammonia leads to its depletion. This halts the TCA cycle, resulting in a failure of aerobic respiration and a subsequent **ATP deficit** in neurons, which manifests as encephalopathy. **Analysis of Incorrect Options:** * **Oxaloacetate:** While it can be converted to Aspartate via transamination, it is not the primary substrate consumed during acute ammonia detoxification. * **Citrate:** This is the product of the first step of the TCA cycle; its levels fall secondary to the lack of α-ketoglutarate and slowed cycle turnover, but it is not the direct target of depletion. * **Succinyl CoA:** This is a downstream intermediate. Its levels decrease as a consequence of the cycle's failure, not as the primary cause. **NEET-PG High-Yield Pearls:** * **Brain's main detoxification route:** Glutamine synthesis (this causes osmotic swelling of astrocytes, leading to cerebral edema). * **Key Enzyme inhibited:** High ammonia also inhibits the **α-ketoglutarate dehydrogenase** complex directly. * **Clinical Presentation:** Asterixis (flapping tremors), altered sensorium, and elevated blood lactate (due to shifted metabolism toward glycolysis).

Question 124: Increased risk of Myocardial infarction is associated with which amino acid?

A. Methionine
B. Homocystine (Correct Answer)
C. Ornithine
D. Melatonin

Explanation: ### Explanation **Correct Answer: B. Homocystine** **Mechanism and Medical Concept:** Hyperhomocysteinemia is a well-established independent risk factor for coronary artery disease (CAD) and myocardial infarction (MI). Homocystine is the oxidized dimer of **homocysteine**. Elevated levels lead to vascular injury through several mechanisms: 1. **Endothelial Dysfunction:** It promotes the production of reactive oxygen species (ROS), damaging the vessel lining. 2. **Pro-thrombotic State:** It increases platelet aggregation and activates the coagulation cascade (Factor V activation). 3. **Plaque Instability:** It interferes with collagen cross-linking and promotes the oxidation of LDL cholesterol, accelerating atherosclerosis. **Analysis of Incorrect Options:** * **A. Methionine:** While homocysteine is derived from methionine via the S-adenosylmethionine (SAM) cycle, methionine itself is an essential amino acid and is not directly vasculotoxic. * **C. Ornithine:** This is an intermediate in the Urea Cycle. Elevated levels (as seen in HHH syndrome) are associated with neurological issues and chorioretinal degeneration (Gyrate atrophy), not MI. * **D. Melatonin:** This is a hormone derived from Tryptophan that regulates sleep-wake cycles. It actually has antioxidant properties and is not associated with an increased risk of MI. **High-Yield Clinical Pearls for NEET-PG:** * **Homocystinuria:** Most commonly caused by a deficiency of **Cystathionine β-synthase**. Clinical features include ectopia lentis (downward dislocation), marfanoid habitus, and premature arterial/venous thrombosis. * **Cofactors:** Deficiencies in **Vitamin B6 (Pyridoxine)**, **B9 (Folate)**, and **B12 (Cobalamin)** lead to elevated homocysteine levels because they are essential cofactors for its metabolism. * **Treatment:** Supplementation with B6, B12, and Folate is used to lower homocysteine levels, though its impact on preventing secondary MI is still debated in clinical trials.

Question 125: Which amino acid undergoes hepatic deamination?

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

Explanation: **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.

Question 126: What is the chemical process involved in the conversion of noradrenaline to adrenaline?

A. Hydroxylation
B. Carboxylation
C. Methylation (Correct Answer)
D. Dehydrogenation

Explanation: **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.

Question 127: In alkaptonuria, what substance is found in the urine?

A. Homogentisic acid (Correct Answer)
B. Phenylalanine
C. Ketones
D. Acetates

Explanation: **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.

Question 128: In a person suffering from phenylketonuria, which of the following should be restricted in their diet?

A. Fat
B. Protein (Correct Answer)
C. Glycine
D. Glutamate

Explanation: **Explanation:** **Phenylketonuria (PKU)** is an autosomal recessive inborn error of metabolism caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)**. This enzyme normally converts the essential amino acid **Phenylalanine** into Tyrosine. When PAH is deficient, phenylalanine accumulates in the blood and tissues, leading to severe intellectual disability, seizures, and a "mousy" body odor. **Why Protein is the Correct Answer:** Phenylalanine is an essential amino acid found in almost all natural **dietary proteins**. Since the body cannot metabolize excess phenylalanine in PKU patients, the primary treatment strategy is a **strict lifelong restriction of natural protein intake**. Patients must meet their protein requirements through specialized medical formulas that are phenylalanine-free but supplemented with Tyrosine (which becomes an "essential" amino acid in these patients). **Analysis of Incorrect Options:** * **A. Fat:** Fat metabolism is not affected in PKU. While a balanced diet is necessary, fat restriction does not impact phenylalanine levels. * **C & D. Glycine and Glutamate:** These are non-essential amino acids. While they are components of protein, restricting them individually is ineffective and unnecessary. The clinical focus is specifically on limiting the intake of Phenylalanine, which is best achieved by limiting total natural protein. **NEET-PG High-Yield Pearls:** * **Deficiency:** Most commonly Phenylalanine Hydroxylase; rarely **Dihydropteridine reductase (BH4 deficiency)**. * **Diagnosis:** Screened via the **Guthrie Test** (bacterial inhibition assay) or Tandem Mass Spectrometry. * **Clinical Feature:** "Mousy" or "Musty" odor due to **phenylacetate** in sweat and urine. * **Dietary Warning:** Avoid **Aspartame** (an artificial sweetener), as it is metabolized into phenylalanine.

Question 129: Valine is an essential amino acid because it cannot be synthesized by the body. Which of the following describes the side chain of valine?

A. Contains a sulfhydryl group
B. Contains a branched chain hydrocarbon (Correct Answer)
C. Contains an aromatic ring
D. Does not have a side chain

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** Valine is one of the three **Branched-Chain Amino Acids (BCAAs)**, along with Leucine and Isoleucine. Its side chain consists of an isopropyl group ($–CH(CH_3)_2$), which is a branched hydrocarbon. These amino acids are non-polar, hydrophobic, and essential, meaning they must be obtained through the diet. In the body, BCAAs are primarily metabolized in the skeletal muscle rather than the liver, serving as a critical energy source during fasting or prolonged exercise. **2. Analysis of Incorrect Options:** * **Option A (Sulfhydryl group):** This describes **Cysteine**. The sulfhydryl (–SH) group is crucial for forming disulfide bonds, which stabilize the tertiary and quaternary structures of proteins. * **Option C (Aromatic ring):** This describes amino acids like **Phenylalanine, Tyrosine, and Tryptophan**. These contain cyclic benzene-like rings and are responsible for UV light absorption at 280 nm. * **Option D (No side chain):** This describes **Glycine**. Glycine is the simplest amino acid where the "side chain" is merely a Hydrogen atom, making it the only achiral amino acid. **3. Clinical Pearls for NEET-PG:** * **Maple Syrup Urine Disease (MSUD):** A high-yield clinical condition caused by a deficiency in the **Branched-chain α-keto acid dehydrogenase** complex. This leads to the accumulation of Valine, Leucine, and Isoleucine, giving urine a characteristic burnt-sugar odor. * **Glucogenic vs. Ketogenic:** Valine is purely **glucogenic**, whereas Leucine is purely ketogenic, and Isoleucine is both. * **Sickle Cell Anemia:** A classic molecular pathology where **Valine replaces Glutamic acid** at the 6th position of the Beta-globin chain, causing the hemoglobin to polymerize under deoxygenated conditions.

Question 130: Ammonia is toxic to the brain because it leads to depletion of which substrate?

A. Succinate
B. Alpha ketoglutarate (Correct Answer)
C. Isocitrate
D. Fumarate

Explanation: **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.

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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