Amino Acid Metabolism Practice Questions

Q: Which of the following is not an enzyme involved in urea biosynthesis?

Carbamoyl phosphate synthetase II. ### Explanation The **Urea Cycle (Ornithine Cycle)** is the primary mechanism for detoxifying ammonia into urea in the liver. To answer this question correctly, one must distinguish between the enzymes of the urea cycle and those of pyrimidine synthesis. **1. Why Carbamoyl Phosphate Synthetase II (CPS-II) is the correct answer:** CPS-II is **not** involved in the urea cycle. It is the rate-limiting enzyme for **De Novo Pyrimidine Synthesis**. It is located in the **cytosol** of all nucleated cells and uses glutamine as a nitrogen source. In contrast, **CPS-I** is the rate-limiting enzyme for the urea cycle, located in the **mitochondria**, and requires N-acetylglutamate (NAG) as an activator. **2. Analysis of Incorrect Options:** * **Ornithine Transcarbamylase (OTC):** This is the second enzyme of the urea cycle (mitochondrial). It combines carbamoyl phosphate with ornithine to form citrulline. Deficiency of OTC is the most common urea cycle disorder (X-linked). * **Argininosuccinase (Argininosuccinate Lyase):** This cytosolic enzyme cleaves argininosuccinate into arginine and fumarate. Fumarate provides a link to the TCA cycle (Krebs' Bi-cycle). * **Arginase:** This is the final enzyme of the cycle. It hydrolyzes arginine into urea and regenerates ornithine. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Urea Cycle Enzymes:** **C**an **O**ur **A**unt **A**lways **A**ssist? (**C**PS-I, **O**TC, **A**rgininosuccinate synthetase, **A**rgininosuccinate lyase, **A**rginase). * **Compartmentalization:** The first two steps occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. * **CPS-I vs. CPS-II:** Remember **"I"** for **Urea** (one "u" in urea) and **"II"** for **Pyrimidines** (two "i"s in pyrimidine). * **Rate-limiting step:** CPS-I (requires N-acetylglutamate as an absolute allosteric activator).

Q: In one-carbon metabolism, when serine is converted to glycine, which carbon atom is transferred to tetrahydrofolate (THFA)?

Beta carbon. The conversion of **Serine to Glycine** is a pivotal reaction in one-carbon metabolism, catalyzed by the enzyme **Serine Hydroxymethyltransferase (SHMT)**. This reaction requires **Pyridoxal Phosphate (PLP)** as a cofactor and **Tetrahydrofolate (THF)** as the one-carbon acceptor. ### Why the Beta Carbon is Correct Serine is a three-carbon amino acid, while glycine is a two-carbon amino acid. During this reaction, the **side chain carbon (the Beta-carbon)** of serine is cleaved and transferred to THF. This results in the formation of **N5, N10-methylene THF**, which is the most important source of one-carbon units in the body. This one-carbon unit is subsequently used for the synthesis of thymidine (DNA) and purines. ### Why Other Options are Incorrect * **Alpha Carbon:** The alpha carbon (C2) remains part of the amino acid structure, becoming the alpha carbon of the resulting glycine molecule. * **Gamma and Delta Carbons:** These options are structurally impossible for serine. Serine is a small amino acid with only three carbons (Carboxyl C1, Alpha C2, and Beta C3). Gamma and Delta carbons are found in longer-chain amino acids like glutamate or lysine. ### High-Yield Clinical Pearls for NEET-PG * **Cofactor Requirement:** SHMT is a **PLP-dependent** enzyme. Do not confuse this with the THF requirement; both are essential. * **The "One-Carbon Pool":** Serine is considered the **major physiological source** of one-carbon units in human metabolism. * **Clinical Link:** Methotrexate (an anticancer drug) inhibits dihydrofolate reductase, depleting the THF pool and indirectly halting the serine-to-glycine conversion, thereby inhibiting DNA synthesis. * **Reversibility:** This reaction is reversible and also serves as a pathway for the de novo synthesis of serine from glycine.

Q: Transfer of an amino group from an amino acid to an alpha keto acid is catalyzed by which class of enzymes?

Transaminases. **Explanation:** **1. Why Transaminases is Correct:** Transamination is the first step in the catabolism of most amino acids. This process involves the transfer of an $\alpha$-amino group from an amino acid to an $\alpha$-keto acid (usually $\alpha$-ketoglutarate), resulting in the formation of a new amino acid (glutamate) and a new $\alpha$-keto acid. This reaction is catalyzed by **Transaminases** (also known as Aminotransferases). These enzymes require **Pyridoxal Phosphate (PLP)**, a derivative of Vitamin B6, as an essential cofactor. **2. Why Other Options are Incorrect:** * **Aminases:** These enzymes catalyze the addition of an amine group (amination), but the term is not used for the specific transfer reaction between amino acids and keto acids. * **Transketolase:** This enzyme is part of the Pentose Phosphate Pathway (HMP Shunt). It transfers two-carbon units between sugars and requires Thiamine Pyrophosphate (TPP) as a cofactor. * **Decarboxylase:** These enzymes catalyze the removal of a carboxyl group (as $CO_2$) from substrates. In amino acid metabolism, decarboxylation converts amino acids into biogenic amines (e.g., Histidine to Histamine). **3. NEET-PG High-Yield Clinical Pearls:** * **Cofactor:** All transaminases require **Vitamin B6 (PLP)**. * **Diagnostic Markers:** AST (SGOT) and ALT (SGPT) are transaminases used as sensitive markers for liver injury. AST is also found in the heart and muscle. * **Exceptions:** Two amino acids, **Lysine and Threonine**, do not undergo transamination. * **The "Funnel":** Most amino acids transfer their amino group to $\alpha$-ketoglutarate to form **Glutamate**, which then undergoes oxidative deamination in the liver to release free ammonia.

Q: Selenocysteine is a derivative of which amino acid?

Serine. **Explanation:** **Selenocysteine (Sec)**, often referred to as the **21st amino acid**, is unique because it is not coded directly by the genetic code but is synthesized while attached to its own specific tRNA (tRNA$^{Sec}$). 1. **Why Serine is Correct:** The synthesis of selenocysteine begins with the "charging" of tRNA$^{Sec}$ with the amino acid **Serine** by seryl-tRNA synthetase. Once attached, the serine residue is enzymatically converted into selenocysteine. Specifically, the hydroxyl group of serine is replaced by a selenol group (-SeH) using selenophosphate as the selenium donor. Thus, the carbon skeleton of selenocysteine is derived entirely from **Serine**. 2. **Why Other Options are Incorrect:** * **Alanine:** While structurally similar (selenocysteine is technically a selenium analog of cysteine, which is a thio-analog of alanine), alanine does not serve as the biosynthetic precursor. * **Arginine & Glycine:** These amino acids are involved in other metabolic pathways (e.g., urea cycle, heme synthesis) but do not contribute to the formation of the selenocysteine-tRNA complex. **High-Yield Clinical Pearls for NEET-PG:** * **UGA Codon:** Selenocysteine is encoded by the **UGA** stop codon. It is incorporated only when a specific mRNA sequence called the **SECIS element** (Selenocysteine Insertion Sequence) is present. * **Essential Selenoproteins:** Key enzymes containing selenocysteine include **Glutathione peroxidase** (antioxidant), **Thioredoxin reductase**, and **Deiodinase** (converts T4 to T3). * **Structure:** It is an analog of Cysteine where the Sulfur (S) atom is replaced by **Selenium (Se)**.

Q: Which neurotransmitter is synthesized from choline and acetyl-CoA?

Acetylcholine. ### Explanation **Correct Answer: A. Acetylcholine** **Mechanism of Synthesis:** Acetylcholine (ACh) is the primary neurotransmitter of the parasympathetic nervous system, neuromuscular junctions, and preganglionic autonomic neurons. It is synthesized in the nerve terminal from two precursors: **Choline** (derived from dietary sources and recycled from the synaptic cleft) and **Acetyl-CoA** (produced in mitochondria during glucose metabolism). The reaction is catalyzed by the enzyme **Choline Acetyltransferase (ChAT)**. Once synthesized, ACh is stored in vesicles by the vesicular acetylcholine transporter (VAChT). **Analysis of Incorrect Options:** * **B. Glutamine:** This is a non-essential amino acid that serves as a precursor for **Glutamate** (the brain's primary excitatory neurotransmitter) via the enzyme glutaminase. * **C. Aspartate:** This is an excitatory amino acid neurotransmitter synthesized from **Oxaloacetate** (a Krebs cycle intermediate) via transamination. * **D. Nitric Oxide (NO):** This is a gaseous neurotransmitter synthesized from the amino acid **L-arginine** by the enzyme Nitric Oxide Synthase (NOS). It is not stored in vesicles but is produced on demand. **High-Yield Clinical Pearls for NEET-PG:** * **Termination of Action:** Unlike most neurotransmitters that undergo reuptake, ACh is primarily inactivated by enzymatic degradation via **Acetylcholinesterase (AChE)** in the synaptic cleft. * **Rate-limiting Step:** The uptake of **Choline** into the presynaptic nerve terminal is the rate-limiting step in ACh synthesis. This step can be inhibited by the drug **Hemicholinium**. * **Vesicular Release:** **Botulinum toxin** prevents the release of ACh by cleaving SNARE proteins, leading to flaccid paralysis. * **Clinical Correlation:** Myasthenia Gravis involves autoantibodies against post-synaptic nicotinic ACh receptors, treated with AChE inhibitors (e.g., Neostigmine).

Q: In Glycinuria, glycine is excreted as what?

Oxalate. **Explanation:** **1. Why Oxalate is Correct:** Glycinuria (specifically Primary Hyperoxaluria Type I) is a metabolic disorder caused by a deficiency of the hepatic enzyme **alanine-glyoxylate aminotransferase (AGT)**. Normally, this enzyme converts glyoxylate into glycine. When this pathway is blocked, glyoxylate accumulates and is alternatively oxidized by lactate dehydrogenase into **oxalate**. Oxalate is highly insoluble and is excreted in the urine, where it precipitates with calcium to form calcium oxalate stones (nephrolithiasis) and causes renal damage (nephrocalcinosis). **2. Why the Other Options are Incorrect:** * **A. Urea:** While glycine is a precursor in the urea cycle (providing nitrogen), it is not the primary excretory form of glycine in this specific pathology. * **B. Glutathione:** Glycine is a constituent amino acid of glutathione (along with glutamate and cysteine), but glutathione is a functional antioxidant, not an excretory product. * **C. Formate:** Glycine can enter the one-carbon pool via the glycine cleavage system to produce CO₂ and NH₄⁺, but formate is not the characteristic excretory product in glycinuria. **3. High-Yield Clinical Pearls for NEET-PG:** * **Primary Hyperoxaluria Type I:** Deficiency of Alanine-glyoxylate aminotransferase (peroxisomal). * **Primary Hyperoxaluria Type II:** Deficiency of Glyoxylate reductase/Hydroxypyruvate reductase. * **Clinical Presentation:** Recurrent calcium oxalate stones in a young child should always raise suspicion of these disorders. * **Glycine Cleavage System:** The major physiological pathway for glycine catabolism; deficiency leads to **Non-ketotic Hyperglycinemia**, characterized by severe neurological distress.

Q: Which amino acid cannot be used for glycogen synthesis?

Leucine. ### Explanation **Core Concept: Glucogenic vs. Ketogenic Amino Acids** Amino acids are classified based on their metabolic end-products. To be used for **glycogen synthesis (gluconeogenesis)**, an amino acid must be converted into pyruvate or a Krebs cycle intermediate (like oxaloacetate). **Why Leucine is the Correct Answer:** Leucine is one of the two **purely ketogenic** amino acids (the other being Lysine). Its catabolism yields only **Acetyl-CoA and Acetoacetate**. Since the Pyruvate Dehydrogenase reaction is irreversible in humans, Acetyl-CoA cannot be converted back into pyruvate or glucose. Therefore, Leucine cannot contribute to glycogen synthesis. **Analysis of Incorrect Options:** * **Alanine (Option A):** A purely glucogenic amino acid. It is the primary substrate for gluconeogenesis via the **Cahill cycle**, where it is transaminated directly into **pyruvate**. * **Threonine (Option B):** Primarily glucogenic. It can be converted into pyruvate or alpha-ketobutyrate (which enters the TCA cycle as Succinyl-CoA). * **Methionine (Option D):** A glucogenic amino acid. It is converted into **Succinyl-CoA** via the propionyl-CoA pathway, allowing it to enter the TCA cycle and contribute to glucose formation. **High-Yield NEET-PG Pearls:** * **Purely Ketogenic:** Leucine and Lysine (Mnemonic: The "L"s are ketogenic). * **Both Glucogenic & Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, and Isoleucine (Mnemonic: **PITTT**). * **Purely Glucogenic:** All other 14 amino acids. * **Clinical Link:** In Maple Syrup Urine Disease (MSUD), there is a defect in the breakdown of branched-chain amino acids (Leucine, Isoleucine, Valine). Leucine accumulation is particularly neurotoxic.

Q: Which amino acid acts as a precursor in the production of urea?

Arginine. **Explanation** The correct answer is **Arginine**. In the final step of the Urea Cycle (Krebs-Henseleit cycle), the enzyme **Arginase** hydrolyzes Arginine into **Urea** and **Ornithine**. This makes Arginine the immediate direct precursor from which the urea molecule is cleaved. **Analysis of Options:** * **Arginine (Correct):** It is the only amino acid in the cycle that directly releases urea. This reaction occurs exclusively in the liver due to the presence of the Arginase enzyme. * **Aspartic acid:** While it provides the second nitrogen atom for the urea molecule (combining with Citrulline to form Argininosuccinate), it is not the direct precursor of urea itself. * **Ornithine:** It acts as a "catalyst" or a carrier in the cycle. It re-enters the mitochondria to start another round but does not produce urea; rather, it is a *product* of the reaction that releases urea. * **Glutamate:** It plays a vital role in collecting nitrogen from other amino acids (via transamination) to form Ammonia and Aspartate, but it is an upstream nitrogen donor, not a direct precursor. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Formation of Carbamoyl Phosphate by **CPS-I** (requires N-acetylglutamate as an activator). * **Site:** The cycle occurs in both the **Mitochondria** (first two steps) and **Cytosol** (remaining steps). * **Hyperammonemia Type II:** The most common urea cycle disorder, caused by **Ornithine Transcarbamoylase (OTC) deficiency** (X-linked). * **BUN (Blood Urea Nitrogen):** Urea is the chief disposal form of amino acid nitrogen; its levels decrease in severe liver disease and increase in renal failure.

Q: Phenylpyruvic acid in the urine is detected by which test?

Ferric chloride test. **Explanation:** The correct answer is **D. Ferric chloride test**. **1. Why Ferric Chloride Test is correct:** Phenylketonuria (PKU) is an autosomal recessive disorder caused by a deficiency of the enzyme **phenylalanine hydroxylase**. This leads to the accumulation of phenylalanine, which is alternatively metabolized into phenylketones like **phenylpyruvic acid**, phenylacetate, and phenyllactate. When a few drops of 10% ferric chloride ($FeCl_3$) are added to the urine of a patient with PKU, the phenylpyruvic acid reacts with the iron ions to produce a characteristic **transient blue-green color**. **2. Analysis of Incorrect Options:** * **A. Guthrie’s test:** This is a **semi-quantitative bacterial inhibition assay** used for neonatal screening of PKU. It detects elevated levels of phenylalanine in the *blood*, not phenylpyruvic acid in the urine. It utilizes the growth of *Bacillus subtilis*. * **B. VMA in urine:** Vanillylmandelic acid (VMA) is a breakdown product of catecholamines (epinephrine/norepinephrine). It is measured in 24-hour urine samples to diagnose **Pheochromocytoma**, not PKU. * **C. Gerhardt’s test:** This is also a ferric chloride-based test, but it is specifically used to detect **acetoacetate (ketone bodies)** in the urine, producing a wine-red color. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mousy/Musty Odor:** The urine of PKU patients has a characteristic "mousy" odor due to phenylacetic acid. * **Clinical Triad:** Intellectual disability, hypopigmentation (fair skin/blue eyes due to decreased melanin), and seizures. * **Dietary Management:** Restriction of phenylalanine and supplementation of **Tyrosine** (which becomes an essential amino acid in PKU). * **Ferric Chloride Test Overlap:** It also gives a **purple** color in Alkaptonuria (homogentisic acid) and a **gray-green** color in Maple Syrup Urine Disease (MSUD).

Q: Ammonia for the urea cycle is supplied by the action of which of the following enzymes?

Glutamate dehydrogenase. **Explanation:** The urea cycle is the primary mechanism for detoxifying ammonia into urea in the liver. To initiate the cycle, ammonia must be provided for the synthesis of Carbamoyl Phosphate. **Why Glutamate Dehydrogenase (GDH) is correct:** GDH plays a pivotal role in **oxidative deamination**. Most amino acids transfer their $\alpha$-amino group to $\alpha$-ketoglutarate to form **Glutamate** (via transamination). GDH then catalyzes the release of the free ammonium ion ($NH_4^+$) from glutamate in the mitochondrial matrix. This free ammonia then combines with $CO_2$ and ATP, catalyzed by *Carbamoyl Phosphate Synthetase-I (CPS-I)*, to enter the urea cycle. **Analysis of Incorrect Options:** * **Aspartate aminotransferase (AST):** While AST is involved, it provides the **second nitrogen** atom of urea by transferring an amino group from glutamate to oxaloacetate to form **Aspartate**. It does not release free ammonia. * **Argininosuccinate synthase:** This is the third enzyme of the urea cycle that condenses citrulline with aspartate. * **Argininosuccinate lyase:** This enzyme cleaves argininosuccinate into arginine and fumarate. **NEET-PG High-Yield Pearls:** * **Two Sources of Nitrogen:** The first nitrogen of urea comes from **free ammonia** (via GDH); the second nitrogen comes from **Aspartate** (via AST). * **Rate-Limiting Step:** CPS-I is the rate-limiting enzyme of the urea cycle and requires **N-acetylglutamate (NAG)** as an absolute allosteric activator. * **Location:** The first two steps (CPS-I and Ornithine Transcarbamoylase) occur in the **mitochondria**, while the remaining steps occur in the **cytosol**.

Question 1

Which of the following is not an enzyme involved in urea biosynthesis?

Accepted Answer

Carbamoyl phosphate synthetase II

Answer

Ornithine transcarbamylase

Answer

Argininosuccinase

Answer

Arginase

Question 2

In one-carbon metabolism, when serine is converted to glycine, which carbon atom is transferred to tetrahydrofolate (THFA)?

Accepted Answer

Beta carbon

Answer

Alpha carbon

Answer

Delta carbon

Answer

Gamma carbon

Question 3

Transfer of an amino group from an amino acid to an alpha keto acid is catalyzed by which class of enzymes?

Accepted Answer

Transaminases

Answer

Aminases

Answer

Transketolase

Answer

Decarboxylase

Question 4

Selenocysteine is a derivative of which amino acid?

Accepted Answer

Serine

Answer

Alanine

Answer

Arginine

Answer

Glycine

Question 5

Which neurotransmitter is synthesized from choline and acetyl-CoA?

Accepted Answer

Acetylcholine

Answer

Glutamine

Answer

Aspartate

Answer

Nitric Oxide

Question 6

In Glycinuria, glycine is excreted as what?

Accepted Answer

Oxalate

Answer

Urea

Answer

Glutathione

Answer

Formate

Question 7

Which amino acid cannot be used for glycogen synthesis?

Accepted Answer

Leucine

Answer

Alanine

Answer

Threonine

Answer

Methionine

Question 8

Which amino acid acts as a precursor in the production of urea?

Accepted Answer

Arginine

Answer

Aspartic acid

Answer

Ornithine

Answer

Glutamate

Question 9

Phenylpyruvic acid in the urine is detected by which test?

Accepted Answer

Ferric chloride test

Answer

Guthrie's test

Answer

VMA in urine

Answer

Gerhardt's test

Question 10

Ammonia for the urea cycle is supplied by the action of which of the following enzymes?

Accepted Answer

Glutamate dehydrogenase

Answer

Aspartate aminotransferase

Answer

Argininosuccinate synthase

Answer

Argininosuccinate lyase

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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