Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 391: Which of the following is not an enzyme involved in urea biosynthesis?

A. Carbamoyl phosphate synthetase II (Correct Answer)
B. Ornithine transcarbamylase
C. Argininosuccinase
D. Arginase

Explanation: ### 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).

Question 392: In one-carbon metabolism, when serine is converted to glycine, which carbon atom is transferred to tetrahydrofolate (THFA)?

A. Alpha carbon
B. Beta carbon (Correct Answer)
C. Delta carbon
D. Gamma carbon

Explanation: 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.

Question 393: Primary Hyperoxaluria occurs due to a defect in the metabolism of which of the following?

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

Explanation: **Explanation:** **Primary Hyperoxaluria Type 1 (PH1)** is an autosomal recessive metabolic disorder caused by a deficiency of the hepatic peroxisomal enzyme **Alanine-Glyoxylate Aminotransferase (AGT)**. 1. **Why Glycine is correct:** In normal metabolism, **Glycine** is converted to **Glyoxylate**. The enzyme AGT (which requires Vitamin B6 as a cofactor) then converts glyoxylate back into glycine. When AGT is defective, glyoxylate cannot be converted back to glycine; instead, it is oxidized into **Oxalate**. Since oxalate is an end-product that cannot be metabolized further in humans, it accumulates and precipitates as calcium oxalate crystals. 2. **Why other options are incorrect:** * **Cysteine:** Metabolism of cysteine is associated with *Cystinuria* (defective renal transport) or *Cystinosis* (lysosomal storage), not oxalate production. * **Tryptophan:** Defects in tryptophan metabolism lead to *Hartnup disease* (niacin deficiency symptoms) or the production of serotonin/melatonin metabolites. * **Tyrosine:** Defects here lead to *Alkaptonuria* (homogentisic acid accumulation) or *Tyrosinemia*, characterized by liver failure or cabbage-like odor. **Clinical Pearls & High-Yield Facts:** * **Clinical Presentation:** PH1 leads to recurrent **nephrolithiasis** (calcium oxalate stones), nephrocalcinosis, and eventually end-stage renal disease (ESRD). * **Systemic Oxalosis:** Once kidneys fail, oxalate deposits in bones, joints, and heart. * **Treatment:** High fluid intake, Vitamin B6 (Pyridoxine) supplementation (as it is a cofactor for AGT), and in severe cases, a combined liver-kidney transplant. * **Ethylene Glycol Poisoning:** This is an *acquired* cause of hyperoxaluria, as ethylene glycol is metabolized to glyoxylate and then oxalate.

Question 394: Transfer of an amino group from an amino acid to an alpha keto acid is catalyzed by which class of enzymes?

A. Transaminases (Correct Answer)
B. Aminases
C. Transketolase
D. Decarboxylase

Explanation: **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.

Question 395: Which of the following amino acids is involved in the one-carbon pool?

A. Glycine (Correct Answer)
B. Proline
C. Threonine
D. Hydroxyproline

Explanation: **Explanation:** The **one-carbon pool** refers to a collection of single-carbon units (such as methyl, methylene, and formyl groups) attached to the carrier **Tetrahydrofolate (THF)**. These units are essential for the synthesis of purines and thymidine (DNA synthesis). **Why Glycine is Correct:** Glycine is a major contributor to the one-carbon pool through two primary pathways: 1. **Glycine Cleavage System:** This multi-enzyme complex decomposes glycine into CO₂, NH₄⁺, and a methylene group, which is transferred to THF to form **N5, N10-methylene THF**. 2. **Conversion to Serine:** Glycine can react with N5, N10-methylene THF to form Serine (catalyzed by serine hydroxymethyltransferase). Serine itself is considered the most important source of one-carbon units. **Analysis of Incorrect Options:** * **Proline & Hydroxyproline:** These are cyclic amino acids. Their metabolism involves conversion to glutamate and subsequently entering the TCA cycle; they do not directly donate single-carbon units to the THF pool. * **Threonine:** While threonine can be degraded to glycine in some organisms, in humans, its primary catabolic pathway leads to alpha-ketobutyrate (entering the TCA cycle as Succinyl-CoA) or acetaldehyde, not directly to the one-carbon pool. **High-Yield Clinical Pearls for NEET-PG:** * **Sources of 1-C units:** Remember the mnemonic **"SHIT"**: **S**erine (major source), **H**istidine, **I**soleucine (incorrect, actually **I**midazole ring of Histidine), and **T**ryptophan. **Glycine** and **Choline** are also vital donors. * **FIGLU Test:** Histidine donates a formimino group. In Vitamin B12 or Folate deficiency, **FIGLU (Formiminoglutamate)** is excreted in the urine, serving as a diagnostic marker. * **The "Methyl Trap":** Vitamin B12 deficiency leads to a functional folate deficiency because folate becomes "trapped" as N5-methyl THF, unable to return to the active pool.

Question 396: The urea cycle is linked to the TCA cycle by which molecule?

A. Arginine
B. Ornithine
C. Oxaloacetate
D. Fumarate (Correct Answer)

Explanation: ### Explanation The urea cycle and the TCA cycle are interconnected via the **"Krebs' Bicycle"** (or the aspartate-argininosuccinate shunt). **Why Fumarate is Correct:** During the fourth step of the urea cycle, the enzyme **argininosuccinate lyase** cleaves argininosuccinate into **arginine** and **fumarate**. While arginine continues the urea cycle, fumarate serves as the bridge to the TCA cycle. Once produced in the cytosol, fumarate is hydrated to malate, which can then enter the mitochondria to join the TCA cycle or be oxidized to oxaloacetate to regenerate aspartate (via transamination), which re-enters the urea cycle. **Analysis of Incorrect Options:** * **Arginine (A):** This is an intermediate within the urea cycle that is subsequently cleaved by arginase to produce urea and ornithine. It does not directly enter the TCA cycle. * **Ornithine (B):** This molecule acts as a carrier that is regenerated at the end of the urea cycle. It re-enters the mitochondria to pick up another carbamoyl phosphate but has no direct role in the TCA cycle. * **Oxaloacetate (C):** While oxaloacetate is a TCA cycle intermediate and a precursor to aspartate (which provides the second nitrogen for urea), it is not the direct molecule produced *by* the urea cycle to link the two. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** The urea cycle occurs in both the **mitochondria** (first two steps) and the **cytosol** (remaining steps) of hepatocytes. * **Rate-limiting enzyme:** Carbamoyl Phosphate Synthetase I (CPS-I), which requires **N-acetylglutamate (NAG)** as an essential allosteric activator. * **Nitrogen Sources:** One nitrogen atom comes from free **Ammonia (NH₃)**, and the second comes from **Aspartate**. * **Energetics:** The synthesis of one molecule of urea consumes **3 ATP** (but breaks 4 high-energy phosphate bonds).

Question 397: Selenocysteine is a derivative of which amino acid?

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

Explanation: **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)**.

Question 398: Glycine is useful in the synthesis of all of the following except?

A. Purine synthesis
B. Creatine synthesis
C. Spermine synthesis (Correct Answer)
D. Heme synthesis

Explanation: **Explanation:** Glycine is the simplest non-essential amino acid and serves as a vital precursor for several high-yield metabolic pathways. **Why Spermine is the correct answer:** Spermine and spermidine are **polyamines** involved in cell proliferation. They are synthesized from **Methionine** (via S-adenosylmethionine/SAM) and **Ornithine** (via putrescine). Glycine does not contribute to the carbon or nitrogen skeleton of polyamines. **Why the other options are incorrect:** * **Purine Synthesis:** Glycine provides the entire **C4, C5, and N7** atoms of the purine ring. It is a major structural contributor to adenine and guanine. * **Creatine Synthesis:** Glycine combines with Arginine (forming guanidinoacetate) and Methionine (as a methyl donor) to synthesize creatine, which is essential for energy storage in muscles. * **Heme Synthesis:** Glycine is the starting substrate for heme. It condenses with Succinyl CoA to form **delta-aminolevulinic acid (d-ALA)**, catalyzed by the rate-limiting enzyme ALA synthase. **High-Yield Clinical Pearls for NEET-PG:** * **Glycine Conjugation:** Glycine is used for the detoxification of benzoic acid (forming hippuric acid) and in the synthesis of **glycocholic acid** (bile salt). * **Glutathione:** Glycine is one of the three amino acids (along with Glutamate and Cysteine) that form the antioxidant tripeptide Glutathione. * **Inhibitory Neurotransmitter:** Glycine acts as an inhibitory neurotransmitter in the spinal cord and brainstem. * **Primary Hyperoxaluria:** A deficiency in glycine metabolism enzymes can lead to excess oxalate production and renal stones.

Question 399: What are the limiting amino acids in wheat?

A. Lysine, arginine
B. Threonine, methionine
C. Lysine, threonine (Correct Answer)
D. Lysine, methionine

Explanation: **Explanation:** The concept of **limiting amino acids** refers to the essential amino acid present in the lowest amount relative to the body's requirements in a specific food source. When an essential amino acid is deficient, it limits the body's ability to synthesize proteins, even if other amino acids are abundant. **Why Option C is Correct:** Cereals, including **wheat**, are characteristically deficient in **Lysine**. While Lysine is the primary limiting amino acid in most cereals, **Threonine** is considered the second limiting amino acid in wheat. Therefore, the combination of Lysine and Threonine represents the most accurate deficiency profile for wheat protein. **Analysis of Incorrect Options:** * **Option A (Lysine, Arginine):** While Lysine is correct, Arginine is a semi-essential amino acid and is generally not a limiting factor in cereal proteins. * **Option B (Threonine, Methionine):** Methionine is the limiting amino acid in **pulses (legumes)** and milk, not wheat. * **Option D (Lysine, Methionine):** This is a common distractor. While Lysine is the primary deficiency in wheat, Methionine is actually relatively abundant in cereals (cereal proteins are used to complement the methionine deficiency in pulses). **High-Yield Clinical Pearls for NEET-PG:** * **Pulses/Legumes:** Limiting amino acid is **Methionine** (rich in Lysine). * **Cereals (Wheat/Rice):** Limiting amino acid is **Lysine** (rich in Methionine). * **Maize (Corn):** Limiting amino acids are **Lysine and Tryptophan**. (Note: Tryptophan deficiency in maize leads to Pellagra because Tryptophan is a precursor for Niacin/Vitamin B3). * **Mutual Supplementation:** This is the nutritional strategy of combining cereals and pulses (e.g., Dal-Chawal) to provide a complete amino acid profile, as they compensate for each other's deficiencies.

Question 400: Which of these amino acids will migrate slowest to the anode end at physiological pH?

A. Aspartic acid
B. Glycine
C. Lysine (Correct Answer)
D. Valine

Explanation: ### Explanation The migration of amino acids in an electric field depends on their **net charge** at a specific pH. At physiological pH (~7.4), the charge of an amino acid is determined by its side chain (R-group). **1. Why Lysine is Correct:** Lysine is a **basic amino acid**. At physiological pH (7.4), its side chain amino group is protonated, giving the molecule a **net positive charge**. Since "opposites attract," positively charged molecules (cations) migrate toward the negative electrode (cathode). Because it is moving away from or resisting the **anode (positive electrode)**, it will migrate the slowest toward it compared to neutral or acidic amino acids. **2. Analysis of Incorrect Options:** * **Aspartic Acid (Option A):** This is an **acidic amino acid**. At pH 7.4, its carboxyl side chain loses a proton, giving it a **net negative charge**. It will migrate the **fastest** toward the anode. * **Glycine (Option B) & Valine (Option D):** These are **neutral, non-polar amino acids**. At physiological pH, they exist primarily as zwitterions (net charge near zero). While they don't migrate as fast as Aspartic acid, they lack the positive repulsion of Lysine, thus moving faster toward the anode than Lysine would. **3. High-Yield Clinical Pearls for NEET-PG:** * **Isoelectric Point (pI):** The pH at which an amino acid has no net charge and does not migrate in an electric field. * **Acidic AAs (Aspartate, Glutamate):** pI is low (~3); they are negatively charged at pH 7.4. * **Basic AAs (Lysine, Arginine, Histidine):** pI is high (>7.5–10); they are positively charged at pH 7.4. * **Mnemonic:** **"Anode is Positive" (Panic: Positive Anode, Negative Is Cathode).** Acidic amino acids love the Anode.

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free