Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 211: Classical phenylketonuria is caused by a deficiency of?

A. Phenylalanine transaminase
B. Phenylalanine hydroxylase (Correct Answer)
C. Tyrosine transaminase
D. Tyrosine hydroxylase

Explanation: **Explanation:** **Classical Phenylketonuria (PKU)** is an autosomal recessive inborn error of metabolism caused by a deficiency of the enzyme **Phenylalanine hydroxylase (PAH)**. 1. **Why Option B is Correct:** Under normal physiological conditions, PAH converts the essential amino acid Phenylalanine into Tyrosine. This reaction requires the cofactor **Tetrahydrobiopterin (BH4)**. A deficiency in PAH leads to the accumulation of Phenylalanine in the blood and tissues, which is then diverted into alternative pathways, forming phenylketones (phenylpyruvate, phenyllactate, and phenylacetate) that are excreted in the urine. 2. **Why Other Options are Incorrect:** * **Option A:** Phenylalanine transaminase is the enzyme that converts phenylalanine into phenylpyruvate. In PKU, this enzyme is actually *overactive* due to the substrate buildup, rather than being deficient. * **Option C:** Tyrosine transaminase deficiency leads to Tyrosinemia Type II (Richner-Hanhart syndrome), characterized by palmoplantar keratoderma and corneal dystrophies. * **Option D:** Tyrosine hydroxylase is involved in the synthesis of catecholamines (Dopamine, Epinephrine). While its activity may be indirectly affected by BH4 levels, it is not the cause of classical PKU. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Features:** Intellectual disability, "mousy" or "musty" body odor, microcephaly, and hypopigmentation (due to decreased melanin synthesis from tyrosine). * **Diagnosis:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry for newborn screening. * **Management:** Dietary restriction of Phenylalanine and supplementation of Tyrosine (which becomes an "essential" amino acid in PKU patients). * **Maternal PKU:** If a mother with PKU does not maintain a strict diet during pregnancy, the fetus may suffer from congenital heart defects and microcephaly, regardless of the fetus's genotype.

Question 212: Which of the following is NOT an enzyme of the urea cycle?

A. Ornithine transcarbamylase
B. Carbamoyl-phosphate synthetase I
C. Argininosuccinase
D. Citrulline synthase (Correct Answer)

Explanation: The urea cycle (Krebs-Henseleit cycle) occurs in the liver to convert toxic ammonia into water-soluble urea. The correct answer is **Citrulline synthase** because no such enzyme exists in the urea cycle; Citrulline is actually synthesized by the enzyme **Ornithine transcarbamylase (OTC)**. ### Why Citrulline synthase is the correct answer: In the second step of the urea cycle, Carbamoyl phosphate reacts with Ornithine to form Citrulline. This reaction is catalyzed by **Ornithine transcarbamylase (OTC)**, not "Citrulline synthase." This is a common distractor in biochemistry exams. ### Analysis of incorrect options: * **A. Ornithine transcarbamylase:** This is a mitochondrial enzyme that catalyzes the formation of Citrulline. It is the most common enzyme deficiency in the urea cycle (X-linked recessive). * **B. Carbamoyl-phosphate synthetase I (CPS-I):** This is the **rate-limiting enzyme** of the urea cycle, located in the mitochondria. It requires N-acetylglutamate (NAG) as an essential allosteric activator. * **C. Argininosuccinase (Argininosuccinate lyase):** This cytosolic enzyme cleaves Argininosuccinate into Arginine and Fumarate. Fumarate then provides a link to the TCA cycle (the "Krebs bicycle"). ### High-Yield Clinical Pearls for NEET-PG: * **Location:** The cycle is "split"—the first two steps occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. * **Mnemonic for Enzymes:** **C**an **O**rdinary **A**nyone **A**lways **A**nywhere (**C**PS-I, **O**TC, **A**rgininosuccinate synthetase, **A**rgininosuccinate lyase, **A**rginase). * **Arginase:** The final enzyme that releases urea and regenerates Ornithine. It is found almost exclusively in the liver. * **Hyperammonemia Type II:** Caused by OTC deficiency; characterized by increased orotic acid in urine.

Question 213: Which amino acid plays a role in the sleep-wake cycle?

A. Tryptophan (Correct Answer)
B. Phenylalanine
C. Glycine
D. Glutamine

Explanation: **Explanation:** **Tryptophan** is the correct answer because it is the primary precursor for **Serotonin** and **Melatonin**, the two key neurotransmitters regulating the sleep-wake cycle (circadian rhythm). 1. **Mechanism:** Tryptophan is converted to 5-hydroxytryptophan (5-HTP) by tryptophan hydroxylase, which is then decarboxylated to Serotonin. In the pineal gland, Serotonin undergoes N-acetylation and O-methylation to form **Melatonin**. Melatonin levels rise in darkness, signaling the body to initiate sleep. 2. **Incorrect Options:** * **Phenylalanine:** A precursor to Tyrosine, which leads to the synthesis of catecholamines (Dopamine, Epinephrine, Norepinephrine). These are generally associated with alertness and the "fight or flight" response rather than sleep induction. * **Glycine:** An inhibitory neurotransmitter in the spinal cord and brainstem. While it has some calming effects, it is not the primary regulator of the circadian rhythm. * **Glutamine:** Primarily acts as a non-toxic carrier of ammonia and a precursor to Glutamate (excitatory) and GABA (inhibitory). It does not directly synthesize melatonin. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Tryptophan hydroxylase is the rate-limiting enzyme for serotonin synthesis. * **Vitamin Cofactors:** The conversion of Tryptophan to Serotonin requires **Vitamin B6 (Pyridoxine)** and **Tetrahydrobiopterin (BH4)**. * **Hartnup Disease:** A defect in the transport of neutral amino acids (like Tryptophan) can lead to niacin deficiency (Pellagra-like symptoms) because Tryptophan is also a precursor for **Niacin (Vitamin B3)**. * **Melatonin Synthesis:** Occurs specifically in the **Pineal Gland**; its secretion is inhibited by light hitting the retina.

Question 214: Which of the following is an essential amino acid?

A. Lysine (Correct Answer)
B. Alanine
C. Asparagine
D. Glycine

Explanation: ### Explanation **Concept Overview:** Amino acids are categorized into **Essential** and **Non-essential** based on the body's ability to synthesize them. Essential amino acids cannot be synthesized *de novo* by the human body at a rate sufficient to meet metabolic demands and must be obtained through the diet. **Why Lysine is Correct:** **Lysine** is one of the 10 essential amino acids. It is strictly ketogenic and plays a vital role in protein synthesis, calcium absorption, and the production of hormones and enzymes. A deficiency can lead to stunted growth and reproductive disorders. **Analysis of Incorrect Options:** * **Alanine (B):** A non-essential amino acid. It is primarily synthesized via the transamination of pyruvate and plays a key role in the Glucose-Alanine cycle (Cahill cycle) for transporting nitrogen to the liver. * **Asparagine (C):** A non-essential amino acid synthesized from aspartate and glutamine by the enzyme asparagine synthetase. * **Glycine (D):** The simplest non-essential amino acid. It is synthesized from serine and is a precursor for heme, purines, and creatine. **High-Yield NEET-PG Pearls:** 1. **Mnemonic for Essential Amino Acids:** **"PVT TIM HALL"** (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine). 2. **Semi-essential:** Arginine and Histidine are considered semi-essential because they are required in larger quantities during periods of rapid growth (infancy/pregnancy). 3. **Purely Ketogenic:** Leucine and Lysine are the only two amino acids that are purely ketogenic. 4. **Limiting Amino Acids:** Lysine is the limiting amino acid in cereals (wheat/rice), while Methionine is limiting in pulses (legumes).

Question 215: Which of the following is NOT true about the transamination reaction?

A. It involves the transfer of an alpha-amino group from an alpha-amino acid to a keto acid.
B. Alpha-ketoglutarate is the most common alpha-keto acid acceptor.
C. Threonine does not undergo transamination.
D. Biotin is required as a coenzyme. (Correct Answer)

Explanation: ### Explanation **1. Why Option D is the Correct Answer (The Concept):** Transamination is the first step in the catabolism of most amino acids. The reaction requires **Pyridoxal Phosphate (PLP)**, a derivative of **Vitamin B6**, as a mandatory coenzyme—not Biotin. PLP acts as a temporary carrier for the amino group, forming a Schiff base intermediate (Pyridoxamine phosphate). Biotin, on the other hand, is a coenzyme for **carboxylation** reactions (e.g., Pyruvate carboxylase). **2. Analysis of Incorrect Options:** * **Option A:** This is the fundamental definition of transamination. It involves the reversible transfer of an amino group from an amino acid to a keto acid, resulting in the formation of a new amino acid and a new keto acid. * **Option B:** Alpha-ketoglutarate is indeed the universal acceptor. It accepts the amino group to become **Glutamate**, which then undergoes oxidative deamination in the liver to release ammonia. * **Option C:** While most amino acids participate in transamination, there are notable exceptions. **Threonine, Lysine, Proline, and Hydroxyproline** do not undergo transamination; they are instead deaminated or metabolized via other pathways. **3. NEET-PG High-Yield Clinical Pearls:** * **Enzymes:** Transaminases (Aminotransferases) are located in both the cytosol and mitochondria. * **Diagnostic Markers:** AST (SGOT) and ALT (SGPT) are clinical markers of liver/cardiac damage. ALT is more specific for the liver. * **The "Funneling" Effect:** Transamination "funnels" nitrogen from various amino acids into Glutamate, which is the only amino acid that undergoes rapid oxidative deamination via **Glutamate Dehydrogenase (GDH)**. * **Equilibrium:** Transamination reactions are freely reversible ($K_{eq} \approx 1$).

Question 216: Taurine is synthesized from which amino acid?

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

Explanation: **Explanation:** **Correct Answer: C. Cysteine** Taurine is a sulfur-containing amino sulfonic acid derived primarily from **Cysteine**. The metabolic pathway involves the oxidation of the sulfhydryl group of cysteine to form cysteine sulfinic acid, which then undergoes decarboxylation (catalyzed by cysteine sulfinate decarboxylase) to form hypotaurine, and finally oxidation to **Taurine**. Taurine is essential for the conjugation of bile acids (forming taurocholic acid), which enhances their solubility and detergent properties for lipid digestion. **Why other options are incorrect:** * **Glycine:** While glycine is also used in bile acid conjugation (forming glycocholic acid), it is a precursor for heme, creatine, and glutathione, but not for taurine. * **Tyrosine:** This is the precursor for catecholamines (Dopamine, Epinephrine, Norepinephrine), Thyroid hormones (T3, T4), and Melanin. * **Phenylalanine:** This is an essential amino acid that is converted to Tyrosine by phenylalanine hydroxylase. It does not enter the sulfur-containing amino acid pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Bile Acid Conjugation:** The ratio of glycine to taurine conjugates in human bile is typically **3:1**. * **Sulfur-containing Amino Acids:** Remember the sequence: **Methionine → Homocysteine → Cystathionine → Cysteine → Taurine.** * **Vitamin B6 Dependency:** The decarboxylation step in taurine synthesis requires Pyridoxal Phosphate (PLP). * **Biological Role:** Beyond bile conjugation, taurine acts as an inhibitory neurotransmitter in the CNS and plays a role in osmoregulation and retinal function.

Question 217: Creatine is synthesized from which of the following amino acids?

A. Methionine (Correct Answer)
B. Histidine
C. Proline
D. Lysine

Explanation: **Explanation:** Creatine synthesis is a high-yield topic in biochemistry, involving three specific amino acids: **Glycine, Arginine, and Methionine.** 1. **Why Methionine is correct:** The synthesis occurs in two main steps. First, Arginine and Glycine combine in the kidney to form guanidinoacetate. Second, in the liver, guanidinoacetate undergoes **methylation** to form Creatine. The methyl group donor for this reaction is **S-adenosylmethionine (SAMe)**, which is derived from **Methionine**. Without Methionine, the final conversion to Creatine cannot occur. 2. **Why other options are incorrect:** * **Histidine:** Primarily serves as a precursor for Histamine (via decarboxylation) and FIGLU. * **Proline:** A non-essential amino acid involved in collagen structure; it does not contribute to the guanidino group or methylation. * **Lysine:** An essential amino acid used for protein synthesis and the production of Carnitine, but not Creatine. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Synthesis:** Starts in the **Kidney** (Guanidinoacetate formation) and completes 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:** Creatine spontaneously cyclizes to **Creatinine**, which is excreted in the urine. Serum creatinine levels are a key clinical marker for Glomerular Filtration Rate (GFR). * **Mnemonic:** Remember **"GAM"** (Glycine, Arginine, Methionine) for Creatine synthesis.

Question 218: In urea synthesis, which amino acid functions solely as an enzyme activator?

A. N-acetyl glutamate (Correct Answer)
B. Ornithine
C. Citrulline
D. Arginine

Explanation: ### Explanation **Correct Answer: A. N-acetyl glutamate (NAG)** **Why it is correct:** The rate-limiting step of the urea cycle is catalyzed by the enzyme **Carbamoyl Phosphate Synthetase I (CPS-I)**. This enzyme is strictly dependent on **N-acetyl glutamate (NAG)** for its activity. NAG acts as an **allosteric activator**, inducing a conformational change in CPS-I that allows it to bind its substrates. Without NAG, the urea cycle cannot initiate. It is synthesized from glutamate and acetyl-CoA by the enzyme *N-acetylglutamate synthase (NAGS)*, which is itself activated by Arginine. **Why the other options are incorrect:** * **B. Ornithine:** This is a **substrate** (intermediate) of the urea cycle. It combines with carbamoyl phosphate to form citrulline. It is regenerated at the end of the cycle. * **C. Citrulline:** This is a **product** of the first mitochondrial reaction and a substrate for the next step in the cytosol. It is an intermediate, not an activator. * **D. Arginine:** While Arginine stimulates the synthesis of NAG, it is primarily a **substrate** (intermediate) in the cycle that is cleaved by Arginase to produce urea and ornithine. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** The first two steps of the urea cycle (CPS-I and Ornithine Transcarbamoylase) occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. * **NAGS Deficiency:** Presents clinically identical to CPS-I deficiency (hyperammonemia) but can be treated with **Carglumic acid** (a synthetic analog of NAG). * **Hyperammonemia Type I:** Caused by CPS-I deficiency. * **Hyperammonemia Type II:** Caused by OTC deficiency (the most common urea cycle disorder; X-linked).

Question 219: The formation of glycine takes place by the transamination of which of the following?

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

Explanation: **Explanation:** The correct answer is **Glyoxylate**. Glycine is a non-essential amino acid that can be synthesized through multiple pathways. One of the primary routes is the **transamination of glyoxylate**. In this reaction, the enzyme **glycine aminotransferase** transfers an amino group (usually from glutamate or alanine) to glyoxylate, converting it into glycine. **Why the other options are incorrect:** * **Alanine:** Alanine undergoes transamination to form **Pyruvate**. While alanine can act as an amino group donor in the synthesis of glycine, it is not the direct carbon skeleton precursor. * **Aspartate:** Transamination of aspartate (via AST/GOT) yields **Oxaloacetate**, a key intermediate in the TCA cycle and gluconeogenesis. * **Glutamate:** Transamination of glutamate (via ALT or AST) yields **$\alpha$-ketoglutarate**. Glutamate typically serves as the universal amino group donor rather than the precursor skeleton for glycine. **High-Yield NEET-PG Clinical Pearls:** 1. **Primary Hyperoxaluria Type I:** This condition is caused by a deficiency of the liver-specific peroxisomal enzyme **alanine-glyoxylate aminotransferase**. When glyoxylate cannot be converted to glycine, it is instead oxidized to **oxalate**, leading to renal stones (nephrolithiasis) and kidney failure. 2. **Other Glycine Sources:** Glycine is also synthesized from **Serine** (via *Serine hydroxymethyltransferase* in a folate-dependent reaction) and from **CO₂ and NH₄⁺** (via the *Glycine Cleavage System*). 3. **Specialized Products:** Glycine is a precursor for **Heme, Purines, Creatine, Glutathione, and Conjugated Bile Salts**.

Question 220: All of the following amino acids are one-carbon donors, EXCEPT?

A. Glycine
B. Serine
C. Histidine
D. Threonine (Correct Answer)

Explanation: **Explanation:** The one-carbon pool refers to the collection of single-carbon units (e.g., methyl, methylene, formyl) attached to the carrier **Tetrahydrofolate (THF)**. These units are essential for the synthesis of purines and thymidine. **Why Threonine is the Correct Answer:** Threonine is primarily metabolized via threonine dehydratase into alpha-ketobutyrate or via threonine aldolase into glycine and acetaldehyde. Unlike the other options, **Threonine does not directly contribute a carbon unit to the THF pool.** While it can be converted into glycine, it is not classified as a primary one-carbon donor in standard biochemical pathways. **Analysis of Incorrect Options:** * **Serine:** The **most important** source of one-carbon units. The enzyme *Serine hydroxymethyltransferase* transfers a hydroxymethyl group to THF, forming N5,N10-methylene-THF and glycine. * **Glycine:** It contributes a carbon unit through the **Glycine Cleavage System**, releasing CO2, NH4+, and a methylene group to THF. * **Histidine:** During its catabolism to glutamate, histidine forms **FIGLU** (formiminoglutamate). The formimino group is then transferred to THF, forming N5-formimino-THF. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for 1-C Donors:** **"H-S-G-T"** (Histidine, Serine, Glycine, Tryptophan). Note that Tryptophan contributes via the formate produced during its breakdown. * **FIGLU Excretion Test:** In **Vitamin B12 or Folate deficiency**, FIGLU cannot transfer its carbon unit to THF and is excreted in the urine. This is a diagnostic marker for folate deficiency. * **Primary Carrier:** Tetrahydrofolate (THF) is the active form of Folic Acid (Vitamin B9).

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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