Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 61: What is the most common cause of tyrosinosis?

A. Fumarylacetoacetate hydrolase deficiency (Correct Answer)
B. Tyrosine transaminase deficiency
C. Parahydroxy phenyl pyruvate hydroxylase deficiency
D. Homogentisate oxidase deficiency

Explanation: **Explanation:** The correct answer is **A. Fumarylacetoacetate hydrolase deficiency.** **Tyrosinemia Type I (Tyrosinosis)** is the most common and severe form of the disorders involving tyrosine catabolism. It is caused by a deficiency of **Fumarylacetoacetate hydrolase (FAH)**, the final enzyme in the tyrosine degradation pathway. This deficiency leads to the accumulation of fumarylacetoacetate and its conversion into **succinylacetone**, which is a pathognomonic marker. Succinylacetone is highly toxic, causing severe liver failure, renal tubular dysfunction (Fanconi syndrome), and an increased risk of hepatocellular carcinoma. **Analysis of Incorrect Options:** * **B. Tyrosine transaminase deficiency:** This causes **Tyrosinemia Type II (Richner-Hanhart syndrome)**, characterized by palmoplantar keratosis and corneal erosions. * **C. Parahydroxy phenyl pyruvate hydroxylase deficiency:** This leads to **Tyrosinemia Type III**, a very rare condition presenting with neurological symptoms and ataxia. * **D. Homogentisate oxidase deficiency:** This causes **Alkaptonuria**, characterized by ochronosis (darkening of tissues) and urine that turns black upon standing. **NEET-PG High-Yield Pearls:** * **Diagnostic Marker:** Succinylacetone in urine is the "gold standard" for Tyrosinemia Type I. * **Treatment:** **Nitisinone (NTBC)** is the drug of choice; it inhibits the pathway upstream to prevent the formation of toxic metabolites. * **Cabbage-like odor:** Patients with Tyrosinemia Type I often have a characteristic "boiled cabbage" body odor. * **Dietary Management:** Restriction of Phenylalanine and Tyrosine is essential.

Question 62: Which of the following amino acids can be synthesized from a glycolytic intermediate in the human body?

A. Aspartate
B. Glutamate
C. Histidine
D. Serine (Correct Answer)

Explanation: ### Explanation **Correct Option: D (Serine)** Serine is synthesized from **3-phosphoglycerate**, which is a key intermediate of the **glycolytic pathway**. The synthesis involves a three-step process: 1. **Oxidation:** 3-phosphoglycerate is converted to 3-phosphohydroxypyruvate by *3-phosphoglycerate dehydrogenase*. 2. **Transamination:** Conversion to 3-phosphoserine using glutamate as an amino donor. 3. **Dephosphorylation:** Final conversion to Serine by *phosphoserine phosphatase*. Since 3-phosphoglycerate is produced during glycolysis, Serine is directly linked to glucose metabolism. **Analysis of Incorrect Options:** * **A. Aspartate:** This is synthesized via transamination of **Oxaloacetate**, which is an intermediate of the **TCA cycle**, not glycolysis. * **B. Glutamate:** This is synthesized from **$\alpha$-ketoglutarate**, another **TCA cycle** intermediate, via the action of glutamate dehydrogenase or transamination. * **C. Histidine:** This is an **essential amino acid** in humans (especially in children/during growth). It cannot be synthesized de novo in the human body and must be obtained from the diet. **High-Yield NEET-PG Pearls:** * **Glycine Link:** Serine is the immediate precursor for **Glycine** (via *serine hydroxymethyltransferase*), making Glycine also indirectly derived from glycolysis. * **Cysteine Link:** Serine provides the carbon skeleton for Cysteine synthesis (via the Cystathionine pathway). * **Alanine:** Another amino acid linked to glycolysis, synthesized directly from **Pyruvate** (the end product of glycolysis) via transamination. * **Mnemonic:** "S-A-G" (Serine, Alanine, Glycine) are the primary amino acids derived from glycolytic intermediates.

Question 63: Which molecule contributes the first carbon atom to urea during its synthesis?

A. Succinyl CoA
B. Malonyl CoA
C. Acetyl CoA
D. CO2 (Correct Answer)

Explanation: **Explanation:** The synthesis of urea occurs via the **Urea Cycle (Ornithine Cycle)**, primarily in the liver. The process begins in the mitochondria with the formation of **Carbamoyl Phosphate**. This reaction is catalyzed by the rate-limiting enzyme **Carbamoyl Phosphate Synthetase I (CPS-I)**. The substrates for this first step are **Ammonia (NH3)**, **Bicarbonate (HCO3⁻)**, and **ATP**. The bicarbonate ion (derived from dissolved **CO2**) provides the single carbon atom that eventually forms the carbonyl group of the urea molecule ($NH_2-CO-NH_2$). Therefore, CO2 is the fundamental source of the carbon atom in urea. **Analysis of Incorrect Options:** * **Succinyl CoA:** An intermediate of the TCA cycle and heme synthesis; it does not contribute carbon to the urea cycle. * **Malonyl CoA:** A key intermediate in fatty acid synthesis; it plays no role in nitrogen excretion. * **Acetyl CoA:** While it acts as an allosteric activator for the production of **N-acetylglutamate (NAG)**—the essential cofactor for CPS-I—it does not donate carbon atoms directly to the urea molecule. **High-Yield Clinical Pearls for NEET-PG:** * **Nitrogen Sources:** Urea contains two nitrogen atoms. The first comes from **free ammonia**, and the second is donated by **Aspartate**. * **Rate-Limiting Step:** CPS-I is the rate-limiting enzyme and requires **N-acetylglutamate (NAG)** as an obligatory allosteric activator. * **Compartmentalization:** The first two reactions occur in the **mitochondria**, while the remaining three occur in the **cytosol**. * **Hyperammonemia:** Defects in any urea cycle enzyme lead to ammonia toxicity, with **Ornithine Transcarbamoylase (OTC) deficiency** being the most common (X-linked).

Question 64: Which of the following amino acids has its pKa value within the physiological range?

A. Glycine
B. Histidine (Correct Answer)
C. Lysine
D. Arginine

Explanation: **Explanation:** The physiological pH of the human body is approximately **7.4**. For an amino acid to act as an effective buffer within this range, its side chain (R-group) dissociation constant (**pKa**) must be close to 7.4. **Why Histidine is Correct:** Histidine is the only amino acid with an imidazole side chain that has a **pKa of approximately 6.0**. While 6.0 is slightly below 7.4, it is the closest among all amino acids. This proximity allows the imidazole ring to exist in both protonated and unprotonated forms at physiological pH, making Histidine a crucial **buffering agent** in proteins, particularly in **Hemoglobin**. **Why the others are Incorrect:** * **Glycine (A):** It is a non-polar amino acid with no ionizable side chain. Its carboxyl group pKa is ~2.3 and its amino group pKa is ~9.6, both far from physiological pH. * **Lysine (C):** It is a basic amino acid with an $\epsilon$-amino side chain. Its pKa is high (**~10.5**), meaning it remains positively charged (protonated) at pH 7.4. * **Arginine (D):** It contains a guanidino group with a very high pKa (**~12.5**). It is the most basic amino acid and remains strongly protonated at physiological pH. **High-Yield NEET-PG Pearls:** 1. **Hemoglobin Buffering:** The buffering capacity of hemoglobin is primarily due to its high **Histidine** content. 2. **Charge at pH 7.4:** At physiological pH, basic amino acids (Lysine, Arginine) are positively charged, while acidic amino acids (Aspartate, Glutamate) are negatively charged. 3. **Isoelectric Point (pI):** Histidine’s pI is 7.59, which is also very close to the physiological pH.

Question 65: Cystinuria is characterized by an excess of which amino acid in the urine?

A. Cysteine (Correct Answer)
B. Tyrosine
C. Glutamine
D. Valine

Explanation: **Explanation:** **Cystinuria** is an autosomal recessive disorder caused by a defect in the high-affinity renal transport system responsible for the reabsorption of dibasic amino acids from the proximal convoluted tubule. **Why Cysteine is Correct:** The defect involves the **COAL** transporter (Cysteine, Ornithine, Arginine, and Lysine). When this transporter fails, these four amino acids are not reabsorbed and are excreted in high amounts in the urine. Among these, **Cysteine** is the most clinically significant because it is relatively insoluble at acidic urinary pH. This leads to the precipitation of cysteine crystals and the formation of radiopaque **hexagonal stones** in the urinary tract. **Why Incorrect Options are Wrong:** * **Tyrosine:** Elevated tyrosine in urine (Tyrosinuria) is associated with Tyrosinemia, a defect in the tyrosine degradation pathway, not a renal transport defect. * **Glutamine:** Glutamine is a neutral amino acid. Its transport is typically unaffected in Cystinuria. * **Valine:** Valine is a branched-chain amino acid. Its accumulation in urine is seen in Maple Syrup Urine Disease (MSUD) due to a decarboxylase deficiency, not a transport defect. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** The **Cyanide-Nitroprusside test** is the screening test of choice (turns purple/magenta in the presence of cysteine). * **Microscopy:** Pathognomonic **hexagonal crystals** in urine sediment. * **Management:** Treatment focuses on high fluid intake, urinary alkalinization (potassium citrate), and chelating agents like **Penicillamine** (which forms a more soluble complex with cysteine). * **Note:** Do not confuse *Cystinuria* (transport defect) with *Cystinosis* (lysosomal storage disorder).

Question 66: Arginine enters the TCA cycle by forming which substrate?

A. Fumarate
B. α-Ketoglutarate (Correct Answer)
C. Oxaloacetate
D. Succinyl CoA

Explanation: **Explanation:** Arginine is a glucogenic amino acid that enters the TCA cycle via **α-Ketoglutarate**. The metabolic pathway follows a specific sequence: Arginine is first converted to **Ornithine** (releasing urea via the enzyme Arginase). Ornithine then undergoes transamination to form **Glutamate-γ-semialdehyde**, which is oxidized to **Glutamate**. Finally, Glutamate is deaminated or transaminated to form **α-Ketoglutarate**, a key intermediate of the TCA cycle. **Analysis of Options:** * **α-Ketoglutarate (Correct):** This is the common entry point for the "5-carbon family" of amino acids, which includes **Arginine, Histidine, Proline, and Glutamine**, all of which are first converted to Glutamate. * **Fumarate (Incorrect):** While Arginine is part of the Urea cycle where Fumarate is released, Arginine *itself* does not enter the TCA cycle as Fumarate. Phenylalanine and Tyrosine are the primary amino acids that enter via Fumarate. * **Oxaloacetate (Incorrect):** Asparagine and Aspartate are the amino acids that enter the TCA cycle at the level of Oxaloacetate. * **Succinyl CoA (Incorrect):** This is the entry point for Methionine, Valine, Isoleucine, and Threonine (M-V-I-T). **High-Yield Clinical Pearls for NEET-PG:** * **Glucogenic vs. Ketogenic:** Arginine is purely glucogenic. * **Urea Cycle Link:** Arginine is a semi-essential amino acid and a vital intermediate of the Urea cycle; its cleavage by Arginase is the final step that produces Urea. * **Precursor Molecule:** Arginine is the precursor for **Nitric Oxide (NO)**, **Creatine**, and **Polyamines** (Spermine/Spermidine). * **Mnemonic:** To remember amino acids entering via α-Ketoglutarate, use **"ROHQ"** (Arginine, Ornithine, Histidine, Glutamine).

Question 67: Which amino acid contains a guanidine group?

A. Histidine
B. Proline
C. Tryptophan
D. Arginine (Correct Answer)

Explanation: **Explanation:** The correct answer is **Arginine**. Arginine is a basic, semi-essential amino acid characterized by the presence of a **guanidino (guanidine) group** in its side chain. This group consists of a central carbon atom bonded to three nitrogen atoms, making Arginine the most basic amino acid (pKa ≈ 12.5) as it remains protonated and positively charged at physiological pH. **Analysis of Options:** * **Histidine (A):** Contains an **imidazole** ring. It is a basic amino acid often involved in enzyme active sites due to its pKa being near physiological pH. * **Proline (B):** Contains a secondary amino group (pyrrolidine ring) and is technically an **imino acid**. It causes "kinks" in alpha-helices. * **Tryptophan (C):** Contains an **indole** ring. It is the precursor for serotonin, melatonin, and niacin. * **Arginine (D):** Contains the **guanidine** group. It is a key intermediate in the Urea Cycle and the precursor for Nitric Oxide (NO). **High-Yield Clinical Pearls for NEET-PG:** * **Urea Cycle:** Arginine is cleaved by the enzyme *Arginase* to produce Urea and Ornithine. * **Nitric Oxide Synthesis:** Arginine is the sole precursor of Nitric Oxide (NO), a potent vasodilator, via the enzyme *Nitric Oxide Synthase (NOS)*. * **Creatine Synthesis:** Arginine, along with Glycine and Methionine (SAM), is required for creatine synthesis. * **Glucogenic nature:** Arginine is a purely glucogenic amino acid as it enters the TCA cycle via α-ketoglutarate.

Question 68: What is the important role of selenocysteine?

A. Hydroxylation of dopamine
B. Oxidation of drugs
C. Antioxidant mechanism (Correct Answer)
D. None of the above

Explanation: **Explanation:** **Selenocysteine** is often referred to as the **21st amino acid**. It is unique because it contains selenium instead of the sulfur atom found in cysteine. Its primary physiological role is serving as a critical component of **selenoproteins**, which are essential for cellular redox homeostasis and protection against oxidative stress. **Why Option C is Correct:** The most prominent selenoprotein is **Glutathione Peroxidase (GPx)**. This enzyme plays a vital role in the **antioxidant mechanism** by reducing hydrogen peroxide ($H_2O_2$) and lipid hydroperoxides into water and alcohols, respectively. By doing so, it prevents the formation of harmful free radicals and protects cell membranes from oxidative damage. Other antioxidant selenoproteins include Thioredoxin Reductase. **Why Other Options are Incorrect:** * **Option A:** The hydroxylation of dopamine to norepinephrine is catalyzed by *Dopamine $\beta$-hydroxylase*, which requires **Vitamin C (Ascorbate)** and Copper as cofactors, not selenocysteine. * **Option B:** The oxidation and detoxification of drugs primarily occur in the liver via the **Cytochrome P450 system**, which utilizes heme-iron complexes, not selenocysteine. **High-Yield NEET-PG Pearls:** * **Genetic Coding:** Unlike other amino acids, selenocysteine is encoded by the **UGA stop codon**. This requires a specific mRNA secondary structure called the **SECIS element** (Selenocysteine Insertion Sequence). * **Deiodinases:** Selenocysteine is also a component of **Iodothyronine deiodinases**, which are responsible for converting the prohormone $T_4$ into the active thyroid hormone $T_3$. * **Synthesis:** It is synthesized while attached to its unique tRNA ($tRNA^{SerSec}$), starting from the amino acid Serine.

Question 69: Consumption of which amino acid reduces the requirement of methionine due to a sparing effect?

A. Homocysteine
B. Cysteine (Correct Answer)
C. Lysine
D. Arginine

Explanation: **Explanation:** The correct answer is **Cysteine**. This is a classic example of the **"Sparing Effect"** in amino acid metabolism. **1. Why Cysteine is correct:** Methionine is a sulfur-containing **essential amino acid**. One of its primary metabolic roles is to provide the sulfur atom required for the synthesis of Cysteine (a non-essential amino acid). This occurs via the **transsulfuration pathway**, where Methionine is converted to Homocysteine, which then combines with Serine to form Cystathionine and eventually Cysteine. If Cysteine is supplied adequately through the diet, the body does not need to divert Methionine toward Cysteine synthesis. This "spares" Methionine for its other vital functions, such as protein synthesis and acting as a methyl donor (via S-adenosylmethionine). **2. Why other options are incorrect:** * **Homocysteine:** This is an intermediate in the methionine cycle. While it can be remethylated back to Methionine (requiring B12 and Folate), it does not "spare" the requirement; rather, it is a metabolic byproduct. * **Lysine:** This is a purely ketogenic essential amino acid with no metabolic link to the sulfur-containing pathway of Methionine. * **Arginine:** This is a semi-essential amino acid involved in the urea cycle and nitric oxide synthesis, unrelated to Methionine sparing. **Clinical Pearls for NEET-PG:** * **The Phenylalanine-Tyrosine Parallel:** Just as Cysteine spares Methionine, **Tyrosine spares Phenylalanine**. * **Cystathionine β-synthase (CBS) deficiency:** This is the most common cause of **Homocystinuria**. In these patients, Cysteine becomes an essential amino acid because the pathway to synthesize it from Methionine is blocked. * **Key Cofactors:** The transsulfuration pathway (Methionine → Cysteine) specifically requires **Vitamin B6 (Pyridoxine)**.

Question 70: Glutamate is not a precursor of which of the following substances?

A. Ammonia
B. Proline
C. Glutathione
D. Histidine (Correct Answer)

Explanation: **Explanation:** The correct answer is **Histidine**. This question tests your knowledge of the metabolic pathways and biosynthetic roles of Glutamate. **1. Why Histidine is the Correct Answer:** Histidine is an **essential amino acid**, meaning it cannot be synthesized by the human body and must be obtained from the diet. Its biosynthesis (in plants and bacteria) is complex and involves **ATP and Phosphoribosyl pyrophosphate (PRPP)**, not glutamate. Glutamate does not serve as a precursor for Histidine; rather, the catabolism of Histidine actually produces Glutamate (via the intermediate FIGLU). **2. Why the other options are incorrect:** * **Ammonia:** Glutamate undergoes **oxidative deamination** catalyzed by *Glutamate Dehydrogenase (GDH)* to release free ammonia ($NH_3$) and $\alpha$-ketoglutarate. This is a primary source of ammonia for the urea cycle. * **Proline:** Glutamate is the direct precursor for proline synthesis. It is first converted to glutamate-$\gamma$-semialdehyde, which then cyclizes to form proline. * **Glutathione:** This is a tripeptide (Glu-Cys-Gly). Glutamate is one of the three constituent amino acids required for its synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **FIGLU Test:** Formiminoglutamate (FIGLU) is an intermediate of histidine catabolism. In **Vitamin B12 or Folic acid deficiency**, FIGLU is excreted in the urine because its conversion to glutamate requires THF. * **GABA:** Glutamate is the precursor for the inhibitory neurotransmitter GABA via the enzyme *Glutamate Decarboxylase* (requires Vitamin B6/PLP). * **Transamination:** Glutamate is the "collection center" for amino groups from most other amino acids during transamination reactions.

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

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