Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 281: Tetrahydrobiopterin is used in the management of which amino acid defect?

A. Alanine
B. Tyrosine
C. Phenylalanine (Correct Answer)
D. Tryptophan

Explanation: **Explanation:** **Tetrahydrobiopterin (BH4)** is a vital cofactor for several hydroxylase enzymes. Its primary clinical application is in the management of **Phenylketonuria (PKU)**, specifically cases involving **Phenylalanine Hydroxylase (PAH)** deficiency or defects in BH4 synthesis/regeneration. 1. **Why Phenylalanine is Correct:** The enzyme **Phenylalanine Hydroxylase** converts Phenylalanine to Tyrosine. This reaction requires BH4 as a co-substrate. In "Sapropterin-responsive PKU," pharmacological doses of BH4 (Sapropterin) can enhance the residual activity of a mutated PAH enzyme, effectively lowering blood phenylalanine levels. Furthermore, in rare cases of **Malignant PKU** (caused by DHPR deficiency), BH4 supplementation is the mainstay of treatment. 2. **Why Other Options are Incorrect:** * **Alanine:** Alanine undergoes transamination via ALT (requiring Vitamin B6/PLP), not hydroxylation requiring BH4. * **Tyrosine:** While Tyrosine hydroxylase requires BH4 to produce DOPA, "Tyrosinemia" (the primary defect of tyrosine metabolism) is managed by dietary restriction or Nitisinone, not BH4. * **Tryptophan:** Tryptophan hydroxylase (for serotonin synthesis) does require BH4; however, BH4 is not used as a standard clinical management strategy for primary tryptophan metabolic defects in the same way it is for PKU. **High-Yield Clinical Pearls for NEET-PG:** * **BH4-dependent enzymes:** Phenylalanine hydroxylase, Tyrosine hydroxylase, Tryptophan hydroxylase, and Nitric Oxide Synthase (NOS). * **Malignant PKU:** Caused by a deficiency of **Dihydropteridine reductase (DHPR)**. Unlike classic PKU, it presents with neurological deterioration despite a phenylalanine-restricted diet because neurotransmitter synthesis (Dopamine/Serotonin) is also impaired. * **Guthrie Test:** A classic bacterial inhibition assay used for neonatal screening of PKU.

Question 282: Which cofactor is involved in the metabolism of sulfur-containing amino acids?

A. Folic acid
B. Biotin
C. Vitamin B1
D. Vitamin B12 (Correct Answer)

Explanation: **Explanation:** The metabolism of sulfur-containing amino acids (Methionine and Cysteine) is heavily dependent on the **Methionine Cycle**. The correct answer is **Vitamin B12 (Cobalamin)** because it acts as an essential cofactor for the enzyme **Methionine Synthase**. This enzyme catalyzes the remethylation of Homocysteine back to Methionine, utilizing Methyl-tetrahydrofolate as a donor. A deficiency in B12 leads to the "Methyl-folate trap," resulting in secondary folate deficiency and hyperhomocysteinemia. **Analysis of Incorrect Options:** * **A. Folic Acid:** While Folate (as N5-methyl THF) is a co-substrate in this reaction, Vitamin B12 is the specific prosthetic group/cofactor required for the enzyme's catalytic activity. * **B. Biotin (B7):** Involved in carboxylation reactions (e.g., Pyruvate carboxylase, Acetyl-CoA carboxylase). It is not directly involved in the sulfur amino acid pathway. * **C. Vitamin B1 (Thiamine):** Acts as a cofactor for oxidative decarboxylation (e.g., Pyruvate dehydrogenase, Alpha-ketoglutarate dehydrogenase). **High-Yield Clinical Pearls for NEET-PG:** * **The "Triple B" Rule:** Metabolism of sulfur amino acids (specifically Homocysteine) requires three B-vitamins: **B6** (for Cystathionine $\beta$-synthase), **B9** (Folate), and **B12**. * **Homocystinuria:** Deficiency of Vitamin B12 or the enzyme Cystathionine $\beta$-synthase leads to elevated homocysteine, which is a significant risk factor for early-onset atherosclerosis and venous thrombosis. * **Propionyl-CoA Pathway:** Vitamin B12 is also a cofactor for **Methylmalonyl-CoA mutase**, which converts Methylmalonyl-CoA to Succinyl-CoA (the catabolic pathway for Isoleucine, Valine, Threonine, and Methionine).

Question 283: Transfer of the carbamoyl moiety of carbamoyl phosphate to ornithine is catalysed by a liver mitochondrial enzyme?

A. Carbamoyl phosphate synthetase
B. Ornithine transcarbamoylase (Correct Answer)
C. N-acetyl glutamate synthetase
D. N-acetyl glutamate hydrolase

Explanation: ### Explanation The question describes the second step of the **Urea Cycle**, which occurs within the **mitochondrial matrix** of hepatocytes. **Correct Option: B. Ornithine transcarbamoylase (OTC)** The enzyme **Ornithine transcarbamoylase** catalyzes the transfer of the carbamoyl group from carbamoyl phosphate to the amino acid ornithine. This reaction produces **citrulline**, which is then transported out of the mitochondria into the cytosol. This is a critical step in the disposal of toxic ammonia. **Analysis of Incorrect Options:** * **A. Carbamoyl phosphate synthetase I (CPS-I):** This is the rate-limiting enzyme of the urea cycle. It catalyzes the formation of carbamoyl phosphate from $NH_4^+$, $CO_2$, and 2 ATP. It does not transfer the moiety to ornithine. * **C. N-acetyl glutamate synthetase (NAGS):** This enzyme produces N-acetylglutamate (NAG) from acetyl-CoA and glutamate. NAG is an essential allosteric activator of CPS-I, not a catalyst for the ornithine reaction. * **D. N-acetyl glutamate hydrolase:** This enzyme breaks down NAG; it is not involved in the primary steps of the urea cycle. **High-Yield Clinical Pearls for NEET-PG:** * **OTC Deficiency:** This is the **most common** urea cycle disorder. Unlike other urea cycle enzyme deficiencies (which are autosomal recessive), OTC deficiency is **X-linked recessive**. * **Biochemical Marker:** In OTC deficiency, carbamoyl phosphate accumulates and is diverted to the pyrimidine synthesis pathway, leading to increased levels of **Orotic acid** in the urine (Orotic aciduria). * **Localization:** Remember that the first two steps (CPS-I and OTC) occur in the **mitochondria**, while the remaining steps occur in the **cytosol** (Mnemonic: **CO** = **C**PS-I and **O**TC are in the mitochondria).

Question 284: Which amino acid produces ammonia in the kidney?

A. Glutamine (Correct Answer)
B. Alanine
C. Methionine
D. Glycine

Explanation: **Explanation:** **Glutamine** is the primary source of ammonia ($NH_3$) in the kidney, a process essential for maintaining acid-base balance. In the renal tubular cells, the enzyme **Glutaminase** hydrolyzes glutamine into glutamate and free ammonia. A second enzyme, **Glutamate Dehydrogenase**, can further deaminate glutamate to produce another molecule of ammonia and $\alpha$-ketoglutarate. The produced ammonia acts as a buffer by trapping hydrogen ions ($H^+$) to form ammonium ($NH_4^+$), which is then excreted in the urine. This mechanism is the kidney's most important adaptive response to **metabolic acidosis**. **Why other options are incorrect:** * **Alanine:** While alanine is the major carrier of nitrogen from muscles to the liver (via the Glucose-Alanine cycle), it does not serve as the primary direct source of renal ammonia. * **Methionine:** This is a sulfur-containing essential amino acid primarily involved in methylation reactions (via S-adenosylmethionine) and cysteine synthesis, not renal ammoniagenesis. * **Glycine:** Although glycine can be deaminated by the glycine cleavage system, its contribution to renal ammonia production is negligible compared to glutamine. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Action:** Renal ammoniagenesis occurs predominantly in the **Proximal Convoluted Tubule (PCT)**. * **Regulation:** During chronic acidosis, the expression of renal Glutaminase increases significantly to enhance $H^+$ excretion. * **The "Ammonia Trap":** $NH_3$ is lipid-soluble and diffuses into the lumen, but once it picks up $H^+$ to become $NH_4^+$, it becomes water-soluble and "trapped," ensuring excretion. * **Glutamine** is the most abundant free amino acid in the blood, serving as a non-toxic transport form of ammonia.

Question 285: Histidine is converted to histamine by which of the following reactions?

A. Transamination
B. Hydroxylation
C. Decarboxylation (Correct Answer)
D. Reduction

Explanation: **Explanation:** The conversion of **Histidine to Histamine** is a classic example of **Decarboxylation**. **1. Why Decarboxylation is Correct:** In this reaction, the enzyme **Histidine decarboxylase** removes a carboxyl group (-COOH) from the amino acid Histidine, releasing it as Carbon Dioxide ($CO_2$). This process converts the amino acid into its corresponding primary amine, Histamine. This reaction requires **Pyridoxal Phosphate (Vitamin B6)** as a mandatory co-factor, a common theme for most amino acid decarboxylases. **2. Why Other Options are Incorrect:** * **Transamination:** This involves the transfer of an amino group to a keto-acid (e.g., Histidine to Imidazole-pyruvate). While important for amino acid catabolism, it does not produce Histamine. * **Hydroxylation:** This involves the addition of a hydroxyl (-OH) group (e.g., Phenylalanine to Tyrosine). Histidine does not undergo hydroxylation to form Histamine. * **Reduction:** This involves the addition of electrons or hydrogen. The pathway to Histamine is oxidative/decarboxylative, not reductive. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **The B6 Connection:** Almost all decarboxylation reactions of amino acids to form biogenic amines require **Vitamin B6 (PLP)**. * **Other Key Decarboxylations:** * Glutamate $\rightarrow$ **GABA** (Inhibitory neurotransmitter) * Tryptophan $\rightarrow$ 5-HT (**Serotonin**) * Tyrosine $\rightarrow$ Tyramine * DOPA $\rightarrow$ **Dopamine** * **Clinical Significance:** Histamine is a potent mediator of **Type I Hypersensitivity** (allergic) reactions, secreted by Mast cells and Basophils. It also stimulates gastric acid secretion via H2 receptors.

Question 286: Which enzyme is responsible for the conversion of norepinephrine to epinephrine?

A. COMT
B. MAO
C. PENMT (Correct Answer)
D. SAM

Explanation: **Explanation:** The conversion of norepinephrine to epinephrine is the final step in the catecholamine biosynthetic pathway. **1. Why PENMT is correct:** The enzyme **Phenylethanolamine N-methyltransferase (PNMT)** catalyzes the transfer of a methyl group from S-adenosylmethionine (SAM) to norepinephrine to form epinephrine. This reaction occurs primarily in the **adrenal medulla**. A key regulatory feature is that **cortisol** (transported via the intra-adrenal portal system) induces the expression of PNMT, ensuring high epinephrine production during stress. **2. Why the other options are incorrect:** * **COMT (Catechol-O-methyltransferase):** This enzyme is involved in the **degradation** (metabolism) of catecholamines, not their synthesis. It transfers a methyl group to the hydroxyl group of the catechol ring. * **MAO (Monoamine Oxidase):** This enzyme is also involved in the **degradation** of catecholamines via oxidative deamination. * **SAM (S-adenosylmethionine):** While SAM is required for this reaction, it is a **co-factor (methyl donor)**, not the enzyme itself. **Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Tyrosine hydroxylase (converts Tyrosine to DOPA). * **Vitamin C requirement:** Dopamine $\beta$-hydroxylase (converts Dopamine to Norepinephrine) requires Vitamin C and Copper. * **Localization:** PNMT is found in the cytosol; therefore, norepinephrine must exit the chromaffin granules to be methylated and then re-enter for storage. * **Pheochromocytoma:** Tumors arising outside the adrenal gland (paragangliomas) often lack PNMT and thus secrete norepinephrine rather than epinephrine.

Question 287: Which amino acid is not present in creatine?

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

Explanation: **Explanation:** The synthesis of **Creatine** (methyl-guanidinoacetate) is a high-yield topic in biochemistry, as it involves the coordination of three specific amino acids across two organs (kidney and liver). **1. Why Lysine is the Correct Answer:** Lysine is not involved in the creatine biosynthetic pathway. While lysine is an essential amino acid involved in carnitine synthesis and collagen cross-linking, it plays no role in the formation of creatine. **2. Analysis of Incorrect Options (The Precursors):** Creatine is synthesized using three specific amino acids: * **Glycine (Option C):** Acts as the backbone. In the kidney, the enzyme *AGAT* transfers an amidino group to glycine to form guanidinoacetate (GAA). * **Arginine (Option A):** Serves as the donor of the amidino group during the first step of synthesis. * **Methionine (Option B):** Specifically in its active form, **S-adenosylmethionine (SAM)**, it provides the methyl group required to convert guanidinoacetate into creatine in the liver. **Clinical Pearls for NEET-PG:** * **Site of Synthesis:** Starts in the **Kidney** (formation of GAA) and is completed in the **Liver** (methylation). * **Storage:** 95% of creatine is stored in skeletal muscle as **Creatine Phosphate**, a high-energy reservoir used to regenerate ATP during the first few seconds of intense exercise. * **Excretion:** Creatinine (the anhydride form) is excreted in the urine. Its excretion rate is constant and proportional to total muscle mass, making it a reliable marker for GFR. * **Mnemonic:** Remember **"GAM"** (Glycine, Arginine, Methionine) to recall the three precursors.

Question 288: Oxaloacetate is formed from which of the following?

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

Explanation: **Explanation:** The conversion of amino acids into metabolic intermediates is a high-yield topic in biochemistry. Amino acids are classified as glucogenic if they can be converted into intermediates of the TCA cycle or gluconeogenesis. **1. Why Aspartate is Correct:** Aspartate is a four-carbon dicarboxylic amino acid. It undergoes a **transamination reaction** (catalyzed by Aspartate Aminotransferase/AST) where it transfers its amino group to $\alpha$-ketoglutarate, directly forming **Oxaloacetate (OAA)**. This reaction is reversible and requires Pyridoxal Phosphate (Vitamin B6) as a cofactor. * *Reaction:* Aspartate + $\alpha$-ketoglutarate $\rightleftharpoons$ Oxaloacetate + Glutamate. **2. Analysis of Incorrect Options:** * **Glutamate:** Undergoes oxidative deamination (via Glutamate dehydrogenase) or transamination to form **$\alpha$-ketoglutarate**, not oxaloacetate. * **Histidine:** This is a basic amino acid that is converted to **Glutamate** (via the intermediate FIGLU), which then enters the TCA cycle as **$\alpha$-ketoglutarate**. * **Alanine:** A three-carbon amino acid that undergoes transamination (via ALT) to form **Pyruvate**. Pyruvate can eventually become oxaloacetate via pyruvate carboxylase, but Alanine’s immediate keto-acid product is pyruvate. **3. NEET-PG Clinical Pearls:** * **AST/ALT Ratio:** AST (found in mitochondria) and ALT (cytoplasm) are key markers of liver injury. * **FIGLU Excretion Test:** In Vitamin B12 or Folic acid deficiency, the conversion of Histidine to Glutamate is blocked, leading to increased urinary excretion of Formiminoglutamic acid (FIGLU). * **Asparagine Connection:** Asparagine is hydrolyzed by asparaginase to form Aspartate, which then forms Oxaloacetate. This is why Asparagine is also a glucogenic amino acid.

Question 289: Serine is produced in humans from which precursor molecule?

A. Glycine (Correct Answer)
B. Methionine
C. Homocysteine
D. Homoserine

Explanation: **Explanation:** **1. Why Glycine is Correct:** Serine and glycine are interconvertible through a reversible reaction catalyzed by the enzyme **Serine Hydroxymethyltransferase (SHMT)**. This reaction requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor and **N5, N10-Methylene Tetrahydrofolate (THF)** as the one-carbon donor/acceptor. In this pathway, glycine receives a hydroxymethyl group to form serine. Additionally, serine can be synthesized de novo from **3-phosphoglycerate** (an intermediate of glycolysis), but among the given options, glycine is the direct precursor. **2. Why the Other Options are Incorrect:** * **B. Methionine:** This is an essential sulfur-containing amino acid. While it provides the sulfur atom for cysteine synthesis, it is not a direct precursor for serine. * **C. Homocysteine:** This is an intermediate in the methionine cycle. It reacts with serine to form cystathionine (via cystathionine β-synthase), meaning serine is a *reactant* used to metabolize homocysteine, not a product derived from it. * **D. Homoserine:** This is an intermediate in the synthesis of threonine and methionine in plants and bacteria; it is not a precursor for serine in human metabolism. **3. NEET-PG High-Yield Pearls:** * **Cofactor Alert:** Always remember that SHMT requires **Vitamin B6**. Deficiency can impair the interconversion of these amino acids. * **One-Carbon Pool:** This reaction is a major entry point for one-carbon units into the folate pool (converting THF to Methylene-THF). * **Glucogenic Status:** Both serine and glycine are **purely glucogenic** amino acids as they can ultimately be converted to pyruvate. * **Special Product:** Serine is a crucial precursor for the synthesis of **sphingosine, phospholipids (phosphatidylserine), and cysteine.**

Question 290: Branched chain keto acids are excreted in urine in which condition?

A. Maple syrup urine disease (Correct Answer)
B. Hanup disease
C. Albinism
D. Alkaptonuria

Explanation: ### Explanation **Maple Syrup Urine Disease (MSUD)** is the correct answer because it is caused by a deficiency in the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD) complex**. This enzyme is responsible for the oxidative decarboxylation of keto acids derived from the branched-chain amino acids (BCAAs): **Leucine, Isoleucine, and Valine**. When this enzyme is defective, these keto acids (and their parent amino acids) accumulate in the blood and are excreted in the urine, giving it a characteristic burnt-sugar or maple syrup odor. **Analysis of Incorrect Options:** * **Hartnup Disease:** A defect in the transport of neutral amino acids (especially Tryptophan) in the intestine and renal tubules. It leads to pellagra-like symptoms, not the excretion of branched-chain keto acids. * **Albinism:** Caused by a deficiency of the enzyme **Tyrosinase**, leading to a failure in melanin synthesis from Tyrosine. * **Alkaptonuria:** An autosomal recessive disorder due to a deficiency of **Homogentisate Oxidase**. It results in the excretion of homogentisic acid, which causes the urine to turn black upon standing. **NEET-PG High-Yield Pearls:** * **Mnemonic for BCAAs:** "LIV" (Leucine, Isoleucine, Valine). * **Diagnostic Test:** The **Dinitrophenylhydrazine (DNPH) test** is positive in MSUD (detects alpha-keto acids). * **Cofactors for BCKAD:** Thiamine (B1), Riboflavin (B2), Niacin (B3), Pantothenic acid (B5), and Lipoic acid. (Mnemonic: **T**ender **R**oving **N**ights **F**or **L**ove). * **Clinical Presentation:** Neonatal onset of poor feeding, vomiting, seizures, and a maple syrup odor in urine/cerumen.

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

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One-Carbon Transfer Reactions

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