One-Carbon Transfer Reactions Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for One-Carbon Transfer Reactions. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
One-Carbon Transfer Reactions Indian Medical PG Question 1: What is the appropriate treatment for megaloblastic anemia with neurological symptoms?
- A. Iron supplementation
- B. Folic acid with Hydroxycobalamin (Correct Answer)
- C. Vitamin B1 supplementation
- D. Folic Acid only
One-Carbon Transfer Reactions Explanation: Folic acid with Hydroxycobalamin
- Neurological symptoms in megaloblastic anemia strongly suggest vitamin B12 deficiency, as folic acid alone can mask this deficiency and worsen neurological sequelae [3].
- Hydroxycobalamin is the preferred treatment for vitamin B12 deficiency, while folic acid addresses the megaloblastic hematopoiesis.
Iron supplementation
- This is used to treat iron deficiency anemia, which presents with microcytic or normocytic red blood cells, not megaloblastic changes [1].
- Iron supplementation would not address the neurological symptoms or the underlying B12 or folate deficiency.
Vitamin B1 supplementation
- Vitamin B1 (thiamine) deficiency is associated with conditions like beriberi and Wernicke-Korsakoff syndrome, characterized by neurological symptoms, but not megaloblastic anemia [4].
- Supplementation would not correct the underlying hematological abnormality or the specific neurological symptoms of B12 deficiency [4].
Folic Acid only
- While folic acid is essential for DNA synthesis and would improve the hematological parameters of megaloblastic anemia, it does not treat vitamin B12 deficiency [2].
- Giving folic acid alone in the presence of B12 deficiency can lead to a worsening of neurological symptoms as it can correct the anemia but allow the neurological damage to progress [3].
One-Carbon Transfer Reactions Indian Medical PG Question 2: Which of the following is not a precursor in the synthesis of pyrimidines?
- A. Glutamine
- B. Carbon dioxide (CO2)
- C. Aspartic acid
- D. Thymidine (Correct Answer)
One-Carbon Transfer Reactions Explanation: ***Thymidine***
- **Thymidine** is a *nucleoside* consisting of deoxyribose and thymine. It is a *product* and a component of DNA, not a precursor in the *de novo synthesis* of pyrimidine bases.
- While it can be incorporated into DNA via the *salvage pathway*, it does not serve as an initial building block for the pyrimidine ring itself.
*Glutamine*
- **Glutamine** provides the **nitrogen atoms** crucial for the formation of the pyrimidine ring, specifically N3 in the pyrimidine base.
- It is a key donor of *amino groups* in various anabolic pathways, including nucleotide synthesis.
*Carbon dioxide (CO2)*
- **Carbon dioxide (CO2)** contributes one of the carbon atoms (C2) to the pyrimidine ring.
- It combines with **ammonia** (derived from glutamine) to form **carbamoyl phosphate**, an essential intermediate.
*Aspartic acid*
- **Aspartic acid** provides four atoms (N1, C4, C5, C6) of the pyrimidine ring.
- Its carbon skeleton and amino group are directly incorporated into the pyrimidine structure during the *de novo synthesis* pathway.
One-Carbon Transfer Reactions Indian Medical PG Question 3: Which vitamin deficiency leads to megaloblastic anemia?
- A. Riboflavin
- B. Folate (Correct Answer)
- C. Vitamin C
- D. Niacin
One-Carbon Transfer Reactions Explanation: ***Folate***
- **Folate** is essential for DNA synthesis; a deficiency impairs erythrocyte maturation, leading to the production of **large, immature red blood cells** (megaloblasts) [3].
- This vitamin deficiency also presents with symptoms like **fatigue, glossitis**, and neurologic manifestations are absent unlike vitamin B12 deficiency [1].
*Riboflavin*
- **Riboflavin (Vitamin B2)** deficiency can cause **normocytic anemia**, but generally not megaloblastic anemia.
- Its deficiency is mainly associated with **angular stomatitis, cheilosis**, and ocular symptoms.
*Vitamin C*
- **Vitamin C** deficiency (scurvy) is associated with impaired collagen synthesis, leading to **gingival bleeding, petechiae**, and poor wound healing.
- While it can cause some anemia, it is typically **microcytic** due to impaired iron absorption if it affects iron metabolism, not megaloblastic [2].
*Niacin*
- **Niacin (Vitamin B3)** deficiency causes **pellagra**, characterized by the "3 D's": **dermatitis, diarrhea, and dementia**.
- It does not directly lead to megaloblastic anemia, as it is not involved in a critical step of DNA synthesis in the same way folate is.
One-Carbon Transfer Reactions Indian Medical PG Question 4: Which drug is metabolized by glutathionation?
- A. Nicotinic acid
- B. Fosfomycin
- C. Benzodiazepines
- D. Dapsone (Correct Answer)
One-Carbon Transfer Reactions Explanation: ***Dapsone***- **Dapsone** undergoes hepatic metabolism via **N-hydroxylation** by CYP450 enzymes (particularly CYP2E1 and CYP3A4), forming reactive **hydroxylamine metabolites**.- These reactive metabolites are toxic and can cause **methemoglobinemia** and **hemolysis**.- **Glutathione conjugation (glutathionation)** serves as an important **detoxification pathway** for these reactive dapsone metabolites [1].- Individuals with **glutathione deficiency** (such as G6PD deficiency) are at increased risk of dapsone-induced hemolytic anemia [2].*Fosfomycin*- **Fosfomycin** is primarily eliminated by the kidneys as an **unchanged drug** (up to 90% excreted unchanged in urine).- It undergoes **minimal hepatic metabolism** and does NOT undergo significant glutathionation.- Its primary route of elimination is **renal excretion** via glomerular filtration.*Benzodiazepines*- **Benzodiazepines** are primarily metabolized in the liver via **CYP450 enzymes** (Phase I oxidation) followed by **glucuronidation** (Phase II conjugation).- They do NOT undergo glutathionation as a significant metabolic pathway.*Nicotinic acid*- **Nicotinic acid** (niacin) undergoes conjugation with **glycine** to form nicotinuric acid and **methylation** to form N-methylnicotinamide.- It does NOT undergo glutathione conjugation.
One-Carbon Transfer Reactions Indian Medical PG Question 5: The cofactor vitamin B12 is required for the following conversion:
- A. Dopamine to Norepinephrine
- B. Propionyl CoA to methyl malonyl CoA
- C. Methyl malonyl CoA to succinyl CoA (Correct Answer)
- D. Homocysteine to cysteine
One-Carbon Transfer Reactions Explanation: ***Methyl malonyl CoA to succinyl CoA***
- **Vitamin B12**, in its active form **adenosylcobalamin**, is a crucial cofactor for the enzyme **methylmalonyl-CoA mutase**, which catalyzes the isomerization of **methylmalonyl-CoA to succinyl-CoA**.
- This conversion is vital for the metabolism of **odd-chain fatty acids** and certain **amino acids**, allowing their entry into the **Krebs cycle**.
*Dopamine to Norepinephrine*
- This conversion is catalyzed by **dopamine beta-hydroxylase**, which requires **vitamin C** (ascorbate) and **copper** as cofactors, not vitamin B12.
- It is a key step in the synthesis of **catecholamines** within the nervous system.
*Propionyl CoA to methyl malonyl CoA*
- This conversion is catalyzed by **propionyl-CoA carboxylase** and requires **biotin** as a cofactor, not vitamin B12.
- This reaction is the first step in the metabolic pathway that leads to succinyl-CoA from odd-chain fatty acids.
*Homocysteine to cysteine*
- This conversion occurs via the **transsulfuration pathway** and requires **vitamin B6** (pyridoxal phosphate) as a cofactor, not vitamin B12.
- The enzymes involved are **cystathionine β-synthase** and **cystathionine γ-lyase**, both B6-dependent.
- Vitamin B12 is involved in the **remethylation** of homocysteine to methionine (not in transsulfuration to cysteine).
One-Carbon Transfer Reactions Indian Medical PG Question 6: All are cofactors for Dehydrogenase except:
- A. SAM (Correct Answer)
- B. NADP
- C. NAD
- D. FAD
One-Carbon Transfer Reactions Explanation: ***SAM***
- **S-adenosylmethionine (SAM)** is a cofactor involved in **methyl group transfer reactions**, carried out by enzymes known as methyltransferases.
- Dehydrogenase enzymes catalyze **redox reactions**, typically involving the transfer of hydride ions, and thus do not utilize SAM as a cofactor.
*NADP*
- **Nicotinamide adenine dinucleotide phosphate (NADP)** is a crucial coenzyme for many **dehydrogenase reactions**, particularly in **anabolic pathways** like fatty acid synthesis and the pentose phosphate pathway.
- It acts as an **electron carrier**, accepting or donating hydride ions.
*NAD*
- **Nicotinamide adenine dinucleotide (NAD)** is a highly common coenzyme for numerous **dehydrogenase enzymes**, especially in **catabolic pathways** such as glycolysis, the Krebs cycle, and oxidative phosphorylation.
- It functions as an **electron acceptor** or donor in redox reactions.
*FAD*
- **Flavin adenine dinucleotide (FAD)** is a coenzyme derived from **riboflavin (Vitamin B2)** and is associated with various dehydrogenase enzymes, particularly those involved in **electron transport** and fatty acid oxidation.
- FAD can accept two hydrogen atoms (one hydride and one proton) to become FADH₂.
One-Carbon Transfer Reactions Indian Medical PG Question 7: What is the mechanism of action of Methotrexate?
- A. Inhibition of Dihydrofolate reductase (Correct Answer)
- B. Inhibits pyrimidine synthesis
- C. Inhibits cell replication by acting on G phase of cell cycle
- D. Inhibits Thymidylate synthase
- E. Inhibits RNA polymerase
One-Carbon Transfer Reactions Explanation: ***Inhibition of Dihydrofolate reductase***
- **Methotrexate** is a **folate analog** that competitively inhibits **dihydrofolate reductase (DHFR)**, an enzyme essential for converting **dihydrofolate** to **tetrahydrofolate**.
- This inhibition blocks the synthesis of **purines** and **pyrimidines**, thereby preventing DNA and RNA synthesis and ultimately inhibiting cell proliferation.
*Inhibits pyrimidine synthesis*
- While methotrexate ultimately inhibits pyrimidine synthesis by depleting tetrahydrofolate, its direct mechanism is not the inhibition of the pyrimidine synthesis pathway enzymes themselves.
- Its primary action is upstream, by inhibiting DHFR.
*Inhibits cell replication by acting on G phase of cell cycle*
- Methotrexate primarily inhibits cells in the **S-phase** of the cell cycle, as it interferes with DNA synthesis.
- It does not specifically target the G phase; rather, it affects cells that are actively attempting to replicate their DNA.
*Inhibits Thymidylate synthase*
- **Thymidylate synthase** is inhibited by drugs like **5-fluorouracil**, which directly blocks the conversion of **deoxyuridine monophosphate (dUMP)** to **deoxythymidine monophosphate (dTMP)**.
- Methotrexate's effect on thymidylate synthesis is indirect, mediated by the depletion of the cofactor **N5,N10-methylene-tetrahydrofolate** due to DHFR inhibition.
*Inhibits RNA polymerase*
- **RNA polymerase** inhibition is the mechanism of drugs like **rifampin** (bacterial RNA polymerase) and **α-amanitin** (eukaryotic RNA polymerase).
- Methotrexate does not directly inhibit RNA polymerase; its effects on RNA synthesis are secondary to depletion of nucleotide precursors through DHFR inhibition.
One-Carbon Transfer Reactions Indian Medical PG Question 8: Amino acid required for conversion of norepinephrine to epinephrine:-
- A. Lysine
- B. Tryptophan
- C. Methionine (Correct Answer)
- D. Phenylalanine
One-Carbon Transfer Reactions Explanation: ***Methionine***
- **Norepinephrine** is converted to **epinephrine** by the enzyme **phenylethanolamine N-methyltransferase (PNMT)**.
- This enzyme uses **S-adenosylmethionine (SAM)** as a **methyl donor**, which is derived from methionine.
*Lysine*
- **Lysine** is an essential amino acid primarily involved in **protein synthesis**, **calcium absorption**, and the production of **carnitine**.
- It does not directly participate in the methylation reaction converting norepinephrine to epinephrine.
*Tryptophan*
- **Tryptophan** is a precursor for **serotonin** and **niacin** synthesis.
- It is not involved in the catecholamine synthesis pathway from norepinephrine to epinephrine.
*Phenylalanine*
- **Phenylalanine** is the initial amino acid in the **catecholamine synthesis pathway**, being converted to **tyrosine**, then to DOPA, dopamine, and norepinephrine.
- While it's crucial for the synthesis *up to* norepinephrine, it is not directly involved in the *conversion of norepinephrine to epinephrine*.
One-Carbon Transfer Reactions Indian Medical PG Question 9: In the metabolism of xenobiotics, which of the following reactions does not occur in phase one?
- A. Reduction
- B. Hydrolysis
- C. Oxidation
- D. Conjugation (Correct Answer)
One-Carbon Transfer Reactions Explanation: ***Correct Answer: Conjugation***
- **Conjugation** reactions are characteristic of **Phase II metabolism**, NOT Phase I
- In Phase II, a polar molecule (glucuronide, sulfate, acetyl, or glutathione) is added to the xenobiotic to increase water solubility and facilitate excretion
- This process typically renders the xenobiotic inactive and more readily eliminated by the kidneys or bile
- Common conjugation reactions include glucuronidation, sulfation, acetylation, and glutathione conjugation
*Incorrect: Oxidation*
- **Oxidation** is a primary **Phase I reaction**, primarily involving the cytochrome P450 (CYP450) enzyme system
- Phase I oxidation introduces or exposes polar functional groups (-OH, -COOH, -NH2)
- This makes the xenobiotic more reactive and prepares it for Phase II conjugation
- Examples include hydroxylation, N-dealkylation, and O-dealkylation
*Incorrect: Reduction*
- **Reduction** reactions are also common in **Phase I metabolism**
- Particularly important for compounds containing nitro groups, carbonyl groups, or azo compounds
- These reactions can occur in various tissues, including the liver
- Catalyzed by reductases such as cytochrome P450 reductase and other enzyme systems
*Incorrect: Hydrolysis*
- **Hydrolysis** is another key **Phase I reaction** that breaks down xenobiotics by adding water
- Especially important for esters, amides, and other compounds with hydrolyzable bonds
- Enzymes like esterases, amidases, and peptidases catalyze these reactions
- Results in more polar metabolites that can undergo Phase II conjugation
One-Carbon Transfer Reactions Indian Medical PG Question 10: Which one among the following essential amino acids is usually the "limiting" amino acid in most of the pulses?
- A. Valine
- B. Threonine
- C. Methionine (Correct Answer)
- D. Lysine
One-Carbon Transfer Reactions Explanation: ***Methionine***
- In **pulses** (legumes), the sulfur-containing amino acids, primarily **methionine** and **cysteine**, are often the first limiting amino acids.
- This means that the amount of protein synthesis that can occur is restricted by the availability of methionine, even if other essential amino acids are abundant.
*Valine*
- **Valine** is an essential branched-chain amino acid, but it is typically not the limiting amino acid in pulses.
- It is more likely to be limiting in certain grains or in specific diets lacking a variety of protein sources.
*Threonine*
- **Threonine** is an essential amino acid that can be limiting in some cereals, but it is not typically the primary limiting amino acid in pulses.
- Its deficiency is less common in a balanced diet including legumes.
*Lysine*
- **Lysine** is often the limiting amino acid in most **cereal grains** (e.g., wheat, corn, rice).
- Pulses, however, are generally good sources of lysine, making its deficiency less likely when consumed.
More One-Carbon Transfer Reactions Indian Medical PG questions available in the OnCourse app. Practice MCQs, flashcards, and get detailed explanations.
cycle and methyl donation pathways)