Riboflavin (B2) and Flavin Coenzymes — MCQs
Which of the following is a consequence of Vitamin B12 deficiency?
Riboflavin deficiency is assessed by?
The cofactor vitamin B12 is required for the following conversion:
A 2-year-old boy weighing 12 kg is diagnosed with vitamin A deficiency. What is the recommended oral dose of vitamin A for him?
How does cyanide affect the electron transport chain and cellular respiration?
A 69-year-old man is given a multivitamin containing vitamin B12. He feels well, reports no symptoms, and his physical examination is normal. His complete blood count is completely normal. Which of the following statements about vitamin B12 absorption is correct?
Which of the following enzymes is NAD+ dependent?
Which cofactor is primarily associated with the activity of glutamate dehydrogenase?
Which of the following coenzymes is directly derived from riboflavin?
All of the following are true about glutathione, except:
Riboflavin (B2) and Flavin Coenzymes Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following is a consequence of Vitamin B12 deficiency?
- A. Enhanced folate absorption
- B. Increased red blood cell production
- C. Decreased metabolic intermediate levels
- D. Neurological complications (Correct Answer)
Explanation: ### Neurological complications - **Vitamin B12** is crucial for maintaining the **myelin sheath** around nerves, and its deficiency leads to demyelination. - This can result in a range of neurological symptoms, including **peripheral neuropathy**, cognitive impairment, and **ataxia**. *Enhanced folate absorption* - **Vitamin B12 deficiency** actually impairs the proper utilization of **folate** within the cells, leading to a "folate trap" phenomenon [1]. - It does not enhance folate absorption; rather, it makes folate functionally deficient. *Increased red blood cell production* - **Vitamin B12 deficiency** leads to **megaloblastic anemia**, characterized by the production of **large, immature red blood cells** that are fewer in number [2]. - This results in a **decrease** in overall red blood cell production and an impaired ability to carry oxygen. *Decreased metabolic intermediate levels* - **Vitamin B12** is a cofactor for enzymes involved in critical metabolic pathways, such as the conversion of **methylmalonyl-CoA** to succinyl-CoA [1]. - Its deficiency leads to the **accumulation of these metabolic intermediates** (e.g., methylmalonic acid and homocysteine), not a decrease [1].
Question 2: Riboflavin deficiency is assessed by?
- A. None of the options
- B. Glutathione reductase activity (Correct Answer)
- C. Pyruvate dehydrogenase activity
- D. Transketolase activity
Explanation: ***Glutathione reductase activity*** - Riboflavin is a precursor to **flavin adenine dinucleotide (FAD)**, a coenzyme for **glutathione reductase**. - Reduced glutathione reductase activity and its activation coefficient in erythrocytes are reliable indicators of **riboflavin deficiency**. *Pyruvate dehydrogenase activity* - **Pyruvate dehydrogenase** complex requires **thiamine pyrophosphate** (from thiamine, vitamin B1), not riboflavin, as a coenzyme. - Its activity is used to assess **thiamine status**, not riboflavin. *None of the options* - This option is incorrect as **glutathione reductase activity** is a valid method for assessing riboflavin deficiency. - The other options provided target different vitamin deficiencies. *Transketolase activity* - **Transketolase** activity is specifically used to assess **thiamine (vitamin B1) status**. - It requires **thiamine pyrophosphate** as a coenzyme, which is derived from thiamine.
Question 3: 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
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).
Question 4: A 2-year-old boy weighing 12 kg is diagnosed with vitamin A deficiency. What is the recommended oral dose of vitamin A for him?
- A. 200,000 I.U. (Correct Answer)
- B. 50,000 I.U.
- C. 100,000 I.U.
- D. 150,000 I.U.
Explanation: ***200,000 I.U.*** - According to **WHO guidelines**, for children aged **12 months and older** with vitamin A deficiency, the recommended oral dose is **200,000 I.U.** - This dose is given immediately upon diagnosis, repeated the next day, and a third dose is given 2-4 weeks later. - Since this child is **2 years old**, he falls into the ≥12 months category requiring 200,000 I.U. *100,000 I.U.* - A dose of **100,000 I.U.** is recommended for infants aged **6-11 months** with vitamin A deficiency. - This child is 2 years old, making 100,000 I.U. an insufficient dose for his age group. *50,000 I.U.* - A dose of **50,000 I.U.** is recommended for infants **younger than 6 months** diagnosed with vitamin A deficiency. - This dose is too low for a 2-year-old child. *150,000 I.U.* - **150,000 I.U.** is not a standard WHO-recommended dose for vitamin A deficiency treatment in any pediatric age group. - This is an incorrect dosing option.
Question 5: How does cyanide affect the electron transport chain and cellular respiration?
- A. Increases mitochondrial permeability
- B. Blocks NADH oxidation
- C. Inhibits complex IV (Correct Answer)
- D. Stimulates ATP production
Explanation: ***Inhibits complex IV*** - Cyanide is a potent **inhibitor** of **cytochrome c oxidase** (Complex IV) in the electron transport chain. - By binding to the ferric iron within cytochrome c oxidase, it prevents the transfer of electrons to oxygen, effectively **halting cellular respiration**. *Increases mitochondrial permeability* - While some toxins can increase mitochondrial permeability, **cyanide's primary mechanism** of action is not through this process. - Increased permeability would lead to uncoupling of oxidative phosphorylation, which is different from direct inhibition of electron transfer. *Blocks NADH oxidation* - **NADH oxidation** occurs primarily at **Complex I** (NADH dehydrogenase) of the electron transport chain. - Cyanide acts much later in the chain, specifically at Complex IV, and does not directly block NADH oxidation. *Stimulates ATP production* - Cyanide directly **inhibits ATP production** by blocking the electron transport chain and subsequently oxidative phosphorylation. - Without a functioning electron transport chain, the proton gradient necessary for **ATP synthase** cannot be established, leading to a severe energy deficit.
Question 6: A 69-year-old man is given a multivitamin containing vitamin B12. He feels well, reports no symptoms, and his physical examination is normal. His complete blood count is completely normal. Which of the following statements about vitamin B12 absorption is correct?
- A. absorbed primarily in the distal ileum
- B. not significantly affected by folic acid deficiency
- C. best absorbed in the presence of intrinsic factor (Correct Answer)
- D. absorbed equally well with or without intrinsic factor
Explanation: ***best absorbed in the presence of intrinsic factor*** - **Intrinsic factor** is a glycoprotein secreted by gastric parietal cells that binds to vitamin B12, forming a complex essential for its absorption in the **terminal ileum**. - This is the **most complete answer** as it identifies the critical requirement for efficient B12 absorption. - In conditions like **pernicious anemia**, where intrinsic factor is deficient, vitamin B12 absorption is severely impaired, leading to **megaloblastic anemia** and neurological symptoms. - Only about **1-2% of oral B12** can be absorbed by passive diffusion without intrinsic factor, which is insufficient for physiological needs. *absorbed primarily in the distal ileum* - While this statement is **technically correct** (B12 is indeed absorbed in the terminal/distal ileum), it is **incomplete** as it does not mention the essential role of intrinsic factor. - The **best answer** identifies not just the location but the critical mechanism (intrinsic factor requirement). - Without specifying intrinsic factor, this option misses the most clinically important aspect of B12 absorption. *not significantly affected by folic acid deficiency* - This is **incorrect** as **vitamin B12** and **folic acid** metabolism are intricately linked in one-carbon metabolism. - Both are crucial for DNA synthesis, particularly in rapidly dividing cells like bone marrow. - Vitamin B12 (as methylcobalamin) is required to convert 5-methyl-tetrahydrofolate to tetrahydrofolate, allowing folate to participate in DNA synthesis. - Deficiency in either can lead to **megaloblastic anemia** with similar hematologic findings. *absorbed equally well with or without intrinsic factor* - This statement is **incorrect** as **intrinsic factor is essential** for efficient vitamin B12 absorption. - The B12-intrinsic factor complex binds to specific receptors (cubilin) in the terminal ileum for absorption. - Without intrinsic factor, only minimal passive diffusion occurs (~1-2%), which is inadequate to meet daily requirements of approximately 2-3 mcg.
Question 7: Which of the following enzymes is NAD+ dependent?
- A. HMG CoA reductase
- B. Glycerol-3-phosphate dehydrogenase (Correct Answer)
- C. Acyl CoA dehydrogenase
- D. Succinate dehydrogenase
Explanation: ***Glycerol-3-phosphate dehydrogenase*** - This enzyme catalyzes the interconversion of **dihydroxyacetone phosphate (DHAP)** and **glycerol-3-phosphate**, using **NAD+** as a cofactor to oxidize glycerol-3-phosphate. - In the **glycerol-phosphate shuttle**, it enables the transfer of reducing equivalents from cytosolic NADH to the mitochondrial electron transport chain. *HMG CoA reductase* - This enzyme is a key regulatory step in **cholesterol biosynthesis** and utilizes **NADPH** as a reducing agent, not NAD+. - It catalyzes the reduction of HMG-CoA to **mevalonate**. *Acyl CoA dehydrogenase* - This enzyme is involved in the first step of **beta-oxidation of fatty acids** and uses **FAD** as a prosthetic group, not NAD+, which is reduced to FADH2. - It catalyzes the formation of a double bond between the alpha and beta carbons of the acyl-CoA. *Succinate dehydrogenase* - This enzyme is part of the **TCA cycle** (Complex II of the electron transport chain) and uses **FAD** as its electron acceptor, converting succinate to fumarate. - It is unique among TCA cycle enzymes as it is membrane-bound and directly links the cycle to the **electron transport chain**.
Question 8: Which cofactor is primarily associated with the activity of glutamate dehydrogenase?
- A. NAD+ (Correct Answer)
- B. FAD
- C. FMN
- D. FADH2
Explanation: ***NAD+*** - Glutamate dehydrogenase catalyzes the oxidative deamination of **glutamate** to **α-ketoglutarate** and ammonia, and this reaction primarily uses **NAD+** as an electron acceptor. - In some organisms and contexts, it can also use **NADP+**, but **NAD+** is the more common and significant cofactor for its catabolic role. *FAD* - **FAD (flavin adenine dinucleotide)** is typically associated with **flavoproteins** and enzymes involved in oxidation-reduction reactions, such as those in the **electron transport chain** and the **Krebs cycle**. - Enzymes like **succinate dehydrogenase** use FAD, not glutamate dehydrogenase. *FMN* - **FMN (flavin mononucleotide)** is another flavin coenzyme, similar to FAD, and is found in various **flavoproteins** and enzymes of the **electron transport chain**, such as **NADH dehydrogenase (Complex I)**. - It does not serve as a primary cofactor for **glutamate dehydrogenase** activity. *FADH2* - **FADH2** is the reduced form of **FAD**, carrying high-energy electrons to the **electron transport chain** for ATP synthesis. - It's a product or reactant of various metabolic pathways, but not a direct cofactor for **glutamate dehydrogenase**.
Question 9: Which of the following coenzymes is directly derived from riboflavin?
- A. FMN (Correct Answer)
- B. NAD
- C. THF
- D. FAD
Explanation: ***FMN (Flavin Mononucleotide)*** - **FMN is the direct derivative** of riboflavin (vitamin B2), formed by phosphorylation of riboflavin - Serves as a prosthetic group in various **flavoproteins** involved in electron transfer reactions - Functions as a redox cofactor in multiple metabolic pathways including the electron transport chain *NAD (Nicotinamide Adenine Dinucleotide)* - Derived from **niacin (vitamin B3)**, not riboflavin - Key coenzyme in redox reactions, particularly in glycolysis and the citric acid cycle *THF (Tetrahydrofolate)* - Active form of **folate (vitamin B9)**, not riboflavin - Essential for one-carbon metabolism, DNA synthesis, and amino acid conversions *FAD (Flavin Adenine Dinucleotide)* - While FAD is also derived from riboflavin, it is a **secondary derivative** formed from FMN + ATP - The conversion pathway is: Riboflavin → FMN → FAD - FMN is the more direct answer to this question
Question 10: All of the following are true about glutathione, except:
- A. It is co-factor of various enzymes
- B. It converts hemoglobin to methemoglobin (Correct Answer)
- C. It is a tripeptide
- D. It conjugates xenobiotics
Explanation: ***It converts hemoglobin to methemoglobin*** - Glutathione is a **reducing agent** that helps protect hemoglobin from oxidation, thus **preventing** the formation of methemoglobin. - **Methemoglobin** occurs when the iron in hemoglobin is oxidized from the ferrous (Fe2+) to the ferric (Fe3+) state, which is a process glutathione actively counters. *It is co-factor of various enzymes* - Glutathione serves as a crucial **co-factor** for several enzymes, including **glutathione peroxidase**, which plays a vital role in antioxidant defense. - It participates in various **detoxification reactions** and catalyzes the reduction of harmful reactive oxygen species. *It is a tripeptide* - Glutathione is indeed a **tripeptide** composed of three amino acids: **glutamate**, **cysteine**, and **glycine**. - Its unique structure enables its diverse biological functions, including its prominent role as an antioxidant. *It conjugates xenobiotics* - Glutathione plays a critical role in **detoxifying xenobiotics** (foreign compounds) by conjugating with them, making them more water-soluble and easier to excrete. - This process is mediated by **glutathione S-transferases**, which attach glutathione to various toxic compounds.
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free