Antioxidant Enzymes — MCQs
Which of the following disorders presents with repeated catalase positive infections?
Which is not a feature of G6PD deficiency?
Which of the following is NOT true regarding the role of NAD+?
The following are major free radical scavengers except:
In G6PD deficiency, which enzyme's function is MOST directly impaired due to decreased NADPH availability, leading to reduced protection against oxidative stress?
All are cofactors for Dehydrogenase except:
Which enzyme joins two substrates?
All of the following are true about glutathione, except:
Which of the following enzymes is not classified as an oxidoreductase?
Glutathione is maintained in reduced state by the help of ?
Antioxidant Enzymes Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following disorders presents with repeated catalase positive infections?
- A. Chediak higashi syndrome
- B. SCID
- C. X linked hypogammaglobulinemia
- D. CGD (Correct Answer)
Explanation: ***CGD*** - Chronic Granulomatous Disease (CGD) is characterized by a defect in **NADPH oxidase**, preventing phagocytes from producing a **respiratory burst** to kill certain bacteria and fungi. - Patients with CGD are particularly susceptible to infections by **catalase-positive organisms** because these organisms degrade hydrogen peroxide, which CGD phagocytes rely on for killing. *Chediak higashi syndrome* - This syndrome involves defective lysosomal trafficking, leading to impaired neutrophil chemotaxis and degranulation, resulting in recurrent infections, but not specifically to **catalase-positive organisms**. - Other features include **partial albinism**, peripheral neuropathy, and normal respiratory burst. *SCID* - Severe Combined Immunodeficiency (SCID) involves a profound defect in both **T-cell and B-cell immunity**, leading to severe and recurrent infections by a wide range of pathogens, not limited to catalase-positive ones [1]. - Patients typically present in infancy with **failure to thrive**, opportunistic infections, and lack of lymphoid tissue [1]. *X linked hypogammaglobulinemia* - Also known as **Bruton's agammaglobulinemia**, this disorder involves a defect in B-cell maturation, leading to the absence of antibodies and recurrent bacterial infections [1]. - The infections are typically with **encapsulated bacteria** and are not specifically linked to catalase-positive organisms [1].
Question 2: Which is not a feature of G6PD deficiency?
- A. Presence of Heinz bodies
- B. Males and females are equally affected (Correct Answer)
- C. Absence of NADPH
- D. Oxidative stress
Explanation: ***Males and females are equally affected*** - G6PD deficiency is an **X-linked recessive disorder**, meaning males are predominantly and more severely affected because they have only one X chromosome [2]. - Females are typically carriers and are less commonly affected, or may experience milder symptoms, due to **X-chromosome inactivation** (Lyonization). *Presence of Heinz bodies* - **Heinz bodies** are formed from denatured hemoglobin precipitates within red blood cells, a characteristic feature of **oxidative stress** in G6PD deficiency [2]. - These bodies are removed by the spleen, contributing to **hemolytic anemia**. *Absence of NADPH* - G6PD is the rate-limiting enzyme in the **pentose phosphate pathway**, which generates **NADPH** [1], [2]. - Without sufficient G6PD, the production of **NADPH** is severely impaired, leading to a deficiency in this critical reducing agent. *Oxidative stress* - **NADPH** is crucial for reducing **glutathione**, which in turn detoxifies reactive oxygen species [2]. - The lack of NADPH makes red blood cells vulnerable to **oxidative damage**, manifesting as hemolytic anemia upon exposure to oxidative agents [3].
Question 3: Which of the following is NOT true regarding the role of NAD+?
- A. Acts as an electron carrier
- B. Functions as an antioxidant (Correct Answer)
- C. Participates in glycolysis
- D. Involved in TCA cycle
Explanation: ***Functions as an antioxidant*** - **NAD+** primarily functions as an **electron carrier** in redox reactions, not as an antioxidant that directly neutralizes reactive oxygen species. - While it plays a role in maintaining cellular redox balance, its direct function is not scavenging free radicals like **glutathione** or **vitamins C and E**. *Acts as an electron carrier* - **NAD+** is a crucial coenzyme that accepts electrons and protons during metabolic reactions, converting into **NADH**. - **NADH** then donates these electrons to the **electron transport chain** to generate **ATP**. *Participates in glycolysis* - In glycolysis, **NAD+** is reduced to **NADH** during the oxidation of **glyceraldehyde-3-phosphate** to **1,3-bisphosphoglycerate**. - This step is vital for producing **ATP** and regenerating **NAD+** for continued glycolytic flux. *Involved in TCA cycle* - **NAD+** is reduced to **NADH** at several steps in the **TCA cycle**, including the conversion of **isocitrate to α-ketoglutarate**, **α-ketoglutarate to succinyl CoA**, and **malate to oxaloacetate**. - These **NADH** molecules are then funneled into the **electron transport chain** for oxidative phosphorylation.
Question 4: The following are major free radical scavengers except:
- A. Glutathione
- B. Catalase
- C. Glutamine (Correct Answer)
- D. Superoxide dismutase
Explanation: ***Glutamine*** - **Glutamine** is an amino acid primarily involved in **protein synthesis**, immune function, and as a precursor for neurotransmitters, but it is not a direct antioxidant or free radical scavenger. - While it plays a role in maintaining cellular health, it does not directly neutralize **reactive oxygen species** like other listed compounds. *Glutathione* - **Glutathione** is a major endogenous antioxidant, directly neutralizing **free radicals** and participating in detoxification processes. - It's a key component of the **glutathione redox cycle**, protecting cells from oxidative damage. *Catalase* - **Catalase** is an enzyme that catalyzes the decomposition of hydrogen peroxide to water and oxygen, thus protecting cells from **oxidative damage**. - It is particularly important in neutralizing **reactive oxygen species** generated during metabolic processes. *Superoxide dismutase* - **Superoxide dismutase (SOD)** is an enzyme that catalyzes the dismutation of the **superoxide radical** into oxygen and hydrogen peroxide. - It is a crucial primary antioxidant defense against **oxidative stress**.
Question 5: In G6PD deficiency, which enzyme's function is MOST directly impaired due to decreased NADPH availability, leading to reduced protection against oxidative stress?
- A. Catalase
- B. Pyruvate kinase
- C. Superoxide dismutase
- D. Glutathione reductase (Correct Answer)
Explanation: ***Glutathione reductase*** - **G6PD deficiency** impairs the production of **NADPH** through the pentose phosphate pathway - **Glutathione reductase** is NADPH-dependent and reduces oxidized glutathione (GSSG) back to reduced glutathione (GSH) - Without adequate NADPH, glutathione reductase cannot maintain sufficient **GSH levels**, which is the primary antioxidant protecting RBCs from oxidative damage - This explains why G6PD deficiency leads to **hemolysis** when exposed to oxidative stressors (antimalarials, sulfonamides, fava beans) *Catalase* - **Catalase** decomposes hydrogen peroxide to water and oxygen, protecting cells from oxidative damage - While important for antioxidant defense, catalase does **not require NADPH** for its function - Its activity is not directly impaired by decreased NADPH in G6PD deficiency *Pyruvate kinase* - **Pyruvate kinase** catalyzes the final step of **glycolysis**, producing ATP - Its function is **completely independent** of NADPH levels - Pyruvate kinase deficiency causes a separate hemolytic anemia unrelated to oxidative stress or G6PD deficiency *Superoxide dismutase* - **Superoxide dismutase (SOD)** converts superoxide radicals to hydrogen peroxide and oxygen - SOD functions **independently of NADPH** and uses metal cofactors (Cu/Zn or Mn) - While part of antioxidant defense, it is not directly affected by G6PD deficiency
Question 6: All are cofactors for Dehydrogenase except:
- A. SAM (Correct Answer)
- B. NADP
- C. NAD
- D. FAD
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.
Question 7: Which enzyme joins two substrates?
- A. Synthase
- B. Lyase
- C. Ligase (Correct Answer)
- D. Isomerase
Explanation: ***Ligase*** - **Ligases** are a class of enzymes that **catalyze the joining of two large molecules** by forming a new chemical bond, typically with the concomitant hydrolysis of a small pendant chemical group on one of the larger molecules or the coupling of a reaction to the cleavage of pyrophosphate on ATP or similar. - This process often involves the use of **ATP or other energy sources** to form a covalent bond. *Lyase* - **Lyases** are enzymes that **catalyze the breaking of chemical bonds** by means other than hydrolysis (e.g., elimination reactions). - They typically form a new double bond or a ring structure during the bond cleavage. *Synthase* - **Synthases** are a type of **lyase enzyme** that **catalyzes synthesis reactions** without the direct involvement of ATP or other nucleoside triphosphates for energy. - While they synthesize molecules, they don't necessarily "join two substrates" in the same way a ligase does, especially without consuming a high-energy phosphate. *Isomerase* - **Isomerases** catalyze the **rearrangement of atoms within a molecule**, converting a compound into one of its isomers. - They do not join two separate substrates; rather, they alter the structure of a single substrate.
Question 8: 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.
Question 9: Which of the following enzymes is not classified as an oxidoreductase?
- A. Alcohol dehydrogenase
- B. Catalase
- C. Peroxidase
- D. Glucokinase (Correct Answer)
Explanation: ***Glucokinase*** - **Glucokinase** is a **transferase** enzyme that catalyzes the transfer of a phosphate group from ATP to glucose, forming glucose-6-phosphate. - Its function is primarily in **glucose metabolism** and **insulin secretion**, not in oxidation or reduction reactions. *Catalase* - **Catalase** is an **oxidoreductase** that catalyzes the decomposition of **hydrogen peroxide** into water and oxygen. - This reaction involves the **oxidation and reduction** of substrates, fitting the definition of an oxidoreductase. *Alcohol dehydrogenase* - **Alcohol dehydrogenase** is an **oxidoreductase** that catalyzes the interconversion between alcohols and aldehydes or ketones with the concomitant reduction and oxidation of **NAD+** to **NADH**. - This enzyme is crucial in **detoxifying alcohol** by oxidizing it and is a classic example of an oxidoreductase. *Peroxidase* - **Peroxidase** is an **oxidoreductase** that catalyzes the oxidation of a substrate by **hydrogen peroxide**. - Peroxidases work by using hydrogen peroxide to accept electrons from another molecule, thereby **oxidizing** that molecule.
Question 10: Glutathione is maintained in reduced state by the help of ?
- A. Transamination
- B. HMP shunt (Correct Answer)
- C. Uronic acid pathway
- D. Glycogenesis
Explanation: ***HMP shunt*** - The **hexose monophosphate (HMP) shunt** produces **NADPH**, which is crucial for reducing **oxidized glutathione** back to its reduced form via **glutathione reductase**. - **Reduced glutathione** protects cells from **oxidative damage** by detoxifying harmful **reactive oxygen species.** *Transamination* - **Transamination** is a process involving the transfer of an **amino group** from an amino acid to a keto acid. - This pathway is primarily involved in **amino acid metabolism** and the synthesis of **non-essential amino acids**, not directly in glutathione reduction. *Uronic acid pathway* - The **uronic acid pathway** is involved in the synthesis of **glycolipids**, **sugars**, and **vitamin C** (in some animals). - It does not directly produce **NADPH** or enzymes necessary for maintaining **glutathione** in its reduced state. *Glycogenesis* - **Glycogenesis** is the process of synthesizing **glycogen** from **glucose** for storage, typically occurring in the liver and muscles. - This pathway is involved in **glucose storage** and **energy regulation**, not in the **redox state of glutathione**.
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free