Antioxidant Enzymes Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for Antioxidant Enzymes. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Antioxidant Enzymes Indian Medical PG 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)
Antioxidant Enzymes 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].
Antioxidant Enzymes Indian Medical PG 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
Antioxidant Enzymes 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].
Antioxidant Enzymes Indian Medical PG 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
Antioxidant Enzymes 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.
Antioxidant Enzymes Indian Medical PG Question 4: The following are major free radical scavengers except:
- A. Glutathione
- B. Catalase
- C. Glutamine (Correct Answer)
- D. Superoxide dismutase
Antioxidant Enzymes 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**.
Antioxidant Enzymes Indian Medical PG 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)
Antioxidant Enzymes 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
Antioxidant Enzymes Indian Medical PG Question 6: All are cofactors for Dehydrogenase except:
- A. SAM (Correct Answer)
- B. NADP
- C. NAD
- D. FAD
Antioxidant Enzymes 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.
Antioxidant Enzymes Indian Medical PG Question 7: Which enzyme joins two substrates?
- A. Synthase
- B. Lyase
- C. Ligase (Correct Answer)
- D. Isomerase
Antioxidant Enzymes 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.
Antioxidant Enzymes Indian Medical PG 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
Antioxidant Enzymes 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.
Antioxidant Enzymes Indian Medical PG Question 9: Which of the following enzymes is not classified as an oxidoreductase?
- A. Alcohol dehydrogenase
- B. Catalase
- C. Peroxidase
- D. Glucokinase (Correct Answer)
Antioxidant Enzymes 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.
Antioxidant Enzymes Indian Medical PG 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
Antioxidant Enzymes 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**.
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