Subtopic: Fundamentals of Biomolecules, enzyme kinetics
Question Statement:
Which of the following are true about bacterial superoxide dismutase?
P. Present in obligate aerobes
Q. Present in facultative anaerobes
R. Present in aerotolerant anaerobes
S. Absent in obligate aerobes
Options:
(A) P and Q only
(B) P, Q and R only
(C) P and R only
(D) Q and S only
GATE 2018 Biotechnology
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2018 Q.44)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Microbiology (Oxygen Toxicity & Superoxide Dismutase Distribution)
**2. Detailed Step-by-Step Analysis**:
- **Superoxide dismutase (SOD)** is an essential antioxidant metalloenzyme that catalyzes the dismutation of toxic superoxide radicals into molecular oxygen and hydrogen peroxide:
$2 ext{O}_2^{\cdot-} + 2 ext{H}^+ \xrightarrow{ ext{SOD}} ext{O}_2 + ext{H}_2 ext{O}_2$
- Let us evaluate its presence across microbial physiological categories with respect to oxygen tolerance:
- **P. Obligate aerobes**: Require oxygen for respiration and continuously generate reactive oxygen species (ROS). They must possess SOD and catalase to survive in air. $\implies$ **P is TRUE**.
- **Q. Facultative anaerobes**: Can grow aerobically via respiration or anaerobically via fermentation; when exposed to oxygen, they rely on SOD to neutralize superoxide. $\implies$ **Q is TRUE**.
- **R. Aerotolerant anaerobes**: Do not utilize oxygen for energy generation (relying solely on fermentation), but they tolerate ambient oxygen because they constitutively synthesize SOD (and peroxidase) to eliminate superoxide radicals. $\implies$ **R is TRUE**.
- **S. Obligate aerobes**: SOD is present (not absent) in obligate aerobes. $\implies$ **S is FALSE**.
- Therefore, statements **P, Q, and R only** are true.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Omits aerotolerant anaerobes (R), which famously express SOD.
- **Option (B) [CORRECT]**: P, Q, and R are all true: obligate aerobes, facultative anaerobes, and aerotolerant anaerobes all possess superoxide dismutase.
- **Option (C) [INCORRECT]**: Omits facultative anaerobes (Q).
- **Option (D) [INCORRECT]**: Statement S is blatantly false.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **B**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).
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