Subtopic: Fundamentals of Biomolecules, enzyme kinetics
Question Statement:
Which one of the following statements is TRUE about leghemoglobin?
Options:
(A) It binds oxygen to protect nitrogenase
(B) It binds hemoglobin to protect oxygenase
(C) It binds oxygen to protect hydrogenase
(D) It binds oxygen to protect oxygenase
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2023 Q.19)**
**1. Governing Concept**:
- **Topic**: Biological Nitrogen Fixation & Plant Physiology
- The nitrogenase enzyme complex in *Rhizobium* bacteroids within legume root nodules is irreversibly inactivated by molecular oxygen ($\text{O}_2$).
- **Leghemoglobin** is a plant-encoded, heme-containing myoglobin homolog with extremely high affinity for $\text{O}_2$ ($K_d \approx 10\text{ nM}$). It acts as an oxygen buffer/scavenger, delivering oxygen for bacteroid respiration while keeping free dissolved $\text{O}_2$ low enough to protect **nitrogenase** from oxidative denaturation.
**2. Option Analysis**:
- **Option (A) [It binds oxygen to protect nitrogenase] is CORRECT**: Fundamental biological function of leghemoglobin.
- **Option (B) is INCORRECT**: Leghemoglobin binds oxygen, not hemoglobin.
- **Option (C) is INCORRECT**: Protects nitrogenase, not hydrogenase.
- **Option (D) is INCORRECT**: Nitrogenase, not oxygenase, is the oxygen-labile target.
**3. Final Answer**:
- **Correct Choice**: **A**
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