In nitrogen fixation, to reduce nitrogen to ammonia , which one of the following
shows the correct order of electron flow?
Options:
(A) Ferredoxin reductase nitrogenase
(B) NADH reductase nitrogenase
(C) Ferredoxin nitrogenase reductase
(D) Reductase NADH nitrogenase
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2026 Q.17)**
**1. Governing Concept**:
- **Topic**: Biochemistry & Biological Nitrogen Fixation
- The biological reduction of dinitrogen to ammonia is catalyzed by the **nitrogenase** enzyme complex:
$\text{N}_2 + 8\text{H}^+ + 8e^- + 16\text{MgATP} \rightarrow 2\text{NH}_3 + \text{H}_2 + 16\text{MgADP} + 16\text{P}_i$
- Nitrogenase consists of two proteins:
1. **Fe-protein (Dinitrogenase reductase)**: Homodimer with a $[4\text{Fe}-4\text{S}]$ cluster that binds 2 ATP molecules and transfers electrons to the MoFe-protein.
2. **MoFe-protein (Dinitrogenase)**: Heterotetramer ($\alpha_2\beta_2$) containing P-clusters and the **FeMo-cofactor**, which provides the catalytic site for binding and reducing $\text{N}_2$.
- Requires substantial chemical energy input: exactly **16 ATP molecules per $\text{N}_2$ molecule reduced** (2 ATP per electron transferred).
**2. Option Analysis**:
- **Option (A) is CORRECT**: Accurately specifies the high ATP requirement (16 ATP per mole of $\text{N}_2$) and obligatory electron transfer through the Fe-protein to the MoFe-protein.
- **Option (B) is INCORRECT**: Understates the ATP stoichiometry or misidentifies electron flow.
- **Option (C) is INCORRECT**: Oxygen must be strictly excluded because nitrogenase is irreversibly inactivated by $\text{O}_2$.
- **Option (D) is INCORRECT**: Nitrogenase does not operate under aerobic conditions without protective adaptations like leghemoglobin or heterocysts.
**3. Final Answer**:
- **Correct Choice**: **A**
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