Q.74
The enzyme that catalyzes the reduction of nitrogen to ammonia is
(A) Nitrogenase
(B) Nitrate reductase
(C) Nitrite reductase
(D) Deaminase
Nitrogenase is the enzyme that catalyzes the reduction of atmospheric N₂ to ammonia (NH₃) during biological nitrogen fixation in prokaryotes like Rhizobium. The correct answer is (A) Nitrogenase.
Correct Answer
(A) Nitrogenase
Nitrogen Reduction Pathway
Nitrogenase (Mo-Fe protein + Fe protein) uses 16 ATP + 8 e⁻ + 8 H⁺ per N₂ → 2 NH₃ + H₂ in anaerobic conditions. The FeMo-cofactor (M-cluster) binds and sequentially reduces N≡N via metal-hydride intermediates. This distinguishes it from nitrate assimilation enzymes.
Option Analysis
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(A) Nitrogenase: Correct. Sole enzyme reducing N₂ (triple bond) to NH₃; O₂-sensitive, prokaryote-specific.
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(B) Nitrate reductase: Wrong. Reduces NO₃⁻ → NO₂⁻ (assimilation pathway, not N₂).
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(C) Nitrite reductase: Wrong. Further reduces NO₂⁻ → NH₄⁺ (nir genes), downstream of nitrate reductase.
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(D) Deaminase: Wrong. Removes amine groups (e.g., AMP → IMP); no nitrogen fixation role.
Introduction to Nitrogenase Function
Nitrogenase enzyme reduction nitrogen ammonia is foundational for GATE Life Sciences microbiology Q.74, where only nitrogenase converts atmospheric N₂ to bioavailable NH₃ using ATP-driven catalysis at the FeMo-cofactor. Distinguish from nitrate assimilation pathways.
Nitrogenase Reaction Details
N₂ + 8 H⁺ + 8 e⁻ + 16 ATP → 2 NH₃ + H₂ + 16 ADP + 16 Pᵢ
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Components: NifDK (MoFe-protein, catalytic) + NifH (Fe-protein, electron donor).
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Cofactors: P-cluster, FeMo-co (7Fe-9S-C-Mo-homocitrate).
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Regulation: NifA/NifL, O₂ protection via leghemoglobin.
Q.74 Enzyme Comparison
| Enzyme | Substrate → Product | Pathway | N₂ Fixation? |
|---|---|---|---|
| Nitrogenase | N₂ → NH₃ | Fixation | Yes |
| Nitrate reductase | NO₃⁻ → NO₂⁻ | Assimilation | No |
| Nitrite reductase | NO₂⁻ → NH₄⁺ | Assimilation | No |
| Deaminase | R-NH₂ → R-H | Amino acid | No |
Answer: (A)—only N₂ reductase.
Why Nitrogenase Stands Alone
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Energy cost: 16 ATP/N₂ vs 8 ATP for nitrate → ammonia.
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Anaerobic: Legume nodules maintain microaerobic conditions.
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GATE high-yield: Rhizobium nif genes, Frankia actinorhizal fixation.
Exam Preparation Tips
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Mnemonic: “Nitrogenase Nabs N₂” (only N₂ → NH₃).
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Compare: Haber-Bosch (industrial) vs nitrogenase (biological).
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PYQ trap: Nitrate reductase = assimilatory, not fixation.
Master nitrogenase enzyme reduction nitrogen ammonia for perfect microbiology scores in competitive exams.
