Q.17 The nitrogenase of diazotrophs
(A) contains Cu‑S center and uses 12 NADH to reduce one N₂
(B) contains one (4Fe‑4S) cluster and uses 8 FADH₂ to reduce one N₂
(C) is a complex of Fe‑protein and MoFe‑protein and uses 16 ATPs to reduce one N₂
(D) is a MoFe‑protein and uses 4 ATP and 4 FMNH₂ to reduce one N₂
Correct Answer: (C)
Nitrogenase in diazotrophs forms a complex of Fe-protein and MoFe-protein, requiring 16 ATPs to reduce one N₂ molecule. This matches standard biochemical knowledge for biological nitrogen fixation.
Option Analysis
Option (A): Nitrogenase lacks a Cu-S center, which is characteristic of other enzymes like cytochrome c oxidase. It uses ferredoxin or flavodoxin as electron donors, not NADH, and the stoichiometry involves ~16 ATP per N₂, not 12.
Option (B): The Fe-protein contains one [4Fe-4S] cluster, but the MoFe-protein has additional P-clusters ([8Fe-7S]) and FeMo-cofactors. Electrons come from reduced ferredoxin, not FADH₂, and the ATP requirement is 16, not 8.
Option (C): Nitrogenase consists of Fe-protein (electron donor with [4Fe-4S]) and MoFe-protein (catalytic with P-clusters and FeMo-co). Reducing N₂ + H⁺ → 2NH₃ + H₂ demands 8 electrons (16 ATP, as 2 ATP per electron transfer).
Option (D): Nitrogenase is not solely MoFe-protein; it requires the Fe-protein complex. It uses ~16 ATP and ferredoxin (not FMNH₂), with much higher energy demand.
Nitrogenase of diazotrophs enables biological N₂ fixation, converting atmospheric nitrogen to ammonia via a Fe-protein and MoFe-protein complex powered by 16 ATPs.
Enzyme Structure
Nitrogenase comprises two proteins: Fe-protein (γ₂ dimer with one [4Fe-4S] cluster) transfers electrons to MoFe-protein (α₂β₂ tetramer with P-clusters [8Fe-7S] and FeMo-cofactors [Mo-7Fe-9S-C-homocitrate]). Diazotrophs like Azotobacter protect this O₂-sensitive system in heterocysts or anaerobically.
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Fe-protein: ATPase, binds 2 MgATP per cycle.
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MoFe-protein: Houses catalytic sites for N₂ reduction.
Reduction Mechanism
The reaction N₂ + 8H⁺ + 8e⁻ → 2NH₃ + H₂ requires 8 electrons from ferredoxin/flavodoxin via Fe-protein (1 e⁻ per ATP hydrolysis cycle, 2 ATP/e⁻). Thus, 16 ATP hydrolyzes per N₂, coupling energy to electron tunneling through clusters.
Energy Stoichiometry
Each Fe-protein:MoFe-protein docking hydrolyzes 2 ATP for 1 electron transfer (Pi/e⁻ ratio ~2.8 observed). Full N₂ reduction demands 8 cycles, confirming 16 ATP minimum despite inefficiencies.
Biological Relevance
In CSIR NET contexts, option (C) distinguishes correct nitrogenase anatomy and energetics from distractors misstating clusters, reductants (no NADH/FADH₂/FMNH₂), or oversimplifying components.
