Q. 81 Nitrogenase reduces 𝐍𝟐 to 𝐍𝐇𝟑. Metal co-factors required for this activity are ____ .
(A) Fe&Cu
(B) Mo&Fe
(C) Mo & Mn
(D) Cu&Mn
Nitrogenase, the enzyme responsible for converting atmospheric N₂ to ammonia (NH₃) in biological nitrogen fixation, requires specific metal cofactors for its activity. The correct answer is (B) Mo & Fe, as confirmed by extensive biochemical research on its structure and function.
Article Content
Nitrogenase operates in symbiotic bacteria like Rhizobium and free-living diazotrophs, using the MoFe protein’s FeMo-cofactor as the active site for N₂ reduction. This cofactor contains molybdenum (Mo) and iron (Fe) atoms, enabling electron transfer and proton addition in an ATP-dependent process. The Fe protein provides electrons to the MoFe protein, where the FeMo-co cluster catalyzes 8 electrons and protons to yield 2 NH₃ and H₂.
Correct Answer: (B) Mo & Fe
The FeMo-cofactor ([MoFe₇S₉C] with homocitrate) houses the N₂ binding site, with Fe atoms facilitating reductive elimination steps. Structural studies via X-ray crystallography confirm Mo and Fe as essential, absent in mutants lacking activity. This combination achieves high specificity for N₂ over other substrates.
Why Not (A) Fe & Cu?
Copper plays no role in standard nitrogenase; it’s absent from Fe protein, P-clusters, or FeMo-co. Some alternative oxidases involve Cu, but nitrogenase relies solely on Fe/Mo/V for catalysis, making this incorrect.
Why Not (C) Mo & Mn?
Manganese supports photosynthesis (e.g., OEC in PSII) but not nitrogenase, which lacks Mn in its metalloclusters. Mutants with Mn show no N₂ fixation, confirming exclusion.
Why Not (D) Cu & Mn?
Neither Cu nor Mn features in nitrogenase components; Cu inhibits some reductases, while Mn is irrelevant. Only Fe/S and Mo/V/Fe cofactors enable activity.
