GATE 2026 BT Q.17: solution and answer

Official answer: ASource: Official answer key, GATE 2026 BT (IIT Guwahati)

1. Nitrogenase is a two-component enzyme complex. The Fe protein is called dinitrogenase reductase. The MoFe protein is called dinitrogenase, and it is the site where ${\text{N}}_2$ is reduced to ${\text{NH}}_3$.

2. Reduced ferredoxin (or flavodoxin) is the physiological electron donor. It gives electrons to the Fe protein (the reductase).

3. The Fe protein binds ATP, and ATP hydrolysis drives electron transfer from the Fe protein to the MoFe protein. Inside the MoFe protein the electrons pass through the P-cluster to the FeMo-cofactor, where ${\text{N}}_2$ is reduced.

4. The order of electron flow is therefore ferredoxin $\to$ reductase $\to$ nitrogenase. This is option (A).

5. Option (B) is wrong because NADH does not donate electrons directly to the Fe protein. Ferredoxin or flavodoxin is the immediate donor, and NADH is at most an indirect source further upstream.

6. Option (C) is wrong because it reverses the last two carriers. The reductase passes electrons to the nitrogenase, because the nitrogenase holds the substrate-binding site.

7. Option (D) is wrong because the reductase is not at the start of the chain, and NADH is not an electron carrier placed between the reductase and the nitrogenase.