Q.(a) Write the role of nitrogenase enzyme in nitrogen fixation. OR

Nitrogenase is the key enzyme of biological nitrogen fixation, reducing atmospheric N2 to ammonia; the reaction is energy-expensive and the enzyme is inactivated by oxygen, so it needs a protected, low-oxygen environment.

Biological nitrogen fixation — the conversion of inert atmospheric nitrogen gas (N2) into ammonia (NH3), a form plants can use — is carried out by certain prokaryotes (like Rhizobium in legume root nodules, and free-living forms like Azotobacter and cyanobacteria) using the enzyme nitrogenase.

Role of nitrogenase in nitrogen fixation:

1. Catalytic role: Nitrogenase is a complex metalloenzyme (containing molybdenum and iron, hence often called an Mo-Fe protein) that catalyses the reduction of atmospheric N2 to NH3, according to the overall reaction:

   N2 + 8H+ + 8e- + 16 ATP → 2NH3 + H2 + 16 ADP + 16 Pi

2. High energy requirement: The triple bond of N2 is extremely stable, so breaking it and reducing it to ammonia requires a very large input of energy — about 16 ATP molecules are consumed for every molecule of N2 reduced, making this one of the most energy-expensive biochemical processes in nature.

3. Need for reducing power (electrons): The reaction also requires a continuous supply of electrons, typically donated by the electron carrier ferredoxin, to gradually reduce N2 (via intermediate stages) all the way to NH3.

   …