The following statements are made regarding the nitrogenase enzyme involved in the reduction of atmospheric nitrogen to ammonia:

A. Nitrogenase enzyme is composed of two components: dinitrogenase and dinitrogenase reductase.

B. MoFe protein component is a homodimer.

C. Fe protein component is dinitrogenase.

D. MoFe protein contains the active site metal cluster where N2 binds.

E. Fe protein delivers electron to MoFe protein component in a reaction coupled to the hydrolysis of MgATP.

Which one of the following options represents the combination of all correct statements?

Correct option is D

​The nitrogenase enzyme complex catalyzes the biologically essential conversion of atmospheric nitrogen (N₂) to ammonia (NH₃) in certain prokaryotes like cyanobacteria and Rhizobium spp. The enzyme consists of two main components: Fe protein (dinitrogenase reductase) and MoFe protein (dinitrogenase).

Statement-wise Evaluation:

1. Statement A: Correct 

   • The nitrogenase complex consists of two protein components:
     • Dinitrogenase reductase (Fe protein) – transfers electrons.
     • Dinitrogenase (MoFe protein) – site of nitrogen reduction.
   • Hence, the statement is correct.

2. Statement B: Incorrect 

   • The MoFe protein is a heterotetramer, not a homodimer.
     • It consists of two α-subunits (encoded by nifD) and two β-subunits (encoded by nifK), forming an α₂β₂ heterotetramer.
   • Hence, this statement is incorrect.

3. Statement C: Incorrect 

   • The Fe protein is the dinitrogenase reductase, not dinitrogenase.
   • Dinitrogenase = MoFe protein, not the Fe protein.
   • Thus, this statement is incorrect.

4. Statement D: Correct 

   • The MoFe protein contains the active site metal clusters, including:
     • FeMo-cofactor (FeMo-co) – the catalytic site where N₂ binds and is reduced to NH₃.
   • Hence, this statement is correct.

5. Statement E: Correct 

   • The Fe protein (dinitrogenase reductase) delivers electrons to the MoFe protein, and this electron transfer is coupled to ATP hydrolysis (MgATP).
   • Typically, 2 ATP molecules are hydrolyzed per electron transfer.
   • Hence, this statement is correct.