Question

How many times the Fe-protein and the Mo-Fe Protein components of the nitrogenase complex have to interact with each other for the reduction of one dinitrogen molecule to one hydrogen and two ammonia molecules?

• A. Four
• B. Eight
• C. Sixteen
• D. Two

Hint:

Magic Numbers of Nitrogenase:
8 Electrons = 8 Interactions.
2 ATP per electron = 16 ATP total.
Always remember that $H_2$ is a "Bystander" that costs 2 electrons, which is why we need 8 instead of 6!

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
Biological Nitrogen Fixation is carried out by the Nitrogenase enzyme complex. The question asks for the number of association-dissociation cycles between the two protein components required to fix a single molecule of atmospheric nitrogen ($N_2$).
Key Formula or Approach:
The overall balanced chemical equation for nitrogen fixation is:
\[ N_2 + 8e^- + 8H^+ + 16 ATP \rightarrow 2NH_3 + H_2 + 16 ADP + 16 P_i \]

Step 2: Detailed Explanation:

• Nitrogenase Components: The complex consists of two parts:
- Fe-Protein (Dinitrogenase reductase): Acts as the electron donor.
- Mo-Fe Protein (Dinitrogenase): Acts as the site where $N_2$ is reduced.

• Electron Transfer Mechanism: Electrons are transferred from the Fe-protein to the Mo-Fe protein one at a time.

• The Interaction Cycle: For each electron transferred, the Fe-protein must bind to the Mo-Fe protein, hydrolyze 2 ATP molecules, transfer 1 electron, and then dissociate (detach) to be reduced again.

• Stoichiometry: According to the balanced equation, a total of 8 electrons are required to reduce $1$ $N_2$ molecule (6 electrons for $N_2 \rightarrow 2NH_3$ and 2 electrons for the obligatory production of $1$ $H_2$).

• Calculating Interactions: Since 1 interaction transfers 1 electron, and 8 electrons are needed, the components must interact exactly eight times to complete the reduction of one nitrogen molecule.

Step 3: Final Answer:
The nitrogenase components must interact eight times because eight separate electron transfer events are necessary to satisfy the chemical requirements of the reaction.