Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): The nitrogenase enzyme is considered to be a sluggish and inefficient enzyme.
Reason (R): The nitrogenase enzyme must go through several catalytic reduction cycles, wherein the two components viz. Fe-Protein and MoFe-Protein get dissociated from each other following each electron transfer, before final product i.e. ammonia appears.
In light of the above statements, choose the correct answer from the options given below:

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Remember that nitrogenase requires a minimum of 8 electron transfers, and each single-electron transfer requires a complete cycle of association, ATP hydrolysis, and dissociation of the Fe and MoFe proteins.
This makes the process slow but precise.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Biological nitrogen fixation is the process by which specialized diazotrophic microorganisms reduce atmospheric dinitrogen (\( \text{N}_2 \)) into two molecules of ammonia (\( \text{NH}_3 \)).
This conversion is catalyzed by the nitrogenase enzyme complex, which consists of two distinct, highly conserved proteins working in tandem: the Fe-protein (dinitrogenase reductase) and the MoFe-protein (dinitrogenase).

Step 2: Detailed Explanation:
1. Analysis of Assertion (A): Nitrogenase is kinetics-limited and exhibits an exceptionally low turnover number (\( k_{\text{cat}} \)), typically ranging from only \( 5 \text{ to } 10 \text{ s}^{-1} \).
This means a single enzyme molecule can only process a few molecules of nitrogen per second, making it incredibly sluggish compared to typical metabolic enzymes.
To compensate, diazotrophs must synthesize large quantities of nitrogenase, sometimes accounting for up to \( 10\% \text{ to } 20\% \) of their total cellular protein. The assertion is true.
2. Analysis of Reason (R): The biochemical explanation for this sluggishness lies in the mechanism of electron transfer.
To reduce \( \text{N}_2 \), the nitrogenase complex requires a total of eight electrons and sixteen ATP molecules:
\[ \text{N}_2 + 8\text{H}^+ + 8e^- + 16\text{ATP} \rightarrow 2\text{NH}_3 + \text{H}_2 + 16\text{ADP} + 16\text{P}_i \] Each electron transfer event is a multi-step cycle:
- The Fe-protein binds two ATP molecules and is reduced by an external donor (ferredoxin or flavodoxin).
- The reduced Fe-protein transiently associates with the MoFe-protein.
- ATP hydrolysis occurs, triggering a conformational change that drives the transfer of a single electron from the Fe-protein to the MoFe-protein.
- Crucially, the two proteins must completely dissociate from each other to allow the Fe-protein to be re-reduced and re-loaded with ATP.
Because this association, electron transfer, and dissociation cycle must occur sequentially eight times to deliver the eight electrons required for one round of nitrogen reduction, the overall reaction rate is heavily restricted.
Therefore, the slow, repetitive nature of these catalytic cycles directly explains why the enzyme is kinetically sluggish.

Step 3: Final Answer:
Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct biochemical explanation of Assertion (A).