Question

The synthesis and/or activity of the nitrogenase enzyme gets stimulated by which of the following conditions?

• A. Low glutamine/$\alpha$-ketoglutarate ratio
• B. Higher expression of Dinitrogenase reductase ADP-ribosyl transferase (DRAT) enzyme
• C. High oxygen concentration
• D. High expression of Nif L protein

Hint:

To protect nitrogenase from oxygen damage, diazotrophic organisms have evolved various adaptations, such as the rapid respiration of *Azotobacter*, specialized heterocysts in cyanobacteria, and oxygen-binding leghaemoglobin in legume root nodules.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Nitrogenase is the enzyme complex responsible for biological nitrogen fixation, catalyzing the reduction of atmospheric nitrogen ($\text{N}_2$) to ammonia ($\text{NH}_3$).
Because nitrogen fixation requires significant ATP and reducing power, the expression and activity of nitrogenase are tightly regulated by environmental nitrogen, oxygen, and energy levels.

Step 2: Detailed Explanation:
Let us evaluate each condition to determine its effect on nitrogenase:
1. Low glutamine/$\alpha$-ketoglutarate ratio (A):
Glutamine is the primary product of ammonia assimilation, and $\alpha$-ketoglutarate is the carbon skeleton acceptor.
A low glutamine/$\alpha$-ketoglutarate ratio indicates that nitrogen levels are low relative to carbon, signaling nitrogen starvation in the cell.
This low ratio triggers a regulatory cascade (involving the P${}_{II}$ signaling protein and the nif-specific transcriptional activator NifA) that activates the transcription of the *nif* operon, stimulating the synthesis and activity of nitrogenase.
Thus, this condition stimulates nitrogenase.
2. Higher expression of DRAT enzyme (B):
The DRAT enzyme catalyzes the ADP-riboxylation (inactivation) of dinitrogenase reductase under high-nitrogen or dark conditions, suppressing nitrogenase activity.
3. High oxygen concentration (C):
Oxygen is a strong inhibitor of nitrogenase, irreversibly denaturing the Fe-protein and MoFe-protein components of the complex.
4. High expression of NifL protein (D):
NifL is a negative regulator that binds and inhibits the transcriptional activator NifA in the presence of oxygen or combined nitrogen, preventing the synthesis of nitrogenase.

Step 3: Final Answer:
The synthesis and activity of nitrogenase are stimulated by a low glutamine/$\alpha$-ketoglutarate ratio, corresponding to option (A).