Question

Which of the following regions of Nitrate reductase (NR) enzyme is extremely important for the 14-3-3 protein-mediated posttranslational regulation of its activity?

• A. Hinge1 (H1) region between Molybdenum cofactor (MoCo) and the Heme domain
• B. Hinge2 (H2) region between Heme and FAD domain
• C. Acidic residues-rich N-terminal region preceding MoCo domain
• D. The C-terminal part of FAD domain

Hint:

Light-induced activation of photosynthesis increases cytosolic glucose and sucrose levels, which promote the dephosphorylation and activation of Nitrate Reductase, coordinating nitrogen assimilation with carbon availability.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Nitrate Reductase (NR) is a key homodimeric enzyme in plants that catalyzes the rate-limiting reduction of nitrate ($\text{NO}_3^-$) to nitrite ($\text{NO}_2^-$) in the cytosol.
To prevent toxic nitrite accumulation, NR activity is tightly regulated by post-translational phosphorylation and protein-protein interactions.

Step 2: Detailed Explanation:
Each monomer of the nitrate reductase enzyme contains three catalytic domains: the N-terminal Molybdenum cofactor (MoCo) domain, the central Heme domain, and the C-terminal FAD domain.
These domains are connected by two flexible linker regions:
- Hinge 1 (H1) connects the MoCo and Heme domains.
- Hinge 2 (H2) connects the Heme and FAD domains.
Under dark conditions or low CO${}_2$ levels, a specific calcium-dependent protein kinase phosphorylates a conserved serine residue (Ser-543 in spinach) located within the Hinge 1 (H1) region.
Once phosphorylated, this site is recognized and bound by a dimeric regulatory protein called 14-3-3.
The binding of the 14-3-3 protein to the phosphorylated Hinge 1 region physically blocks electron transfer from the Heme domain to the MoCo domain, inactivating the enzyme.
When the plant is exposed to light, a specific protein phosphatase dephosphorylates the serine residue, releasing the 14-3-3 protein and restoring NR activity.
Therefore, the Hinge 1 region is the critical site for this post-translational regulation.

Step 3: Final Answer:
The region extremely important for 14-3-3 protein-mediated regulation of NR is the Hinge1 (H1) region between the MoCo and Heme domains, corresponding to option (A).