Question

What is the role of NAD(P)H as a component of Nitrate reductase in Nitrogen fixation?

• A. As a prosthetic group
• B. As a cofactor
• C. As an electron donor
• D. As an electron accepter

Hint:

Remember the mnemonic OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). In the reaction, NAD(P)H becomes NAD(P)⁺, losing electrons (Oxidation), so it must be the electron donor.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The question asks for the specific function of NAD(P)H in the enzymatic reaction catalyzed by Nitrate reductase. This enzyme is crucial for nitrogen assimilation (not fixation) in plants, where nitrate is converted to a usable form.
Step 2: Detailed Explanation:
The enzyme Nitrate reductase catalyzes the reduction of nitrate (NO₃⁻) to nitrite (NO₂⁻). This is the first step in converting nitrate absorbed from the soil into ammonia for use in amino acid synthesis. The chemical reaction is a redox reaction: \[ \text{NO}_3^- + \text{NAD(P)H} + \text{H}^+ \xrightarrow{\text{Nitrate reductase}} \text{NO}_2^- + \text{NAD(P)}^+ + \text{H}_2\text{O} \] In this reaction:

Nitrate (NO₃⁻) gains electrons, so it is reduced. It acts as the electron accepter.
NAD(P)H loses electrons (and a proton) to become NAD(P)⁺, so it is oxidized. A substance that gets oxidized in a reaction provides electrons to another substance.
Therefore, the role of NAD(P)H is to provide the reducing power, i.e., to act as the electron donor.
Step 3: Analysis of Options:


(A) As a prosthetic group: A prosthetic group is a non-protein component that is tightly and permanently bound to an enzyme. NAD(P)H is a coenzyme (or cosubstrate) that binds transiently to the enzyme. Thus, this is incorrect.

(B) As a cofactor: While NAD(P)H is a type of cofactor (specifically, a coenzyme), this term is general. "Electron donor" describes its specific chemical role in this particular reaction, which is more precise.

(C) As an electron donor: This accurately describes its function of providing electrons to reduce nitrate. This is the correct and most specific answer.

(D) As an electron accepter: This is incorrect. Nitrate is the electron accepter.

Step 4: Final Answer:
The specific role of NAD(P)H in the nitrate reductase reaction is to donate electrons for the reduction of nitrate.