Q.51 In the symbiotic nitrogen fixation process, Leghemoglobin present in the nodule helps in fixing
nitrogen in presence of the enzyme
(A) Nitrate synthetase (B) Nitrate synthase
(C) Glutathione synthetase (D) Nitrogenase

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Leghemoglobin in Symbiotic Nitrogen Fixation: Key Role with Nitrogenase Enzyme

In symbiotic nitrogen fixation, plants like legumes form partnerships with Rhizobium bacteria in root nodules to convert atmospheric nitrogen (N2N2) into usable forms. A key player here is leghemoglobin, a protein that gives nodules their pinkish color. But which enzyme does it assist? This article breaks down a common exam question (Q.51) on the topic, reveals the correct answer, and explains all options for clarity.

The Correct Answer: (D) Nitrogenase

Leghemoglobin helps fix nitrogen in the presence of nitrogenase.

Leghemoglobin acts like hemoglobin in blood—it binds oxygen reversibly. In nodules, it maintains low oxygen levels (around 10−610−6 atm) to protect the oxygen-sensitive nitrogenase enzyme. Nitrogenase catalyzes the reduction of N2N2 to ammonia (NH3NH3):

N2+8H++8e−+16ATP→2NH3+H2+16ADP+16PiN2+8H++8e−+16ATP→2NH3+H2+16ADP+16Pi

Without leghemoglobin, oxygen would inactivate nitrogenase, halting fixation. This makes (D) the clear choice for exams in microbiology, plant physiology, or biochemistry.

Why Not the Other Options? Full Breakdown

Let’s examine each incorrect option to build deeper understanding—perfect for competitive exams like NEET, CSIR NET, or GATE.

• (A) Nitrate synthetase: This enzyme (also called nitrate reductase) reduces nitrate (NO3−NO3−) to nitrite (NO2−NO2−) in plants and microbes. It’s part of nitrogen assimilation, not fixation from N2N2. Leghemoglobin isn’t involved here, as this step occurs in cytoplasm, not oxygen-controlled nodules.

• (B) Nitrate synthase: This isn’t a standard enzyme name in nitrogen metabolism. “Synthase” implies synthesis without net ATP hydrolysis, but nitrate reduction uses reductase enzymes. It’s a distractor option—real pathways use nitrate reductase, unrelated to leghemoglobin or N2N2 fixation.

• (C) Glutathione synthetase: This enzyme synthesizes glutathione (GSH), a key antioxidant from gamma-glutamylcysteine and glycine. Glutathione protects cells from oxidative stress but plays no direct role in nitrogen fixation. Leghemoglobin handles oxygen regulation, not redox buffering via glutathione.

Quick Visual: Nitrogen Fixation Process

Here’s a simple flowchart of the symbiotic process:

1. Rhizobia enter roots → Form nodules.

2. Leghemoglobin binds O₂ → Protects nitrogenase.

3. Nitrogenase fixes N2N2 → Ammonia produced.

4. Plant uses ammonia for amino acids/proteins.

Example: Soybean nodules rely on this—leghemoglobin levels correlate with fixation efficiency.

This mechanism boosts soil fertility naturally, reducing fertilizer needs.

Optimizing your study? Focus on enzyme-oxygen interactions and pathway distinctions for similar MCQs.