# DBT JRF 2022 — Question 80

Exam: DBT JRF 2022 · Section: B · Topic: Plant Biology › Nitrogen Fixation - Leghaemoglobin · Difficulty: Medium

## Question

Which one among the following is the correct function of leghaemoglobin?

## Options

- **A.** It sequesters oxygen to facilitate nitrogen fixation
- **B.** It protects nitrogenase from $CO_2$
- **C.** It facilitates the supply of $CO_2$ to nitrogen fixing bacteria
- **D.** It binds to nitrogen and moves it out of the nodule

## Answer

Option A

## Explanation

Leghemoglobin in root nodules: Root nodules formed by Rhizobium in legume roots are the site of nitrogen fixation. A critical problem: nitrogenase (the N-fixing enzyme) is extremely sensitive to oxygen ( $O_2$ inactivates it), yet aerobic respiration is needed to power nitrogen fixation. Solution — Leghemoglobin: • A plant hemoglobin (the globin part is plant-encoded; the heme is Rhizobium-encoded). • Has exceptionally HIGH oxygen affinity (Kd ~10 nM, much higher than myoglobin). • Functions as an oxygen BUFFER/SCAVENGER: maintains free $[O_2]$ at a very low level (~10–50 nM) in the nodule interior — microaerobic conditions. • This protects nitrogenase from $O_2$ damage while still allowing enough $O_2$ for ATP generation by Rhizobium. The pink/red colour of healthy root nodules comes from leghemoglobin (an indicator of active nitrogen fixation). Answer: Leghemoglobin has high O₂ affinity and sequesters O₂ to maintain microaerobic conditions protecting nitrogenase.

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