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GATE 2023 BT – Question 19

General Biology · Microbiology: Microbial growth and nutrition, operons (lac, trp, ara), nitrogen fixation · 1 mark · Multiple choice

Which one of the following statements is TRUE about leghemoglobin?

  1. It binds oxygen to protect nitrogenase
  2. It binds hemoglobin to protect oxygenase
  3. It binds oxygen to protect hydrogenase
  4. It binds oxygen to protect oxygenase

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Correct answer: (A) It binds oxygen to protect nitrogenase

Explanation

**Leghemoglobin** is an oxygen-binding haem protein found in the root nodules of legumes (the plant makes the globin and the bacterium makes the haem).

**Problem:** the enzyme **nitrogenase**, which fixes $N_2$ in *Rhizobium*, is irreversibly damaged by oxygen. But the bacteroids also need oxygen for respiration to make ATP.

**Solution:** leghemoglobin binds free $O_2$ with very high affinity, keeping its free concentration very low (protecting nitrogenase) while still delivering enough $O_2$ to the bacteroids' respiratory chain.

Answer **A**: it binds oxygen to protect nitrogenase.