GATE 2023 BT Q.19: solution and answer

Official answer: ASource: Official answer key, GATE 2023 BT (IIT Kanpur)

1. Leghemoglobin is a heme-containing, oxygen-binding protein made in the root nodules of legumes, where rhizobia (as bacteroids) fix nitrogen.

2. Nitrogenase, which reduces ${\text{N}}_2$ to ${\text{NH}}_3$, is irreversibly inactivated by free ${\text{O}}_2$, so the nodule interior must keep free ${\text{O}}_2$ very low.

3. The bacteroids still need ${\text{O}}_2$ for aerobic respiration, which supplies the large ATP demand of nitrogen fixation. Leghemoglobin binds ${\text{O}}_2$ with high affinity, so it holds free ${\text{O}}_2$ low and still delivers ${\text{O}}_2$ to the bacteroid respiratory chain.

4. Option (A) is correct: leghemoglobin binds oxygen and so protects nitrogenase.

5. Option (B) is wrong: leghemoglobin binds oxygen, not hemoglobin, and no oxygenase is the protected enzyme.

6. Option (C) is wrong: the enzyme protected is nitrogenase. Hydrogenase in the nodule only recycles the ${\text{H}}_2$ released by nitrogenase and is not what leghemoglobin protects.

7. Option (D) is wrong: oxygenases use ${\text{O}}_2$ as a substrate and are not damaged by it, so they need no protection.