Q.19 Which one of the following statements is TRUE about leghemoglobin?
(A) It binds oxygen to protect nitrogenase
(B) It binds hemoglobin to protect oxygenase
(C) It binds oxygen to protect hydrogenase
(D) It binds oxygen to protect oxygenase

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

Leghemoglobin is a hemoglobin-like protein in root nodules of leguminous plants. It binds oxygen to maintain low levels, shielding the oxygen-sensitive nitrogenase enzyme during symbiotic nitrogen fixation with Rhizobium bacteria.​

Option Analysis

(A) It binds oxygen to protect nitrogenase
This is true. Leghemoglobin acts as an oxygen scavenger, binding free oxygen to create an anaerobic environment. Nitrogenase requires this protection to convert atmospheric N₂ into ammonia without inactivation.​

(B) It binds hemoglobin to protect oxygenase
False. Leghemoglobin does not bind hemoglobin (an animal protein) and oxygenase enzymes do not need such protection here; they function in oxygen-present environments.​

(C) It binds oxygen to protect hydrogenase
Incorrect. Hydrogenase is not the primary target; nitrogenase is the key enzyme protected in nitrogen-fixing nodules.​

(D) It binds oxygen to protect oxygenase
Wrong. Oxygenase thrives with oxygen, unlike nitrogenase, so no protection is required.​

Leghemoglobin plays a vital role in nitrogen fixation by binding oxygen to protect nitrogenase, the enzyme sensitive to oxygen in leguminous plant root nodules. This process supports symbiotic bacteria like Rhizobium in converting atmospheric nitrogen to usable forms.​

Key Role in Nitrogen Fixation

Leghemoglobin, a pink-red pigment similar to hemoglobin, maintains low oxygen levels in nodules. It binds oxygen rapidly, preventing nitrogenase denaturation while allowing controlled respiration for energy.​

• Ensures anaerobic conditions for N₂ reduction to ammonia.

• Gives nodules their characteristic pink color during active fixation.

• Boosts soil fertility in crops like soy, peas, and alfalfa.​

Exam Relevance for IIT JAM

In competitive exams like IIT JAM Biotechnology, questions test leghemoglobin’s protective function against incorrect options involving hydrogenase or oxygenase. Understanding its oxygen-binding specificity clarifies choices.​

Biological Significance

This mechanism enhances plant nutrition and reduces fertilizer needs. Leghemoglobin knockout studies confirm impaired fixation without it.​