Q.68 Which one of the following proteins does NOT bind to a gaseous ligand?
(A) Leghemoglobin (B) Carbonic anhydrase
(C) Nitrogenase (D) NADPH oxidase

Nitrogenase is the protein among the options that does not bind to a gaseous ligand. Carbonic anhydrase, however, interacts with CO₂ during its catalytic cycle, while leghemoglobin binds O₂.

What is a Gaseous Ligand?

Gaseous ligands are small gas molecules like O₂, CO, NO, or CO₂ that bind proteins to regulate function, often at heme or metal centers. In biology, such binding supports processes like respiration, nitrogen fixation, or catalysis.
This MCQ tests knowledge of enzyme-substrate interactions in microbiology and biochemistry.

Correct Answer: (C) Nitrogenase

Nitrogenase catalyzes N₂ → NH₃ conversion using Fe-Mo cofactors and ATP hydrolysis but does not bind gaseous ligands directly; N₂ acts as a substrate reduced at the active site without stable binding like heme-O₂. No gas coordinates reversibly to regulate it.[ from prior]
Its mechanism involves electron transfer to N₂ without gaseous ligand affinity, distinguishing it from oxygen carriers or sensors.

Why Not the Other Options?

Option (A) Leghemoglobin

Leghemoglobin in legume root nodules binds O₂ tightly at its heme iron, maintaining low free O₂ to protect O₂-sensitive nitrogenase while facilitating respiration.[ from prior]
Similar to myoglobin but pink, it exemplifies gaseous ligand binding.

Option (B) Carbonic Anhydrase

Carbonic anhydrase uses Zn²⁺ to catalyze CO₂ + H₂O ⇌ HCO₃⁻ + H⁺; CO₂ enters the hydrophobic pocket near Zn-OH, undergoing nucleophilic attack without direct coordination but clear gaseous substrate binding.
The Zn polarizes water for CO₂ hydration, essential for CO₂ transport and pH balance.

Option (D) NADPH Oxidase

NADPH oxidase generates superoxide (O₂⁻) from O₂ in phagocytes during respiratory burst; O₂ binds at the heme or flavin sites in its subunits for reduction to reactive oxygen species.[ from prior]
This O₂-dependent activity underscores its gaseous ligand interaction.

Exam Tips

• Remember: Gaseous ligands imply reversible binding (e.g., O₂, CO₂); substrates like N₂ in nitrogenase count as catalytic turnover, not binding.​

• Common in NEET/CSIR biotech questions on metalloproteins.