Q.34 The total number of ATP is required for the conversion of one
N2 to 2 NH4+ during biological
N2 fixation?
- 8 ATP
- 10 ATP
- 12 ATP
- 16 ATP
ATP Requirement in Biological N2 Fixation: 16 ATP for N2 to 2 NH4+
Biological nitrogen fixation converts atmospheric N2 into usable ammonium (2 NH4+) using the nitrogenase enzyme complex, demanding significant energy input. The correct answer is 16 ATP, as this matches the standard biochemical requirement for reducing one N2 molecule.
Why 16 ATP?
Nitrogenase catalyzes N2 + 8H+ + 8e- → 2 NH3 + H2, but the actual product is 2 NH4+ after protonation, with H2 as a byproduct. This process hydrolyzes 16 ATP molecules to provide energy for breaking the stable N≡N triple bond and transferring electrons from ferredoxin or flavodoxin. The reaction involves two key proteins: dinitrogenase (with FeMo cofactor) and dinitrogenase reductase, where ATP binding enables electron transfer in a stepwise manner.
Uptake hydrogenase in some bacteria recycles H2, potentially saving 1-2 ATP (about 9-10% efficiency), but the baseline remains 16 ATP per N2 fixed.
Option Analysis
Option Explanation Correct? 8 ATP Matches ATP for one NH3 if ignoring stoichiometry; some sources cite ~8 ATP/N atom, but full reaction requires double for 2 NH3/N2. No 10 ATP Incorrect; no standard reference supports this—possibly confuses with H2 recycling savings, but baseline exceeds it. No 12 ATP Not aligned with nitrogenase mechanism; underestimates electron transfers (8 e-) and activations needed. No 16 ATP Precise match: 16 ATP hydrolyzed per N2 → 2 NH3 (or 2 NH4+), confirmed across microbiology texts. Yes
