Biological nitrogen fixation — How many ATP molecules are hydrolyzed by nitrogenase to reduce one N2 to two molecules of ammonia (2 NH3)?

Introduction / Context:
Nitrogen fixation converts atmospheric nitrogen (N2) into ammonia (NH3), making nitrogen bioavailable. The enzyme complex nitrogenase catalyzes this energetically demanding reduction, consuming large amounts of ATP. Understanding the ATP cost is key to appreciating the bioenergetic burden of fixation.

Given Data / Assumptions:

• The overall stoichiometry for Mo–Fe nitrogenase is commonly summarized as requiring at least 16 ATP to reduce 1 N2 to 2 NH3, with H2 as a byproduct.
• Reaction occurs in diazotrophs under reducing conditions, often protected from oxygen.
• ATP hydrolysis occurs at the Fe protein during electron transfer cycles.

Concept / Approach:
Nitrogenase operates via repeated cycles of electron transfer from a reduced electron donor (such as ferredoxin) through the Fe protein to the MoFe protein, where N2 is bound and reduced. Each electron transfer step is coupled to ATP hydrolysis, and multiple electrons are required to fully reduce N2 to NH3. The canonical minimal requirement is 16 ATP per N2 reduced to 2 NH3.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical and genetic studies of Mo–Fe nitrogenase consistently report the 16 ATP minimal requirement, though actual cellular costs may be higher due to ancillary processes.

Why Other Options Are Wrong:

• 10, 5, or 15 ATP underestimate the energy cost of the nitrogenase reaction.

Common Pitfalls:
Confusing the urea cycle’s ATP use with nitrogen fixation; these are distinct processes. The urea cycle uses 3 ATP per urea, not per ammonia formation from N2.

Final Answer:
16