Energetics of biological nitrogen fixation: approximately how many ATP molecules are hydrolyzed by nitrogenase to reduce one molecule of N2 to NH3 under standard stoichiometry?

Introduction / Context:
Nitrogenase is the key enzyme complex that reduces atmospheric nitrogen (N2) to ammonia (NH3) in diazotrophs. The process is energetically expensive and oxygen-sensitive. Knowing the approximate ATP cost helps appreciate the metabolic burden of fixation and why organisms regulate the process tightly.

Given Data / Assumptions:

• Balanced reaction (minimum requirement): N2 + 8 H+ + 8 e− + 16 ATP → 2 NH3 + H2 + 16 ADP + 16 Pi.
• Real cellular costs can be higher due to inefficiencies and protection mechanisms.
• Question asks for the approximate, canonical ATP requirement.

Concept / Approach:
Nitrogenase couples electron transfer from reduced ferredoxin/flavodoxin to N2 reduction with ATP hydrolysis at the Fe protein. Each electron transfer step consumes ATP, summing to about 16 ATP per N2 under minimal stoichiometry. Additional energy may be spent to maintain microaerobic conditions (e.g., leghemoglobin) or to regenerate reductant.

Step-by-Step Solution:

Verification / Alternative check:
Microbiology and biochemistry references consistently present 16 ATP as the minimal stoichiometric cost, with practical ranges sometimes higher.

Why Other Options Are Wrong:

• A/C/D/E: 12, 18, 24, or 8 ATP are not the canonical minimal value; 18 or 24 may appear in broader energetic discussions but are not the standard stoichiometry for the nitrogenase reaction equation.

Common Pitfalls:
Confusing total cellular energy expenditure with the enzyme’s minimal requirement; always cite the balanced reaction unless otherwise specified.

Final Answer:
16 ATP