Nitrogenase ATP Requirement

Nitrogenase is an enzyme complex responsible for nitrogen fixation in certain microorganisms. Nitrogen fixation is the process by which atmospheric nitrogen ($\text{N}_2$) is converted into ammonia ($\text{NH}_3$), which is a form usable by plants.

This biological nitrogen fixation process requires a significant amount of energy, which is supplied by the hydrolysis of ATP (adenosine triphosphate), typically in the form of Mg-ATP.

The overall balanced chemical equation catalyzed by the nitrogenase complex for the reduction of one mole of atmospheric nitrogen is:

\( \text{N}_2 + 8\text{e}^- + 8\text{H}^+ + 16\text{Mg-ATP} \rightarrow 2\text{NH}_3 + \text{H}_2 + 16\text{Mg-ADP} + 16\text{Pi} \)

Let's break down the components of this reaction:

• \( \text{N}_2 \): One mole of atmospheric nitrogen is reduced.
• \( 8\text{e}^- \): Eight electrons are transferred. These electrons ultimately come from a reductant like ferredoxin or flavodoxin.
• \( 8\text{H}^+ \): Eight protons are consumed.
• \( 16\text{Mg-ATP} \): Sixteen molecules of Mg-ATP are hydrolyzed to provide the energy required for the reaction. Note that 2 ATP molecules are hydrolyzed for each electron transferred (8 electrons × 2 ATP/electron = 16 ATP).
• \( 2\text{NH}_3 \): Two molecules of ammonia are produced.
• \( \text{H}_2 \): One molecule of hydrogen gas is also produced as a byproduct; some energy is consumed in reducing protons to hydrogen.
• \( 16\text{Mg-ADP} \) and \( 16\text{Pi} \): The products of ATP hydrolysis are Mg-ADP and inorganic phosphate.

Based on the stoichiometry of the reaction equation, for the reduction of one mole of nitrogen ($\text{N}_2$), 16 moles of Mg-ATP are required.

Therefore, the number of moles of Mg-ATP needed for the reduction of one mole of nitrogen by the nitrogenase enzyme is 16.