Question

Describe why the nitrogenase enzyme complex has to cycle electrons through 3 separate cycles during nitrogen fixation and explain what role ATP hydrolysis plays each time the electrons cycle through the complex.

Answer 1

Biological nitrogen fixation can be represented by the following equation, in which two moles of ammonia are produced from one mole of nitrogen gas, at the expense of 16 moles of ATP and a supply of electrons and hydrogen ions.

N2 + 8H+ + 8e- + 16 ATP = 2NH3 + H2 + 16ADP + 16 Pi

This reaction is performed by nitrogen fixing bacteria which lives in the root nodules of leguminous plants using an enzyme complex called nitrogenase.

This enzyme consists of two proteins - an iron protein and a molybdenum-iron protein.

The reactions occur while N₂ is bound to the nitrogenase enzyme complex. The Fe protein is first reduced by electrons donated by Ferredoxin. Then the reduced Fe protein binds ATP and reduces the molybdenum-iron protein, which donates electrons to N₂, producing HN=NH. In two further cycles of this process (each requiring electrons donated by Ferredoxin) HN=NH is reduced to H₂N-NH₂, and this in turn is reduced to 2 NH₃.

Ferredoxin which supplies electrons for this process is generated by photosynthesis, respiration or fermentation by the nitrogen fixing bacteria in the root nodules.