Nitrogenase is an enzyme system made up of several proteins. It consumes ATP to make ammonia from dinitrogen. The reduction of N2 is relatively "energy-intense", and two molecules of ATP must be hydrolyzed for each electron that is transferred.
The overall reaction can be described as: N2 + 8 H+ + 16 Mg-ATP + 8 e- → 2 NH3 + H2 + 16 Mg-ADP + 16 Phosphate
One main components of nitrogenase is dinitrogenase-reductase. This iron-based enzyme is responsible for binding Mg-ATP and thus lowering its redox potential by ca. 100 mV. This is necessary to allow electron transfer to a second component, an iron-molybdenum protein, which can be considered the actual active site. Nitrogenase is a good example of cooperation of active sites with metal components which are mainly responsible for electron transfer processes.