The fixation of atmospheric nitrogen is an energy-intensive process, as it involves breaking the very stable triple bond between the nitrogen atoms. The nitrogenases catalyze the process. One such enzyme occurs in Rhizobium bacteria. There are three components to its action: a molybdenum atom at the active site, iron–sulfur clusters that are involved in transporting the electrons needed to reduce the nitrogen, and an abundant energy source in the form of magnesium ATP. This last is provided by a mutualistic symbiosis between the bacteria and a host plant, often a legume. The reaction may be written symbolically as
:N + 16 MgATP + 8 e → 2 NH + 16 MgADP +16 P + H
where P stands for inorganic phosphate. The precise structure of the active site has been difficult to determine. It appears to contain a MoFeS cluster that is able to bind the dinitrogen molecule and, presumably, enable the reduction process to begin. The electrons are transported by the associated "P" cluster, which contains two cubical FeS clusters joined by sulfur bridges.