Illustration of the active site of a molybdenum nitrogenase enzyme. Nitrogenase enzymes convert nitrogen gas to ammonia, a catalytic process called nitrogen fixation. This process is vital for life on earth. The nitrogenase enzyme has several active sites. The active site illustrated here shows nitrogen (blue) interacting with an iron sulphur molybdenum molecule. Catalysis involves the flow of electrons to the nitrogen molecule, producing ammonia.