Leghemoglobin:

The nitrogenase complex is extremely sensitive to inactivation through O₂, so the enzyme must be protected from this reactive substance. During the root nodules of leguminous plants, protection is afforded through the symbiotic synthesis of leghemoglobin. The globin part of this monomeric oxygen-binding protein is synthesized via the plant, while the heme group is synthesized through the Rhizobium. Leghemoglobin has a very high affinity for O₂ for maintaining a low enough concentration to protect the nitrogenase.