- Nitrogenase is an enzyme that is used to "fix" nitrogen gas. It pulls nitrogen from the atmosphere and converts it to a form that can be used by other forms of life. It is the only enzyme that can perform this chemical reaction.
- One of the unusual features of nitrogenase is that it is inhibited by a form of oxygen called dioxygen. This gas, which is made up of two oxygen atoms bonded together, is present in the atmosphere and comprises the oxygen that we breathe. Dioxygen is able to inhibit nitrogenase because it reacts with a critical iron molecule. When dioxygen encounters this iron atom, it oxidizes it, which deactivates the nitrogenase enzyme.
- In order to avoid this inhibitory regulation of dioxygen, many nitrogen-fixing organisms live anaerobically. This means that they live in environments in which there is no dioxygen. While anaerobic organisms are unable to utilize sugars as efficiently as organisms that can use oxygen gas ("aerobic" organisms), they are able to utilize enzymes like nitrogenase, which are not activated by dioxygen.
- For organisms that require limited exposure to oxygen, their nitrogenase enzymes have two ways of protecting themselves. One technique is called conformational protection. With this kind of protection, the enzyme actually changes its shape so that the dioxygen cannot access the iron atom and oxidize it. When in this protected state, the nitrogenase enzyme is unable to process nitrogen gas. The other method is called respiratory protection. It involves the use of other molecules, called scavengers, to bind to and disable the dioxygen before it can reach the iron atom.
- Despite the effects of dioxygen on nitrogenase, the enzyme is not affected by other forms of oxygen. Compounds like water and carbon dioxide, which both contain oxygen atoms, do not deactivate nitrogenase. Additional forms of pure oxygen, such as ozone, also do not interact with nitrogenase.
