Nitrogenase is a biological enzyme found in specific microorganisms. It converts inert atmospheric nitrogen gas (N₂) into ammonia (NH₃). This transformation is fundamental, as nitrogen in its gaseous form is unusable by most living organisms. Nitrogenase makes this scarce nutrient available to support life across various ecosystems.
The Nitrogen Fixation Process
Nitrogen fixation, the process catalyzed by nitrogenase, is highly energy-intensive. It requires a substantial input of energy in the form of ATP. Specifically, the enzyme consumes approximately 16 to 20 molecules of ATP for every molecule of nitrogen gas converted. Electrons are also supplied, often by electron carriers like ferredoxin, alongside hydrogen ions.
The overall reaction involves one molecule of nitrogen gas combining with eight electrons and eight hydrogen ions to produce two molecules of ammonia and one molecule of hydrogen gas. This complex conversion breaks the strong triple bond in N₂, a bond difficult to disrupt. Nitrogenase acts as a catalyst, significantly reducing the activation energy required for this challenging chemical transformation.
Structure and Cofactors
The nitrogenase enzyme complex consists of two distinct protein components: the iron protein (dinitrogenase reductase) and the molybdenum-iron protein (dinitrogenase). Both are necessary for the enzyme’s activity.
The Fe protein binds and hydrolyzes ATP, providing energy for the reaction. It also transfers electrons to the MoFe protein. This component contains [4Fe-4S] clusters that facilitate electron transport.
The MoFe protein is the catalytic core where nitrogen gas reduction occurs. It is a larger protein, typically a heterotetramer composed of two alpha and two beta subunits. Within the MoFe protein, there are two types of metal clusters: the P-cluster and the iron-molybdenum cofactor (FeMoco). The P-cluster, an [8Fe-7S] cluster, mediates electron transfer between the Fe protein and FeMoco.
FeMoco, also known as the M-cluster, is the active site where nitrogen gas binds and is reduced to ammonia. This cofactor has a complex structure, typically described as a [7Fe-9S-Mo-C-homocitrate] cluster. It consists of two fused subclusters, one containing iron and sulfur, and the other containing molybdenum, iron, and sulfur, connected by bridging sulfide ligands and a central carbon atom. This intricate arrangement enables nitrogenase to cleave the stable triple bond of N₂.
Biological Sources and Regulation
Nitrogenase is produced exclusively by a diverse group of microorganisms called diazotrophs, including certain bacteria and archaea. Examples include free-living soil bacteria such as Azotobacter and Azospirillum.
Symbiotic diazotrophs, such as Rhizobium, form root nodules on leguminous plants like soybeans and peas. Cyanobacteria, including Anabaena and Nostoc, also produce nitrogenase. The enzyme’s presence is restricted to these microbial groups due to its sensitivity to molecular oxygen.
Oxygen can irreversibly damage and deactivate nitrogenase’s metal clusters, particularly the Fe protein. To overcome this, diazotrophs have evolved various protective mechanisms.
Aerobic organisms like Azotobacter employ “respiratory protection,” maintaining high rates of respiration to consume oxygen rapidly and create a low-oxygen environment. Some filamentous cyanobacteria develop thick-walled cells called heterocysts, which provide an anaerobic compartment where nitrogen fixation can occur away from oxygen produced during photosynthesis. In legume root nodules, the plant produces an oxygen-scavenging protein called leghemoglobin, which binds oxygen and maintains low concentrations to protect the bacterial nitrogenase. Certain diazotrophs also utilize proteins like FeSII, which can temporarily bind to nitrogenase under high oxygen conditions, protecting it from damage while halting its activity.
Agricultural and Environmental Significance
Nitrogenase activity holds significant importance for both agriculture and the global environment. Biological nitrogen fixation provides a natural and sustainable alternative to the industrial Haber-Bosch process, which synthesizes ammonia for fertilizers. The Haber-Bosch process is highly energy-intensive, relying on fossil fuels and contributing to carbon dioxide emissions, estimated at around 275 million tons annually. Much synthetic nitrogen fertilizer applied in agriculture is lost to the environment, leading to pollution.
By contrast, biological nitrogen fixation is powered by renewable energy, primarily through the plant’s photosynthesis in symbiotic relationships. Promoting the use of nitrogen-fixing organisms, particularly legumes in crop rotation, can naturally enrich soil fertility and reduce the need for synthetic nitrogen fertilizers. This practice not only lowers agricultural costs but also mitigates environmental impacts like greenhouse gas emissions and water pollution.
Nitrogenase’s role in the global nitrogen cycle is fundamental, second only to photosynthesis for sustaining the biosphere. It is the primary biological mechanism that converts atmospheric nitrogen into a usable form, making it available for the synthesis of biomolecules like proteins and nucleic acids in plants, and subsequently in animals. This continuous natural supply of reactive nitrogen supports nearly all terrestrial and aquatic ecosystems, underpinning global food webs.