Life on Earth depends on nitrogen, an element found in all proteins, DNA, and chlorophyll. Despite its abundance, making up about 78% of the atmosphere, atmospheric nitrogen gas (N2) is unreactive and cannot be directly used by most living organisms. This presents a significant challenge for biological systems. The solution lies with nitrogenase, a specialized enzyme that converts inert atmospheric nitrogen into a usable form.
The Enzyme’s Core Function
Nitrogenase is an enzyme complex found in certain prokaryotes, primarily bacteria and archaea, that catalyzes the conversion of atmospheric nitrogen gas (N2) into ammonia (NH3). This process is known as biological nitrogen fixation and is the initial stage of the nitrogen cycle. Ammonia is the first stable product of this conversion, and it is a form of nitrogen that plants and other organisms can readily absorb and use.
The conversion of N2 into ammonia is a complex biochemical reaction that requires a substantial amount of energy, supplied as adenosine triphosphate (ATP). Nitrogenase acts as a catalyst, reducing the activation energy required for this transformation. Without this enzyme, the strong triple bond in N2 would remain largely unbroken, severely limiting nitrogen availability for biological processes. The fixed nitrogen is then used to synthesize essential biomolecules, including amino acids, nucleotides for DNA and RNA, and proteins.
Biological Homes and Partnerships
Nitrogenase is produced by certain microorganisms, bacteria and archaea, collectively known as diazotrophs. These microorganisms are categorized into two groups: those that form symbiotic relationships with plants and those that are free-living in soil or aquatic environments. A well-known symbiotic example involves Rhizobium bacteria, which inhabit root nodules of leguminous plants such as peas, beans, and clovers.
Within these root nodules, Rhizobium bacteria convert atmospheric nitrogen into ammonia, which the host plant then utilizes. In return, the plant provides the bacteria with carbohydrates produced through photosynthesis, illustrating a mutually beneficial partnership. Free-living nitrogen-fixing bacteria, such as Azotobacter, Beijerinckia, and cyanobacteria like Anabaena and Nostoc, also contribute to nitrogen fixation in various ecosystems. These free-living bacteria obtain the necessary energy for nitrogen fixation from organic matter in their surroundings, such as decomposing crop residues.
How Nitrogenase Works
The nitrogenase enzyme complex consists of two main protein components: the iron (Fe) protein and the molybdenum-iron (MoFe) protein. The Fe protein acts as a reductase, transferring electrons to the MoFe protein, which is the site where nitrogen reduction occurs. This process requires significant energy, with approximately 16 ATP molecules hydrolyzed for each molecule of N2 converted to ammonia. The MoFe protein contains a specific metal cluster, the iron-molybdenum cofactor (FeMo-cofactor), where nitrogen binding and reduction occur.
A challenge for nitrogenase is its sensitivity to molecular oxygen, which can damage the enzyme and inhibit its activity. Nitrogen-fixing organisms have evolved various strategies to protect nitrogenase from oxygen exposure. For instance, in symbiotic relationships like those between Rhizobium and legumes, specialized proteins such as leghemoglobin are produced within root nodules to bind oxygen and maintain a low-oxygen environment suitable for nitrogenase. Other free-living bacteria may employ high respiration rates to consume oxygen or reside in anaerobic (oxygen-free) conditions within the soil.
Impact on Life and Agriculture
Nitrogenase plays an important role in global ecosystems by facilitating biological nitrogen fixation, a process that underpins the nitrogen cycle and sustains life. This natural conversion of atmospheric nitrogen into usable forms ensures that plants, and subsequently animals, have access to this element for synthesizing proteins, nucleic acids, and other organic compounds. Without nitrogenase, the vast reservoir of nitrogen in the atmosphere would remain largely inaccessible, severely limiting biological productivity.
In agriculture, nitrogenase activity is important, as it provides a natural source of nitrogen fertilizer for crops. Leguminous plants, through their symbiotic relationships with nitrogen-fixing bacteria, enrich the soil with fixed nitrogen, reducing the need for synthetic nitrogen fertilizers. This natural fertilization offers environmental benefits, such as minimizing runoff of excess synthetic fertilizers into waterways, which can lead to pollution and eutrophication. Ongoing research aims to enhance the efficiency of natural nitrogen fixation or even engineer non-nitrogen-fixing crops to utilize nitrogenase, contributing to more sustainable food production systems globally.