Nitrogenase is the key enzyme for biological nitrogen fixation. Nitrogenase converts nitrogen to ammonia. It is present in some prokaryotes. The reduction of dinitrogen to ammonia by living organisms is called biological nitrogen fixation.
Nitrogenase catalyses the conversion of nitrogen to ammonia. Ammonia is the first stable product in nitrogen fixation. Nitrogenase acts as a catalyst and reduces the activation energy.
The conversion of Nitrogen to ammonia utilises energy in the form of ATPs. The reaction catalysed by nitrogenase enzyme is as follows:
N2 + 8e_ + 8H+ + 16ATP → 2NH3 + H2 + 16ADP + 16Pi
Some of the important characteristics of the nitrogenase enzyme are:
- Nitrogenase is exclusively present in prokaryotes.
- It carries out the nitrogen fixation in the living organism.
- Nitrogenase converts atmospheric nitrogen to ammonia.
- It is a Mo-Fe protein. It is a heterotetramer and consists of two alpha and two beta subunits.
- The oxidation state of Mo in the nitrogenase enzyme is Mo(III).
- It also contains Fe-S clusters between the alpha and beta subunits.
- It is highly sensitive to molecular oxygen. The nitrogenase enzyme requires anaerobic conditions for its activity.
- The leg-haemoglobin present in the nodules acts as the oxygen scavenger and helps in maintaining the anaerobic conditions required for the activity of the nitrogenase enzyme.
- The energy in the form of ATP required for the conversion of nitrogen to ammonia comes from the respiration of host cells.
Nitrogenase Enzyme Types
Nitrogenase catalyses nitrogen fixation. It is a key enzyme for sustaining life on the earth. There are three types of nitrogenase found in various types of nitrogen-fixing bacteria. They are:
- Molybdenum (Mo) nitrogenase
- Vanadium (V) nitrogenase
- Iron-only (Fe) nitrogenase
Out of these three, molybdenum (Mo) nitrogenase is the most studied and characterised. It is present in Rhizobium, Azotobacter, etc.
Vanadium (V) nitrogenase and iron-only (Fe) nitrogenase are present as alternative nitrogenase in some species of Azotobacter.
General Structure and Mechanism
Nitrogenase is a heterotetramer and consists of two alpha and two beta subunits. Nitrogenases are made up of two components.
Component I – The first component or component I is known as dinitrogenase. It is Mo-Fe protein (or V-Fe or Fe-only). It contains P clusters and two Fe-Mo cofactors within alpha subunits.
Component II – The second component or component II is known as dinitrogenase reductase. It is an iron protein and contains Fe-S clusters.
The Fe-S clusters of component II transfer electrons to the component I. The electrons flow from ATP molecules to the Fe-S clusters and then to P clusters. Electrons are finally transferred to the Fe-Mo cofactor and that lead to the reduction of N2 to NH3.
The reduction of N2 to NH3 takes place at the Fe-Mo cofactor of component I with the sequential addition of protons and transfer of electrons from component II.
Other than nitrogen fixation, nitrogenase also carries out various other reactions. It acts as a dehydrogenase and reduces protons to dihydrogen.
Dihydrogen acts as a competitive inhibitor of nitrogenase and carbon monoxide acts as a non-competitive inhibitor.
Biological Nitrogen Fixation
Nitrogen is present abundantly in the atmosphere yet only a few organisms can utilise the nitrogen in the dinitrogen form present in the air. The reduction of nitrogen (N2) to ammonia is called nitrogen fixation.
Some prokaryotic organisms have the capability to fix atmospheric nitrogen and convert it to ammonia, which can be utilised by plants and various organisms. The reduction of nitrogen (N2) to ammonia by living organisms is called biological nitrogen fixation.
Biological nitrogen fixation is an important phenomenon for the utilisation of nitrogen by living organisms. Nitrogen is an essential constituent of biomolecules such as nucleotides, proteins (amino acids), etc.
Organisms that perform biological nitrogen fixation are symbiotic as well as free-living. E.g. cyanobacteria (Anabaena and Nostoc)
Some examples of bacteria performing nitrogen fixation are:
Free-living aerobic nitrogen-fixing bacteria: Azotobacter and Beijernickia
Free-living anaerobic nitrogen-fixing bacteria: Rhodospirillum
Symbiotic Nitrogen fixers: Rhizobium in the root nodules of leguminous plants, Frankia in the root nodules of non-leguminous plants such as Alnus, etc.
Ammonia is converted to nitrites, nitrates by soil bacteria by the process of nitrification and gets absorbed by the plants for further use.
This was in detail about the Nitrogenase enzyme. Check the notes on Nodule Formation to learn in detail about symbiotic nitrogen fixation.
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