Nitrogen fixation
Contents :
- Introduction
- Nitrogen fixation
- Non – biological nitrogen fixation
- Biological nitrogen fixation
- Nitrogenase complex
- Symbiotic nitrogen fixation
- Asymbiotic nitrogen fixation
- Energetics and regulation
- Conclusion
- References
Introduction:
- Nitrogen is an essential constituent of all biomolecules both in plants and in animals
- Most of the plants obtain nitrogen from the soil in the form of nitrate or ammonium ion, but it is limited
- The atmosphere consists of 78% of molecular nitrogen but plants unable to convert this molecular nitrogen into a useful form because of the lack the enzyme nitrogenase
- Only prokaryote species possess this enzyme
- Nitrogen fixation is the first step of nitrogen cycle where molecular nitrogen is reduced by nitrogen-fixing bacteria to yield ammonia
- Nitrogen cycle has 3 process
- Ammonification
- Nitrification
- Denitrification
Nitrogen fixation:
- The process of reducing dinitrogen to ammonia so that plants can absorb nitrogen is known as nitrogen fixation
Types of nitrogen fixation
Non – biological nitrogen fixation:
- Nitrogen may be fixed by the electrical discharge of lightning in the atmosphere
- The nitrous oxide formed combines with oxygen to form nitric oxide
- Nitric oxide readily dissolves in water to produce nitric and nitrous acids
- These acids readily release the hydrogen, forming nitrate and nitrite ions.
- The nitrate can be readily utilized by plants and microorganisms.
Biological nitrogen fixation :
- It’s a prokaryote domain because of the presence of nitrogenase enzyme
- prokaryotes which fix nitrogen – nitrogen fixers
- It includes both free-living and symbiotic associations with plants
- Diazatrophs – certain strains of bacteria of genus Rhizobium that shows symbiotic association with legumes via root nodules
- These species convert molecular nitrogen to ammonia
- Ammonia thus produced is incorporated either into glutamate by glutamate dehydrogenase or into glutamine by glutamine synthetase
Basic requirements of nitrogen fixation:
- Basic requirements for Nitrogen fixation are
- Nitrogenase enzyme complex
- Protective mechanism against Oxygen – leghaemoglobin
- Ferredoxin
- Hydrogen releasing system or electron donor (Pyruvic acid )
- The constant supply of ATP
- Coenzymes and cofactors like CoA, inorganic phosphate and Mg+2
- Cobalt and Molybdenum
Nitrogenase complex :
- Biological nitrogen fixation is carried out by a highly conserved complex of proteins called as nitrogenase complex
- Which is mainly consists of 2 important proteins
- Fe protein (dinitrogenase reductase)
- Mo-Fe protein (dinitrogenase)
- Structure
- Dinitrogenase reductase – is a dimer of 2 identical subunits
- It contains a single 4Fe – 4S redox center bound between the subunits
- This can be oxidized and reduced by 1 electron
- Also, it has 2 binding sites for ATP or ADP
- Dinitrogenase – is a tetramer with 2 copies of 2 different subunits (𝜶𝟐 − 𝜷𝟐 heterodimer)
- Contains both iron and molybdenum
- Its redox centers have 2 MO, 32 Fe and 30 S per tetramer
- And it has 2 binding sites for reductase
- About half of the iron and sulfur is present as 2 bridged pairs of 4Fe – 4S centers called as P cluster
- P cluster – consists of 2[ 4Fe – 4S] clusters linked through additional sulfide ion
The action of the nitrogenase enzyme :
- For reducing nitrogen into ammonia nitrogenase requires 8 electrons
- At first, dinitrogen is reduced by transferring of electrons to dintrogenase reductase
- Dinitrogenase has 2 binding sites for reductase
- The 8 electrons are transferred from reductase to dinitrigenase one at a time :
- Reduced reductase binds to dinitrogenase and transfers a single electron, oxidized form dissociates in a repeating cycle
- Each cycle requires the hydrolysis of ATP molecule by dinitrogenase reductase
- Immediate source of electrons to reduce reductase is reduced ferredoxin
- Ultimate source of electrons to reduce ferredoxin is pyruvate
Symbiotic nitrogen fixation :
- Symbiotic nitrogen fixation occurs in plants that harbor nitrogen-fixing bacteria within their tissues
- The best-studied example is the symbiotic association between roots of legumes and bacteria of the genus Rhizobium
- This association results from the root nodules in legumes
- Root nodules – it is an enlarged multicellular structure on roots
Legume – rhizobium association will fix 25 – 60 kg of molecular nitrogen annually
Root nodule formation :
Asymbiotic nitrogen fixation:
Asymbiotic nitrogen fixation:
- The free-living nitrogen-fixing organisms are called are symbiotic – organisms.
- It includes Aerobic bacteria, anaerobic bacteria, and blue-green algae
- Bacteria: types –
- Free-living aerobic: Azotobacter
- Free-living anaerobic: Clostridium
- Blue-green algae: types –
- Filamentous (in heterocystous) -Oscillatoria
- Filamentous (heterocystous) – Nostoc, Anabaena
Energetics and regulation :
- Nitrogen fixation is energetically costly – because it requires
16 ATPs to reduce one molecule of nitrogen
- Thus to avoid this wastage of energy it must be regulated
- When soil nitrogen (NO3 or NH4) levels are high, the formation of nodules is inhibited thus regulating nitrogen fixation
- Some of the inhibitors also regulate nitrogen fixation – hydrogen, nitrous oxide, and nitric oxide
- Nitrogenase enzyme also regulated at its genetic level
- It is achieved by transcriptional level modification
- Transcriptional level regulation
- Here the Nif genes are mainly regulated
- If genes – are the genes encode for nitrogenase enzyme
- Regulation of nif genes transcription is done by the nitrogen sensitive NifA protein
- When there aren’t enough fixed nitrogen available NtrC triggers NifA expression
- And NifA activates the rest of the Nif genes
- If there is a sufficient amount of reduced nitrogen or oxygen is present, another protein is activated: NFL
- NFL inhibits NifA activity resulting in the inhibition of nitrogenase formation
- NFL is regulated by the products of glnB and glnK
- The Nif genes can be found on bacterial chromosomes, but in symbiotic bacteria, they are often found on plasmids
- Post-translational regulation
- In free-living diazotrophs, this is the additional level of nitrogenase regulation
- During energy limiting or nitrogen sufficient condition, the nitrogenase complex is rapidly, reversibly inactivated by ADP – ribosylation of Fe protein
- It occurs at a specific arginine residue. i.e. Arg 101
- The presence of ADP ribose group prevents the association of Fe protein with MO – Fe protein
- Thus it results in regulating the nitrogen fixation
Conclusion :
- Nitrogen is a limiting nutrient for plants, even though molecular nitrogen is readily available in the atmosphere
- Plants do not have the nitrogenase enzyme thus have to depend on prokaryotes to absorb nitrogen
- Nitrogen fixation is the first step of the nitrogen cycle and it is of two types biological and nonbiological nitrogen fixation
- Biological nitrogen fixation occurs in two ways i. e symbiotic, where it involves the prokaryotic interaction with plants via root nodules and a symbiotic where free-living microorganisms fix the atmospheric nitrogen into the soil
- Nonbiological nitrogen fixation involves the lighting process to fix atmospheric nitrogen into the soil
- Because of the high demand for energy, this nitrogen fixation is tightly regulated by various methods
References :
- Introduction to plant physiology – Hopkins .W, Hunter. N
- Lehninger Principles of Biochemistry – Nelson, Cox
- Biochemistry – Donald Voet, Judith G. Voet
Nitrogen fixation – Wikipedia
