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Biological nitrogen fixation

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Biological Nitrogen Fixation

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Biological nitrogen fixation

  2. 2. INTRODUCTION  Nitrogen is an essential element of many biomolecules, most important being nucleic acid & aminoacid.  Although nitrogen is the most abundant gas (about 80%) in the atmosphere, neither animals nor plants can use this nitrogen to synthesize biological compounds.  Animals can use nitrogen either in inorganic forms (NH3, nitrites, nitrates) or in organic forms (urea, protein, nucleic acid).  A regular supply of nitrogen to plant is maintained through nitrogen cycle.
  3. 3.  Nitrogen can be fixed by 2 methods Nitrogen fixation Biological Industrial Natural Abiological Symbiotic Asymbiotic
  4. 4. BIOLOGICAL NITROGEN FIXATION Nitrogen fixing bacteria – 2 types of nitrogen fixing microorganisms 1. ASYMBIOTC N2 FIXING BACTERIA  The gaseous nitrogen in the atmosphere is directly & independently utilized to produce nitrogen rich compounds.  When these non symbiotic organisms die, they enrich the soil with nitrogenous compounds.
  5. 5.  the mechanism of nitrogen fixation by asymbiotic bacteria is not clearly understood.  Ex: Clostridium pasturianum , Azotobacter chrooccum 2. SYMBIOTIC N2 FIXING BACTERIA  These organisms live together with the plants in a mutually beneficial relationship (symbiosis)  Anabaena & Nostoc species are common symbionts in lichens, azolla, cycad roots etc.  The most important microorganism involved in symbiosis belongs to 2 related genera namely Rhizobium & Bradyrhizobium
  6. 6. NITROGENASE ENZYME  Is a complex enzyme containing 2 oxygen sensitive components.  Component 1 has two α protein subunits & two β protein subunits, 24 molecules of Fe, 2 molecules of Mo & an iron-molybdenum cofactor.  Component 2 possesses two α subunits (different from that of component 1) & a large number of Fe molecules.  The MoFe protein & Fe protein combine together in the presence of Na+ ions to form an active nitrogenase complex.
  7. 7.  Component 1 of nitrogenase catalyses the actual conversion of nitrogen to ammonia, while component 2 donates electron to component 1.  They readily dissociates after nitrogen reduction
  9. 9. MECHANISM OF N2 FIXATION  Symbiotically bacteria are found in the root nodules of the members of the family leguminosae (beans, gram, soyabean, groundnut etc).  The best known nitrogen fixing symbiotic bacterium is Rhizobium leguminosarum .  Rhizobium penetrates to the cortex of the root through infection thread.  Simultaneous cortical cells of root are stimulated to divide more vigorously to form nodules on the root.
  10. 10. Roots of legume secrete chemical attractants (flavanoids,betains) Bacteria collect over the root hairs & release nod factors cause curling of root hairs around the bacteria & degradation of cell wall Formation of infection thread & infection thread grows at its tips Deeper into the root cortex, near the xylem, cortical cells differentiate & start diving forming nodule primordium from which the nodule will develop
  11. 11. Nodule formation is stimulated by auxin produced by the cortical cells & cytokinin liberated by the invading bacteria Infected cells enlarge Bacteria stop dividing & form bacterioids
  12. 12.  When a section of root nodule is observed the presence of a pigment, leg haemoglobin (LHb) is seen to impart pink colour to it.  This pigment is closely related to Hb & helpful in creating optimal condition for N2 fixation.  Like Hb, LHb is an oxygen scavenger.  Fixation of nitrogen is done with the help of enzyme nitrogenase, which function under anaerobic condition.  LHb combines with oxygen & protects nitrogenase.  Symbiotic nitrogen fixation requires cooperation of Nod genes of legumes, nod, nif & fix gene cluster of bacteria
  13. 13.  Ammonification  Carried out by decay causing organisms.  They act upon nitrogenous excreations & proteins of dead bodies of living organisms.  Ex: Bacillus ramosus, Bacillus vulgaris proteins + H2O R-NH2 + H2O ROH + NH3  NH does not remain in gaseous state in the soil but is changed to ionic form (NH4) it can be used by plants.  Nitrification  Phenomenon of conversion of ammonia nitrogen to nitrate nitrogen.
  14. 14.  It is performed in 2 steps Nitrite formation Nitrate formation 2 NH3 + 3O2 nitrococcus /nitrosomonas 2NO2- + 2H+ + 2H2O 2NO2- + O2 nitrobacter 2NO- + energy  Denitrification  Under anaerobic condition some microorganism use nitrate & other oxidised ions as a source of O2  In this, nitrates are reduced to gaseous compound of nitrogen & the latter escape from the soil.  Ex: Pseudomonas denitrificans, Thiobacillus denitrificans, Micrococcus denitrificans
  15. 15.  Nitrate Assimilation  The nitrates produced by nitrification are absorbed by higher plants & assimilated by the process called nitrate assimilation.  The nitrates absorbed by plant roots get converted to amino acids & amides besides incorporating them into proteins & other macromolecules.  Reduction of nitrate to ammonia is called nitrogen assimilation.  Ammonia produced is unstable at the physiological pH, so it is readily converted to ammonium  To avoid ammonium toxicity plants rapidly convert it into amino acids.
  16. 16. NO3- + 8 electron +10 H+ nitrate reductase NH4+ +3 H2O NO3- + NAD(P)H + H+ NO2- + H2O + NADP+ 2NO2- + 7NAD(P)H + 7H+ 2NH3 + 4H2O+7NADP+ NH4+ + α KG + NAD(P)H Glutamate + H2O + NADP+ Glutamate + NH4+ + ATP GS Glutamine + ADP + Pi Glutamine + α KG + NAD(P)H GS 2 Glutamine + NADP Glutamine + 2 Oxoglutarate GOGAT 2 Glutamate GS – Glutamine synthase GOGAT – Glutamine 2 oxoglutarate aminotransferase
  17. 17. THANK YOU