Biological nitrogen fixation

Biological Nitrogen Fixation

1. Biological Nitrogen Fixation DILSHANA FATHIMA M.Sc. BIOCHEMISTRY

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 (NH 3 , nitrites, nitrates) or in organic forms (urea, protein, nucleic acid). A regular supply of nitrogen to plant is maintained through nitrogen cycle.

3. Nitrogen can be fixed by 2 methods

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. 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. 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. 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

8. Mechanism Of Nitrogenase Enzyme

9. Mechanism Of N 2 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. 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

11. 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 + H 2 O R-NH 2 + H 2 O ROH + NH 3 NH does not remain in gaseous state in the soil but is changed to ionic form (NH 4 ) it can be used by plants. Nitrification Phenomenon of conversion of ammonia nitrogen to nitrate nitrogen.

12. It is performed in 2 steps Nitrite formation Nitrate formation 2 NH 3 + 3O 2 nitrococcus / nitrosomonas 2NO 2 - + 2H + + 2H 2 O 2NO 2 - + O 2 nitrobacter 2NO- + energy Denitrification Under anaerobic condition some microorganism use nitrate & other oxidised ions as a source of O 2 In this, nitrates are reduced to gaseous compound of nitrogen & the latter escape from the soil. Ex: Pseudomonas denitrificans , Thiobacillus denitrificans , Micrococcus denitrificans

13. 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.

14. NO 3 - + 8 electron +10 H + nitrate reductase NH 4+ +3 H 2 O NO 3 - + NAD(P)H + H + NO 2 - + H 2 O + NADP + 2NO 2 - + 7NAD(P)H + 7H + 2NH 3 + 4H 2 O+7NADP + NH 4+ + α KG + NAD(P)H Glutamate + H 2 O + NADP + Glutamate + NH 4+ + ATP GS Glutamine + ADP + P i Glutamine + α KG + NAD(P)H GS 2 Glutamine + NADP Glutamine + 2 Oxoglutarate GOGAT 2 Glutamate

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