Mechanism of Nitrogenase Action

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Nitrogen Fixation Nitrogen Fixation Part 1 Structure of Nitrogenase Components Fe Protein Mo-Fe Protein Mechanism of Nitrogenase Action Hydrogen Evolution Factors Affecting Nitrogenase Activity Nitrogen Fixation Genes Nitrification Nitrate Reduction Glutamate Synthase Pathway

Mechanism of Nitrogenase Action

Mechanism of Nitrogenase Action -
(i) In most organisms, the physiologically functional reductant of nitrogenase is ferredoxin. Other natural reductants include flavodoxin and NADPH.

Artificial reductants include Na₂S₂O₄ and reduced methyl viologen. Reduced ferredoxin, the electron donor, reduces the Fe protein of nitrogenase.

Fe Protein (Oxidized )+ e^- -->Fe Protein (reduced)
(ii) The reduction of N₂ to NH₄ is exothermic. Yet nitrogen fixation requires energy in the form of adenosine 5'-triphosphate (ATP), because of a high activation energy.

The Fe protein of nitrogenase specifically binds to Mg ATP and lowers its redox potential. A complex containing both Fe and Mo-Fe proteins and Mg ATP is assembled.

In the absence of Mg ATP the midpoint potential (Em) of Fe protein is about -250 to -295 m V. After binding with Mg ATP the Em is about -400m V.

Fe protein + 2Mg ATP + Mo-Fe protein
Fe protein. 2MgATP. Mo-Fe protein

(iii) The Fe protein transfers an electron to the Mo-Fe protein. This results in the oxidation of the Fe protein and the reduction of the Mo-Fe protein. This reaction is coupled to ATP--->ADP hydrolysis.

ATP is not hydrolyzed to A TP until the Fe protein transfer an electron to the Mo-Fe protein. There are 12 or more ATP hydrolyzed for each N₂ reduced, or 4 A TP per pair of electrons transferred to the Mo- Fe protein or to the substrate.

Thus, there appear to be 2 ATPs hydrolyzed for each electron transferred. In vivo and in vitro A TP requirements are not necessarily the same. Growth yield experiments indicate that in Azotobacter only 4 or 5 ATPs are required for each N₂ fixed. On the other hand 29 ATPs are required in K. pneumoniae and 20 ATPs in C. pasteurianum.

(iv) The reduced Mo-Fe protein can in turn reduce the substrate. A number of substrates other than N₂can be reduced by nitrogenase. Both Fe and Mo-Fe proteins are required for all these reductions, which are coupled to Mg ATP hydrolysis.

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Nitrogen Fixation Part 1 Ammonification Denitrification Denitrifying Bacteria Ecological Importance of Denitrification Nitrogen Assimilation Transamination Glutamate Dehydrogenase (GDH) Glutamate Synthase Pathway Glutamine Synthetase (GS) Reaction Glutamate Synthase (GOGAT) Reaction Transaminase Reactions Glutamine Synthtase Adenylylation and Deadenylylation

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