Friday, March 18, 2011

Nitrogenase: Not Just Another "Bosch-up"

Atmospheric nitrogen is extremely stable. Even the traditional Lewis representation of the homodiatomic molecule, :N=N:, gives some indication of the enormous amounts of energy required to break this triple bond. Industrially the degradation of nitrogen was accomplished through the Haber-Bosch process, which employs the use of an iron catalyst along with calcium, potassium, and aluminum oxides, temperatures from between 300-500˚C, and pressures of 15-20 MPa. The Haber-Bosch process was hailed for its use in fertilizer production, as many believe the current human population could not have been sustained without it, but its harsh conditions and its potential use in explosives manufacturing cause some to view this reaction with disdain.
N2 + 3 H2  2 NH3
The biological fixation of nitrogen in to ammonia, however, should not be viewed with distaste as the Haber-Bosch process perhaps warrants. For millions of years bacterial species have fixed the extremely stable atmospheric nitrogen - the results being nothing but beneficial. The process is energetically taxing, but is efficiently accomplished by the work of an enzyme known as Nitrogenase. 

Nitrogenase isolated from Azotobacter vinelandii was shown to be a dimer, with one subunit containing a Fe cofactor and the other containing a Mo-Fe cofactor. The Fe subunit functions to provide a constant electron flow from ATP breakdown in order to fuel the Mo-Fe subunit's scission of the :N=N: bond[1]. These two subunits are shown to covalently cross-link between a glutamine residue on the Fe subunit and a lysine residue on the MoFe subunit during the catalytic mechanism[2]. Other mechanistic studies have isolated a hexa-coordinated ligand within the Fe-Mo complex, which is argued to be a single nitrogen atom[3]. The implication is that the enzymatic stabilization of the lone nitrogen atom greatly contributes to the efficacy of the nitrogenase mechanism.


[1] Schindelin, H., Kisker, C., Schlessman, J., Howard, J., and Rees, D. (1997) Structure of ADP x AIF4(-)-stabilized nitrogenase complex and its implications for signal transduction. Nature 387, 370-376.
[2] Schmid, B., Einsle, O., Chiu, H., Willing, A., Yoshida, M., Howard, J., Rees, D. (2002) Biochemistry 41,15557-15565.
[3] Einsle, O., Tezcan, F., Andrade, S., Schmid, B., Yoshida, M., Howard, J., Rees, D. Nitrogenase MoFe-Protein at 1.16 Å Resolution: A Central Ligand in the Fe-Mo cofactor. Science 297, 1696-1699.

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