Biological N2 fixation

Roger PA, Ladha JK. 1992. Biological N2 fixation in wetland rice fields-estimation and contribution to nitrogen balance. Plant and Soil 141 41-55. 

Extensive studies on nitrogenases, and in particular on the Fe-Mo protein (1) component of these complex enzymic systems, have revealed the presence of a unique Fe/Mo/S aggregate intimately involved in biological N2 fixation. 

Biological N2 fixation (1), i.e., the reduction of N2 to NH3 catalyzed by FeMo, FeV, or FeFe nitrogenases, is one of the fundamental synthetic processes of nature (2-4). In spite of intense efforts over the last decades, its molecular mechanism is poorly understood, in particular because the pivotal chemical question has remained unanswered how do nitrogenases achieve to activate and convert the inert N2 molecule to ammonia under ambient conditions and mild redox potentials. 

All mechanistic proposals for biological N2 fixation are left with the problem to explain how nitrogenases enable the catalytic N2 reduction at mild biological reduction potentials (6,8). These reduction potentials probably represent the biggest challenge in the search for synthetic competitive catalysts that exhibit nitrogenase-like activity. 

Sellmann, D. Sutter, J. Biological N2 fixation Molecular mechanism of the nitrogen-ase catalyzed N2 dependent HD-formation, the N2 fixation inhibition and the open-side FeMoco model, Perspectives in Coordination Chemistry , Vol. 5 Eds. Trzeciak, A. M. Sobota, D. Ziolkowski, J. University of Wroclaw Poland, 2000. 

Figure 10.5 Major processes involved in the biogeochemical cycling of N in estuaries and the coastal ocean (1) biological N2 fixation (2) ammonia assimilation (3) nitrification (4) assimilatory NC>3 reduction (5) ammonification or N remineralization (6) ammonium oxidation (speculative at this time) (7) denitrification and dissimilatory NO3 reduction to NH4+ and (8) assimilation of dissolved organic nitrogen (DON). (Modified from Libes, 1992.)...

Heterocysts specialized well-sealed cells capable of biological N2 fixation. 

Chikowo, R. Mapfumo, P. Nyamugafata, P. and K.E. Giller (2004) Woody legume fallow productivity, biological N2-fixation and residual benefits to two successive maize crops in Zimbabwe.- Plant and Soil 262, 303-315. 

Hairiah, K. van Noordwijk, M. and G. Cadisch (2000) Quantification of biological N2 fixation of hedgerow trees in Northern Lampung.- Netherlands Journal of Agricultural Science 48, 47-59. 

Biological N2 fixation is catalyzed by Fe/Mo, Fe/V, or FeFe (the Fe-only) nitrogenases (150, 151). The extremely different reaction conditions of the biological and the Haber-Bosch processes of N2 reduction, that is, standard temperature and pressure and biological redox potentials on the one hand, red-hot temperatures and high pressures on the other hand, make the quest for low molecular weight competitive catalysts particularly challenging. 

Nitrogenases represent a class of complex metalloen-zymes that catalyze the process of biological N2 fixation, during which dinitrogen is reduced to ammonia in a nucleotide-dependent process. The ammonia produced is subsequently used for cell growth or maintenance. The overall reaction catalyzed by nitrogenases is usually depicted as follows... 

White, A. E., Prahl, P. G., LeteKer, R. M., and Popp, B. N. (2007). Summer surface waters in the Gulf of California Prime habitat for biological N2 fixation. Global Biogeochem. Cycles 21, doi 10.1029/ 2006GB002779. 

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