Cleaving the N,N Triple Bond: The Transformation of Dinitrogen to Ammonia by Nitrogenases

Abstract

Biological nitrogen fixation is a natural process that converts atmospheric nitrogen (N2) to bioavailable ammonia (NH3). This reaction not only plays a key role in supplying bio-accessible nitrogen to all life forms on Earth, but also embodies the powerful chemistry of cleaving the inert N,N triple bond under ambient conditions. The group of enzymes that carry out this reaction are called nitrogenases and typically consist of two redox active protein components, each containing metal cluster(s) that are crucial for catalysis. In the past decade, a number of crystal structures, including several at high resolutions, have been solved. However, the catalytic mechanism of nitrogenase, namely, how the N,N triple bond is cleaved by this enzyme under ambient conditions, has remained elusive. Nevertheless, recent biochemical and spectroscopic studies have led to a better understanding of the potential intermediates of N2 reduction by the molybdenum (Mo)-nitrogenase. In addition, it has been demonstrated that carbon monoxide (CO), which was thought to be an inhibitor of N2 reduction, could also be reduced by the vanadium (V)-nitrogenase to small alkanes and alkenes. This chapter will begin with an introduction to biological nitrogen fixation and Mo-nitrogenase, continue with a discussion of the catalytic mechanism of N2 reduction by Mo-nitrogenase, and conclude with a survey of the current knowledge of N2- and CO-reduction by V-nitrogenase and how V-nitrogenase compares to its Mo-counterpart in these catalytic activities.

Elucidation of the biosynthesis of the methane catalyst coenzyme F430

“…It is interesting to note that Nature employs nitrogenase-like proteins (NifD, H, K) to catalyse difficult reduction reactions, or at least reactions that require a low redox potential, including the reduction of N 2 to NH 3 31, protochlorophyllide to chlorophyllide26 and Ni 2+ -sirohydrochlorin diamide to Ni 2+ -hexahydrosirohydrochlorin diamide. Clearly, the role of CfbC/D more closely parallels the stereospecific reduction of the C17-C18 double bond catalysed by the orthologous DPOR during chlorophyll and bacteriochlorophyll biosynthesis26, but the requirement in F 430 biosynthesis for only the NifD and NifH homologues suggests that this system may provide a simpler model for the coupling of ATP hydrolysis to such biological reduction processes.…”

Purple Sulfur Bacteria Dominate Microbial Community in Brazilian Limestone Cave

“…Indeed, the level of potential nitrogen fixating bacteria observed in the present study was 100-times higher than a previous study from our group in another limestone cave [4] and supported the idea of PSB related with nitrogenase activity in the studied chamber of Sopradeira cave. The surprisingly high level of nifH was unexpected because of the amount of energy spent to fix one N 2 molecule [33]. However, it is plausible to assume that part of the nifH detected in the samples may not form an active nitrogenase, since the presence of nifH does not guarantee expression and full function of this multigenic enzyme [34].…”

Steric Switching From Photochemical to Thermal N2 Splitting: A Computational Analysis of the Isomerization Reaction {(Cp*)(Am)Mo}2(μ-η1:η1-N2) → {(Cp*)(Am)Mo}2(μ-N)2

“…Catalytic nitrogen fixation with well-defined molecular complexes remains a grand challenge despite decades of research in this field. The research field is driven by the vision that a molecular catalyst capable of catalytically transforming nitrogen atoms from the dinitrogen molecule into ammonia or chemicals of higher economic value would contribute to a more sustainable chemical industry not dependent on fossil resources (Crossland and Tyler, 2010; Broda et al, 2013; Tanabe and Nishibayashi, 2013; Lee et al, 2014; Burford and Fryzuk, 2017; Burford et al, 2017; Connor and Holland, 2017; Creutz and Peters, 2017; Eizawa and Nishibayashi, 2017; Kuriyama and Nishibayashi, 2017; Roux et al, 2017). Industrial ammonia production with the Haber-Bosch process is overall energy efficient, but relies on fossil H 2 for the steam reforming step (Schlögl, 2008).…”