Structural Enzymology of Nitrogenase Enzymes

Abstract

The reduction of dinitrogen to ammonia by nitrogenase reflects a complex choreography involving two component proteins, MgATP, and reductant. At center stage of this process resides the active site cofactor, a complex metallocluster organized around a trigonal prismatic arrangement of iron sites surrounding an interstitial carbon. As a consequence of the choreography, electrons and protons are delivered to the active site for transfer to the bound N 2 . While the detailed mechanism for the substrate reduction remains enigmatic, recent developments highlight the role of hydrides and the privileged role for two irons of the trigonal prism in the binding of exogenous ligands. Outstanding questions concern the precise nature of the intermediates between N 2 and NH 3 , and whether the cofactor undergoes significant rearrangement during turnover; resolution of these issues will require the convergence of biochemistry, structure, spectroscopy, computation and model chemistry.

Dinitrogen Fixation: Rationalizing Strategies Utilizing Molecular Complexes

“… [31] In that complex, the electrons required for the reaction are formally not only delivered by the metal centers but also resulted from the reductive elimination of two equivalents of H 2 per dinitrogen molecule. This system is relevant in the understanding of the biological dinitrogen fixation process, since bridging hydride ligands and analogous H 2 elimination prior to N 2 binding have been shown to be of key importance in the nitrogenase enzyme [11, 32, 33] . The cleavage of N 2 by the related chromium(II) and chromium(III) hydride complexes 7 and 8 was demonstrated very recently (Scheme 5 d).…”

“…Mechanistic investigations of N 2 reduction at the nitrogenase co‐factor and identification of the role of iron sites in N 2 activation [8, 11, 33] have been the driving force for the intense development of iron‐based catalysts for dinitrogen reduction. In 2013, Peters et al.…”

Operando Infrared Spectroscopy for the Analysis of Gas-processing Metalloenzymes

“…Nitrogenase forms water-soluble, multi-subunit protein complexes that catalyse the reduction of N2 into NH4 and H2 at the expense of 16 -40 equivalents of ATP, depending on the metal composition of the active site cofactor. 27 The enzyme comprises the so-called 'Fe protein', a homodimer that binds and hydrolyses ATP upon reduction of an all-ferrous [4Fe-4S] cluster. 28 Electrons are transferred from the Fe protein to an [8Fe-7S] compound, the 'P-cluster', which charges a [X-7Fe-9S-C] cluster, where X may be M, V, or Fe ( Figure 2).…”

Preparation and spectroscopic characterization of lyophilized Mo nitrogenase

“…Of the three known nitrogenases, the Mo-dependent enzyme is by far the widest spread, with production of the V and Fe homologs only appearing in certain organisms under Molimiting conditions [9,10]. As a result, the study of these enzymes has heavily focused on the Mo-dependent nitrogenase [11][12][13][14][15][16][17][18][19]. To accomplish this reaction, Mo nitrogenase employs a series of iron-sulfur metallocofactors are employed, specifically the catalytic M-cluster (FeMoco) and P-cluster of MoFe, and the [4Fe-4S] F-cluster of FeP, shown in Fig.…”

“…Although extensive spectroscopic, crystallographic, and reactivity studies have helped to shed light on the elusive mechanism of Mo nitrogenase, much still remains to be understood [15][16][17][18][19]22]. A significant limitation in studying this enzymatic system is the concentrations that can be reached before these proteins precipitate in solution.…”