How many metals does it take to fix N ₂ ? A mechanistic overview of biological nitrogen fixation

Abstract

During the process of biological nitrogen fixation, the enzyme nitrogenase catalyzes the ATP-dependent reduction of dinitrogen to ammonia. Nitrogenase consists of two component metalloproteins, the iron (Fe) protein and the molybdenum-iron (MoFe) protein; the Fe protein mediates the coupling of ATP hydrolysis to interprotein electron transfer, whereas the active site of the MoFe protein contains the polynuclear FeMo cofactor, a species composed of seven iron atoms, one molybdenum atom, nine sulfur atoms, an interstitial light atom, and one homocitrate molecule. This Perspective provides an overview of biological nitrogen fixation and introduces three contributions to this special feature that address central aspects of the mechanism and assembly of nitrogenase. Biological nitrogen fixation, as defined by the reduction of dinitrogen to ammonia under physiological conditions, is thermodynamically favorable (1). Even at low intracellular dissolved gas pressure, the reaction has a large negative free energy when reduced ferredoxin (Fd red ) serves as the electron donor. Nevertheless, the enzymatic reaction, as catalyzed by the complex metalloclustercontaining nitrogenase system, does not proceed without the additional input of substantial quantities of energy in the specific form of ATP hydrolysis. These requirements may be summarized by the following equation:where n is the ratio of ATP hydrolyzed per electron transferred. This expression encompasses three central questions relevant to the mechanism of nitrogenase:What is the role of nucleotide hydrolysis in electron transfer and substrate reduction? How are substrates bound and reduced at the active site? How are the nitrogenase metalloclusters assembled and inserted?These issues are addressed in the contributions to this Nitrogen Fixation Special Feature by the groups of Watt (2); Seefeldt, Dean, and Hoffman (3); and Ribbe, Hodgson, and Hedman (4). The purpose of this Perspective is to provide both a background for these contributions and a summary of our views on the progress made in deciphering the molecular mechanisms of this fascinating process. Intermolecular Electron Transfer and the Role of ATP Hydrolysis in NitrogenaseThe overall reaction mechanism of biological nitrogen fixation (Eq. 1) may be divided into two parts (5, 6): (i) the control or regulation of electron transfer to the substrate reduction site and (ii) the substrate reduction process itself. What sets nitrogenase apart from essentially all other enzymatically catalyzed redox processes is the number of electrons (eight) that must ultimately be delivered to the substrates each turnover cycle, with the consequent demand for precise timing of the underlying electron transfer events. The first part of this mechanism consists of a cycle involving the ATP-dependent electron transfer between the two protein components of nitrogenase, named Fe protein and MoFe protein, as diagrammatically shown in Scheme 1. The second part of the process, discussed subsequently, involves substrate reduction on the MoF...

Diverse syntrophic partnerships from deep-sea methane vents revealed by direct cell capture and metagenomics

“…Nitrogen fixation is one of the most energetically expensive anabolic processes, requiring a significant cellular investment in both ATP and reducing equivalents (34). Preliminary 15 N 2 incubation experiments designed to test for nitrogen fixation by the consortia within the ERB methane seep sediments were conducted by using a combination of FISH and secondary ion mass spectrometry (FISH-SIMS) (SI Text and ref.…”

Nitrogenase-mimic iron-containing chalcogels for photochemical reduction of dinitrogen to ammonia

“…A nitrogenase-inspired biomimetic chalcogel system comprising double-cubane [Mo 2 Fe 6 S 8 (SPh) 3 ] and single-cubane (Fe 4 S 4 ) biomimetic clusters demonstrates photocatalytic N 2 fixation and conversion to NH 3 in ambient temperature and pressure conditions. Replacing the Fe 4 S 4 clusters in this system with other inert ions such as Sb 3+ , Sn 4+ , Zn 2+ also gave chalcogels that were photocatalytically active.…”

“…Replacing the Fe 4 S 4 clusters in this system with other inert ions such as Sb 3+ , Sn 4+ , Zn 2+ also gave chalcogels that were photocatalytically active. Finally, molybdenum-free chalcogels containing only Fe 4 S 4 clusters are also capable of accomplishing the N 2 fixation reaction with even higher efficiency than their Mo 2 Fe 6 S 8 (SPh) 3 -containing counterparts. Our results suggest that redox-active iron-sulfide-containing materials can activate the N 2 molecule upon visible light excitation, which can be reduced all of the way to NH 3 using protons and sacrificial electrons in aqueous solution.…”

“…Our results suggest that redox-active iron-sulfide-containing materials can activate the N 2 molecule upon visible light excitation, which can be reduced all of the way to NH 3 using protons and sacrificial electrons in aqueous solution. Evidently, whereas the Mo 2 Fe 6 S 8 (SPh) 3 is capable of N 2 fixation, Mo itself is not necessary to carry out this process. The initial binding of N 2 with chalcogels under illumination was observed with in situ diffuse-reflectance Fourier transform infrared spectroscopy (DRIFTS).…”

“…Currently, roughly half of the fixed nitrogen is supplied biologically by nitrogenase, while nearly the other half is from the industrial Haber-Bosch process, which operates under high temperature (400-500°C) and high pressure (200-250 bar) in the presence of a metallic iron catalyst (1). Nitrogenase, a two-component protein system comprising a MoFe protein and an associated Fe protein, carries out this "fixation" in nature under ambient temperature and pressure (2)(3)(4). N 2 substrate binding and activation take place at the ironmolybdenum-sulfur cofactor (FeMoco), and in some cases, Mofree iron-sulfur cofactor FeFeco and iron-vanadium-sulfur cofactor FeVco cofactors.…”