Protein interaction with and higher oxidation states of the nitrogenase molybdenum-iron protein from Azotobacter vinelandii

How many metals does it take to fix N2? A mechanistic overview of biological nitrogen fixation

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.

Evidence that MgATP accelerates primary electron transfer in a Clostridium pasteurianum Fe protein-Azotobacter vinelandii MoFe protein nitrogenase tight complex

The nitrogenase catalytic cycle involves binding of the iron (Fe) protein to the molybdenum-iron (MoFe) protein, transfer of a single electron from the Fe protein to the MoFe protein concomitant with the hydrolysis of at least two MgATP molecules, followed by dissociation of the two proteins. Earlier studies found that combining the Fe protein isolated from the bacterium Clostridium pasteurianum with the MoFe protein isolated from the bacterium Azotobacter vinelandii resulted in an inactive, nondissociating Fe protein-MoFe protein complex. In the present work, it is demonstrated that primary electron transfer occurs within this nitrogenase tight complex in the absence of MgATP (apparent first-order rate constant k = 0.007 s-1) and that MgATP accelerates this electron transfer reaction by more than 10,000-fold to rates comparable to those observed within homologous nitrogenase complexes (k = 100 s-1). Electron transfer reactions were confirmed by EPR spectroscopy. Finally, the midpoint potentials (Em) for the Fe protein [4Fe-4S]2+/+ cluster and the MoFe protein P2+/N cluster were determined for both the uncomplexed and complexed proteins and with or without MgADP. Calculations from electron transfer theory indicate that the measured changes in Em are not likely to be sufficient to account for the observed nucleotide-dependent rate accelerations for electron transfer.

An all-ferrous state of the Fe protein of nitrogenase. Interaction with nucleotides and electron transfer to the MoFe protein

The MoFe protein of nitrogenase catalyzes the six-electron reduction of dinitrogen to ammonia. It has long been believed that this protein receives the multiple electrons it requires one at a time, from the [4Fe-4S]2+/+ couple of the Fe protein. Recently an all-ferrous [4Fe-4S]0 state of the Fe protein was demonstrated suggesting instead a series of two electron steps involving the [4Fe-4S]2+/0 couple. We have examined the interactions of the [4Fe-4S]0 Fe protein with nucleotides and its ability to transfer electrons to the MoFe protein. The [4Fe-4S]0 Fe protein binds both MgATP and MgADP and undergoes the MgATP induced conformational change and then binds properly to the MoFe protein, as evidenced by the fact that the behavior of the 0 and +1 oxidation states in the chelation and chelation protection assays are indistinguishable. Nucleotide binding does not effect the distinctive UV/Vis, CD, or Mössbauer spectra exhibited by the [4Fe-4S]0 Fe protein; however, because the intensity of the g = 16.4 EPR signal of the [4Fe-4S]0 Fe protein is extremely sensitive to minor variations of the rhombicity parameter E/D, the EPR signal is sensitive to the binding of nucleotides. A 50:50 mixture of [4Fe-4S]2+ and [4Fe-4S]0 Fe protein results in electron self-exchange and 100% production of [4Fe-4S]+ Fe protein, demonstrating that the +1/0 couple is fully reversible. MgATP is absolutely required for electron transfer from the [4Fe-4S]0 Fe protein to the reduced state of the MoFe protein. In that reaction both electrons are transferred and are used to reduce substrate.

Purification, composition, charge, and molecular weight of the FeMo cofactor from Azotobacter vinelandii nitrogenase

A procedure has been developed for purifying NMF and NMF/DMF solutions of the FeMo cofactor (FeMoco) derived from the molybdenum iron protein of nitrogenase. This procedure consists of anaerobic chromatography of FeMoco solutions on two consecutive anaerobic molecular sizing columns followed by electrophoretic migration through a third sizing column. FeMoco prepared by this procedure is homogeneous as evidenced by chromatographic, electrophoretic, and compositional criteria. The minimal elemental composition was found to be MoFe6S6 using chemical colorimetric, inductively coupled plasma (ICP), and proton induced x-ray emission (PIXE) analytical procedures. Molecular weight measurements of NMF and DMF solutions of FeMoco using calibrated columns containing various molecular sizing matrices gave values of 1395 +/- 130 daltons for the molecular weight of FeMoco. The measured MW of FeMoco is about twice the value expected from the minimal stoichiometry, suggesting that FeMoco may exist as Mo2Fe12S12 in NMF and DMF solutions. The charge of FeMoco in its EPR silent state was determined to be 2- per Mo by passing NMF solutions of FeMoco containing excess salts of Na+, K+, Rb+, and Mg2+ through long columns equilibrated with pure NMF and then measuring the M/Mo ratio of the emerging FeMoco. Decomposition of purified FeMoco by acid or O2-exposure followed by exhaustive methylation or silanation of the resulting mixture failed to yield any methylated or silanated homocitric acid as measured by tandem gas chromatography-mass spectrometry (GC-MS) analysis. The GC-MS procedure applied to standard homocitric acid samples and various controls readily detects methylated homocitric acid at the sub-nanomole level. We conclude that the minimum molecular formula for active oxidized (EPR silent) FeMoco in NMF and in NMF-DMF mixtures is [Mo2Fe12S12]4-, but that other small organic anions such as NMF- may be present.