Isolation of two forms of the nitrogenase VFe protein from Azotobacter vinelandii

Enzymatic Fischer-Tropsch-Type Reactions

The Fischer-Tropsch (FT) process converts a mixture of CO and H2 into liquid hydrocarbons as a major component of the gas-to-liquid technology for the production of synthetic fuels. Contrary to the energy-demanding chemical FT process, the enzymatic FT-type reactions catalyzed by nitrogenase enzymes, their metalloclusters, and synthetic mimics utilize H+ and e- as the reducing equivalents to reduce CO, CO2, and CN- into hydrocarbons under ambient conditions. The C1 chemistry exemplified by these FT-type reactions is underscored by the structural and electronic properties of the nitrogenase-associated metallocenters, and recent studies have pointed to the potential relevance of this reactivity to nitrogenase mechanism, prebiotic chemistry, and biotechnological applications. This review will provide an overview of the features of nitrogenase enzymes and associated metalloclusters, followed by a detailed discussion of the activities of various nitrogenase-derived FT systems and plausible mechanisms of the enzymatic FT reactions, highlighting the versatility of this unique reactivity while providing perspectives onto its mechanistic, evolutionary, and biotechnological implications.

The electronic structure of FeV-cofactor in vanadium-dependent nitrogenase

The electronic structure of the active-site metal cofactor (FeV-cofactor) of resting-state V-dependent nitrogenase has been an open question, with earlier studies indicating that it exhibits a broad S = 3/2 EPR signal (Kramers state) having g values of ∼4.3 and 3.8, along with suggestions that it contains metal-ions with valencies [1V3+, 3Fe3+, 4Fe2+]. In the present work, genetic, biochemical, and spectroscopic approaches were combined to reveal that the EPR signals previously assigned to FeV-cofactor do not correlate with active VFe-protein, and thus cannot arise from the resting-state of catalytically relevant FeV-cofactor. It, instead, appears resting-state FeV-cofactor is either diamagnetic, S = 0, or non-Kramers, integer-spin (S = 1, 2 etc.). When VFe-protein is freeze-trapped during high-flux turnover with its natural electron-donating partner Fe protein, conditions which populate reduced states of the FeV-cofactor, a new rhombic S = 1/2 EPR signal from such a reduced state is observed, with g = [2.18, 2.12, 2.09] and showing well-defined 51V (I = 7/2) hyperfine splitting, a iso = 110 MHz. These findings indicate a different assignment for the electronic structure of the resting state of FeV-cofactor: S = 0 (or integer-spin non-Kramers state) with metal-ion valencies, [1V3+, 4Fe3+, 3Fe2+]. Our findings suggest that the V3+ does not change valency throughout the catalytic cycle.

Expression, Isolation, and Characterization of Vanadium Nitrogenase from Azotobacter vinelandii

Nitrogenases are the sole enzymes known to mediate biological nitrogen fixation, an essential process for sustaining life on earth. Among the three known variants, molybdenum nitrogenase is the best-studied to date. Recent work on the alternative vanadium nitrogenase provided important insights into the mechanism of nitrogen fixation since this enzyme differs from its molybdenum counterpart in some important aspects. Here, we present a protocol to obtain unmodified vanadium nitrogenase in high yield and purity from the paradigmatic diazotroph Azotobacter vinelandii, including procedures for cell cultivation, purification, and protein characterization.

Reactivity, Mechanism, and Assembly of the Alternative Nitrogenases

Biological nitrogen fixation is catalyzed by the enzyme nitrogenase, which facilitates the cleavage of the relatively inert triple bond of N2. Nitrogenase is most commonly associated with the molybdenum-iron cofactor called FeMoco or the M-cluster, and it has been the subject of extensive structural and spectroscopic characterization over the past 60 years. In the late 1980s and early 1990s, two "alternative nitrogenase" systems were discovered, isolated, and found to incorporate V or Fe in place of Mo. These systems are regulated by separate gene clusters; however, there is a high degree of structural and functional similarity between each nitrogenase. Limited studies with the V- and Fe-nitrogenases initially demonstrated that these enzymes were analogously active as the Mo-nitrogenase, but more recent investigations have found capabilities that are unique to the alternative systems. In this review, we will discuss the reactivity, biosynthetic, and mechanistic proposals for the alternative nitrogenases as well as their electronic and structural properties in comparison to the well-characterized Mo-dependent system. Studies over the past 10 years have been particularly fruitful, though key aspects about V- and Fe-nitrogenases remain unexplored.

Characterization of an M-Cluster-Substituted Nitrogenase VFe Protein

The Mo- and V-nitrogenases are two homologous members of the nitrogenase family that are distinguished mainly by the presence of different heterometals (Mo or V) at their respective cofactor sites (M- or V-cluster). However, the V-nitrogenase is ~600-fold more active than its Mo counterpart in reducing CO to hydrocarbons at ambient conditions. Here, we expressed an M-cluster-containing, hybrid V-nitrogenase in Azotobacter vinelandii and compared it to its native, V-cluster-containing counterpart in order to assess the impact of protein scaffold and cofactor species on the differential reactivities of Mo- and V-nitrogenases toward CO. Housed in the VFe protein component of V-nitrogenase, the M-cluster displayed electron paramagnetic resonance (EPR) features similar to those of the V-cluster and demonstrated an ~100-fold increase in hydrocarbon formation activity from CO reduction, suggesting a significant impact of protein environment on the overall CO-reducing activity of nitrogenase. On the other hand, the M-cluster was still ~6-fold less active than the V-cluster in the same protein scaffold, and it retained its inability to form detectable amounts of methane from CO reduction, illustrating a fine-tuning effect of the cofactor properties on this nitrogenase-catalyzed reaction. Together, these results provided important insights into the two major determinants for the enzymatic activity of CO reduction while establishing a useful framework for further elucidation of the essential catalytic elements for the CO reactivity of nitrogenase.

This is the first report on the in vivo generation and in vitro characterization of an M-cluster-containing V-nitrogenase hybrid. The “normalization” of the protein scaffold to that of the V-nitrogenase permits a direct comparison between the cofactor species of the Mo- and V-nitrogenases (M- and V-clusters) in CO reduction, whereas the discrepancy between the protein scaffolds of the Mo- and V-nitrogenases (MoFe and VFe proteins) housing the same cofactor (M-cluster) allows for an effective assessment of the impact of the protein environment on the CO reactivity of nitrogenase. The results of this study provide a first look into the “weighted” contributions of protein environment and cofactor properties to the overall activity of CO reduction; more importantly, they establish a useful platform for further investigation of the structural elements attributing to the CO-reducing activity of nitrogenase.

Nitrogenase and homologs

Nitrogenase catalyzes biological nitrogen fixation, a key step in the global nitrogen cycle. Three homologous nitrogenases have been identified to date, along with several structural and/or functional homologs of this enzyme that are involved in nitrogenase assembly, bacteriochlorophyll biosynthesis and methanogenic process, respectively. In this article, we provide an overview of the structures and functions of nitrogenase and its homologs, which highlights the similarity and disparity of this uniquely versatile group of enzymes.

Coordinated expression of fdxD and molybdenum nitrogenase genes promotes nitrogen fixation by Rhodobacter capsulatus in the presence of oxygen

Rhodobacter capsulatus is able to grow with N2 as the sole nitrogen source using either a molybdenum-dependent or a molybdenum-free iron-only nitrogenase whose expression is strictly inhibited by ammonium. Disruption of the fdxD gene, which is located directly upstream of the Mo-nitrogenase genes, nifHDK, abolished diazotrophic growth via Mo-nitrogenase at oxygen concentrations still tolerated by the wild type, thus demonstrating the importance of FdxD under semiaerobic conditions. In contrast, FdxD was not beneficial for diazotrophic growth depending on Fe-nitrogenase. These findings suggest that the 2Fe2S ferredoxin FdxD specifically supports the Mo-nitrogenase system, probably by protecting Mo-nitrogenase against oxygen, as previously shown for its Azotobacter vinelandii counterpart, FeSII. Expression of fdxD occurred under nitrogen-fixing conditions, but not in the presence of ammonium. Expression of fdxD strictly required NifA1 and NifA2, the transcriptional activators of the Mo-nitrogenase genes, but not AnfA, the transcriptional activator of the Fe-nitrogenase genes. Expression of the fdxD and nifH genes, as well as the FdxD and NifH protein levels, increased with increasing molybdate concentrations. Molybdate induction of fdxD was independent of the molybdate-sensing regulators MopA and MopB, which repress anfA transcription at micromolar molybdate concentrations. In this report, we demonstrate the physiological relevance of an fesII-like gene, fdxD, and show that the cellular nitrogen and molybdenum statuses are integrated to control its expression.

Vanadium nitrogenase: a two-hit wonder?

Nitrogenase catalyzes the biological conversion of atmospheric dinitrogen to bioavailable ammonia. The molybdenum (Mo)- and vanadium (V)-dependent nitrogenases are two homologous members of this metalloenzyme family. However, despite their similarities in structure and function, the characterization of V-nitrogenase has taken a much longer and more winding path than that of its Mo-counterpart. From the initial discovery of this nitrogen-fixing system, to the recent finding of its CO-reducing capacity, V-nitrogenase has proven to be a two-hit wonder in the over-a-century-long research of nitrogen fixation. This perspective provides a brief account of the catalytic function and structural basis of V-nitrogenase, as well as a short discussion of the theoretical and practical potentials of this unique metalloenzyme.

Essential metals for nitrogen fixation in a free-living N₂-fixing bacterium: chelation, homeostasis and high use efficiency

Biological nitrogen fixation, the main source of new nitrogen to the Earth's ecosystems, is catalysed by the enzyme nitrogenase. There are three nitrogenase isoenzymes: the Mo-nitrogenase, the V-nitrogenase and the Fe-only nitrogenase. All three types require iron, and two of them also require Mo or V. Metal bioavailability has been shown to limit nitrogen fixation in natural and managed ecosystems. Here, we report the results of a study on the metal (Mo, V, Fe) requirements of Azotobacter vinelandii, a common model soil diazotroph. In the growth medium of A. vinelandii, metals are bound to strong complexing agents (metallophores) excreted by the bacterium. The uptake rates of the metallophore complexes are regulated to meet the bacterial metal requirement for diazotrophy. Under metal-replete conditions Mo, but not V or Fe, is stored intracellularly. Under conditions of metal limitation, intracellular metals are used with remarkable efficiency, with essentially all the cellular Mo and V allocated to the nitrogenase enzymes. While the Mo-nitrogenase, which is the most efficient, is used preferentially, all three nitrogenases contribute to N₂ fixation in the same culture under metal limitation. We conclude that A. vinelandii is well adapted to fix nitrogen in metal-limited soil environments.

Unique features of the nitrogenase VFe protein from Azotobacter vinelandii

Nitrogenase is an essential metalloenzyme that catalyzes the biological conversion of dinitrogen (N(2)) to ammonia (NH(3)). The vanadium (V)-nitrogenase is very similar to the "conventional" molybdenum (Mo)-nitrogenase, yet it holds unique properties of its own that may provide useful insights into the general mechanism of nitrogenase catalysis. So far, characterization of the vanadium iron (VFe) protein of Azotobacter vinelandii V-nitrogenase has been focused on 2 incomplete forms of this protein: alphabeta(2) and alpha(2)beta(2), both of which contain the small delta-subunit in minor amounts. Although these studies provided important information about the V-dependent nitrogenase system, they were hampered by the heterogeneity of the protein samples. Here, we report the isolation and characterization of a homogeneous, His-tagged form of VFe protein from A. vinelandii. This VFe protein has a previously-unsuspected, alpha(2)beta(2)delta(4)-heterooctameric composition. Further, it contains a P-cluster that is electronically and, perhaps, structurally different from the P-cluster of molybdenum iron (MoFe) protein. More importantly, it is catalytically distinct from the MoFe protein, particularly with regard to the mechanism of H(2) evolution. A detailed EPR investigation of the origins and interplays of FeV cofactor- and P-cluster-associated signals is presented herein, which lays the foundation for future kinetic and structural analysis of the VFe protein.

Molecular insights into nitrogenase FeMoco insertion--the role of His 274 and His 451 of MoFe protein alpha subunit

The final step of FeMo cofactor (FeMoco) assembly involves the insertion of FeMoco into its binding site in the molybdenum-iron (MoFe) protein of nitrogenase. Here we examine the role of His alpha274 and His alpha451 of Azotobacter vinelandii MoFe protein in this process. Our results from combined metal, activity, EPR, stability and insertion analyses show that mutations of His alpha274 and/or His alpha451, two of the histidines that belong to a so-called His triad, to small uncharged Ala specifically reduce the accumulation of FeMoco in MoFe protein. This observation indicates that the enrichment of histidines at the His triad is important for FeMoco insertion and that the His triad potentially serves as an intermediate docking point for FeMoco through transitory ligand coordination and/or electrostatic interaction.

Molecular insights into nitrogenase FeMo cofactor insertion: the role of His 362 of the MoFe protein alpha subunit in FeMo cofactor incorporation

The assembly of the complex iron-molybdenum cofactor (FeMoco) of nitrogenase molybdenum-iron (MoFe) protein has served as one of the central topics in the field of bioinorganic chemistry for decades. Here we examine the role of a MoFe protein residue (His alpha362) in FeMoco insertion, the final step of FeMoco biosynthesis where FeMoco is incorporated into its binding site in the MoFe protein. Our data from combined metal, activity and electron paramagnetic resonance analyses show that mutations of His alpha362 to small uncharged Ala or negatively charged Asp result in significantly reduced FeMoco accumulation in MoFe protein, indicating that His alpha362 plays a key role in the process of FeMoco insertion. Given the strategic location of His alpha362 at the entry point of the FeMoco insertion funnel, this residue may serve as one of the initial docking points for FeMoco insertion through transient ligand coordination and/or electrostatic interaction.

Molecular insights into nitrogenase FeMoco insertion: TRP-444 of MoFe protein alpha-subunit locks FeMoco in its binding site

Biosynthesis of the FeMo cofactor (FeMoco) of nitrogenase MoFe protein is arguably one of the most complex processes in metalloprotein biochemistry. Here we investigate the role of a MoFe protein residue (Trp-alpha444) in the final step of FeMoco assembly, which involves the insertion of FeMoco into its binding site. Mutations of this aromatic residue to small uncharged ones result in significantly decreased levels of FeMoco insertion/retention and drastically reduced activities of MoFe proteins, suggesting that Trp-alpha444 may lock the FeMoco tightly in its binding site through the sterically restricting effect of its bulky, aromatic side chain. Additionally, these mutations cause partial conversion of the P-cluster to a more open conformation, indicating a potential connection between FeMoco insertion and P-cluster assembly. Our results provide some of the initial molecular insights into the FeMoco insertion process and, moreover, have useful implications for the overall scheme of nitrogenase assembly.

Azotobacter vinelandii vanadium nitrogenase: formaldehyde is a product of catalyzed HCN reduction, and excess ammonia arises directly from catalyzed azide reduction

The Mo-nitrogenase-catalyzed reduction of both cyanide and azide results in the production of excess NH3, which is an amount of NH3 over and above that expected to be formed from the well-recognized reactions. Several suggestions about the possible sources of excess NH3 have been made, but previous attempts to characterize these reactions have met with either limited (or no) success or controversy. Because V-nitrogenase has a propensity to release partially reduced intermediates, e.g., N2H4 during N2 reduction, it was selected to probe the reduction of cyanide and azide. Sensitive assay procedures were developed and employed to monitor the production of either HCHO or CH3OH (its further two-electron-reduced product) from HCN. Like Mo-nitrogenase, V-nitrogenase suffered electron-flux inhibition by CN- (but was much less sensitive than Mo-nitrogenase), but unlike the case for Mo-nitrogenase, MgATP hydrolysis was also inhibited by CN-. V-Nitrogenase also released more of the four-electron-reduced intermediate, CH3NH2, than did Mo-nitrogenase. At high NaCN concentrations, V-nitrogenase directed a significant percentage of electron flux into excess NH3, and under these conditions, substantial amounts of HCHO, but no CH3OH, were detected for the first time. With azide, in contrast to the case for Mo-nitrogenase, both total electron flux and MgATP hydrolysis with V-nitrogenase were inhibited. V-Nitrogenase, unlike Mo-nitrogenase, showed no preference between the two-electron reduction to N2-plus-NH3 and the six-electron reduction to N2H4-plus-NH3. V-Nitrogenase formed more excess NH3, but reduction of the N2 produced by the two-electron reduction of N3(-) was not its source. Rather, it was formed directly by the eight-electron reduction of N3(-). Unlike Mo-nitrogenase, CO could not completely eliminate either cyanide or azide reduction by V-nitrogenase. CO did, however, eliminate the inhibition of both electron flux and MgATP hydrolysis by CN-, but not that caused by azide. These different responses to CO suggest different sites or modes of interaction for these two substrates with V-nitrogenase.

Functional NifD-K fusion protein in Azotobacter vinelandii is a homodimeric complex equivalent to the native heterotetrameric MoFe protein

The MoFe protein of the complex metalloenzyme nitrogenase folds as a heterotetramer containing two copies each of the homologous alpha and beta subunits, encoded by the nifD and the nifK genes respectively. Recently, the functional expression of a fusion NifD-K protein of nitrogenase was demonstrated in Azotobacter vinelandii, strongly implying that the MoFe protein is flexible as it could accommodate major structural changes, yet remain functional [M.H. Suh, L. Pulakat, N. Gavini, J. Biol. Chem. 278 (2003) 5353-5360]. This finding led us to further explore the type of interaction between the fused MoFe protein units. We aimed to determine whether an interaction exists between the two fusion MoFe proteins to form a homodimer that is equivalent to native heterotetrameric MoFe protein. Using the Bacteriomatch Two-Hybrid System, translationally fused constructs of NifD-K (fusion) with the full-length lambdaCI of the pBT bait vector and also NifD-K (fusion) with the N-terminal alpha-RNAP of the pTRG target vector were made. To compare the extent of interaction between the fused NifD-K proteins to that of the beta-beta interactions in the native MoFe protein, we proceeded to generate translationally fused constructs of NifK with the alpha-RNAP of the pTRG vector and lambdaCI protein of the pBT vector. The strength of the interaction between the proteins in study was determined by measuring the beta-galactosidase activity and extent of ampicillin resistance of the colonies expressing these proteins. This analysis demonstrated that direct protein-protein interaction exists between NifD-K fusion proteins, suggesting that they exist as homodimers. As the interaction takes place at the beta-interfaces of the NifD-K fusion proteins, we propose that these homodimers of NifD-K fusion protein may function in a similar manner as that of the heterotetrameric native MoFe protein. The observation that the extent of protein-protein interaction between the beta-subunits of the native MoFe protein in BacterioMatch Two-Hybrid System is comparable to the extent of protein-protein interaction observed between the NifD-K fusion proteins in the same system further supports this idea.

Nitrogenase reactivity with P-cluster variants

Nitrogenase is a multicomponent metalloenzyme that catalyzes the conversion of atmospheric dinitrogen to ammonia. For decades, it has been generally believed that the [8Fe-7S] P-cluster of nitrogenase component 1 is indispensable for nitrogenase activity. In this study, we identified two catalytically active P-cluster variants by activity assays, metal analysis, and EPR spectroscopic studies. Further, we showed that both P-cluster variants resemble [4Fe-4S]-like centers based on x-ray absorption spectroscopic experiments. We believe that our findings challenge the dogma that the standard P-cluster is the only cluster species capable of supporting substrate reduction at the FeMo cofactor and provide important insights into the general mechanism of nitrogenase catalysis and assembly.

Characterization of Azotobacter vinelandii nifZ deletion strains. Indication of stepwise MoFe protein assembly

The nifZ gene product (NifZ) of Azotobacter vinelandii has been implicated in MoFe protein maturation. However, its exact function in this process remains largely unknown. Here, we report a detailed biochemical/biophysical characterization of His-tagged MoFe proteins purified from A. vinelandii nifZ and nifZ/nifB deletion strains DJ1182 and YM6A (Delta nifZ and Delta nifZ Delta nifB MoFe proteins, respectively). Our data from EPR, metal, activity, and stability analyses indicate that one alpha beta subunit pair of the Delta nifZ MoFe protein contains a P cluster ([8Fe-7S]) and an iron-molybdenum cofactor (FeMoco) ([Mo-7Fe-9S-X-homocitrate]), whereas the other contains a presumed P cluster precursor, possibly comprising a pair of [4Fe-4S]-like clusters, and a vacant FeMoco site. Likewise, the Delta nifZ Delta nifB MoFe protein has the same composition as the Delta nifZ MoFe protein except for the absence of FeMoco, an effect caused by the deletion of the nifB gene. These results suggest that the MoFe protein is likely assembled stepwise, i.e. one alpha beta subunit pair of the tetrameric MoFe protein is assembled prior to the other, and that NifZ might act as a chaperone in the assembly of the second alpha beta subunit pair by facilitating a conformational rearrangement that is required for the formation of the P cluster through the condensation of two [4Fe-4S]-like clusters. The possibility of NifZ exercising its effect through the Fe protein was ruled out because the Fe proteins from nifZ and nifZ/nifB deletion strains are not defective in their normal functions. However, the detailed mechanism of how NifZ carries out its exact function in MoFe protein maturation awaits further investigation.

Comparison of Iron-Molybdenum Cofactor-deficient Nitrogenase MoFe Proteins by X-ray Absorption Spectroscopy

Nitrogenase, the enzyme system responsible for biological nitrogen fixation, is believed to utilize two unique metalloclusters in catalysis. There is considerable interest in understanding how these metalloclusters are assembled in vivo. It has been presumed that immature iron-molybdenum cofactor-deficient nitrogenase MoFe proteins contain the P-cluster, although no biosynthetic pathway for the assembly of this complex cluster has been identified as yet. Through the comparison by iron K-edge x-ray absorption edge and extended fine structure analyses of cofactor-deficient MoFe proteins resulting from nifH and nifB deletion strains of Azotobacter vinelandii, a novel [Fe-S] cluster is identified in the DeltanifH MoFe protein. The iron-iron scattering displayed by the DeltanifH MoFe protein is more similar to that of a standard [Fe(4)S(4)]-containing protein than that of the DeltanifB MoFe protein, which is shown to contain a "normal" P-cluster. The iron-sulfur scattering of the DeltanifH MoFe protein, however, indicates differences in its cluster from an [Fe(4)S(4)](Cys)(4) site that may be consistent with the presence of either oxygenic or nitrogenic ligation. Based on these results, models for the [Fe-S] center in the DeltanifH MoFe protein are constructed, the most likely of which consist of two separate [Fe(4)S(4)] sites, each with some non-cysteinyl coordination. This type of model suggests that the P-cluster is formed by the condensation of two [Fe(4)S(4)] fragments, possibly concomitant with Fe protein (NifH)-induced conformational change.

Oxidation of organic sulfides by vanadium haloperoxidase model complexes

In addition to halide oxidation, the vanadium haloperoxidases are capable of oxidizing sulfides to sulfoxides. Four vanadium complexes with tripodal amine ligands, K[VO(O(2))(heida)] (1), VO(2)(bpg) (2), K[VO(2)(ada)] (3), and K(2)[VO(O(2))(nta)] (4), previously shown to perform bromide oxidation (Colpas, G. J.; Hamstra, B. J.; Kampf, J. W.; Pecoraro, V. L. J. Am. Chem. Soc. 1996, 118, 3469-3477), have now been shown to oxidize aryl alkyl sulfides to the corresponding sulfoxides. The oxidation was observed by the disappearance of thioanisole's ultraviolet absorption at 290 nm, by the change in the aromatic region of the (1)H NMR spectrum of the sulfides, and by changes in the complexes' (51)V NMR spectra. The amount of methyl phenyl sulfide oxidized in 3 h was 1000 equiv (per metal complex). The oxidation product is almost exclusively sulfoxide, with very little sulfone (less than 3% over a 3 h experiment) formed. This is consistent with an electrophilic oxidation mechanism, as had been proposed for oxidation of bromide by 1-4. The rate was found to be first order in substrate concentration, similar to the rate law observed for bromide oxidation. Unlike the bromide oxidation, the equivalent of acid required for peroxovanadium complex activation is not consumed. The complexes 1-4 are not reactive with styrene or cyclooctene. The relevance of these reactions to the mechanism of the vanadium haloperoxidases and, more generally, peroxovanadium oxygenation of sulfides will be discussed.

The FeMoco-deficient MoFe protein produced by a nifH deletion strain of Azotobacter vinelandii shows unusual P-cluster features

The His-tag MoFe protein expressed by the nifH deletion strain Azotobacter vinelandii DJ1165 (Delta(nifH) MoFe protein) was purified in large quantity. The alpha(2)beta(2) tetrameric Delta(nifH) MoFe protein is FeMoco-deficient based on metal analysis and the absence of the S = 3/2 EPR signal, which arises from the FeMo cofactor center in wild-type MoFe protein. The Delta(nifH) MoFe protein contains 18.6 mol Fe/mol and, upon reduction with dithionite, exhibits an unusually strong S = 1/2 EPR signal in the g approximately 2 region. The indigo disulfonate-oxidized Delta(nifH) MoFe protein does not show features of the P(2+) state of the P-cluster of the Delta(nifB) MoFe protein. The oxidized Delta(nifH) MoFe protein is able to form a specific complex with the Fe protein containing the [4Fe-4S](1+) cluster and facilitates the hydrolysis of MgATP within this complex. However, it is not able to accept electrons from the [4Fe-4S](1+) cluster of the Fe protein. Furthermore, the dithionite-reduced Delta(nifH) MoFe can be further reduced by Ti(III) citrate, which is quite unexpected. These unusual catalytic and spectroscopic properties might indicate the presence of a P-cluster precursor or a P-cluster trapped in an unusual conformation or oxidation state.

Thiofunctional vanadium complexes

The neutral tetradentate ligand 1,6-bis(2'-pyridyl)-2,5-dithiahexane (N(2)S(2)), containing two thioether functions, reacts with [VX(2)L(4)] (X = Br, L(4) = 2 tmeda (tmeda = Me(2)NCH(2)CH(2)NMe(2)); X = I, L = tetrahydrofuran (THF)) and [VX(3)(THF)(3)] (X = Br, I) to form the complexes [VX(2)(N(2)S(2))] (1) and [VX(2)(N(2)S(2))]X (2), respectively. [V(2)(mu-Cl)(3)(THF)(6)]I and N(2)S(2) yield the V(IV) complex [VOCl(N(2)S(2)]I (3). The pentadentate, dianionic ligand 2,6-bis(2'-mercaptophenylthio)dimethylpyridine, NS(2)S'(2)(2-), which contains two thioether (S) and two thiophenolate (S') functions, reacts with [VBr(3)(THF)(3)] to afford [VBr(NS(2)S'(2))] (4). The complex [VO(Cl)S'NS'] (5; H(2)S'NS' is the Schiff base formed between o-mercaptoaniline and o-mercaptobenzaldehyde) is obtained by redox interaction between [VCl(3)(THF)(3)] and 2,2'-dithiodibenzaldehyde in the presence of o-mercaptoaniline. The crystal and molecular structures have been obtained for 3. THF, 4. THF, and 5. n-C(5)H(12). The relevance of these compounds and their formation for the interaction between vanadium and thiofunctional biomolecules is addressed.

The Fe-only nitrogenase and the Mo nitrogenase from Rhodobacter capsulatus: a comparative study on the redox properties of the metal clusters present in the dinitrogenase components

The dinitrogenase component proteins of the conventional Mo nitrogenase (MoFe protein) and of the alternative Fe-only nitrogenase (FeFe protein) were both isolated and purified from Rhodobacter capsulatus, redox-titrated according to the same procedures and subjected to an EPR spectroscopic comparison. In the course of an oxidative titration of the MoFe protein (Rc1Mo) three significant S = 1/2 EPR signals deriving from oxidized states of the P-cluster were detected: (1) a rhombic signal (g = 2.07, 1.96 and 1.83), which showed a bell-shaped redox curve with midpoint potentials (Em) of -195 mV (appearance) and -30 mV (disappearance), (2) an axial signal (g(parallel) = 2.00, g perpendicular = 1.90) with almost identical redox properties and (3) a second rhombic signal (g = 2.03, 2.00, 1.90) at higher redox potentials (> 100 mV). While the 'low-potential' rhombic signal and the axial signal have been both attributed to the one-electron-oxidized P-cluster (P1+) present in two conformationally different proteins, the 'high-potential' rhombic signal has been suggested rather to derive from the P3+ state. Upon oxidation, the FeFe protein (Rc1Fe) exhibited three significant S = 1/2 EPR signals as well. However, the Rc1Fe signals strongly deviated from the MoFe protein signals, suggesting that they cannot simply be assigned to different P-cluster states. (a) The most prominent feature is an unusually broad signal at g = 2.27 and 2.06, which proved to be fully reversible and to correlate with catalytic activity. The cluster giving rise to this signal appears to be involved in the transfer of two electrons. The midpoint potentials determined were: -80 mV (appearance) and 70 mV (disappearance). (b) Under weakly acidic conditions (pH 6.4) a slightly altered EPR signal occurred. It was characterized by a shift of the g values to 2.22 and 2.05 and by the appearance of an additional negative absorption-shaped peak at g = 1.86. (c) A very narrow rhombic EPR signal at g = 2.00, 1.98 and 1.96 appeared at positive redox potentials (Em = 80 mV, intensity maximum at 160 mV). Another novel S = 1/2 signal at g = 1.96, 1.92 and 1.77 was observed on further, enzymatic reduction of the dithionite-reduced state of Rc1Fe with the dinitrogenase reductase component (Rc2Fe) of the same enzyme system (turnover conditions in the presence of N2 and ATP). When the Rc1Mo protein was treated analogously, neither this 'turnover signal' nor any other S = 1/2 signal were detectable. All Rc1Fe-specific EPR signals detected are discussed and tentatively assigned with special consideration of the reference spectra obtained from Rc1Mo preparations.

Activation of vanadium nitrogenase expression in Azotobacter vinelandii DJ54 revertant in the presence of molybdenum

Azotobacter vinelandii carries three different and genetically distinct nitrogenase systems on its chromosome. Expression of all three nitrogenases is repressed by high concentrations of fixed nitrogen. Expression of individual nitrogenase systems is under the control of specific metal availability. We have isolated a novel type of A. vinelandii DJ54 revertant, designated A. vinelandii BG54, which carries a defined deletion in the nifH gene and is capable of diazotrophic growth in the presence of molybdenum. Inactivation of nifDK has no effect on growth of this mutant strain in nitrogen-free medium suggesting that products of the nif system are not involved in supporting diazotrophic growth of A. vinelandii BG54. Similar to the wild type, A. vinelandii BG54 is also sensitive to 1 mM tungsten. Tn5-B21 mutagenesis to inactivate the genes specific to individual systems revealed that the structural genes for vnf nitrogenase are required for diazotrophic growth of A. vinelandii BG54. Analysis of promoter activity of different nif systems revealed that the vnf promoter is activated in A. vinelandii BG54 in the presence of molybdenum. Based on these data we conclude that A. vinelandii BG54 strain utilizes vnf nitrogenase proteins to support its diazotrophic growth.

Identification of an Fe protein residue (Glu146) of Azotobacter vinelandii nitrogenase that is specifically involved in FeMo cofactor insertion

The Fe protein of nitrogenase has three separate functions. Much is known about the regions of the protein that are critical to its function as an electron donor to the MoFe protein, but almost nothing is known about the regions of the protein that are critical to its functions in either FeMo cofactor biosynthesis or FeMo cofactor insertion. Using computer modeling and information obtained from Fe protein mutants that were made decades ago by chemical mutagenesis, we targeted a surface residue Glu(146) as potentially being involved in FeMo cofactor biosynthesis and/or insertion. The Azotobacter vinelandii strain expressing an E146D Fe protein variant grows at approximately 50% of the wild type rate. The purified E146D Fe protein is fully functional as an electron donor to the MoFe protein, but the MoFe protein synthesized by that strain is partially ( approximately 50%) FeMo cofactor-deficient. The E146D Fe protein is fully functional in an in vitro FeMo cofactor biosynthesis assay, and the strain expressing this protein accumulates "free" FeMo cofactor. Assays that compared the ability of wild type and E146D Fe proteins to participate in FeMo cofactor insertion demonstrate, however, that the mutant is severely altered in this last reaction. This is the first known mutation that only influences the insertion reaction.

Vanadium nitrogenase

The topic, vanadium nitrogenase, is reviewed with respect to biological characteristics and findings on its structure and functions. Structural models (vanadium complexes containing ligands related to the active center in the iron-vanadium cofactor) and functional models for the reductive protonation of dinitrogen, the activation of alkynes and reductive C-C coupling of isocyanides are addressed.

N2 fixation by Streptomyces thermoautotrophicus involves a molybdenum-dinitrogenase and a manganese-superoxide oxidoreductase that couple N2 reduction to the oxidation of superoxide produced from O2 by a molybdenum-CO dehydrogenase

N2 fixation by Streptomyces thermoautotrophicus follows the equation N2 + 4-12MgATP + 8H+ + 8e- --> 2NH3 + H2 + 4-12MgADP + 4-12Pi and exhibits features which are not obvious in the diazotrophic bacteria studied so far. The reaction is coupled to the oxidation of carbon monoxide (CO) by a molybdenum-containing CO dehydrogenase that transfers the electrons derived from CO oxidation to O2, thereby producing superoxide anion radicals (O-2). A manganese-containing superoxide oxidoreductase reoxidizes the O-2 anions to O2 and transfers the electrons to a MoFeS-dinitrogenase for the reduction of N2 to ammonium. Among the most striking properties of the S. thermoautotrophicus nitrogenase system are the dependence on O2 and O-2, the complete insensitivity of all components involved toward O2 and H2O2, the inability to reduce ethine or ethene, and a low MgATP requirement. In addition, the subunit structure of the S. thermoautotrophicus nitrogenase components and the polypeptides involved seem to be dissimilar from the known nitrogenases.

Characterization of VNFG, the delta subunit of the vnf-encoded apodinitrogenase from Azotobacter vinelandii. Implications for its role in the formation of functional dinitrogenase 2

The vnf-encoded apodinitrogenase (apodinitrogenase 2) from Azotobacter vinelandii is an alpha2beta2delta2 hexamer. The delta subunit (the VNFG protein) has been characterized in order to further delineate its function in the nitrogenase 2 enzyme system. Two species of VNFG were observed in cell-free extracts resolved on anoxic native gels; one is composed of VNFG associated with the VNFDK polypeptides, and the other is a homodimer of the VNFG protein. Both species of VNFG are observed in extracts of A. vinelandii strains that accumulate dinitrogenase 2, whereas extracts of strains impaired in the biosynthetic pathway of the iron-vanadium cofactor (FeV-co) that accumulate apodinitrogenase 2 (a catalytically inactive form of dinitrogenase 2 that lacks FeV-co) exhibit only the VNFG dimer on native gels. FeV-co and nucleotide are required for the stable association of VNFG with the VNFDK polypeptides; this stable association can be correlated with the formation of active dinitrogenase 2. The iron-molybdenum cofactor was unable to replace FeV-co in promoting the stable association of VNFG with VNFDK. FeV-co specifically associates with the VNFG dimer in vitro to form a complex of unknown stoichiometry; combination of this VNFG-FeV-co species with apodinitrogenase 2 results in its reconstitution to dinitrogenase 2. The results presented here suggest that VNFG is required for processing apodinitrogenase 2 to functional dinitrogenase 2.