A new type of metalloprotein: The Mo storage protein from azotobacter vinelandii contains a polynuclear molybdenum-oxide cluster

Chlamydomonas reinhardtii-A Reference Microorganism for Eukaryotic Molybdenum Metabolism

Molybdenum (Mo) is vital for the activity of a small but essential group of enzymes called molybdoenzymes. So far, specifically five molybdoenzymes have been discovered in eukaryotes: nitrate reductase, sulfite oxidase, xanthine dehydrogenase, aldehyde oxidase, and mARC. In order to become biologically active, Mo must be chelated to a pterin, forming the so-called Mo cofactor (Moco). Deficiency or mutation in any of the genes involved in Moco biosynthesis results in the simultaneous loss of activity of all molybdoenzymes, fully or partially preventing the normal development of the affected organism. To prevent this, the different mechanisms involved in Mo homeostasis must be finely regulated. Chlamydomonas reinhardtii is a unicellular, photosynthetic, eukaryotic microalga that has produced fundamental advances in key steps of Mo homeostasis over the last 30 years, which have been extrapolated to higher organisms, both plants and animals. These advances include the identification of the first two molybdate transporters in eukaryotes (MOT1 and MOT2), the characterization of key genes in Moco biosynthesis, the identification of the first enzyme that protects and transfers Moco (MCP1), the first characterization of mARC in plants, and the discovery of the crucial role of the nitrate reductase-mARC complex in plant nitric oxide production. This review aims to provide a comprehensive summary of the progress achieved in using C. reinhardtii as a model organism in Mo homeostasis and to propose how this microalga can continue improving with the advancements in this field in the future.

Overview of physiological, biochemical, and regulatory aspects of nitrogen fixation in Azotobacter vinelandii

Understanding how Nature accomplishes the reduction of inert nitrogen gas to form metabolically tractable ammonia at ambient temperature and pressure has challenged scientists for more than a century. Such an understanding is a key aspect toward accomplishing the transfer of the genetic determinants of biological nitrogen fixation to crop plants as well as for the development of improved synthetic catalysts based on the biological mechanism. Over the past 30 years, the free-living nitrogen-fixing bacterium Azotobacter vinelandii emerged as a preferred model organism for mechanistic, structural, genetic, and physiological studies aimed at understanding biological nitrogen fixation. This review provides a contemporary overview of these studies and places them within the context of their historical development.

Interweaving Disciplines to Advance Chemistry: Applying Polyoxometalates in Biology

This Viewpoint brings awareness of the challenges and subsequent breakthroughs at the intersection of different disciplines, illustrated by the example of the influence biological entities exerted on a huge class of inorganic coordination compounds, called polyoxometalates (POMs). We highlight the possible effects of biological systems on POMs that need to be considered, thereby emphasizing the depth and complexity of interdisciplinary work. We map POMs’ structural, electrochemical, and stability properties in the presence of biomolecules and stress the potential challenges related to inorganic coordination chemistry carried out in biological systems. This Viewpoint shows that new chemistry is available at the intersections between disciplines and aims to guide the community toward a discussion about current as well as future trends in truly interdisciplinary work.

We discuss the investigation of polyoxometalates in biological systems as one future direction of chemistry. Highly interesting, new, and sometimes spectacular findings and applications can be obtained from correctly carried out interdisciplinary research. In this Viewpoint, the challenges of truly interdisciplinary work and concepts for overcoming boundaries while working on intertwining disciplines are discussed.

Biosynthesis of Nitrogenase Cofactors

Nitrogenase harbors three distinct metal prosthetic groups that are required for its activity. The simplest one is a [4Fe-4S] cluster located at the Fe protein nitrogenase component. The MoFe protein component carries an [8Fe-7S] group called P-cluster and a [7Fe-9S-C-Mo-R-homocitrate] group called FeMo-co. Formation of nitrogenase metalloclusters requires the participation of the structural nitrogenase components and many accessory proteins, and occurs both in situ, for the P-cluster, and in external assembly sites for FeMo-co. The biosynthesis of FeMo-co is performed stepwise and involves molecular scaffolds, metallochaperones, radical chemistry, and novel and unique biosynthetic intermediates. This review provides a critical overview of discoveries on nitrogenase cofactor structure, function, and activity over the last four decades.

Molybdate pumping into the molybdenum storage protein via an ATP-powered piercing mechanism

The molybdenum storage protein (MoSto) deposits large amounts of molybdenum as polyoxomolybdate clusters in a heterohexameric (αβ)₃ cage-like protein complex under ATP consumption. Here, we suggest a unique mechanism for the ATP-powered molybdate pumping process based on X-ray crystallography, cryoelectron microscopy, hydrogen-deuterium exchange mass spectrometry, and mutational studies of MoSto from Azotobacter vinelandii First, we show that molybdate, ATP, and Mg²⁺ consecutively bind into the open ATP-binding groove of the β-subunit, which thereafter becomes tightly locked by fixing the previously disordered N-terminal arm of the α-subunit over the β-ATP. Next, we propose a nucleophilic attack of molybdate onto the γ-phosphate of β-ATP, analogous to the similar reaction of the structurally related UMP kinase. The formed instable phosphoric-molybdic anhydride becomes immediately hydrolyzed and, according to the current data, the released and accelerated molybdate is pressed through the cage wall, presumably by turning aside the Metβ149 side chain. A structural comparison between MoSto and UMP kinase provides valuable insight into how an enzyme is converted into a molecular machine during evolution. The postulated direct conversion of chemical energy into kinetic energy via an activating molybdate kinase and an exothermic pyrophosphatase reaction to overcome a proteinous barrier represents a novelty in ATP-fueled biochemistry, because normally, ATP hydrolysis initiates large-scale conformational changes to drive a distant process.

Genetic and Biochemical Analysis of the Azotobacter vinelandii Molybdenum Storage Protein

The N₂ fixing bacterium Azotobacter vinelandii carries a molybdenum storage protein, referred to as MoSto, able to bind 25-fold more Mo than needed for maximum activity of its Mo nitrogenase. Here we have investigated a plausible role of MoSto as obligate intermediate in the pathway that provides Mo for the biosynthesis of nitrogenase iron-molybdenum cofactor (FeMo-co). The in vitro FeMo-co synthesis and insertion assay demonstrated that purified MoSto functions as Mo donor and that direct interaction with FeMo-co biosynthetic proteins stimulated Mo donation. The phenotype of an A. vinelandii strain lacking the MoSto subunit genes (ΔmosAB) was analyzed. Consistent with its role as storage protein, the ΔmosAB strain showed severe impairment to accumulate intracellular Mo and lower resilience than wild type to Mo starvation as demonstrated by decreased in vivo nitrogenase activity and competitive growth index. In addition, it was more sensitive than the wild type to diazotrophic growth inhibition by W. The ΔmosAB strain was found to readily derepress vnfDGK upon Mo step down, in contrast to the wild type that derepressed Vnf proteins only after prolonged Mo starvation. The ΔmosAB mutation was then introduced in a strain lacking V and Fe-only nitrogenase structural genes (Δvnf Δanf) to investigate possible compensations from these alternative systems. When grown in Mo-depleted medium, the ΔmosAB and mosAB ⁺ strains showed low but similar nitrogenase activities regardless of the presence of Vnf proteins. This study highlights the selective advantage that MoSto confers to A. vinelandii in situations of metal limitation as those found in many soil ecosystems. Such a favorable trait should be included in the gene complement of future nitrogen fixing plants.

Genomic profiling of four cultivated Candidatus Nitrotoga spp. predicts broad metabolic potential and environmental distribution

Nitrite-oxidizing bacteria (NOB) play a critical role in the mitigation of nitrogen pollution by metabolizing nitrite to nitrate, which is removed via assimilation, denitrification, or anammox. Recent studies showed that NOB are phylogenetically and metabolically diverse, yet most of our knowledge of NOB comes from only a few cultured representatives. Using cultivation and genomic sequencing, we identified four putative Candidatus Nitrotoga NOB species from freshwater sediments and water column samples in Colorado, USA. Genome analyses indicated highly conserved 16S rRNA gene sequences, but broad metabolic potential including genes for nitrogen, sulfur, hydrogen, and organic carbon metabolism. Genomic predictions suggested that Ca. Nitrotoga can metabolize in low oxygen or anoxic conditions, which may support an expanded environmental niche for Ca. Nitrotoga similar to other NOB. An array of antibiotic and metal resistance genes likely allows Ca. Nitrotoga to withstand environmental pressures in impacted systems. Phylogenetic analyses highlighted a deeply divergent nitrite oxidoreductase alpha subunit (NxrA), suggesting a novel evolutionary trajectory for Ca. Nitrotoga separate from any other NOB and further revealing the complex evolutionary history of nitrite oxidation in the bacterial domain. Ca. Nitrotoga-like 16S rRNA gene sequences were prevalent in globally distributed environments over a range of reported temperatures. This work considerably expands our knowledge of the Ca. Nitrotoga genus and suggests that their contribution to nitrogen cycling should be considered alongside other NOB in wide variety of habitats.

The Molybdenum Storage Protein: A soluble ATP hydrolysis-dependent molybdate pump

A continuous FeMo cofactor supply for nitrogenase maturation is ensured in Azotobacter vinelandii by developing a cage-like molybdenum storage protein (MoSto) capable to store ca. 120 molybdate molecules ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mtext>MoO</mml:mtext> <mml:mrow><mml:mn>4</mml:mn></mml:mrow> <mml:mrow><mml:mn>2</mml:mn> <mml:mo>-</mml:mo></mml:mrow> </mml:msubsup> </mml:math> ) as discrete polyoxometalate (POM) clusters. To gain mechanistic insight into this process, MoSto was characterized by Mo and ATP/ADP content, structural, and kinetic analysis. We defined three functionally relevant states specified by the presence of both ATP/ADP and POM clusters (MoSto_funct ), of only ATP/ADP (MoSto_basal ) and of neither ATP/ADP nor POM clusters (MoSto_zero ), respectively. POM clusters are only produced when ATP is hydrolyzed to ADP and phosphate. V_max was ca. 13 μmol_phosphate ·min^-1 ·mg^-1 and K_m for molybdate and ATP/Mg²⁺ in the low micromolar range. ATP hydrolysis presumably proceeds at subunit α, inferred from a highly occupied α-ATP/Mg²⁺ and a weaker occupied β-ATP/no Mg²⁺ -binding site found in the MoSto_funct structure. Several findings indicate that POM cluster storage is separated into a rapid ATP hydrolysis-dependent molybdate transport across the protein cage wall and a slow molybdate assembly induced by combined auto-catalytic and protein-driven processes. The cage interior, the location of the POM cluster depot, is locked in all three states and thus not rapidly accessible for molybdate from the outside. Based on V_max , the entire Mo storage process should be completed in less than 10 s but requires, according to the molybdate content analysis, ca. 15 min. Long-time incubation of MoSto_basal with nonphysiological high molybdate amounts implicates an equilibrium in and outside the cage and POM cluster self-formation without ATP hydrolysis. DATABASES: The crystal structures MoSto in the MoSto-F6, MoSto-F7, MoSto_basal , MoSto_zero , and MoSto-F1_vitro states were deposited to PDB under the accession numbers PDB 6GU5, 6GUJ, 6GWB, 6GWV, and 6GX4.

Bacterial PerO Permeases Transport Sulfate and Related Oxyanions

Rhodobacter capsulatus synthesizes the high-affinity ABC transporters CysTWA and ModABC to specifically import the chemically related oxyanions sulfate and molybdate, respectively. In addition, R. capsulatus has the low-affinity permease PerO acting as a general oxyanion transporter, whose elimination increases tolerance to molybdate and tungstate. Although PerO-like permeases are widespread in bacteria, their function has not been examined in any other species to date. Here, we present evidence that PerO permeases from the alphaproteobacteria Agrobacterium tumefaciens, Dinoroseobacter shibae, Rhodobacter sphaeroides, and Sinorhizobium meliloti and the gammaproteobacterium Pseudomonas stutzeri functionally substitute for R. capsulatus PerO in sulfate uptake and sulfate-dependent growth, as shown by assimilation of radioactively labeled sulfate and heterologous complementation. Disruption of perO genes in A. tumefaciens, R. sphaeroides, and S. meliloti increased tolerance to tungstate and, in the case of R. sphaeroides, to molybdate, suggesting that heterometal oxyanions are common substrates of PerO permeases. This study supports the view that bacterial PerO permeases typically transport sulfate and related oxyanions and, hence, form a functionally conserved permease family.IMPORTANCE Despite the widespread distribution of PerO-like permeases in bacteria, our knowledge about PerO function until now was limited to one species, Rhodobacter capsulatus In this study, we showed that PerO proteins from diverse bacteria are functionally similar to the R. capsulatus prototype, suggesting that PerO permeases form a conserved family whose members transport sulfate and related oxyanions.

Molybdenum and phosphorus limitation of moss-associated nitrogen fixation in boreal ecosystems

Biological nitrogen fixation (BNF) performed by moss-associated cyanobacteria is one of the main sources of new nitrogen (N) input in pristine, high-latitude ecosystems. Yet, the nutrients that limit BNF remain elusive. Here, we tested whether this important ecosystem function is limited by the availability of molybdenum (Mo), phosphorus (P), or both. BNF in dominant mosses was measured with the acetylene reduction assay (ARA) at different time intervals following Mo and P additions, in both laboratory microcosms with mosses from a boreal spruce forest and field plots in subarctic tundra. We further used a ¹⁵ N₂ tracer technique to assess the ARA to N₂ fixation conversion ratios at our subarctic site. BNF was up to four-fold higher shortly after the addition of Mo, in both the laboratory and field experiments. A similar positive response to Mo was found in moss colonizing cyanobacterial biomass. As the growing season progressed, nitrogenase activity became progressively more P limited. The ARA : ¹⁵ N₂ ratios increased with increasing Mo additions. These findings show that N₂ fixation activity as well as cyanobacterial biomass in dominant feather mosses from boreal forests and subarctic tundra are limited by Mo availability.

A survey of the different roles of polyoxometalates in their interaction with amino acids, peptides and proteins

This perspective provides a comprehensive description of the different roles of POMs in their interaction with relevant biological molecules.

Molybdenum and Tungsten

Molybdenum (Mo) is an essential micronutrient for the majority of organisms ranging from bacteria to animals. To fulfil its biological role, it is incorporated into a pterin-based Mo-cofactor (Moco) and can be found in the active centre of more than 50 enzymes that are involved in key reactions of carbon, nitrogen and sulfur metabolism. Five of the Mo-enzymes are present in eukaryotes: nitrate reductase (NR), sulfite oxidase (SO), aldehyde oxidase (AO), xanthine oxidase (XO) and the amidoxime-reducing component (mARC). Cells acquire Mo in form of the oxyanion molybdate using specific molybdate transporters. In bacteria, molybdate transport is an extensively studied process and is mediated mainly by the ATP-binding cassette system ModABC. In contrast, in eukaryotes, molybdate transport is poorly understood since specific molybdate transporters remained unknown until recently. Two rather distantly related families of proteins, MOT1 and MOT2, are involved in eukaryotic molybdate transport. They each feature high-affinity molybdate transporters that regulate the intracellular concentration of Mo and thus control activity of Mo-enzymes. The present chapter presents an overview of the biological functions of Mo with special focus on recent data related to its uptake, binding and storage.

Structural diversity of polyoxomolybdate clusters along the three-fold axis of the molybdenum storage protein

The molybdenum storage protein (MoSto) can store more than 100 Mo or W atoms as discrete polyoxometalate (POM) clusters. Here, we describe the three POM cluster sites along the threefold axis of the protein complex based on four X-ray structures with slightly different polyoxomolybdate compositions between 1.35 and 2 Å resolution. In contrast to the Moα-out binding site occupied by an Mo3 cluster, the Moα-in and Moβ binding sites contain rather weak and non-uniform electron density for the Mo atoms (but clearly identifiable by anomalous data), suggesting the presence of POM cluster ensembles and/or degradation products of larger aggregates. The "Moα-in cluster ensemble" was interpreted as an antiprism-like Mo6 species superimposed with an Mo7 pyramide and the "Moβ cluster ensemble" as an Mo13 cluster (present mostly in a degraded form) composed of a pyramidal Mo7 and a Mo3 building block linked by three spatially separated MoOx units. Inside the ball-shaped Mo13 cluster sits an occluded central atom, perhaps a metal ion. POM cluster formation at the Moα-in and Moβ sites appears to be driven by filtering out and binding/protecting self-assembled transient species complementary to the protein template.

Redox chemistry of molybdenum in natural waters and its involvement in biological evolution

The transition element molybdenum (Mo) possesses diverse valances (+II to +VI), and is involved in forming cofactors in more than 60 enzymes in biology. Redox switching of the element in these enzymes catalyzes a series of metabolic reactions in both prokaryotes and eukaryotes, and the element therefore plays a fundamental role in the global carbon, nitrogen, and sulfur cycling. In the present oxygenated waters, oxidized Mo(VI) predominates thermodynamically, whilst reduced Mo species are mainly confined within specific niches including cytoplasm. Only recently has the reduced Mo(V) been separated from Mo(VI) in sulfidic mats and even in some reducing waters. Given the presence of reduced Mo(V) in contemporary anaerobic habitats, it seems that reduced Mo species were present in the ancient reducing ocean (probably under both ferruginous and sulfidic conditions), prompting the involvement of Mo in enzymes including nitrogenase and nitrate reductase. During the global transition to oxic conditions, reduced Mo species were constrained to specific anaerobic habitats, and efficient uptake systems of oxidized Mo(VI) became a selective advantage for current prokaryotic and eukaryotic cells. Some prokaryotes are still able to directly utilize reduced Mo if any exists in ambient environments. In total, this mini-review describes the redox chemistry and biogeochemistry of Mo over the Earth’s history.

Molybdenum limitation of microbial nitrogen assimilation in aquatic ecosystems and pure cultures

Molybdenum (Mo) is an essential micronutrient for biological assimilation of nitrogen gas and nitrate because it is present in the cofactors of nitrogenase and nitrate reductase enzymes. Although Mo is the most abundant transition metal in seawater (107 nM), it is present in low concentrations in most freshwaters, typically <20 nM. In 1960, it was discovered that primary productivity was limited by Mo scarcity (2–4 nM) in Castle Lake, a small, meso-oligotrophic lake in northern California. Follow up studies demonstrated that Mo also limited primary productivity in lakes in New Zealand, Alaska, and the Sierra Nevada. Research in the 1970s and 1980s showed that Mo limited primary productivity and nitrate uptake in Castle Lake only during periods of the growing season when nitrate concentrations were relatively high because ammonium assimilation does not require Mo. In the years since, research has shifted to investigate whether Mo limitation also occurs in marine and soil environments. Here we review studies of Mo limitation of nitrogen assimilation in natural microbial communities and pure cultures. We also summarize new data showing that the simultaneous addition of Mo and nitrate causes increased activity of proteins involved in nitrogen assimilation in the hypolimnion of Castle Lake when ammonium is scarce. Furthermore, we suggest that meter-scale Mo and oxygen depth profiles from Castle Lake are consistent with the hypothesis that nitrogen-fixing cyanobacteria in freshwater periphyton communities have higher Mo requirements than other microbial communities. Finally, we present topics for future research related to Mo bioavailability through time and with changing oxidation state.

Nature's polyoxometalate chemistry: X-ray structure of the Mo storage protein loaded with discrete polynuclear Mo-O clusters

Some N(2)-fixing bacteria prolong the functionality of nitrogenase in molybdenum starvation by a special Mo storage protein (MoSto) that can store more than 100 Mo atoms. The presented 1.6 Å X-ray structure of MoSto from Azotobacter vinelandii reveals various discrete polyoxomolybdate clusters, three covalently and three noncovalently bound Mo(8), three Mo(5-7), and one Mo(3) clusters, and several low occupied, so far undefinable clusters, which are embedded in specific pockets inside a locked cage-shaped (αβ)(3) protein complex. The structurally identical Mo(8) clusters (three layers of two, four, and two MoO(n) octahedra) are distinguishable from the [Mo(8)O(26)](4-) cluster formed in acidic solutions by two displaced MoO(n) octahedra implicating three kinetically labile terminal ligands. Stabilization in the covalent Mo(8) cluster is achieved by Mo bonding to Hisα156-N(ε2) and Gluα129-O(ε1). The absence of covalent protein interactions in the noncovalent Mo(8) cluster is compensated by a more extended hydrogen-bond network involving three pronounced histidines. One displaced MoO(n) octahedron might serve as nucleation site for an inhomogeneous Mo(5-7) cluster largely surrounded by bulk solvent. In the Mo(3) cluster located on the 3-fold axis, the three accurately positioned His140-N(ε2) atoms of the α subunits coordinate to the Mo atoms. The formed polyoxomolybdate clusters of MoSto, not detectable in bulk solvent, are the result of an interplay between self- and protein-driven assembly processes that unite inorganic supramolecular and protein chemistry in a host-guest system. Template, nucleation/protection, and catalyst functions of the polypeptide as well as perspectives for designing new clusters are discussed.

Molybdenum isotope fractionation by cyanobacterial assimilation during nitrate utilization and N₂ fixation

We measured the δ⁹⁸Mo of cells and media from molybdenum (Mo) assimilation experiments with the freshwater cyanobacterium Anabaena variabilis, grown with nitrate as a nitrogen (N) source or fixing atmospheric N₂. This organism uses a Mo-based nitrate reductase during nitrate utilization and a Mo-based dinitrogenase during N₂ fixation under culture conditions here. We also demonstrate that it has a high-affinity Mo uptake system (ModABC) similar to other cyanobacteria, including marine N₂-fixing strains. Anabaena variabilis preferentially assimilated light isotopes of Mo in all experiments, resulting in fractionations of -0.2‰ to -1.0‰ ± 0.2‰ between cells and media (ε(cells-media)), extending the range of biological Mo fractionations previously reported. The fractionations were internally consistent within experiments, but varied with the N source utilized and for different growth phases sampled. During growth on nitrate, A. variabilis consistently produced fractionations of -0.3 ± 0.1‰ (mean ± standard deviation between experiments). When fixing N₂, A. variabilis produced fractionations of -0.9 ± 0.1‰ during exponential growth, and -0.5 ± 0.1‰ during stationary phase. This pattern is inconsistent with a simple kinetic isotope effect associated with Mo transport, because Mo is likely transported through the ModABC uptake system under all conditions studied. We present a reaction network model for Mo isotope fractionation that demonstrates how Mo transport and storage, coordination changes during enzymatic incorporation, and the distribution of Mo inside the cell could all contribute to the total biological fractionations. Additionally, we discuss the potential importance of biologically incorporated Mo to organic matter-bound Mo in marine sediments.

A Rhodobacter capsulatus member of a universal permease family imports molybdate and other oxyanions

Molybdenum (Mo) is an important trace element that is toxic at high concentrations. To resolve the mechanisms underlying Mo toxicity, Rhodobacter capsulatus mutants tolerant to high Mo concentrations were isolated by random transposon Tn5 mutagenesis. The insertion sites of six independent isolates mapped within the same gene predicted to code for a permease of unknown function located in the cytoplasmic membrane. During growth under Mo-replete conditions, the wild-type strain accumulated considerably more Mo than the permease mutant. For mutants defective for the permease, the high-affinity molybdate importer ModABC, or both transporters, in vivo Mo-dependent nitrogenase (Mo-nitrogenase) activities at different Mo concentrations suggested that ModABC and the permease import molybdate in nanomolar and micromolar ranges, respectively. Like the permease mutants, a mutant defective for ATP sulfurylase tolerated high Mo concentrations, suggesting that ATP sulfurylase is the main target of Mo inhibition in R. capsulatus. Sulfate-dependent growth of a double mutant defective for the permease and the high-affinity sulfate importer CysTWA was reduced compared to those of the single mutants, implying that the permease plays an important role in sulfate uptake. In addition, permease mutants tolerated higher tungstate and vanadate concentrations than the wild type, suggesting that the permease acts as a general oxyanion importer. We propose to call this permease PerO (for oxyanion permease). It is the first reported bacterial molybdate transporter outside the ABC transporter family.

Molybdenum trafficking for nitrogen fixation

The molybdenum nitrogenase is responsible for most biological nitrogen fixation, a prokaryotic metabolic process that determines the global biogeochemical cycles of nitrogen and carbon. Here we describe the trafficking of molybdenum for nitrogen fixation in the model diazotrophic bacterium Azotobacter vinelandii. The genes and proteins involved in molybdenum uptake, homeostasis, storage, regulation, and nitrogenase cofactor biosynthesis are reviewed. Molybdenum biochemistry in A. vinelandii reveals unexpected mechanisms and a new role for iron-sulfur clusters in the sequestration and delivery of molybdenum.

Coevolution of metal availability and nitrogen assimilation in cyanobacteria and algae

Marine primary producers adapted over eons to the changing chemistry of the oceans. Because a number of metalloenzymes are necessary for N assimilation, changes in the availability of transition metals posed a particular challenge to the supply of this critical nutrient that regulates marine biomass and productivity. Integrating recently developed geochemical, biochemical, and genetic evidence, we infer that the use of metals in N assimilation - particularly Fe and Mo - can be understood in terms of the history of metal availability through time. Anoxic, Fe-rich Archean oceans were conducive to the evolution of Fe-using enzymes that assimilate abiogenic NH(4)(+) and NO(2)(-). The N demands of an expanding biosphere were satisfied by the evolution of biological N(2) fixation, possibly utilizing only Fe. Trace O(2) in late Archean environments, and the eventual 'Great Oxidation Event' c. 2.3 Ga, mobilized metals such as Mo, enabling the evolution of Mo (or V)-based N(2) fixation and the Mo-dependent enzymes for NO(3)(-) assimilation and denitrification by prokaryotes. However, the subsequent onset of deep-sea euxinia, an increasingly-accepted idea, may have kept ocean Mo inventories low and depressed Fe, limiting the rate of N(2) fixation and the supply of fixed N. Eukaryotic ecosystems may have been particularly disadvantaged by N scarcity and the high Mo requirement of eukaryotic NO(3)(-) assimilation. Thorough ocean oxygenation in the Neoproterozoic led to Mo-rich oceans, possibly contributing to the proliferation of eukaryotes and thus the Cambrian explosion of metazoan life. These ideas can be tested by more intensive study of the metal requirements in N assimilation and the biological strategies for metal uptake, regulation, and storage.

Biosynthesis of the iron-molybdenum cofactor of nitrogenase

The iron-molybdenum cofactor (FeMo-co), located at the active site of the molybdenum nitrogenase, is one of the most complex metal cofactors known to date. During the past several years, an intensive effort has been made to purify the proteins involved in FeMo-co synthesis and incorporation into nitrogenase. This effort is starting to provide insights into the structures of the FeMo-co biosynthetic intermediates and into the biochemical details of FeMo-co synthesis.

Metal trafficking for nitrogen fixation: NifQ donates molybdenum to NifEN/NifH for the biosynthesis of the nitrogenase FeMo-cofactor

The molybdenum nitrogenase, present in a diverse group of bacteria and archea, is the major contributor to biological nitrogen fixation. The nitrogenase active site contains an iron-molybdenum cofactor (FeMo-co) composed of 7Fe, 9S, 1Mo, one unidentified light atom, and homocitrate. The nifQ gene was known to be involved in the incorporation of molybdenum into nitrogenase. Here we show direct biochemical evidence for the role of NifQ in FeMo-co biosynthesis. As-isolated NifQ was found to carry a molybdenum-iron-sulfur cluster that serves as a specific molybdenum donor for FeMo-co biosynthesis. Purified NifQ supported in vitro FeMo-co synthesis in the absence of an additional molybdenum source. The mobilization of molybdenum from NifQ required the simultaneous participation of NifH and NifEN in the in vitro FeMo-co synthesis assay, suggesting that NifQ would be the physiological molybdenum donor to a hypothetical NifEN/NifH complex.

Azotobacter vinelandii metal storage protein: "classical" inorganic chemistry involved in Mo/W uptake and release processes

The release of Mo (as molybdate) from the Mo storage protein (MoSto), which is unique among all existing metalloproteins, is strongly influenced by temperature and pH value; other factors (incubation time, protein concentration, degree of purity) have minor, though significant effects. A detailed pH titration at 12 degrees C revealed that three different steps can be distinguished for the Mo-release process. A proportion of approximately 15% at pH 6.8-7.0, an additional 25% at pH 7.2-7.5 and ca. 50% (up to 90% in total) at pH 7.6-7.8. This triphasic process supports the assumption of the presence of different types of molybdenum-oxide-based clusters that exhibit different pH lability. The complete release of Mo was achieved by increasing the temperature to 30 degrees C and the pH value to >7.5. The Mo-release process does not require ATP; on the contrary, ATP prevents, or at least reduces the degree of metal release, depending on the concentration of the nucleotide. From this point of view, the intracellular ATP concentration is suggested to play-in addition to the pH value-an indirect but crucial role in controlling the extent of Mo release in the cell. The binding of molybdenum to the apoprotein (reconstitution process) was confirmed to be directly dependent on the presence of a nucleotide (preferably ATP) and MgCl2. Maximal reincorporation of Mo required 1 mM ATP, which could partly be replaced by GTP. When the storage protein was purified in the presence of ATP and MgCl2 (1 mM each), the final preparation contained 80 Mo atoms per protein molecule. Maximal metal loading (110-115 atoms/MoSto molecule) was only achieved, if Mo was first completely released from the native protein and subsequently (re-) bound under optimal reconstitution conditions: 1 h incubation at pH 6.5 and 12 degrees C in the presence of ATP, MgCl2 and excess molybdate. A corresponding tungsten-containing storage protein ("WSto") could not only be synthesized in vivo by growing cells, but could also be constructed in vitro by a metalate-ion exchange procedure by using the isolated MoSto protein. The high W content of the isolated cell-made WSto (approximately 110 atoms/protein molecule) and the relatively low amount of tungstate that was released from the protein under optimal "release conditions", demonstrates that the W-oxide-based clusters are more stable inside the protein cavity than the Mo-oxide analogues, as expected from the corresponding findings in polyoxometalate chemistry. The optimized isolation of the W-loaded protein form allowed us to get single crystals, and to determine the crystal X-ray structure. This proved that the protein contains remarkably different types of polyoxotungstates, the formation of which is templated in an unprecedented process by the different protein pockets. (Angew. Chem. Int. Ed. 2007, 46, 2408-2413).

Biosynthesis of the iron-molybdenum cofactor of nitrogenase

The iron-molybdenum cofactor (FeMo-co), located at the active site of the molybdenum nitrogenase, is one of the most complex metal cofactors known to date. During the past several years, an intensive effort has been made to purify the proteins involved in FeMo-co synthesis and incorporation into nitrogenase. This effort is starting to provide insights into the structures of the FeMo-co biosynthetic intermediates and into the biochemical details of FeMo-co synthesis.

Tungsten, the surprisingly positively acting heavy metal element for prokaryotes

The history and changing function of tungsten as the heaviest element in biological systems is given. It starts from an inhibitory element/anion, especially for the iron molybdenum-cofactor (FeMoCo)-containing enzyme nitrogenase involved in dinitrogen fixation, as well as for the many "metal binding pterin" (MPT)-, also known as tricyclic pyranopterin- containing classic molybdoenzymes, such as the sulfite oxidase and the xanthine dehydrogenase family of enzymes. They are generally involved in the transformation of a variety of carbon-, nitrogen- and sulfur-containing compounds. But tungstate can serve as a potential positively acting element for some enzymes of the dimethyl sulfoxide (DMSO) reductase family, especially for CO(2)-reducing formate dehydrogenases (FDHs), formylmethanofuran dehydrogenases and acetylene hydratase (catalyzing only an addition of water, but no redox reaction). Tungsten even becomes an essential element for nearly all enzymes of the aldehyde oxidoreductase (AOR) family. Due to the close chemical and physical similarities between molybdate and tungstate, the latter was thought to be only unselectively cotransported or cometabolized with other tetrahedral anions, such as molybdate and also sulfate. However, it has now become clear that it can also be very selectively transported compared to molybdate into some prokaryotic cells by two very selective ABC-type of transporters that contain a binding protein TupA or WtpA. Both proteins exhibit an extremely high affinity for tungstate (K(D) < 1 nM) and can even discriminate between tungstate and molybdate. By that process, tungsten finally becomes selectively incorporated into the few enzymes noted above.