The In Vivo Potential-Regulated Protective Protein of Nitrogenase in Azotobacter vinelandii Supports Aerobic Bioelectrochemical Dinitrogen Reduction In Vitro

Microbial electrochemical Cr(VI) reduction in a soil continuous flow system

Microbial electrochemical technologies represent innovative approaches to contaminated soil and groundwater remediation and provide a flexible framework for removing organic and inorganic contaminants by integrating electrochemical and biological techniques. To simulate in situ microbial electrochemical treatment of groundwater plumes, this study investigates Cr(VI) reduction within a bioelectrochemical continuous flow (BECF) system equipped with soil-buried electrodes, comparing it to abiotic and open-circuit controls. Continuous-flow systems were tested with two chromium-contaminated solutions (20-50 mg Cr(VI)/L). Additional nutrients, buffers, or organic substrates were introduced during the tests in the systems. With an initial Cr(VI) concentration of 20 mg/L, 1.00 mg Cr(VI)/(L day) bioelectrochemical removal rate in the BECF system was observed, corresponding to 99.5% removal within nine days. At the end of the test with 50 mg Cr(VI)/L (156 days), the residual Cr(VI) dissolved concentration was two orders of magnitude lower than that in the open circuit control, achieving 99.9% bioelectrochemical removal in the BECF. Bacteria belonging to the orders Solirubrobacteriales, Gaiellales, Bacillales, Gemmatimonadales, and Propionibacteriales characterized the bacterial communities identified in soil samples; differently, Burkholderiales, Mycobacteriales, Cytophagales, Rhizobiales, and Caulobacterales characterized the planktonic bacterial communities. The complexity of the microbial community structure suggests the involvement of different microorganisms and strategies in the bioelectrochemical removal of chromium. In the absence of organic carbon, microbial electrochemical removal of hexavalent chromium was found to be the most efficient way to remove Cr(VI), and it may represent an innovative and sustainable approach for soil and groundwater remediation. Integr Environ Assess Manag 2024;00:1-17. © 2024 The Author(s). Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Toward nitrogen recovery: Co-cultivation of microalgae and bacteria enhances the production of high-value nitrogen-rich cyanophycin

The algal-bacterial wastewater treatment process has been proven to be highly efficient in removing nutrients and recovering nitrogen (N). However, the recovery of the valuable N-rich biopolymer, cyanophycin, remains limited. This research explored the synthesis mechanism and recovery potential of cyanophycin within two algal-bacterial symbiotic reactors. The findings reveal that the synergy between algae and bacteria enhances the removal of N and phosphorus. The crude contents of cyanophycin in the algal-bacterial consortia reached 115 and 124 mg/g of mixed liquor suspended solids (MLSS), respectively, showing an increase of 11.7 %-20.4 % (p < 0.001) compared with conventional activated sludge. Among the 170 metagenome-assembled genomes (MAGs) analyzed, 50 were capable of synthesizing cyanophycin, indicating that cyanophycin producers are common in algal-bacterial systems. The compositions of cyanophycin producers in the two algal-bacterial reactors were affected by different lighting initiation time. The study identified two intracellular synthesis pathways for cyanophycin. Approximately 36 MAGs can synthesize cyanophycin de novo using ammonium and glucose, while the remaining 14 MAGs require exogenous arginine for production. Notably, several MAGs with high abundance are capable of assimilating both nitrate and ammonium into cyanophycin, demonstrating a robust N utilization capability. This research also marks the first identification of potential horizontal gene transfer of the cyanophycin synthase encoding gene (cphA) within the wastewater microbial community. This suggests that the spread of cphA could expand the population of cyanophycin producers. The study offers new insights into recycling the high-value N-rich biopolymer cyanophycin, contributing to the advancement of wastewater resource utilization.

The Mononuclear Metal-Binding Site of Mo-Nitrogenase Is Not Required for Activity

The biological N2-fixation process is catalyzed exclusively by metallocofactor-containing nitrogenases. Structural and spectroscopic studies highlighted the presence of an additional mononuclear metal-binding (MMB) site, which can coordinate Fe in addition to the two metallocofactors required for the reaction. This MMB site is located 15-Å from the active site, at the interface of two NifK subunits. The enigmatic function of the MMB site and its implications for metallocofactor installation, catalysis, electron transfer, or structural stability are investigated in this work. The axial ligands coordinating the additional Fe are almost universally conserved in Mo-nitrogenases, but a detailed observation of the available structures indicates a variation in occupancy or a metal substitution. A nitrogenase variant in which the MMB is disrupted was generated and characterized by X-ray crystallography, biochemistry, and enzymology. The crystal structure refined to 1.55-Å revealed an unambiguous loss of the metal site, also confirmed by an absence of anomalous signal for Fe. The position of the surrounding side chains and the overall architecture are superposable with the wild-type structure. Accordingly, the biochemical and enzymatic properties of the variant are similar to those of the wild-type nitrogenase, indicating that the MMB does not impact nitrogenase's activity and stability in vitro.

Near ambient N2 fixation on solid electrodes versus enzymes and homogeneous catalysts

The Mo/Fe nitrogenase enzyme is unique in its ability to efficiently reduce dinitrogen to ammonia at atmospheric pressures and room temperature. Should an artificial electrolytic device achieve the same feat, it would revolutionize fertilizer production and even provide an energy-dense, truly carbon-free fuel. This Review provides a coherent comparison of recent progress made in dinitrogen fixation on solid electrodes, homogeneous catalysts and nitrogenases. Specific emphasis is placed on systems for which there is unequivocal evidence that dinitrogen reduction has taken place. By establishing the cross-cutting themes and synergies between these systems, we identify viable avenues for future research.

Emerging Electrochemical Processes to Decarbonize the Chemical Industry

Electrification is a potential approach to decarbonizing the chemical industry. Electrochemical processes, when they are powered by renewable electricity, have lower carbon footprints in comparison to conventional thermochemical routes. In this Perspective, we discuss the potential electrochemical routes for chemical production and provide our views on how electrochemical processes can be matured in academic research laboratories for future industrial applications. We first analyze the CO₂ emission in the manufacturing industry and conduct a survey of state of the art electrosynthesis methods in the three most emission-intensive areas: petrochemical production, nitrogen compound production, and metal smelting. Then, we identify the technical bottlenecks in electrifying chemical productions from both chemistry and engineering perspectives and propose potential strategies to tackle these issues. Finally, we provide our views on how electrochemical manufacturing can reduce carbon emissions in the chemical industry with the hope to inspire more research efforts in electrifying chemical manufacturing.

Structural insight into [Fe-S2-Mo] motif in electrochemical reduction of N2 over Fe1-supported molecular MoS2

The catalytic synthesis of NH₃ from the thermodynamically challenging N₂ reduction reaction under mild conditions is currently a significant problem for scientists. Accordingly, herein, we report the development of a nitrogenase-inspired inorganic-based chalcogenide system for the efficient electrochemical conversion of N₂ to NH₃, which is comprised of the basic structure of [Fe-S₂-Mo]. This material showed high activity of 8.7 mg_NH3 mg_Fe ^-1 h^-1 (24 μg_NH3 cm^-2 h^-1) with an excellent faradaic efficiency of 27% for the conversion of N₂ to NH₃ in aqueous medium. It was demonstrated that the Fe₁ single atom on [Fe-S₂-Mo] under the optimal negative potential favors the reduction of N₂ to NH₃ over the competitive proton reduction to H₂. Operando X-ray absorption and simulations combined with theoretical DFT calculations provided the first and important insights on the particular electron-mediating and catalytic roles of the [Fe-S₂-Mo] motifs and Fe₁, respectively, on this two-dimensional (2D) molecular layer slab.

Insights into Nitrogenase Bioelectrocatalysis for Green Ammonia Production

There is a growing interest in using ammonia as a liquid carrier of hydrogen for energy applications. Currently, ammonia is produced industrially by the Haber-Bosch process, which requires high temperature and high pressure. In contrast, bacteria have naturally evolved an enzyme known as nitrogenase, that is capable of producing ammonia and hydrogen at ambient temperature and pressure. Therefore, nitrogenases are attractive as a potentially more efficient means to produce ammonia via harnessing the unique properties of this enzyme. In recent years, exciting progress has been made in bioelectrocatalysis using nitrogenases to produce ammonia. Here, the prospects for developing biological ammonia production are outlined, key advances in bioelectrocatalysis by nitrogenases are highlighted, and possible solutions to the obstacles faced in realising this goal are discussed.

Electroenzymatic Nitrogen Fixation Using a MoFe Protein System Immobilized in an Organic Redox Polymer

We report an organic redox-polymer-based electroenzymatic nitrogen fixation system using a metal-free redox polymer, namely neutral-red-modified poly(glycidyl methacrylate-co-methylmethacrylate-co-poly(ethyleneglycol)methacrylate) with a low redox potential of -0.58 V vs. SCE. The stable and efficient electric wiring of nitrogenase within the redox polymer matrix enables mediated bioelectrocatalysis of N3- , NO2- and N2 to NH3 catalyzed by the MoFe protein via the polymer-bound redox moieties distributed in the polymer matrix in the absence of the Fe protein. Bulk bioelectrosynthetic experiments produced 209±30 nmol NH3  nmol MoFe-1  h-1 from N2 reduction. 15 N2 labeling experiments and NMR analysis were performed to confirm biosynthetic N2 reduction to NH3 .

Electron Transfer in Nitrogenase

Nitrogenase is the only enzyme capable of reducing N2 to NH3. This challenging reaction requires the coordinated transfer of multiple electrons from the reductase, Fe-protein, to the catalytic component, MoFe-protein, in an ATP-dependent fashion. In the last two decades, there have been significant advances in our understanding of how nitrogenase orchestrates electron transfer (ET) from the Fe-protein to the catalytic site of MoFe-protein and how energy from ATP hydrolysis transduces the ET processes. In this review, we summarize these advances, with focus on the structural and thermodynamic redox properties of nitrogenase component proteins and their complexes, as well as on new insights regarding the mechanism of ET reactions during catalysis and how they are coupled to ATP hydrolysis. We also discuss recently developed chemical, photochemical, and electrochemical methods for uncoupling substrate reduction from ATP hydrolysis, which may provide new avenues for studying the catalytic mechanism of nitrogenase.

Gold nanoclusters cause selective light-driven biochemical catalysis in living nano-biohybrid organisms

Living nano-biohybrid organisms or nanorgs combine the specificity and well-designed surface chemistry of an enzyme catalyst site, with the strong light absorption and efficient charge injection (for biocatalytic reaction) from inorganic materials. Previous efforts in harvesting sunlight for renewable and sustainable photochemical conversion of inexpensive feedstocks to biochemicals using nanorgs focused on the design of semiconductor nanoparticles or quantum dots (QDs). However, metal nanoparticles and nanoclusters (NCs), such as gold (Au), offer strong light absorption properties and biocompatibility for potential application in living nanorgs. Here we show that optimized, sub-1 nanometer Au NCs-nanorgs can carry out selective biochemical catalysis with high turnover number (10⁸ mol mol^-1 of cells) and turnover frequency (>2 × 10⁷ h^-1). While the differences of size, light absorption, and electrochemical properties between these NCs (with 18, 22, and 25 atoms) are small, large differences in their light-activated properties dictate that 22 atom Au NCs are best suited for forming living nanorgs to drive photocatalytic ammonia production from air. Based on our experiments, these Au₂₂ NC-nanorgs demonstrate 29.3% quantum efficiency of converting absorbed photons to the desired chemical, and 12.9% efficiency of photon-to-fuel conversion based on energy input-output. Further, by comparing the light-driven ammonia production yield between strains producing Mo-Fe nitrogenase with and without histidine tags, we demonstrate that preferential coupling of Au NCs to the nitrogenase through Au-histidine interactions is crucial for effective electron transfer and subsequent product generation. Together, these results provide the design rules for forming Au NCs-nanorgs and can have important implications for carrying out light-driven biochemical catalysis for renewable solar fuel generation.

Evolutionary adaptations that enable enzymes to tolerate oxidative stress

Biochemical mechanisms emerged and were integrated into the metabolic plan of cellular life long before molecular oxygen accumulated in the biosphere. When oxygen levels finaly rose, they threatened specific types of enzymes: those that use organic radicals as catalysts, and those that depend upon iron centers. Nature has found ways to ensure that such enzymes are still used by contemporary organisms. In some cases they are restricted to microbes that reside in anoxic habitats, but in others they manage to function inside aerobic cells. In the latter case, it is frequently true that the ancestral enzyme has been modified to fend off poisoning. In this review we survey a range of protein adaptations that permit radical-based and low-potential iron chemistry to succeed in oxic environments. In many cases, accessory domains shield the vulnerable radical or metal center from oxygen. In others, the structures of iron cofactors evolved to less oxidizable forms, or alternative metals replaced iron altogether. The overarching view is that some classes of biochemical mechanism are intrinsically incompatible with the presence of oxygen. The structural modification of target enzymes is an under-recognized response to this problem.

Metalloprotein switches that display chemical-dependent electron transfer in cells

Biological electron transfer is challenging to directly regulate using environmental conditions. To enable dynamic, protein-level control over energy flow in metabolic systems for synthetic biology and bioelectronics, we created ferredoxin logic gates that utilize transcriptional and post-translational inputs to control energy flow through a synthetic electron transfer pathway that is required for bacterial growth. These logic gates were created by subjecting a thermostable, plant-type ferredoxin to backbone fission and fusing the resulting fragments to a pair of proteins that self-associate, a pair of proteins whose association is stabilized by a small molecule, and to the termini of a ligand-binding domain. We show that the latter domain insertion design strategy yields an allosteric ferredoxin switch that acquires an oxygen-tolerant [2Fe-2S] cluster and can use different chemicals, including a therapeutic drug and an environmental pollutant, to control the production of a reduced metabolite in Escherichia coli and cell lysates.

Interfacing nature's catalytic machinery with synthetic materials for semi-artificial photosynthesis

Semi-artificial photosynthetic systems aim to overcome the limitations of natural and artificial photosynthesis while providing an opportunity to investigate their respective functionality. The progress and studies of these hybrid systems is the focus of this forward-looking perspective. In this Review, we discuss how enzymes have been interfaced with synthetic materials and employed for semi-artificial fuel production. In parallel, we examine how more complex living cellular systems can be recruited for in vivo fuel and chemical production in an approach where inorganic nanostructures are hybridized with photosynthetic and non-photosynthetic microorganisms. Side-by-side comparisons reveal strengths and limitations of enzyme- and microorganism-based hybrid systems, and how lessons extracted from studying enzyme hybrids can be applied to investigations of microorganism-hybrid devices. We conclude by putting semi-artificial photosynthesis in the context of its own ambitions and discuss how it can help address the grand challenges facing artificial systems for the efficient generation of solar fuels and chemicals.

Beyond fossil fuel-driven nitrogen transformations

Nitrogen is fundamental to all of life and many industrial processes. The interchange of nitrogen oxidation states in the industrial production of ammonia, nitric acid, and other commodity chemicals is largely powered by fossil fuels. A key goal of contemporary research in the field of nitrogen chemistry is to minimize the use of fossil fuels by developing more efficient heterogeneous, homogeneous, photo-, and electrocatalytic processes or by adapting the enzymatic processes underlying the natural nitrogen cycle. These approaches, as well as the challenges involved, are discussed in this Review.