A Voltammetric Study of Nitrogenase Catalysis Using Electron Transfer Mediators

Light-driven, bias-free nitrogenase-based bioelectrochemical cell for ammonia generation

Nitrogen fixation is a key process that sustains life on Earth. Nitrogenase is the sole enzyme capable of fixing nitrogen under ambient conditions. Extensive research efforts have been dedicated to elucidating the enzyme mechanism and its artificial activation through high applied voltage, photochemistry, or strong reducing agents. Harnessing light irradiation to minimize the required external bias can lower the process's high energy investment. Herein, we present the development of photo-bioelectrochemical cells (PBECs) utilizing BiVO4/CoP or CdS/NiO photoanodes for nitrogenase activation toward N2 fixation. The constructed PBEC based on BiVO4/CoP photoanode requires minimal external bias (200 mV) and suppresses O2 generation that allows efficient activation of the nitrogenase enzyme, using glucose as an electron donor. In a second developed PBEC configuration, CdS/NiO photoanode was used, enabling bias-free activation of the nitrogenase-based cathode to produce 100 μM of ammonia at a faradaic efficiency (FE) of 12%. The ammonia production was determined by a commonly used fluorescence probe and further validated using 1H-NMR spectroscopy. The presented PBECs lay the foundation for biotic-abiotic systems to directly activate enzymes toward value-added chemicals by light-driven reactions.

Structural comparison of (hyper-)thermophilic nitrogenase reductases from three marine Methanococcales

The nitrogenase reductase NifH catalyses ATP-dependent electron delivery to the Mo-nitrogenase, a reaction central to biological dinitrogen (N2) fixation. While NifHs have been extensively studied in bacteria, structural information about their archaeal counterparts is limited. Archaeal NifHs are considered more ancient, particularly those from Methanococcales, a group of marine hydrogenotrophic methanogens, which includes diazotrophs growing at temperatures near 92 °C. Here, we structurally and biochemically analyse NifHs from three Methanococcales, offering the X-ray crystal structures from meso-, thermo-, and hyperthermophilic methanogens. While NifH from Methanococcus maripaludis (37 °C) was obtained through heterologous recombinant expression, the proteins from Methanothermococcus thermolithotrophicus (65 °C) and Methanocaldococcus infernus (85 °C) were natively purified from the diazotrophic archaea. The structures from M. thermolithotrophicus crystallised as isolated exhibit high flexibility. In contrast, the complexes of NifH with MgADP obtained from the three methanogens are superposable, more rigid, and present remarkable structural conservation with their homologues. They retain key structural features of P-loop NTPases and share similar electrostatic profiles with the counterpart from the bacterial model organism Azotobacter vinelandii. In comparison to the NifH from the phylogenetically distant Methanosarcina acetivorans, these reductases do not cross-react significantly with Mo-nitrogenase from A. vinelandii. However, they associate with bacterial nitrogenase when ADP·AlF 4 - $$ {\mathrm{AlF}}_4^{-} $$ is added to mimic a transient reactive state. Accordingly, detailed surface analyses suggest that subtle substitutions would affect optimal binding during the catalytic cycle between the NifH from Methanococcales and the bacterial nitrogenase, implying differences in the N2-machinery from these ancient archaea.

Advanced Electroanalysis for Electrosynthesis

Electrosynthesis is a popular, environmentally friendly substitute for conventional organic methods. It involves using charge transfer to stimulate chemical reactions through the application of a potential or current between two electrodes. In addition to electrode materials and the type of reactor employed, the strategies for controlling potential and current have an impact on the yields, product distribution, and reaction mechanism. In this Review, recent advances related to electroanalysis applied in electrosynthesis were discussed. The first part of this study acts as a guide that emphasizes the foundations of electrosynthesis. These essentials include instrumentation, electrode selection, cell design, and electrosynthesis methodologies. Then, advances in electroanalytical techniques applied in organic, enzymatic, and microbial electrosynthesis are illustrated with specific cases studied in recent literature. To conclude, a discussion of future possibilities that intend to advance the academic and industrial areas is presented.

A voltammetric study of nitrogenase MoFe-protein using low-potential electron transfer mediators

The molybdenum-iron protein (MoFeP), a component of the enzyme nitrogenase, catalyzes the reduction of an array of small molecules, including N2 to NH3. In microorganisms, during the catalytic cycle, MoFeP receives electrons from the obligate biological redox partner iron protein (FeP) in a process coupled to the hydrolysis of two MgATP per one electron transferred. Despite the favorable redox properties of the cofactors, the requirement of the MgATP hydrolysis significantly decreases the energy efficiency of MoFeP. Therefore, remarkable efforts have been devoted to electrochemically activating MoFeP without FeP and MgATP. Previously, MoFeP was adsorbed on an electrode surface and revealed a slow catalysis with and without electron transfer mediators. However, enzyme adsorption can cause conformational and structural changes in a fragile protein molecule and alter its catalytic activity. In this work, MoFeP was electrochemically studied in solution. Various electron transfer mediators with potentials ranging from -0.3 V to -1 V (vs. NHE) were examined with MoFeP using cyclic voltammetry. No significant catalytic activity of the MoFeP was observed with any of the tested mediators. This indicates that efficient electrochemical activation of MoFeP cannot be achieved exclusively by increasing the driving force between the MoFeP redox cofactors and an electron donor.

Bridging Together Theoretical and Experimental Perspectives in Single-Atom Alloys for Electrochemical Ammonia Production

Ammonia is an essential commodity in the food and chemical industry. Despite the energy-intensive nature, the Haber-Bosch process is the only player in ammonia production at large scales. Developing other strategies is highly desirable, as sustainable and decentralized ammonia production is crucial. Electrochemical ammonia production by directly reducing nitrogen and nitrogen-based moieties powered by renewable energy sources holds great potential. However, low ammonia production and selectivity rates hamper its utilization as a large-scale ammonia production process. Creating effective and selective catalysts for the electrochemical generation of ammonia is critical for long-term nitrogen fixation. Single-atom alloys (SAAs) have become a new class of materials with distinctive features that may be able to solve some of the problems with conventional heterogeneous catalysts. The design and optimization of SAAs for electrochemical ammonia generation have recently been significantly advanced. This comprehensive review discusses these advancements from theoretical and experimental research perspectives, offering a fundamental understanding of the development of SAAs for ammonia production.

Bioelectrocatalytic Synthesis: Concepts and Applications

Bioelectrocatalytic synthesis is the conversion of electrical energy into value-added products using biocatalysts. These methods merge the specificity and selectivity of biocatalysis and energy-related electrocatalysis to address challenges in the sustainable synthesis of pharmaceuticals, commodity chemicals, fuels, feedstocks and fertilizers. However, the specialized experimental setups and domain knowledge for bioelectrocatalysis pose a significant barrier to adoption. This review introduces key concepts of bioelectrosynthetic systems. We provide a tutorial on the methods of biocatalyst utilization, the setup of bioelectrosynthetic cells, and the analytical methods for assessing bioelectrocatalysts. Key applications of bioelectrosynthesis in ammonia production and small-molecule synthesis are outlined for both enzymatic and microbial systems. This review serves as a necessary introduction and resource for the non-specialist interested in bioelectrosynthetic research.

Nitrogen Fixation and Hydrogen Evolution by Sterically Encumbered Mo-Nitrogenase

The substrate-reducing proteins of all nitrogenases (MoFe, VFe, and FeFe) are organized as α₂ß₂(γ₂) multimers with two functional halves. While their dimeric organization could afford improved structural stability of nitrogenases in vivo, previous research has proposed both negative and positive cooperativity contributions with respect to enzymatic activity. Here, a 1.4 kDa peptide was covalently introduced in the proximity of the P cluster, corresponding to the Fe protein docking position. The Strep-tag carried by the added peptide simultaneously sterically inhibits electron delivery to the MoFe protein and allows the isolation of partially inhibited MoFe proteins (where the half-inhibited MoFe protein was targeted). We confirm that the partially functional MoFe protein retains its ability to reduce N₂ to NH₃, with no significant difference in selectivity over obligatory/parasitic H₂ formation. Our experiment concludes that wild-type nitrogenase exhibits negative cooperativity during the steady state regarding H₂ and NH₃ formation (under Ar or N₂), with one-half of the MoFe protein inhibiting turnover in the second half. This emphasizes the presence and importance of long-range (>95 Å) protein-protein communication in biological N₂ fixation in Azotobacter vinelandii.

Promotion of Nitrogen Fixation of Diverse Heterotrophs by Solid-Phase Humin

Although biological nitrogen fixation (BNF) proceeds under mild conditions compared to the energy-intensive Haber–Bosch process, the slow kinetics of BNF necessitate the promotion of BNF activity in its practical application. The BNF promotion using purified nitrogenases and using genetically modified microorganisms has been studied, but these enzymes are unstable and expensive; moreover, designing genetically modified microorganisms is also a difficult task. Alternatively, the BNF promotion in non-modified (wild-type) microorganisms (enriched consortia) with humin has been shown, which is a humic substance insoluble at any pH and functions as an extracellular electron mediator. However, the taxonomic distribution of the diazotrophs promoted by humin, the levels of BNF promotion, and the underlying mechanism in BNF promotion with humin remain unknown. In this study, we show that taxonomically diverse heterotrophic diazotrophs, harboring nifH clusters I, II, and III, promoted their BNF by accepting extracellular electrons from humin, based on the characterization of the individual responses of isolated diazotrophs to humin. The reduced humin increased the acetylene reduction activity of the diazotrophs by 194–916% compared to the level achieved by the organic carbon source, causing adenosine triphosphate (ATP) synthesis in the diazotroph cells without increase in the CO₂ production and direct electron donation to the MoFe protein of the nitrogenase in the cells without relying on the biological electron transfer system. These would result in BNF promotion in the wild-type diazotroph cells beyond their biochemical capacity. This significant promotion of BNF with humin would serve as a potential basis for sustainable technology for greener nitrogen fixation.

Analysis of the Ammonia Production Rates by Nitrogenase

Ammonia (NH3) is produced industrially by the Haber–Bosch process from dinitrogen (N2) and dihydrogen (H2) using high temperature and pressure with an iron catalyst. In contrast to the extreme conditions used in the Haber–Bosch process, biology has evolved nitrogenase enzymes, which operate at ambient temperature and pressure. In biological settings, nitrogenase requires large amounts of energy in the form of ATP, using at least 13 GJ ton−1 of ammonia. In 2016, Brown et al. reported ATP-free ammonia production by nitrogenase. This result led to optimism that the energy demands of nitrogenase could be reduced. More recent reports confirmed the ATP-free production of ammonia; however, the rates of reaction are at least an order of magnitude lower. A more detailed understanding of the role of ATP in nitrogenase catalysis is required to develop ATP-free catalytic systems with higher ammonia production rates. Finally, we calculated the theoretical maximal ammonia production rate by nitrogenase and compared it to currently used Haber–Bosch catalysts. Somewhat surprisingly, nitrogenase has a similar theoretical maximum rate to the Haber–Bosch catalysts; however, strategies need to be developed to allow the enzyme to maintain operation at its optimal rate.

Electrochemical Trimethylamine N-Oxide Biosensor with Enzyme-Based Oxygen-Scavenging Membrane for Long-Term Operation under Ambient Air

An amperometric trimethylamine N-oxide (TMAO) biosensor is reported, where TMAO reductase (TorA) and glucose oxidase (GOD) and catalase (Cat) were immobilized on the electrode surface, enabling measurements of mediated enzymatic TMAO reduction at low potential under ambient air conditions. The oxygen anti-interference membrane composed of GOD, Cat and polyvinyl alcohol (PVA) hydrogel, together with glucose concentration, was optimized until the O₂ reduction current of a Clark-type electrode was completely suppressed for at least 3 h. For the preparation of the TMAO biosensor, Escherichia coli TorA was purified under anaerobic conditions and immobilized on the surface of a carbon electrode and covered by the optimized O₂ scavenging membrane. The TMAO sensor operates at a potential of -0.8 V vs. Ag/AgCl (1 M KCl), where the reduction of methylviologen (MV) is recorded. The sensor signal depends linearly on TMAO concentrations between 2 µM and 15 mM, with a sensitivity of 2.75 ± 1.7 µA/mM. The developed biosensor is characterized by a response time of about 33 s and an operational stability over 3 weeks. Furthermore, measurements of TMAO concentration were performed in 10% human serum, where the lowest detectable concentration is of 10 µM TMAO.

Promotion of biological nitrogen fixation activity of an anaerobic consortium using humin as an extracellular electron mediator

Nitrogen fertiliser is manufactured using the industrial Haber–Bosch process, although it is extremely energy-consuming. One sustainable alternative technology is the electrochemical promotion of biological nitrogen fixation (BNF). This study reports the promotion of BNF activity of anaerobic microbial consortia by humin, a solid-phase humic substance, at any pH, functioning as an extracellular electron mediator, to levels of 5.7–11.8 times under nitrogen-deficient conditions. This was evidenced by increased acetylene reduction activity and total nitrogen content of the consortia. Various humins from different origins promoted anaerobic BNF activity, although the degree of promotion differed. The promotion effected by humin differed from the effects of chemical reducing agents and the effects of supplemental micronutrients and vitamins. The promotion of anaerobic BNF activity by only reduced humin without any other electron donor suggested that humin did not serve as organic carbon source but as extracellular electron mediator, for electron donation to the nitrogen-fixing microorganisms. The next generation sequencing (NGS) of partial 16S rRNA genes showed the predominance of Clostridiales (Firmicutes) in the consortia. These findings suggest the effectiveness of humin as a solid-phase extracellular electron mediator for the promotion of anaerobic BNF activity, potentially to serve for the basis for a sustainable technology.

Investigating the use of conducting oligomers and redox molecules in CdS-MoFeP biohybrids

In this work we report the effect of incorporating conducting oligophenylenes and a cobaltocene-based redox mediator on photodriven electron transfer between thioglycolic acid (TGA) capped CdS nanorods (NR) and the native nitrogenase MoFe protein (MoFeP) by following the reduction of H⁺ to H₂. First, we demonstrate that the addition of benzidine-a conductive diphenylene- to TGA-CdS and MoFeP increased catalytic activity by up to 3-fold as compared to CdS-MoFeP alone. In addition, in comparing the use of oligophenylenes composed of one (p-phenylenediamine), two (benzidine) or three (4,4''-diamino-p-terphenyl)phenylene groups, the largest gain in H₂ was observed with the addition of benzidine and the lowest with phenylenediamine. As a comparison to the conductive oligophenylenes, a cobaltocene-based redox mediator was also tested with the TGA-CdS NRs and MoFeP. However, adding either cobaltocene diacid or diamine caused negligible gains in H₂ production and at higher concentrations, caused a significant decrease. Agarose gel electrophoresis revealed little to no detectable interaction between benzidine and TGA-CdS but strong binding between cobaltocene and TGA-CdS. These results suggest that the tight binding of the cobaltocene mediator to CdS may hinder electron transfer between CdS and MoFe and cause the mediator to undergo continuous reduction/oxidation events at the surface of CdS.

Molybdenum-Containing Metalloenzymes and Synthetic Catalysts for Conversion of Small Molecules

The energy deficiency and environmental problems have motivated researchers to develop energy conversion systems into a sustainable pathway, and the development of catalysts holds the center of the research endeavors. Natural catalysts such as metalloenzymes have maintained energy cycles on Earth, thus proving themselves the optimal catalysts. In the previous research results, the structural and functional analogs of enzymes and nano-sized electrocatalysts have shown promising activities in energy conversion reactions. Mo ion plays essential roles in natural and artificial catalysts, and the unique electrochemical properties render its versatile utilization as an electrocatalyst. In this review paper, we show the current understandings of the Mo-enzyme active sites and the recent advances in the synthesis of Mo-catalysts aiming for high-performing catalysts.

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.

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.

Natural and Engineered Electron Transfer of Nitrogenase

As the only enzyme currently known to reduce dinitrogen (N2) to ammonia (NH3), nitrogenase is of significant interest for bio-inspired catalyst design and for new biotechnologies aiming to produce NH3 from N2. In order to reduce N2, nitrogenase must also hydrolyze at least 16 equivalents of adenosine triphosphate (MgATP), representing the consumption of a significant quantity of energy available to biological systems. Here, we review natural and engineered electron transfer pathways to nitrogenase, including strategies to redirect or redistribute electron flow in vivo towards NH3 production. Further, we also review strategies to artificially reduce nitrogenase in vitro, where MgATP hydrolysis is necessary for turnover, in addition to strategies that are capable of bypassing the requirement of MgATP hydrolysis to achieve MgATP-independent N2 reduction.

Reduction of Substrates by Nitrogenases

Nitrogenase is the enzyme that catalyzes biological N₂ reduction to NH₃. This enzyme achieves an impressive rate enhancement over the uncatalyzed reaction. Given the high demand for N₂ fixation to support food and chemical production and the heavy reliance of the industrial Haber-Bosch nitrogen fixation reaction on fossil fuels, there is a strong need to elucidate how nitrogenase achieves this difficult reaction under benign conditions as a means of informing the design of next generation synthetic catalysts. This Review summarizes recent progress in addressing how nitrogenase catalyzes the reduction of an array of substrates. New insights into the mechanism of N₂ and proton reduction are first considered. This is followed by a summary of recent gains in understanding the reduction of a number of other nitrogenous compounds not considered to be physiological substrates. Progress in understanding the reduction of a wide range of C-based substrates, including CO and CO₂, is also discussed, and remaining challenges in understanding nitrogenase substrate reduction are considered.

Nitrogenase Bioelectrochemistry for Synthesis Applications

The fixation of atmospheric dinitrogen to ammonia by industrial technologies (such as the Haber Bosch process) has revolutionized humankind. In contrast to industrial technologies, a single enzyme is known for its ability to reduce or "fix" dinitrogen: nitrogenase. Nitrogenase is a complex oxidoreductase enzymatic system that includes a catalytic protein (where dinitrogen is reduced) and an electron-transferring reductase protein (termed the Fe protein) that delivers the electrons necessary for dinitrogen fixation. The catalytic protein most commonly contains a FeMo cofactor (called the MoFe protein), but it can also contain a VFe or FeFe cofactor. Besides their ability to fix dinitrogen to ammonia, these nitrogenases can also reduce substrates such as carbon dioxide to formate. Interestingly, the VFE nitrogenase can also form carbon-carbon bonds. The vast majority of research surrounding nitrogenase employs the Fe protein to transfer electrons, which is also associated with the rate-limiting step of nitrogenase catalysis and also requires the hydrolysis of adenosine triphosphate. Thus, there is significant interest in artificially transferring electrons to the catalytic nitrogenase proteins. In this Account, we review nitrogenase electrocatalysis whereby electrons are delivered to nitrogenase from electrodes. We first describe the use of an electron mediator (cobaltocene) to transfer electrons from electrodes to the MoFe protein. The reduction of protons to molecular hydrogen was realized, in addition to azide and nitrite reduction to ammonia. Bypassing the rate-limiting step within the Fe protein, we also describe how this approach was used to interrogate the rate-limiting step of the MoFe protein: metal-hydride protonolysis at the FeMo-co. This Account next reviews the use of cobaltocene to mediate electron transfer to the VFe protein, where the reduction of carbon dioxide and the formation of carbon-carbon bonds (yielding the formation of ethene and propene) was realized. This approach also found success in mediating electron transfer to the FeFe catalytic protein, which exhibited improved carbon dioxide reduction in comparison to the MoFe protein. In the final example of mediated electron transfer to the catalytic protein, this Account also reviews recent work where the coupling of infrared spectroscopy with electrochemistry enabled the potential-dependent binding of carbon monoxide to the FeMo-co to be studied. As an alternative to mediated electron transfer, recent work that has sought to transfer electrons to the catalytic proteins in the absence of electron mediators (by direct electron transfer) is also reviewed. This approach has subsequently enabled a thermodynamic landscape to be proposed for the cofactors of the catalytic proteins. Finally, this Account also describes nitrogenase electrocatalysis whereby electrons are first transferred from an electrode to the Fe protein, before being transferred to the MoFe protein alongside the hydrolysis of adenosine triphosphate. In this way, increased quantities of ammonia can be electrocatalytically produced from dinitrogen fixation. We discuss how this has led to the further upgrade of electrocatalytically produced ammonia, in combination with additional enzymes (diaphorase, alanine dehydrogenase, and transaminase), to selective production of chiral amine intermediates for pharmaceuticals. This Account concludes by discussing current and future research challenges in the field of electrocatalytic nitrogen fixation by nitrogenase.

An Efficient Viologen-Based Electron Donor to Nitrogenase

Nitrogenase catalyzes the reduction of N₂ to NH₃, supporting all biological nitrogen fixation. Electron donors to this enzyme are ferredoxin or flavodoxin (in vivo) and sodium dithionite (in vitro). Features of these electron donors put a limit on spectrophotometric studies and electrocatalytic applications of nitrogenase. Although it is common to use methyl viologen as an electron donor for many low-potential oxidoreductases, decreased nitrogenase activity is observed with an increasing concentration of methyl viologen, limiting its utility under many circumstances. In this work, we suggest that this concentration-dependent decrease in activity can be explained by the formation of a dimer of the radical cation of methyl viologen (Me₂V^•+)₂ at higher methyl viologen concentrations. In addition, viologens functionalized with positively and negatively charged groups were synthesized and studied using spectroscopy and cyclic voltammetry. A sulfonated viologen derivative, 1,1'-bis(3-sulfonatopropyl)-4,4'-bipyridinium radical {[(SPr)₂V^•]^-}, was found to support full nitrogenase activity up to a mediator concentration of 3 mM, while the positively charged viologen derivative was not an efficient reductant of nitrogenase due to the high standard redox potential. The utility of [(SPr)₂V^•]^- as an electron donor for nitrogenase was demonstrated by a simple, sensitive spectrophotometric assay for nitrogenase activity that can provide accurate values for the specific activity and turnover rate constant under argon. Under N₂, the formation of ammonia was confirmed. Because of the observed full activity of nitrogenase and low overpotential, [(SPr)₂V^•]^- should also prove to be valuable for nitrogenase electrocatalysis, including bioelectrosynthetic N₂ reduction.

Upgraded Bioelectrocatalytic N2 Fixation: From N2 to Chiral Amine Intermediates

Enantiomerically pure chiral amines are of increasing value in the preparation of bioactive compounds, pharmaceuticals, and agrochemicals. ω-Transaminase (ω-TA) is an ideal catalyst for asymmetric amination because of its excellent enantioselectivity and wide substrate scope. To shift the equilibrium of reactions catalyzed by ω-TA to the side of the amine product, an upgraded N₂ fixation system based on bioelectrocatalysis was developed to realize the conversion from N₂ to chiral amine intermediates. The produced NH₃ was in situ reacted with l-alanine dehydrogenase to generate alanine with NADH as a coenzyme. ω-TA transferred the amino group from alanine to ketone substrates and finally produced the desired chiral amine intermediates. The cathode of the upgraded N₂ fixation system supplied enough reducing power to synchronously realize the regeneration of reduced methyl viologen (MV^•+) and NADH for the nitrogenase and l-alanine dehydrogenase. The coproduct, pyruvate, was consumed by l-alanine dehydrogenase to regenerate alanine and push the equilibrium to the side of amine. After 10 h of reaction, the concentration of 1-methyl-3-phenylpropylamine achieved 0.54 mM with the 27.6% highest faradaic efficiency and >99% enantiomeric excess (ee_p). Because of the wide substrate scope and excellent enantioselectivity of ω-TA, the upgraded N₂ fixation system has great potential to produce a variety of chiral amine intermediates for pharmaceuticals and other applications.