Electrochemical NADH regeneration and electroenzymatic CO2 reduction on Cu nanorods/glassy carbon electrode prepared by cyclic deposition

Advanced strategies for enzyme-electrode interfacing in bioelectrocatalytic systems

Advances in protein engineering-enabled enzyme immobilization technologies have significantly improved enzyme-electrode wiring in enzymatic electrochemical systems, which harness natural biological machinery to either generate electricity or synthesize biochemicals. In this review, we provide guidelines for designing enzyme-electrodes, focusing on how performance variables change depending on electron transfer (ET) mechanisms. Recent advancements in enzyme immobilization technologies are summarized, highlighting their contributions to extending enzyme-electrode sustainability (up to months), enhancing biosensor sensitivity, improving biofuel cell performance, and setting a new benchmark for turnover frequency in bioelectrocatalysis. We also highlight state-of-the-art protein-engineering approaches that enhance enzyme-electrode interfacing through three key principles: protein-protein, protein-ligand, and protein-inorganic interactions. Finally, we discuss prospective avenues in strategic protein design for real-world applications.

Closing the Loop in the Carbon Cycle: Enzymatic Reactions Housed in Metal-Organic Frameworks for CO2 Conversion to Methanol

The preparation of value-added chemicals from carbon dioxide (CO2) can act as a way of reducing the greenhouse gas from the atmosphere. Industrially significant C1 chemicals like methanol (CH3OH), formic acid (HCOOH), and formaldehyde (HCHO) can be formed from CO2. One sustainable way of achieving this is by connecting the reactions catalyzed by the enzymes formate dehydrogenase (FDH), formaldehyde dehydrogenase (FALDH), and alcohol dehydrogenase (ADH) into a single cascade reaction where CO2 is hydrogenated to CH3OH. For this to be adaptable for industrial use, the enzymes should be immobilized in materials that are extraordinarily protective of the enzymes, inexpensive, stable, and of ultra-large surface area. Metal-organic frameworks (MOFs) meet these criteria and are expected to usher in the much-awaited dispensation of industrial biocatalysis. Unfortunately, little is known about the molecular behaviour of MOF-immobilized FDH, FALDH, and ADH. It is also yet not known which MOFs are most promising for industrial enzyme-immobilization since the field of reticular chemistry is growing exponentially with millions of hypothetical and synthesized MOF structures reported at present. This review initially discusses the properties of the key enzymes required for CO2 hydrogenation to methanol including available cofactor regeneration strategies. Later, the characterization techniques of enzyme-MOF composites and the successes or lack thereof of enzyme-MOF-mediated CO2 conversion to CH3OH and intermediate products are discussed. We also discuss reported multi-enzyme-MOF systems for CO2 conversion cognizant of the fact that at present, these systems are the only chance of housing cascade-type biochemical reactions where strict substrate channelling and operational conditions are required. Finally, we delve into future perspectives.

Product Distribution of Steady-State and Pulsed Electrochemical Regeneration of 1,4-NADH and Integration with Enzymatic Reaction

The direct electrochemical reduction of nicotinamide adenine dinucleotide (NAD+) results in various products, complicating the regeneration of the crucial 1,4-NADH cofactor for enzymatic reactions. Previous research primarily focused on steady-state polarization to examine potential impacts on product selectivity. However, this study explores the influence of dynamic conditions on the selectivity of NAD+ reduction products by comparing two dynamic profiles with steady-state conditions. Our findings reveal that the main products, including 1,4-NADH, several dimers, and ADP-ribose, remained consistent across all conditions. A minor by-product, 1,6-NADH, was also identified. The product distribution varied depending on the experimental conditions (steady state vs. dynamic) and the concentration of NAD+, with higher concentrations and overpotentials promoting dimerization. The optimal yield of 1,4-NADH was achieved under steady-state conditions with low overpotential and NAD+ concentrations. While dynamic conditions enhanced the 1,4-NADH yield at shorter reaction times, they also resulted in a significant amount of unidentified products. Furthermore, this study assessed the potential of using pulsed electrochemical regeneration of 1,4-NADH with enoate reductase (XenB) for cyclohexenone reduction.

Renewable Hydrogen Production and Storage Via Enzymatic Interconversion of CO2 and Formate with Electrochemical Cofactor Regeneration

The urgent need to reduce CO2 emissions has motivated the development of CO2 capture and utilization technologies. An emerging application is CO2 transformation into storage chemicals for clean energy carriers. Formic acid (FA), a valuable product of CO2 reduction, is an excellent hydrogen carrier. CO2 conversion to FA, followed by H2 release from FA, are conventionally chemically catalyzed. Biocatalysts offer a highly specific and less energy-intensive alternative. CO2 conversion to formate is catalyzed by formate dehydrogenase (FDH), which usually requires a cofactor to function. Several FDHs have been incorporated in bioelectrochemical systems where formate is produced by the biocathode and the cofactor is electrochemically regenerated. H2 production from formate is also catalyzed by several microorganisms possessing either formate hydrogenlyase or hydrogen-dependent CO2 reductase complexes. Combination of these two processes can lead to a CO2 -recycling cycle for H2 production, storage, and release with potentially lower environmental impact than conventional methods.

Direct Biocatalytic Processes for CO2 Capture as a Green Tool to Produce Value-Added Chemicals

Direct biocatalytic processes for CO₂ capture and transformation in value-added chemicals may be considered a useful tool for reducing the concentration of this greenhouse gas in the atmosphere. Among the other enzymes, carbonic anhydrase (CA) and formate dehydrogenase (FDH) are two key biocatalysts suitable for this challenge, facilitating the uptake of carbon dioxide from the atmosphere in complementary ways. Carbonic anhydrases accelerate CO₂ uptake by promoting its solubility in water in the form of hydrogen carbonate as the first step in converting the gas into a species widely used in carbon capture storage and its utilization processes (CCSU), particularly in carbonation and mineralization methods. On the other hand, formate dehydrogenases represent the biocatalytic machinery evolved by certain organisms to convert CO₂ into enriched, reduced, and easily transportable hydrogen species, such as formic acid, via enzymatic cascade systems that obtain energy from chemical species, electrochemical sources, or light. Formic acid is the basis for fixing C₁-carbon species to other, more reduced molecules. In this review, the state-of-the-art of both methods of CO₂ uptake is assessed, highlighting the biotechnological approaches that have been developed using both enzymes.

Enzymatic cofactor regeneration systems: A new perspective on efficiency assessment

Nowadays, the specificity of enzymatic processes makes them more and more important every year, and their usage on an industrial scale seems to be necessary. Enzymatic cofactors, however, play a crucial part in the prospective applications of enzymes, because they are indispensable for conducting highly effective biocatalytic activities. Due to the relatively high cost of these compounds and their consumption during the processes carried out, it has become crucial to develop systems for cofactor regeneration. Therefore, in this review, an attempt was made to summarize current knowledge on enzymatic regeneration methods, which are characterized by high specificity, non-toxicity and reported to be highly efficient. The regeneration of cofactors, such as nicotinamide dinucleotides, coenzyme A, adenosine 5'-triphosphate and flavin nucleotides, which are necessary for the proper functioning of a large number of enzymes, is discussed, as well as potential directions for further development of these systems are highlighted. This review discusses a range of highly effective cofactor regeneration systems along with the productive synthesis of many useful chemicals, including the simultaneous renewal of several cofactors at the same time. Additionally, the impact of the enzyme immobilization process on improving the stability and the potential for multiple uses of the developed cofactor regeneration systems was also presented. Moreover, an attempt was made to emphasize the importance of the presented research, as well as the identification of research gaps, which mainly result from the lack of available literature on this topic.

Electroenzymatic Reduction of Furfural to Furfuryl Alcohol by an Electron Mediator and Enzyme Orderly Assembled Biocathode

The electroenzymatic valorization of biomass derivatives into valuable biochemicals has a promising outlook. However, bottlenecks including poor electron transfer between the electrode surface and oxidoreductase, inefficient regeneration of cofactors, and high cost of enzymes and electron mediators hindered the realistic applications of the technique. Herein, to address the above technical barriers, a novel bio-electrocatalytic system that integrates the electrochemical NADH regeneration and enzymatic reaction was constructed, using an orderly assembled composite bioelectrode consisting of an outer immobilized enzyme layer and a sandwiched redox mediator rhodium complex layer. The as-prepared composite bioelectrode was further applied for the highly selective hydrogenation of furfural into furfural alcohol. Results indicated that the enzyme activity was significantly improved, while the furfural valorization was promoted by effective interfacial electron transition and co-factor regeneration on the composite bioelectrode. Considerable high furfural conversion (96.4%) can be achieved accompanied by a furfural alcohol selectivity of 90.0% at -1.2 V (vs Ag/AgCl). The novel composite bioelectrode also showed good stability and reusability. Up to 85.1% of the original furfural alcohol selectivity can be preserved after 10 times of recycling. This work presents a promising green alternative for the valorization of furfural, which also shows great potential extending to the valorization of other biomass compounds.

Selective Furfuryl Alcohol Production from Furfural via Bio-Electrocatalysis

The catalytic reduction of renewable furfural into furfuryl alcohol for various applications is in the ascendant. Nonetheless, the conventional chemo-catalysis hydrogenation of furfural always suffers from poor selectivity, harsh conditions, and expensive catalysts. Herein, to overcome the serious technical barriers of conventional furfuryl alcohol production, an alternative bio-electrocatalytic hydrogenation system was established under mild and neutral conditions, where the dissolved cofactor (NADH) and the alcohol dehydrogenase (ADH) participated in a tandem reaction driven by the electron from a novel Rh (III) complex fixed cathode. Under the optimized conditions, 81.5% of furfural alcohol selectivity can be realized at −0.43 V vs. RHE. This contribution presents a ‘green’ and promising route for the valorization of furfural and other biomass compounds.

Influence of electrode potential, pH and NAD+ concentration on the electrochemical NADH regeneration

Electrochemical NAD⁺ reduction is a promising method to regenerate NADH for enzymatic reactions. Many different electrocatalysts have been tested in the search for high yields of the 1,4-isomer of NADH, the active NADH, but aside from electrode material, other system parameters such as pH, electrode potential and educt concentration also play a role in NADH regeneration. The effect of these last three parameters and the mechanisms behind their influence on NADH regeneration was systematically studied and presented in this paper. With percentages of active NADH ranging from 10 to 70% and faradaic efficiencies between 1 and 30%, it is clear that all three system parameters drastically affect the reaction outcome. As a proof of principle, the NAD+ reduction in the presence of pyruvate and lactate dehydrogenase was performed. It could be shown that the electrochemical NADH regeneration can also be done successfully in parallel to enzymatically usage of the regenerated cofactor.

Improving the Enzymatic Cascade of Reactions for the Reduction of CO2 to CH3OH in Water: From Enzymes Immobilization Strategies to Cofactor Regeneration and Cofactor Suppression

The need to decrease the concentration of CO2 in the atmosphere has led to the search for strategies to reuse such molecule as a building block for chemicals and materials or a source of carbon for fuels. The enzymatic cascade of reactions that produce the reduction of CO2 to methanol seems to be a very attractive way of reusing CO2; however, it is still far away from a potential industrial application. In this review, a summary was made of all the advances that have been made in research on such a process, particularly on two salient points: enzyme immobilization and cofactor regeneration. A brief overview of the process is initially given, with a focus on the enzymes and the cofactor, followed by a discussion of all the advances that have been made in research, on the two salient points reported above. In particular, the enzymatic regeneration of NADH is compared to the chemical, electrochemical, and photochemical conversion of NAD+ into NADH. The enzymatic regeneration, while being the most used, has several drawbacks in the cost and life of enzymes that suggest attempting alternative solutions. The reduction in the amount of NADH used (by converting CO2 electrochemically into formate) or even the substitution of NADH with less expensive mimetic molecules is discussed in the text. Such an approach is part of the attempt made to take stock of the situation and identify the points on which work still needs to be conducted to reach an exploitation level of the entire process.

Strategies for overcoming the limitations of enzymatic carbon dioxide reduction

The overexploitation of fossil fuels has led to a significant increase in atmospheric carbon dioxide (CO₂) concentrations, thereby causing problems, such as the greenhouse effect. Rapid global climate change has caused researchers to focus on utilizing CO₂ in a green and efficient manner. One of the ways to achieve this is by converting CO₂ into valuable chemicals via chemical, photochemical, electrochemical, or enzymatic methods. Among these, the enzymatic method is advantageous because of its high specificity and selectivity as well as the mild reaction conditions required. The reduction of CO₂ to formate, formaldehyde, and methanol using formate dehydrogenase (FDH), formaldehyde dehydrogenase (F_aldDH), and alcohol dehydrogenase (ADH) are attractive routes, respectively. In this review, strategies for overcoming the common limitations of enzymatic CO₂ reduction are discussed. First, we present a brief background on the importance of minimizing of CO₂ emissions and introduce the three bottlenecks limiting enzymatic CO₂ reduction. Thereafter, we explore the different strategies for enzyme immobilization on various support materials. To solve the problem of cofactor consumption, different state-of-the-art cofactor regeneration strategies as well as research on the development of cofactor substitutes and cofactor-free systems are extensively discussed. Moreover, aiming at improving CO₂ solubility, biological, physical, and engineering measures are reviewed. Finally, conclusions and future perspectives are presented.

Efficient Nicotinamide Adenine Dinucleotide Regeneration with a Rhodium-Carbene Catalyst and Isolation of a Hydride Intermediate

Regeneration of nicotinamide adenine dinucleotide (NADH) has been the primary interest in the field of enzymatic transformation, especially associating oxidoreductases given the stoichiometric consumption. The synthesized carbene-ligated rhodium complex [(η⁵-Cp*)Rh(MDI)Cl]⁺ [Cp* = pentamethylcyclopentadienyl; MDI = 1,1'-methylenebis(3,3'-dimethylimidazolium)] acts as an exceptional catalyst in the reduction of NAD⁺ to NADH with a turnover frequency of 1730 h^-1, which is over twice that of the higher catalytic activity of the commercially available catalyst [Cp*Rh(bpy)Cl]⁺ (bpy = 2,2'-bipyridine). Offsetting the contentious atmosphere currently taking place over the specific intermediate of the NADH regeneration, this study presents pivotal evidence of a metal hydride intermediate with a bis(carbene) ligand: a stable form of the rhodium hydride intermediate, [(η⁵-Cp*)Rh(MDI)H]⁺, was isolated and fully characterized. This enables thorough insight into the possible mechanism and exact intermediate structure in the NAD⁺ reduction process.

The challenges of using NAD+-dependent formate dehydrogenases for CO2 conversion

In recent years, CO₂ reduction and utilization have been proposed as an innovative solution for global warming and the ever-growing energy and raw material demands. In contrast to various classical methods, including chemical, electrochemical, and photochemical methods, enzymatic methods offer a green and sustainable option for CO₂ conversion. In addition, enzymatic hydrogenation of CO₂ into platform chemicals could be used to produce economically useful hydrogen storage materials, making it a win-win strategy. The thermodynamic and kinetic stability of the CO₂ molecule makes its utilization a challenging task. However, Nicotine adenine dinucleotide (NAD⁺)-dependent formate dehydrogenases (FDHs), which have high selectivity and specificity, are attractive catalysts to overcome this issue and convert CO₂ into fuels and renewable chemicals. It is necessary to improve the stability, cofactor necessity, and CO₂ conversion efficiency of these enzymes, such as by combining them with appropriate hybrid systems. However, metal-independent, NAD⁺-dependent FDHs, and their CO₂ reduction activity have received limited attention to date. This review outlines the CO₂ reduction ability of these enzymes as well as their properties, reaction mechanisms, immobilization strategies, and integration with electrochemical and photochemical systems for the production of formic acid or formate. The biotechnological applications of FDH, future perspectives, barriers to CO₂ reduction with FDH, and aspects that must be further developed are briefly summarized. We propose that constructing hybrid systems that include NAD⁺-dependent FDHs is a promising approach to convert CO₂ and strengthen the sustainable carbon bio-economy.

Zirconia supported gold-palladium nanocatalyst for NAD(P)H regeneration via two-step mechanism

The regeneration cycle of expensive cofactor, NAD(P)H, is of paramount importance for the bio-catalyzed redox reactions. Here a ZrO₂supported bimetallic nanocatalyst of gold-palladium (Au-Pd/ZrO₂) was prepared to catalyze the regeneration of NAD(P)H without using electron mediators and extra energy input. Over 98% of regeneration efficiency can be achieved catlyzed by Au-Pd/ZrO₂using TEOA as the electron donor. Mechanism study showed that the regeneration of NAD(P)H took place through a two-step process: Au-Pd/ZrO₂nanocatalyst first catalyzed the oxidation of triethanolamine (TEOA) to glycolaldehyde (GA), then the generated GA induced the non-catalytic reducing of NAD(P)⁺to NAD(P)H under an alkaline environment maintained by TEOA. This two-step mechanism enables the decoupling of the regeneration of NAD(P)H in space and time into a catalytic oxidation and non-catalytic reducing cascade process which has been further verified using a variety of electron donors. The application significance of this procedure is further demonstrated both by the favorable stability of Au-Pd/ZrO₂nanocatalyst in 5 successive cycles preserving over 90% of its original activity, and by the excellent performance of the regenerated NADH as the cofactor in the catalytic hydrogenation of acetaldehyde using an ethanol dehydrogenase.

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.

Recent Progress and Perspectives on Electrochemical Regeneration of Reduced Nicotinamide Adenine Dinucleotide (NADH)

NAD is a cofactor that maintains cellular redox homeostasis and has immense industrial and biological significance. It acts as an enzymatic mediator in several biocatalytic electrochemical reactions and undergoes oxidation/reduction to form NAD⁺ or NADH, respectively. The NAD redox couple (NAD⁺ /NADH) mostly exists in enzyme-assisted metabolic reactions as a coenzyme during which electrons and protons are transferred. NADH shuttles these charges between the enzyme and the substrate. In order to understand such complex metabolic reactions, it is vital to study the bio-electrochemistry of NADH. In addition, the regeneration of NADH in industries has attracted significant attention due to its vast usage and high cost. To make biocatalysis economically viable, primary methods of NADH regeneration including enzymatic, chemical, photochemical and electrochemical methods are widely used. This review is mainly focused on the electrochemical reduction of NAD⁺ to NADH with specific details on the mechanism and kinetics of the reaction. It provides emphasis on the different routes (direct and mediated) to electrochemically regenerate NADH from NAD⁺ highlighting the NAD dimer formation. Also, it describes the electrocatalysts developed until now and the scope for development in this area of research.

Surface Modification of a MOF-based Catalyst with Lewis Metal Salts for Improved Catalytic Activity in the Fixation of CO2 into Polymers

The catalyst zinc glutarate (ZnGA) is widely used in the industry for the alternating copolymerization of CO2 with epoxides. However, the activity of this heterogeneous catalyst is restricted to the outer surface of its particles. Consequently, in the current study, to increase the number of active surface metal centers, ZnGA was treated with diverse metal salts to form heterogeneous, surface-modified ZnGA-Metal chloride (ZnGA-M) composite catalysts. These catalysts were found to be highly active for the copolymerization of CO2 and propylene oxide. Among the different metal salts, the catalysts treated with ZnCl2 (ZnGA-Zn) and FeCl3 (ZnGA-Fe) exhibited ~38% and ~25% increased productivities, respectively, compared to untreated ZnGA catalysts. In addition, these surface-modified catalysts are capable of producing high-molecular-weight polymers; thus, this simple and industrially viable surface modification method is beneficial from an environmental and industrial perspective.

Photoreduction of CO2 with a Formate Dehydrogenase Driven by Photosystem II Using a Semi-artificial Z-Scheme Architecture

Solar-driven coupling of water oxidation with CO₂ reduction sustains life on our planet and is of high priority in contemporary energy research. Here, we report a photoelectrochemical tandem device that performs photocatalytic reduction of CO₂ to formate. We employ a semi-artificial design, which wires a W-dependent formate dehydrogenase (FDH) cathode to a photoanode containing the photosynthetic water oxidation enzyme, Photosystem II, via a synthetic dye with complementary light absorption. From a biological perspective, the system achieves a metabolically inaccessible pathway of light-driven CO₂ fixation to formate. From a synthetic point of view, it represents a proof-of-principle system utilizing precious-metal-free catalysts for selective CO₂-to-formate conversion using water as an electron donor. This hybrid platform demonstrates the translatability and versatility of coupling abiotic and biotic components to create challenging models for solar fuel and chemical synthesis.

Novel 96-well quantitative bioelectrocatalytic analysis platform reveals highly efficient direct electrode regeneration of cytochrome P450 BM3 on indium tin oxide

Enzymes are the most effective catalysts for a broad range of difficult chemical reactions e.g. hydroxylation of non-activated C-H Bonds and stereoselective synthesis. Nevertheless, a lot of enzymes are not accessible for the biotechnological applications or industrial use. One reason is the prerequisite of expensive cofactors. In this context, we developed a bioelectrocatalytic analysis platform for the electrochemical and photonic quantification of the direct electron transfer from the electrode to redox enzymes and therefore, bypass the need of soluble cofactors that had to be continuously exchanged or regenerated. As reference enzyme, we chose cytochrome P450 BM3 that is restricted by NADPH dependence. We optimized the substrate spectrum for aromatic compounds by introduction of the triple mutation A74G/F87V/L188Q and established a sensitive fluorimetric product formation assay to monitor the enzymatic conversion of 7-ethoxycoumarine to 7-hydroxycoumarine. Gold and indium tin oxide electrodes were characterized with respect to surface morphology, charge-transfer resistance and P450 BM3 immobilization as well as activity. Using gold electrodes, no significant product formation by electrode mediated direct electron transfer could be detected. In contrast, P450 BM3 adsorbed on unmodified indium tin oxide electrodes revealed 36% activity by electrode mediated direct electron transfer in comparison to enzyme regeneration by NADPH. Since the reaction volumes are in the microliter range and upscaling of the measurement system is easily possible, our analysis platform is a useful tool for bioelectrocatalytic enzyme characterization and library screening based optimization for applications in the field of enzyme catalyzed chemical synthesis but also enzyme based fuel cells.

Enhancement of formic acid production from CO2 in formate dehydrogenase reaction using nanoparticles

In an enzyme process using a gas substrate, the enhanced gas liquid mass transfer rate of the gas substrate by methyl-functionalized mesoporous nanoparticles could improve the productivity.