Photocatalytic regeneration of nicotinamide cofactor biomimetics drives biocatalytic reduction by Old Yellow enzymes

A key approach in developing green chemistry involves converting solar energy into chemical energy of biomolecules through photocatalysis. Photocatalysis can facilitate the regeneration of nicotinamide cofactors during redox processes. Nicotinamide cofactor biomimetics (NCBs) are economical substitutes for natural cofactors. Here, photocatalytic regeneration of NADH and reduced NCBs (NCBsred) using graphitic carbon nitride (g-C3N4) was developed. The process involves g-C3N4 as the photocatalyst, Cp*Rh(bpy)H2O2+ as the electron mediator, and Triethanolamine as the electron donor, facilitating the reduction of NAD+ and various oxidative NCBs (NCBsox) under light irradiation. Notably, the highest reduction yield of 48.32 % was achieved with BANA+, outperforming the natural cofactor NAD+. Electrochemical analysis reveals that the reduction efficiency and capacity of cofactors relies on their redox potentials. Additionally, a coupled photo-enzymatic catalysis system was explored for the reduction of 4-Ketoisophorone by Old Yellow Enzyme XenA. Among all the NCBsox and NAD+, the highest conversion ratio of over 99 % was obtained with BANA+. After recycled for 8 times, g-C3N4 maintained over 93.6 % catalytic efficiency. The photocatalytic cofactor regeneration showcases its outstanding performance with NAD+ as well as NCBsox. This work significantly advances the development of photocatalytic cofactor regeneration for artificial cofactors and its potential application.

Visible light-driven oxidation of non-native substrate by laccase attached on Ru-based metal-organic frameworks

Light-induced electron transfer can broaden the substrate range of metalloenzyme. However, the efficiency of photo-enzyme coupling is limited by the poor combination of photosensitizer or photocatalyst with enzyme. Herein, we prepared the nano-photocatalyst MIL-125-NH2@Ru(bpy) by in site embedding ruthenium pyridine-diimine complex [Ru(bpy)3]2+ into metal organic frameworks MIL-125-NH2 and associated it with multicopper oxidase (MCO) laccase. Compared to [Ru(bpy)3]2+, the coupling efficiency of MIL-125-NH2@Ru(bpy)3 for enzymatic oxygen reduction increased by 35.7%. A series of characterizations confirmed that the amino group of laccase formed chemical bonds with the surface defects or hydrophobic groups of MIL-125-NH2@Ru(bpy)3. Consequently, the tight binding accelerated the quenching process and electron transfer between laccase and the immobilized ruthenium pyridine-diimine complex. This work would open an avenue for the synthesis of MOFs photocatalyst towards photo-enzyme coupling.

Light-Driven CO2 Reduction with a Surface-Displayed Enzyme Cascade-C3N4 Hybrid

Efficient and cost-effective conversion of CO2 to biomass holds the potential to address the climate crisis. Light-driven CO2 conversion can be realized by combining inorganic semiconductors with enzymes or cells. However, designing enzyme cascades for converting CO2 to multicarbon compounds is challenging, and inorganic semiconductors often possess cytotoxicity. Therefore, there is a critical need for a straightforward semiconductor biohybrid system for CO2 conversion. Here, we used a visible-light-responsive and biocompatible C3N4 porous nanosheet, decorated with formate dehydrogenase, formaldehyde dehydrogenase, and alcohol dehydrogenase to establish an enzyme-photocoupled catalytic system, which showed a remarkable CO2-to-methanol conversion efficiency with an apparent quantum efficiency of 2.48% in the absence of externally added electron mediator. To further enable the in situ transformation of methanol into biomass, the enzymes were displayed on the surface of Komagataella phaffii, which was further coupled with C3N4 to create an organic semiconductor-enzyme-cell hybrid system. Methanol was produced through enzyme-photocoupled CO2 reduction, achieving a rate of 4.07 mg/(L·h), comparable with reported rates from photocatalytic systems employing mediators or photoelectrochemical cells. The produced methanol can subsequently be transported into the cell and converted into biomass. This work presents a sustainable, environmentally friendly, and cost-effective enzyme-photocoupled biocatalytic system for efficient solar-driven conversion of CO2 within a microbial cell.

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.

Artificial Photoenzymatic Reduction of Carbon Dioxide to Methanol by Using Electron Mediator and Co-factorAssembled ZnIn2 S4 Nanoflowers

Increased absorption of visible light, low electron-hole recombination, and fast electron transfer are the major objectives for highly effective photocatalysts in biocatalytic artificial photosynthetic systems. In this study, a polydopamine (PDA) layer containing electron mediator, [M], and NAD⁺ cofactor was assembled on the outer surface of ZnIn₂ S₄ nanoflower, and the as-prepared nanoparticle, ZnIn₂ S₄ /PDA@poly/[M]/NAD⁺ , was used for photoenzymatic methanol production from CO₂ . Because of effective capturing of visible light, reduced distance of electron transfer, and elimination of electron-holes recombination, a high NADH regeneration of 80.7±1.43 % could be obtained using the novel ZnIn₂ S₄ /PDA@poly/[M]/NAD⁺ . In the artificial photosynthesis system, a maximum methanol production of 116.7±11.8 μm was obtained. The enzymes and nanoparticles in the hybrid bio-photocatalysis system could be easily recovered using the ultrafiltration membrane at the bottom of the photoreactor. This is due to the successful immobilization of the small blocks including the electron mediator and cofactor on the surface of the photocatalyst. The ZnIn₂ S₄ /PDA@poly/[M]/NAD⁺ photocatalyst exhibited good stability and recyclability for methanol production. The novel concept presented in this study shows great promise for other sustainable chemical productions through artificial photoenzymatic catalysis.

A Comprehensive Review on Graphitic Carbon Nitride for Carbon Dioxide Photoreduction

Inspired by natural photosynthesis, harnessing the wide range of natural solar energy and utilizing appropriate semiconductor-based catalysts to convert carbon dioxide into beneficial energy species, for example, CO, CH₄ , HCOOH, and CH₃ COH have been shown to be a sustainable and more environmentally friendly approach. Graphitic carbon nitride (g-C₃ N₄ ) has been regarded as a highly effective photocatalyst for the CO₂ reduction reaction, owing to its cost-effectiveness, high thermal and chemical stability, visible light absorption capability, and low toxicity. However, weaker electrical conductivity, fast recombination rate, smaller visible light absorption window, and reduced surface area make this catalytic material unsuitable for commercial photocatalytic applications. Therefore, certain procedures, including elemental doping, structural modulation, functional group adjustment of g-C₃ N₄ , the addition of metal complex motif, and others, may be used to improve its photocatalytic activity towards effective CO₂ reduction. This review has investigated the scientific community's perspectives on synthetic pathways and material optimization approaches used to increase the selectivity and efficiency of the g-C₃ N₄ -based hybrid structures, as well as their benefits and drawbacks on photocatalytic CO₂ reduction. Finally, the review concludes a comparative discussion and presents a promising picture of the future scope of the improvements.

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.

Synthesis of highly efficient selenium oxide hybridized g-C3N4 photocatalyst for NADH/NADPH regeneration to facilitate solar-to-chemical reaction

An inexpensive graphitic carbon nitrite (g-C3N4) photocatalyst was hybridized with selenium oxide (SeO2) photocatalyst by a monolayer-dispersed technique. After hybridization of g-C3N4 with SeO2, the NADH/NADPH regeneration efficiency of SeO2 photocatalyst was enhanced under solar light illumination was observed. The photocatalytic activity of SeO2/g-C3N4 photocatalyst under solar light illumination was enhanced by 3-fold higher than g-C3N4 photocatalyst, the solar light photocatalytic activity was produced and the photo-decomposition of SeO2 photocatalyst was completely stifled after hybridized SeO2 photocatalyst by g-C3N4 photocatalyst. The improvement in performance and photo-decomposition inhibition under solar light illumination was persuaded by efficiency separation of photo-persuaded holes from SeO2 to the valence bond (V.B.)/highest occupied molecular orbital (HOMO) of g-C3N4 under solar light illumination, the electron jumped from the V.B. to the conduction band (C.B.)/lowest unoccupied molecular orbital (LUMO) of g-C3N4 could directly insert into the C.B. of SeO2 photocatalyst, synthesized SeO2/g-C3N4 photocatalyst is highly active for NADH/NADPH regeneration under solar light.

Modulating Anodic Electrochemiluminescence of Graphitic Carbon Nitride by Thiophene Doping

As an attractive electrochemiluminescence (ECL) emitter, graphitic carbon nitride (g-CN) still suffers from weak anodic ECL signal. In this work, a simple molecular engineering strategy is employed to modulate anodic...

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.

Photoenzymatic Catalytic Cascade System of a Pyromellitic Diimide/g-C3N4 Heterojunction to Efficiently Regenerate NADH for Highly Selective CO2 Reduction toward Formic Acid

Photocatalytic reduction of carbon dioxide (CO₂) holds great promise for both clean energy and environment protection. However, the low activity and poor selectivity of photocatalysts are the main bottlenecks. Herein, inspired by artificial photosynthesis and taking advantages of high efficiency and specificity of bioenzymes, we marry photo with enzyme to synergistically solve the above problems. A metal-free heterojunction of pyromellitic diimide/g-C₃N₄ (PDI/CN) with an excellent visible light response (λ < 660 nm) is fabricated for achieving a photoenzymatic catalytic cascade system, which efficiently regenerates nicotinamide adenine dinucleotide (NADH) and selectively reduces CO₂ to formic acid (HCOOH). The highest NADH yield of the PDI/CN hybrid achieved is 75%, and the HCOOH generation rate achieved is 1.269 mmol g^-1 h^-1 with nearly 100% selectivity, which is much higher than those of the reported materials. The excellent photocatalytic performance is attributed to the unique photoenzymatic catalytic cascade system, heterointerface effect, good conductivity, and a wide sunlight response range of the PDI/CN heterojunction. This work provides an efficient strategy and a corresponding photocatalyst for the directional conversion of CO₂ to HCOOH.

General framework for enzyme-photo-coupled catalytic system toward carbon dioxide conversion

High emission of carbon dioxide (CO₂) has aroused global concern due to the 'greenhouse effect'. The conversion of CO₂ to valuable chemicals/materials is an indispensable route toward 'carbon neutrality'. Enzyme-photo-coupled catalytic systems (EPCCSs), integrating synthetic library of semiconductor photocatalyst and natural database of enzyme, have emerged as a green and powerful platform toward CO₂ conversion. Herein, we discuss the recent progress in design and application of EPCCSs for CO₂ conversion from the perspective of pathway engineering, reaction engineering and system engineering. We firstly summarize the explored pathways of EPCCSs for converting CO₂ to C₁ and C₂₊ products. Secondly, we discuss the matching of kinetics between photocatalytic and enzymatic reactions in EPCCSs. Thirdly, we unveil the complex interplay between photocatalytic and enzymatic modules, and further demonstrate the strategy of compartmentalization to eliminate the negative interactions. Lastly, we conclude with the perspective on the opportunities and challenges of EPCCSs for CO₂ conversion.

Photocatalyst-enzyme hybrid systems for light-driven biotransformation

Enzymes catalyse target reactions under mild conditions with high efficiency, as well as excellent regional-, stereo-, and enantiomeric selectivity. Photocatalysis utilises sustainable and environment-friendly light power to realise efficient chemical conversion. By combining the interdisciplinary advantages of photo- and enzymatic catalysis, the photocatalyst-enzyme hybrid systems have proceeded various light-driven biotransformation with high efficiency under environmentally benign conditions, thus, attracting unparalleled focus during the last decades. It has also been regarded as a promising pathway towards green chemistry utilising ubiquitous solar energy. This systematic review gives insight into this research field by classifying the existing photocatalyst-enzyme hybrid systems into three sections based on different hybridizing modes between photo- and enzymatic catalysis. Furthermore, existing challenges and proposed strategies are discussed within this context. The first system summarised is the cofactor-mediated hybrid system, in which natural/artificial cofactors act as reducing equivalents that connect photocatalysts with enzymes for light-driven enzymatic biotransformation. Second, the direct contact-based photocatalyst-enzyme hybrid systems are described, including two different kinds of electron exchange sites on the enzyme molecules. Third, some cases where photocatalysts and enzymes are integrated into a reaction cascade with specific intermediates will be discussed in the following chapter. Finally, we provide perspective concerning the future of this field.

Highly Efficient S-g-CN/Mo-368 Catalyst for Synergistically NADH Regeneration Under Solar Light

Sulfur-doped graphitic carbon nitride (S-g-CN) has gained significant attention in recent years. Sulfur-doped graphitic carbon nitride (S-g-CN) is a promising metal-free photocatalyst because of its band orientation, natural abundance and groundwork. Improved photocatalytic activity of S-g-CN material for solar chemical production persists a hot yet challenging problem. Herein, we provide an adaptable method for the synthesis of S-g-CN nanocomposite decorated with the moiety of giant polyoxometalate (S-g-CN/Mo-368) that subsequently showed highly efficient photocatalytic activity. The as-synthesized S-g-CN/Mo-368 as a recyclable artificial photocatalyst revealed excellent activity for solar chemical production, that is nicotinamide adenine dinucleotide (NADH) regeneration under visible light. The immobilized Mo-368 on the S-g-CN surface increased the visible light adsorption capacity of the S-g-CN/Mo-368 photocatalyst. The visible light absorption activity, morphology, element compositions, particle size and zeta potential of S-g-CN powder and S-g-CN/Mo-368 were thoroughly investigated. From the application point of view, S-g-CN/Mo-368 was applied to determine the solar chemical production (i.e. NADH regeneration) under visible light with a higher yield% of about ~ 94.85%.

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.

Construction of Functionally Compartmental Inorganic Photocatalyst-Enzyme System via Imitating Chloroplast for Efficient Photoreduction of CO2 to Formic Acid

Inorganic photocatalyst-enzyme systems are a prominent platform for the photoreduction of CO₂ to value-added chemicals and fuels. However, poor electron transfer kinetics and enzyme deactivation by reactive oxygen species in the photoexcitation process severely limit catalytic efficiency. In chloroplast, enzymatic CO₂ reduction and photoexcitation are compartmentalized by the thylakoid membrane, which protects enzymes from photodamage, while the tightly integrated photosystem facilitates electron transfer, promoting photocatalysis. By mimicking this strategy, we constructed a novel functionally compartmental inorganic photocatalyst-enzyme system for CO₂ reduction to formate. To accomplish efficient electron transfer, we first synthesized an integrated artificial photosystem by conjugation of the cocatalyst (a Rh complex) onto thiophene-modified C₃N₄ (TPE-C₃N₄), demonstrating an NADH regeneration rate of 9.33 μM·min^-1, 2.33 times higher than that of a homogeneous counterpart. The enhanced NADH regeneration activity was caused by the tightly conjugated structure of the artificial photosystem, enabling rapid electron transfer from TPE-C₃N₄ to the Rh complex. To protect formate dehydrogenase (FDH) from photoinduced deactivation, FDH was encapsulated into MAF-7, a metal-organic framework (MOF) material, to compartmentalize FDH from the toxic photoexcitation process, similar to the function of the thylakoid membrane. Moreover, the triazole linkers of MAF-7 possess both hydrophilicity and pH-buffering capacity providing a stable microenvironment for FDH, which could enhance enzyme stability in photosynthesis. The synergy between the enhanced electron transfer of TPE-C₃N₄ for NADH cofactor regeneration and MOF-protection of the redox enzyme enables the construction of a functionally compartmental inorganic photocatalyst-enzyme association system, promoting CO₂ photoconversion to formic acid with a yield of 16.75 mM after 9 h of illumination, 3.24 times greater than that of the homogeneous reaction counterpart.

Catalytic recycling of NAD(P)H

A large number of industrially relevant enzymes depend upon dihydronicotinamide adenine dinucleotide (NADH) and dihydronicotinamide adenine dinucleotide phosphate (NADPH) cofactors, which are too expensive to be added in stoichiometric amounts. Existing NAD(P)H-recycling systems suffer from low activity, or the generation of side products. This review focuses on NAD(P)H cofactor regeneration catalyzed by transition metal complexes such as rhodium, ruthenium and iridium complexes using cheap reducing agents such as hydrogen (H₂) and ethanol, which have attracted increasing attention as sustainable energy carriers. The catalytic mechanisms for the regioselective reduction of NAD(P)⁺ are discussed with emphasis on identification of catalytically active intermediates such as transition metal hydride complexes. Applications of NAD(P)H-recycling systems to develop artificial photosynthesis are also discussed.