Photobiocatalysis: Activating Redox Enzymes by Direct or Indirect Transfer of Photoinduced Electrons

Light-driven carbon-carbon bond formation via CO2 reduction catalyzed by complexes of CdS nanorods and a 2-oxoacid oxidoreductase

Redox enzymes are capable of catalyzing a vast array of useful reactions, but they require redox partners that donate or accept electrons. Semiconductor nanocrystals provide a mechanism to convert absorbed photon energy into redox equivalents for enzyme catalysis. Here, we describe a system for photochemical carbon-carbon bond formation to make 2-oxoglutarate by coupling CO₂ with a succinyl group. Photoexcited electrons from cadmium sulfide nanorods (CdS NRs) transfer to 2-oxoglutarate:ferredoxin oxidoreductase from Magnetococcus marinus MC-1 (MmOGOR), which catalyzes a carbon-carbon bond formation reaction. We thereby decouple MmOGOR from its native role in the reductive tricarboxylic acid cycle and drive it directly with light. We examine the dependence of 2-oxoglutarate formation on a variety of factors and, using ultrafast transient absorption spectroscopy, elucidate the critical role of electron transfer (ET) from CdS NRs to MmOGOR. We find that the efficiency of this ET depends strongly on whether the succinyl CoA (SCoA) cosubstrate is bound at the MmOGOR active site. We hypothesize that the conformational changes due to SCoA binding impact the CdS NR-MmOGOR interaction in a manner that decreases ET efficiency compared to the enzyme with no cosubstrate bound. Our work reveals structural considerations for the nano-bio interfaces involved in light-driven enzyme catalysis and points to the competing factors of enzyme catalysis and ET efficiency that may arise when complex enzyme reactions are driven by artificial light absorbers.

Biocatalysis Fueled by Light: On the Versatile Combination of Photocatalysis and Enzymes

Enzymes catalyze a plethora of highly specific transformations under mild and environmentally benign reaction conditions. Their fascinating performances attest to high synthetic potential that is often hampered by operational obstacles such as in vitro cofactor supply and regeneration. Exploiting light and combining it with biocatalysis not only helps in overcoming these drawbacks, but the fruitful liaison of these two fields of "green chemistry" also offers opportunities to unlock new synthetic reactivities. In this review we provide an overview of the wide variety of photo-biocatalysis, ranging from the photochemical delivery of electrons required in redox biocatalysis and photochemical cofactor and reagent (re)generation to direct photoactivation of enzymes enabling reactions unknown in nature. We highlight synthetically relevant transformations such as asymmetric reactions facilitated by the combination of light as energy source and enzymes' catalytic power.

Nicotinamide adenine dinucleotide as a photocatalyst

Nicotinamide adenine dinucleotide (NAD⁺) is a key redox compound in all living cells responsible for energy transduction, genomic integrity, life-span extension, and neuromodulation. Here, we report a new function of NAD⁺ as a molecular photocatalyst in addition to the biological roles. Our spectroscopic and electrochemical analyses reveal light absorption and electronic properties of two π-conjugated systems of NAD⁺. Furthermore, NAD⁺ exhibits a robust photostability under UV-Vis-NIR irradiation. We demonstrate photocatalytic redox reactions driven by NAD⁺, such as O₂ reduction, H₂O oxidation, and the formation of metallic nanoparticles. Beyond the traditional role of NAD⁺ as a cofactor in redox biocatalysis, NAD⁺ executes direct photoactivation of oxidoreductases through the reduction of enzyme prosthetic groups. Consequently, the synergetic integration of biocatalysis and photocatalysis using NAD⁺ enables solar-to-chemical conversion with the highest-ever-recorded turnover frequency and total turnover number of 1263.4 hour^-1 and 1692.3, respectively, for light-driven biocatalytic trans-hydrogenation.

Multiplexed Readout of Enzymatic Reactions by Means of Laterally Resolved Illumination of Quantum Dot Electrodes

Triggering electrochemical reactions with light provides a powerful tool for the control of complex reaction schemes on photoactive electrodes. Here, we report on the light-directed, multiplexed detection of enzymatic substrates using a nonstructured gold electrode modified with CdSe/ZnS quantum dots (QDs) and two enzymes, glucose oxidase (GOx) and sarcosine oxidase (SOx). While QDs introduce visible-light sensitivity into the electrode architecture, GOx and SOx allow for a selective conversion of glucose and sarcosine, respectively. For the QD immobilization to the gold electrode, a linker-assisted approach using trans-4,4'-stilbenedithiol has been used, resulting in the generation of a photocurrent. Subsequently, GOx and SOx have been immobilized in spatially separated spots onto the QD electrode. For the local readout of the QD electrode, a new measurement setup has been developed by moving a laser pointer across the surface to defined positions on the chip surface. The amplitudes of the photocurrents upon illumination of the GOx or SOx spot depend in a concentration-dependent manner on the presence of glucose and sarcosine, respectively. This measurement also allows for a selective detection in the presence of other substances. The setup demonstrates the feasibility of multiplexed measurements of enzymatic reactions using a focused light pointer, resulting in an illumination area with a diameter of 0.3 mm for analyzing spots of different enzymes. Moving the laser pointer in the x- and y-direction and simultaneously detecting the local photocurrent also allow a spatial imaging of enzyme immobilization. Here, not only the spot dimensions but also the activity of the enzyme can be verified.

Discovery, Characterisation, Engineering and Applications of Ene Reductases for Industrial Biocatalysis

Recent studies of multiple enzyme families collectively referred to as ene-reductases (ERs) have highlighted potential industrial application of these biocatalysts in the production of fine and speciality chemicals. Processes have been developed whereby ERs contribute to synthetic routes as isolated enzymes, components of multi-enzyme cascades, and more recently in metabolic engineering and synthetic biology programmes using microbial cell factories to support chemicals production. The discovery of ERs from previously untapped sources and the expansion of directed evolution screening programmes, coupled to deeper mechanistic understanding of ER reactions, have driven their use in natural product and chemicals synthesis. Here we review developments, challenges and opportunities for the use of ERs in fine and speciality chemicals manufacture. The ER research field is rapidly expanding and the focus of this review is on developments that have emerged predominantly over the last 4 years.

Discovery, Characterisation, Engineering and Applications of Ene Reductases for Industrial Biocatalysis

Recent studies of multiple enzyme families collectively referred to as ene-reductases (ERs) have highlighted potential industrial application of these biocatalysts in the production of fine and speciality chemicals. Processes have been developed whereby ERs contribute to synthetic routes as isolated enzymes, components of multi-enzyme cascades, and more recently in metabolic engineering and synthetic biology programmes using microbial cell factories to support chemicals production. The discovery of ERs from previously untapped sources and the expansion of directed evolution screening programmes, coupled to deeper mechanistic understanding of ER reactions, have driven their use in natural product and chemicals synthesis. Here we review developments, challenges and opportunities for the use of ERs in fine and speciality chemicals manufacture. The ER research field is rapidly expanding and the focus of this review is on developments that have emerged predominantly over the last 4 years.

Light-induced formation of NO in endothelial cells by photoactivatable NADPH analogues targeting nitric-oxide synthase

BACKGROUND

Nitric-oxide synthases (NOS) catalyze the formation of NO using NADPH as electron donor. We have recently designed and synthesized a new series of two-photon absorbing and photoactivatable NADPH analogues (NT). These compounds bear one or two carboxymethyl group(s) on the 2'- or/and 3'-position(s) of the ribose in the adenosine moiety, instead of a 2'-phosphate group, and differ by the nature of the electron donor in their photoactivatable chromophore (replacing the nicotinamide moiety). Here, we addressed the ability of NTs to photoinduce eNOS-dependent NO production in endothelial cells.

METHODS

The cellular fate of NTs and their photoinduced effects were studied using multiphoton fluorescence imaging, cell viability assays and a BODIPY-derived NO probe for NO measurements. The eNOS dependence of photoinduced NO production was addressed using two NOS inhibitors (NS1 and L-NAME) targeting the reductase and the oxygenase domains, respectively.

RESULTS

We found that, two compounds, those bearing a single carboxymethyl group on the 3'-position of the ribose, colocalize with the Golgi apparatus (the main intracellular location of eNOS) and display high intracellular two-photon brightness. Furthermore, a eNOS-dependent photooxidation was observed for these two compounds only, which is accompanied by a substantial intracellular NO production accounting for specific photocytotoxic effects.

CONCLUSIONS

We show for the first time that NT photoactivation efficiently triggers electron flow at the eNOS level and increases the basal production of NO by endothelial cells.

GENERAL SIGNIFICANCE

Efficient photoactivatable NADPH analogues targeting NOS could have important implications for generating apoptosis in tumor cells or modulating NO-dependent physiological processes.

Morpholine-based buffers activate aerobic photobiocatalysis via spin correlated ion pair formation

The use of enzymes for synthetic applications is a powerful and environmentally-benign approach to increase molecular complexity. Oxidoreductases selectively introduce oxygen and hydrogen atoms into myriad substrates, catalyzing the synthesis of chemical and pharmaceutical building blocks for chemical production. However, broader application of this class of enzymes is limited by the requirements of expensive cofactors and low operational stability. Herein, we show that morpholine-based buffers, especially 3-(N-morpholino)propanesulfonic acid (MOPS), promote photoinduced flavoenzyme-catalyzed asymmetric redox transformations by regenerating the flavin cofactor via sacrificial electron donation and by increasing the operational stability of flavin-dependent oxidoreductases. The stabilization of the active forms of flavin by MOPS via formation of the spin correlated ion pair 3[flavin˙--MOPS˙+] ensemble reduces the formation of hydrogen peroxide, circumventing the oxygen dilemma under aerobic conditions detrimental to fragile enzymes.

Structure-Activity Relationships of Hierarchical Three-Dimensional Electrodes with Photosystem II for Semiartificial Photosynthesis

Semiartificial photosynthesis integrates photosynthetic enzymes with artificial electronics, which is an emerging approach to reroute the natural photoelectrogenetic pathways for sustainable fuel and chemical synthesis. However, the reduced catalytic activity of enzymes in bioelectrodes limits the overall performance and further applications in fuel production. Here, we show new insights into factors that affect the photoelectrogenesis in a model system consisting of photosystem II and three-dimensional indium tin oxide and graphene electrodes. Confocal fluorescence microscopy and in situ surface-sensitive infrared spectroscopy are employed to probe the enzyme distribution and penetration within electrode scaffolds of different structures, which is further correlated with protein film-photoelectrochemistry to establish relationships between the electrode architecture and enzyme activity. We find that the hierarchical structure of electrodes mainly influences the protein loading but not the enzyme activity. Photoactivity is more limited by light intensity and electronic communication at the biointerface. This study provides guidelines for maximizing the performance of semiartificial photosynthesis and also presents a set of methodologies to probe the photoactive biofilms in three-dimensional electrodes.

Artificial Organelles for Energy Regeneration

One of the critical steps in sustaining life-mimicking processes in synthetic cells is energy, i.e., adenosine triphosphate (ATP) regeneration. Previous studies have shown that the simple addition of ATP or ATP regeneration systems, which do not regenerate ATP directly from ADP and P_i , have no or only limited success due to accumulation of ATP hydrolysis products. In general, ATP regeneration can be achieved by converting light or chemical energy into ATP, which may also involve redox transformations of cofactors. The present contribution provides an overview of the existing ATP regeneration strategies and the related nicotinamide adenine dinucleotide (NAD⁺ ) redox cycling, with a focus on compartmentalized systems. Special attention is being paid to those approaches where so-called artificial organelles are developed. They comprise a semipermeable membrane functionalized by biological or man-made components and employ external energy in the form of light or nutrients in order to generate a transmembrane proton gradient, which is further utilized for ATP synthesis.

Interfacing Formate Dehydrogenase with Metal Oxides for the Reversible Electrocatalysis and Solar-Driven Reduction of Carbon Dioxide

The integration of enzymes with synthetic materials allows efficient electrocatalysis and production of solar fuels. Here, we couple formate dehydrogenase (FDH) from Desulfovibrio vulgaris Hildenborough (DvH) to metal oxides for catalytic CO₂ reduction and report an in‐depth study of the resulting enzyme–material interface. Protein film voltammetry (PFV) demonstrates the stable binding of FDH on metal‐oxide electrodes and reveals the reversible and selective reduction of CO₂ to formate. Quartz crystal microbalance (QCM) and attenuated total reflection infrared (ATR‐IR) spectroscopy confirm a high binding affinity for FDH to the TiO₂ surface. Adsorption of FDH on dye‐sensitized TiO₂ allows for visible‐light‐driven CO₂ reduction to formate in the absence of a soluble redox mediator with a turnover frequency (TOF) of 11±1 s⁻¹. The strong coupling of the enzyme to the semiconductor gives rise to a new benchmark in the selective photoreduction of aqueous CO₂ to formate.

Contribution of substrate reorganization energies of electron transfer to laccase activity

Laccase substrate reorganization energies computed by DFT show that electronic structure changes of these substrates contribute to enzymatic proficiency.

Boosting photobioredox catalysis by morpholine electron donors under aerobic conditions

Morpholine derivatives expand the applicability of photobiocatalysis towards stabilization of flavin-based bio- and photocatalysts.

Photoenzymatic epoxidation of styrenes

Photochemical reduction of flavin adenine dinucleotide (FAD) enables the direct, non-enzymatic regeneration of styrene monooxygenase for enantiospecific epoxidation reactions.

Towards compartmentalized photocatalysis: multihaem proteins as transmembrane molecular electron conduits

We show a proof-of-concept for using MtrCAB as a lipid membrane-spanning building block for compartmentalised photocatalysis that mimics photosynthesis.

Combining Photo-Organo Redox- and Enzyme Catalysis Facilitates Asymmetric C-H Bond Functionalization

In this study, we combined photo‐organo redox catalysis and biocatalysis to achieve asymmetric C–H bond functionalization of simple alkane starting materials. The photo‐organo catalyst anthraquinone sulfate (SAS) was employed to oxyfunctionalise alkanes to aldehydes and ketones. We coupled this light‐driven reaction with asymmetric enzymatic functionalisations to yield chiral hydroxynitriles, amines, acyloins and α‐chiral ketones with up to 99 % ee. In addition, we demonstrate functional group interconversion to alcohols, esters and carboxylic acids. The transformations can be performed as concurrent tandem reactions. We identified the degradation of substrates and inhibition of the biocatalysts as limiting factors affecting compatibility, due to reactive oxygen species generated in the photocatalytic step. These incompatibilities were addressed by reaction engineering, such as applying a two‐phase system or temporal and spatial separation of the catalysts. Using a selection of eleven starting alkanes, one photo‐organo catalyst and 8 diverse biocatalysts, we synthesized 26 products and report for the model compounds benzoin and mandelonitrile > 97 % ee at gram scale.

Concurrent Asymmetric Reactions Combining Photocatalysis and Enzyme Catalysis: Direct Enantioselective Synthesis of 2,2-Disubstituted Indol-3-ones from 2-Arylindoles

The combination of photoredox and enzymatic catalysis for the direct asymmetric one-pot synthesis of 2,2-disubstituted indol-3-ones from 2-arylindoles through concurrent oxidization and alkylation reactions is described. 2-Arylindoles can be photocatalytically oxidized to 2-arylindol-3-one with subsequent enantioselective alkylation with ketones catalyzed by wheat germ lipase (WGL). The chiral quaternary carbon center at C2 of the indoles was directly constructed. This mode of concurrent photobiocatalysis provides a mild and powerful strategy for one-pot enantioselective synthesis of complex compounds. The experiments proved that other lipases containing structurally analogous catalytic triad in the active site also can catalyze the reaction in the same way. This reaction is the first example of combining the non-natural catalytic activity of hydrolases with visible-light catalysis for enantioselective organic synthesis and it does not require any cofactors.

A Photoenzymatic NADH Regeneration System

A photoenzymatic NADH regeneration system was established. The combination of deazariboflavin as a photocatalyst with putidaredoxin reductase enabled the selective reduction of NAD+ into the enzyme-active 1,4-NADH to promote an alcohol dehydrogenase catalysed stereospecific reduction reaction. The catalytic turnover of all the reaction components was demonstrated. Factors influencing the efficiency of the overall system were identified.

Unbiased biocatalytic solar-to-chemical conversion by FeOOH/BiVO4/perovskite tandem structure

Redox enzymes catalyze fascinating chemical reactions with excellent regio- and stereo-specificity. Nicotinamide adenine dinucleotide cofactor is essential in numerous redox biocatalytic reactions and needs to be regenerated because it is consumed as an equivalent during the enzymatic turnover. Here we report on unbiased photoelectrochemical tandem assembly of a photoanode (FeOOH/BiVO4) and a perovskite photovoltaic to provide sufficient potential for cofactor-dependent biocatalytic reactions. We obtain a high faradaic efficiency of 96.2% and an initial conversion rate of 2.4 mM h-1 without an external applied bias for the photoelectrochemical enzymatic conversion of α-ketoglutarate to L-glutamate via L-glutamate dehydrogenase. In addition, we achieve a total turnover number and a turnover frequency of the enzyme of 108,800 and 6200 h-1, respectively, demonstrating that the tandem configuration facilitates redox biocatalysis using light as the only energy source.

Thioflavin T-Amyloid Hybrid Nanostructure for Biocatalytic Photosynthesis

Amyloidogenic peptides can self-assemble into highly ordered nanostructures consisting of cross β-sheet-rich networks that exhibit unique physicochemical properties and high stability. Light-harvesting amyloid nanofibrils are constructed by employing insulin as a building block and thioflavin T (ThT) as a amyloid-specific photosensitizer. The ability of the self-assembled amyloid scaffold to accommodate and align ThT in high density on its surface allows for efficient energy transfer from the chromophores to the catalytic units in a similar way to natural photosystems. Insulin nanofibrils significantly enhance the photoactivity of ThT by inhibiting nonradiative conformational relaxation around the central CC bonds and narrowing the distance between ThT molecules that are bound to the β-sheet-rich amyloid structure. It is demonstrated that the ThT-amyloid hybrid nanostructure is suitable for biocatalytic solar-to-chemical conversion by integrating the light-harvesting amyloid module (for nicotinamide cofactor regeneration) with a redox biocatalytic module (for enzymatic reduction).

Semiconducting Polymer Nanobiocatalysts for Photoactivation of Intracellular Redox Reactions

An organic semiconducting polymer nanobiocatalyst (SPNB) composed of a semiconducting polymer core conjugated with microsomal cytochrome P450 (CYP) has been developed for photoactivation of intracellular redox. The core serves as the light-harvesting unit to initiate photoinduced electron transfer (PET) and facilitate the regeneration of dihydronicotinamide adenine dinucleotide phosphate (NADPH), while CYP is the catalytic center for intracellular redox. Under light irradiation, the semiconducting core can efficiently catalyze the generation of NADPH with a turnover frequency (TOF) 75 times higher than the reported nanosystems, ensuring the supply of the cofactor for intracellular redox. SPNB-mediated intracellular redox thus can be efficiently activated by light in living cells to convert the model substrate and also to trigger the bioactivation of anticancer drugs. This study provides an organic nanobiocatalytic system that allows light to remotely control intracellular redox in living systems.

Biocatalytic C=C Bond Reduction through Carbon Nanodot-Sensitized Regeneration of NADH Analogues

Light-driven activation of redox enzymes is an emerging route for sustainable chemical synthesis. Among redox enzymes, the family of Old Yellow Enzyme (OYE) dependent on the nicotinamide adenine dinucleotide cofactor (NADH) catalyzes the stereoselective reduction of α,β-unsaturated hydrocarbons. Here, we report OYE-catalyzed asymmetric hydrogenation through light-driven regeneration of NADH and its analogues (mNADHs) by N-doped carbon nanodots (N-CDs), a zero-dimensional photocatalyst. Our spectroscopic and photoelectrochemical analyses verified the transfer of photo-induced electrons from N-CDs to an organometallic electron mediator (M) for highly regioselective regeneration of cofactors. Light triggered the reduction of NAD⁺ and mNAD⁺ s with the cooperation of N-CDs and M, and the reduction behaviors of cofactors were dependent on their own reduction peak potentials. The regenerated cofactors subsequently delivered hydrides to OYE for stereoselective conversions of a broad range of substrates with excellent biocatalytic efficiencies.

Nonconventional regeneration of redox enzymes - a practical approach for organic synthesis?

Oxidoreductases have become useful tools in the hands of chemists to perform selective and mild oxidation and reduction reactions. Instead of mimicking native catalytic cycles, generally involving costly and unstable nicotinamide cofactors, more direct, NAD(P)-independent methodologies are being developed. The promise of these approaches not only lies with simpler and cheaper reaction schemes but also with higher selectivity as compared to whole cell approaches and their mimics.