Rational Design of a Miniature Photocatalytic CO2-Reducing Enzyme

Noncanonical Amino Acids: Bringing New-to-Nature Functionalities to Biocatalysis

Biocatalysis has become an important component of modern organic chemistry, presenting an efficient and environmentally friendly approach to synthetic transformations. Advances in molecular biology, computational modeling, and protein engineering have unlocked the full potential of enzymes in various industrial applications. However, the inherent limitations of the natural building blocks have sparked a revolutionary shift. In vivo genetic incorporation of noncanonical amino acids exceeds the conventional 20 amino acids, opening new avenues for innovation. This review provides a comprehensive overview of applications of noncanonical amino acids in biocatalysis. We aim to examine the field from multiple perspectives, ranging from their impact on enzymatic reactions to the creation of novel active sites, and subsequent catalysis of new-to-nature reactions. Finally, we discuss the challenges, limitations, and promising opportunities within this dynamic research domain.

Electron spin resonance analysis of photoenzymatic catalysis

This Perspective highlights recent research progress and prospects in elucidating the catalytic mechanism of photoenzymes using ESR (electron spin resonance) spectroscopy, which is emerging as a unique and crucial method for identifying radical intermediates, illustrating electron transfer events and the underlying mechanisms of photoenzymatic catalysis.

Light-Driven Hydrogen Evolution Reaction Catalyzed by a Molybdenum-Copper Artificial Hydrogenase

Orange protein (Orp) is a small bacterial metalloprotein of unknown function that harbors a unique molybdenum/copper (Mo/Cu) heterometallic cluster, [S₂MoS₂CuS₂MoS₂]^3-. In this paper, the performance of Orp as a catalyst for the photocatalytic reduction of protons into H₂ has been investigated under visible light irradiation. We report the complete biochemical and spectroscopic characterization of holo-Orp containing the [S₂MoS₂CuS₂MoS₂]^3- cluster, with docking and molecular dynamics simulations suggesting a positively charged Arg, Lys-containing pocket as the binding site. Holo-Orp exhibits excellent photocatalytic activity, in the presence of ascorbate as the sacrificial electron donor and [Ru(bpy)₃]Cl₂ as the photosensitizer, for hydrogen evolution with a maximum turnover number of 890 after 4 h irradiation. Density functional theory (DFT) calculations were used to propose a consistent reaction mechanism in which the terminal sulfur atoms are playing a key role in promoting H₂ formation. A series of dinuclear [S₂MS₂M'S₂MS₂]^(4n)- clusters, with M = Mo^VI, W^VI and M'⁽ⁿ⁺⁾ = Cu^I, Fe^I, Ni^I, Co^I, Zn^II, Cd^II were assembled in Orp, leading to different M/M'-Orp versions which are shown to display catalytic activity, with the Mo/Fe-Orp catalyst giving a remarkable turnover number (TON) of 1150 after 2.5 h reaction and an initial turnover frequency (TOF°) of 800 h^-1 establishing a record among previously reported artificial hydrogenases.

Study and design of amino acid-based radical enzymes using unnatural amino acids

Radical enzymes harness the power of reactive radical species by placing them in a protein scaffold, and they are capable of catalysing many important reactions. New native radical enzymes, especially those with amino acid-based radicals, in the category of non-heme iron enzymes (including ribonucleotide reductases), heme enzymes, copper enzymes, and FAD-radical enzymes have been discovered and characterized. We discussed recent research efforts to discover new native amino acid-based radical enzymes, and to study the roles of radicals in processes such as enzyme catalysis and electron transfer. Furthermore, design of radical enzymes in a small and simple scaffold not only allows us to study the radical in a well-controlled system and test our understanding of the native enzymes, but also allows us to create powerful enzymes. In the study and design of amino acid-based radical enzymes, the use of unnatural amino acids allows precise control of pKa values and reduction potentials of the residue, as well as probing the location of the radical through spectroscopic methods, making it a powerful research tool. Our understanding of amino acid-based radical enzymes will allow us to tailor them to create powerful catalysts and better therapeutics.

Enzymatic Conversion of CO2: From Natural to Artificial Utilization

Enzymatic carbon dioxide fixation is one of the most important metabolic reactions as it allows the capture of inorganic carbon from the atmosphere and its conversion into organic biomass. However, due to the often unfavorable thermodynamics and the difficulties associated with the utilization of CO2, a gaseous substrate that is found in comparatively low concentrations in the atmosphere, such reactions remain challenging for biotechnological applications. Nature has tackled these problems by evolution of dedicated CO2-fixing enzymes, i.e., carboxylases, and embedding them in complex metabolic pathways. Biotechnology employs such carboxylating and decarboxylating enzymes for the carboxylation of aromatic and aliphatic substrates either by embedding them into more complex reaction cascades or by shifting the reaction equilibrium via reaction engineering. This review aims to provide an overview of natural CO2-fixing enzymes and their mechanistic similarities. We also discuss biocatalytic applications of carboxylases and decarboxylases for the synthesis of valuable products and provide a separate summary of strategies to improve the efficiency of such processes. We briefly summarize natural CO2 fixation pathways, provide a roadmap for the design and implementation of artificial carbon fixation pathways, and highlight examples of biocatalytic cascades involving carboxylases. Additionally, we suggest that biochemical utilization of reduced CO2 derivates, such as formate or methanol, represents a suitable alternative to direct use of CO2 and provide several examples. Our discussion closes with a techno-economic perspective on enzymatic CO2 fixation and its potential to reduce CO2 emissions.

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.

Repurposing Photosensitizer Proteins Through Genetic Code Expansion to Facilitate Photo-Biocatalysis

Artificial photoenzymes with noncanonical photo-redox cofactors have paved the way for enzyme rational design and the creation of new-to-nature biocatalysts. Genetically encoded photo-redox cofactors endow photoenzymes with enhanced or novel activities that catalyze numerous transformations with high efficiency. Herein, we describe a protocol of repurposing photosensitizer proteins (PSP) through genetic code expansion to facilitate multiple photocatalytic conversions including photo-activated dehalogenation of aryl halides, CO₂ to CO and CO₂ to formic acid reduction. The methods for expression, purification, and characterization of the PSP are detailed. The installation of the catalytic modules and the utilization of PSP-based artificial photoenzymes for photoenzymatic CO₂ reduction and dehalogenation are also described.

A designed photoenzyme for enantioselective [2+2] cycloadditions

The ability to program new modes of catalysis into proteins would allow the development of enzyme families with functions beyond those found in nature. To this end, genetic code expansion methodology holds particular promise, as it allows the site-selective introduction of new functional elements into proteins as noncanonical amino acid side chains1-4. Here we exploit an expanded genetic code to develop a photoenzyme that operates by means of triplet energy transfer (EnT) catalysis, a versatile mode of reactivity in organic synthesis that is not accessible to biocatalysis at present5-12. Installation of a genetically encoded photosensitizer into the beta-propeller scaffold of DA_20_00 (ref. 13) converts a de novo Diels-Alderase into a photoenzyme for [2+2] cycloadditions (EnT1.0). Subsequent development and implementation of a platform for photoenzyme evolution afforded an efficient and enantioselective enzyme (EnT1.3, up to 99% enantiomeric excess (e.e.)) that can promote intramolecular and bimolecular cycloadditions, including transformations that have proved challenging to achieve selectively with small-molecule catalysts. EnT1.3 performs >300 turnovers and, in contrast to small-molecule photocatalysts, can operate effectively under aerobic conditions and at ambient temperatures. An X-ray crystal structure of an EnT1.3-product complex shows how multiple functional components work in synergy to promote efficient and selective photocatalysis. This study opens up a wealth of new excited-state chemistry in protein active sites and establishes the framework for developing a new generation of enantioselective photocatalysts.

Artificial photosynthesis systems for solar energy conversion and storage: platforms and their realities

In natural photosynthesis, photosynthetic organisms such as green plants realize efficient solar energy conversion and storage by integrating photosynthetic components on the thylakoid membrane of chloroplasts. Inspired by natural photosynthesis, researchers have developed many artificial photosynthesis systems (APS's) that integrate various photocatalysts and biocatalysts to convert and store solar energy in the fields of resource, environment, food, and energy. To improve the system efficiency and reduce the operation cost, reaction platforms are introduced in APS's since they allow for great stability and continuous processing. A systematic understanding of how a reaction platform affects the performance of artificial photosynthesis is conducive for designing an APS with superb solar energy utilization. In this review, we discuss the recent APS's researches, especially those confined on/in platforms. The importance of different platforms and their influences on APS's performance are emphasized. Generally, confined platforms can enhance the stability and repeatability of both photocatalysts and biocatalysts in APS's as well as improve the photosynthetic performance due to the proximity effect. For functional platforms that can participate in the artificial photosynthesis reactions as active parts, a high integration of APS's components on/in these platforms can lead to efficient electron transfer, enhanced light-harvesting, or synergistic catalysis, resulting in superior photosynthesis performance. Therefore, the integration of APS's components is beneficial for the transfer of substrates and photoexcited electrons in artificial photosynthesis. We finally summarize the current challenges of APS's development and further efforts on the improvement of APS's.

Synergy of cyano groups and cobalt single atoms in graphitic carbon nitride for enhanced bio-denitrification

Bio-denitrification plays a crucial role in the purification of nitrogen contaminated water, yet the low efficiency of the pure biological system often leads to the accumulation of harmful intermediates. Semi-biological catalysis provides an effective approach to improving the reaction efficiency through hybridizing artificial nanomaterials with natural organisms, yet the application of this strategy in bio-denitrification is limited. In this study, the effect of surface engineered carbon nitride on the denitrification capability of denitrifying bacteria was investigated. We found that cyano groups availed the biotic-abiotic interactions, while immobilized cobalt single atoms attenuated the local electrostatic repulsion. This synergistic effect endowed carbon nitride modified with cobalt atoms and cyano groups (Co/C₃N₄-C) with the unexpected acceleration of bio-denitrification reaction, without the accumulation of harmful intermediates. According to the metabolomics analysis, this improvement was attributed to the moderate metabolic adaptation caused by nanoelicitor, which induced dramatically boosted electron transfer and energy supply for extracellular polymeric substance (EPS) secretion. The elevation of intracellular iron level increased the activities of denitrification reductase, which was evidenced by metatranscriptomic analysis. Our results not only demonstrate the great potential of carbon nitride as an artificial elicitor for biological regulation, but also shed light on comprehending the complicated biotic-abiotic interactions for versatile application.

Photobiocatalysis for Abiological Transformations

Harnessing biocatalysts for novel abiological transformations is a longstanding goal of synthetic chemistry. Combining the merits of biocatalysis and photocatalysis allows for selective transformations fueled by visible light and offers many advantages including new reactivity, high enantioselectivity, greener syntheses, and high yields. Photoinduced electron or energy transfer enables synthetic methodologies that complement conventional two electron processes or offer orthogonal pathways for developing new reactions. Enzymes are well suited and can be tuned by directed evolution to exert control over open-shell intermediates, thereby suppressing undesirable reactions and delivering high chemo- and stereoselectivities. Within the past decade, the combination of biocatalysis and photocatalysis was mainly focused on exploiting light-regenerated cofactors to function native enzymatic activity. However, recent developments have demonstrated that the combination can unlock new-to-nature chemistry. Particularly, the discovery and application of new strategies are well poised to expand the applications of photobiocatalysis.In the past five years, our lab has been studying the combinations of photocatalysis and biocatalysis that can be applied to create new synthetic methodologies and solve challenges in synthetic organic chemistry. Our efforts have expanded the strategies for combining external photocatalysts with enzymes through the construction of a synergistic cooperative stereoconvergent reduction system consisting of photosensitized energy transfer and ene-reductase-catalyzed alkene reduction. Additionally, our efforts have also extended the capability of cofactor-dependent photoenzymatic systems to include enantioselective bimolecular radical hydroalkylations of alkenes by irradiating electron donor-acceptor complexes comprised of enzymatic redox active cofactors and unnatural substrates.In this Account, we highlight strategies developed by our group and others for combining biocatalysis and photocatalysis with the aim of introducing non-natural reactivity to enzymes. Presently, strategies applied to achieve this goal include the repurposing of natural photoenzymes, the elucidation of new photoreactivity within cofactor-dependent enzymes, the combination of external photocatalysts with enzymes, and the construction of artificial photoenzymes. By demonstrating the successful applications of these strategies for achieving selective new-to-nature transformations, we hope to spur interest in expanding the scope of photobiocatalytic systems through the use and extension of these strategies and creation of new strategies. Additionally, we hope to elucidate the intuition in synergizing the unique capabilities of biocatalysis and photocatalysis so that photobiocatalysis can be recognized as a potential solution to difficult challenges in synthetic organic chemistry.

Constructing Schottky junctions via Pd nanosheets on DUT-67 surfaces to accelerate charge transfer

The separation, transfer and recombination of charge often affect the rate of photocatalytic reduction of CO₂. Schottky junctions can promote the rapid separation of space charge. Therefore, in this paper, Pd nanosheets were grown on the surface of DUT-67 by a hydrothermal method, and a Schottky junction was constructed between DUT-67 and Pd. Under the action of the Schottky junction, the CO yield of 0.3-Pd/DUT-67 reached 12.15 μmol/g/h, which was 17 times higher than that of DUT-67. Efficient charge transfer was demonstrated in photochemical experiments. The large specific surface area and the increased light utilization rate also contributed to the increase in the CO₂ reduction efficiency. In addition, the mechanism of Pd/DUT-67 photocatalytic reduction of CO₂ was proposed.