Navigating the Unnatural Reaction Space: Directed Evolution of Heme Proteins for Selective Carbene and Nitrene Transfer

Photobiocatalytic Enantioselective Benzylic C(sp3)-H Acylation Enabled by Thiamine-Dependent Enzymes via Intermolecular Hydrogen Atom Transfer

Hydrogen atom transfer (HAT) constitutes a powerful mechanism exploited in biology and chemistry to functionalize ubiquitous C(sp3)-H bonds in organic molecules. Despite its synthetic potential, achieving stereocontrol in chemical HAT-mediated C-H functionalization transformations remains challenging. By merging the radical reactivity of thiamine (ThDP)-dependent enzymes with chemical hydrogen atom transfer, we report here a photobiocatalytic strategy for the enantioselective C(sp3)-H acylation of an organic substrate, a transformation not found in nature nor currently attainable by chemical means. This method enables the direct functionalization of benzylic C(sp3)-H sites in a broad range of substrates to furnish valuable enantioenriched ketone motifs with good to high enantioselectivity (up to 96% ee). Mechanistic and spectroscopic studies support the involvement of radical species derived from the Breslow intermediate and C-H substrate, highlight the critical role of the photocatalyst and hydrogen atom abstraction reagents for productive catalysis, and reveal a specific enzyme/photocatalyst interaction favoring single electron transfer during catalysis. Further insights into how the enantioselectivity of the C-C bond-forming reaction is controlled by the enzyme and influenced by active site mutations were gained via molecular modeling. This study illustrates the productive integration of ThDP-mediated biocatalysis with chemical HAT, expanding the range of asymmetric C(sp3)-H functionalization transformations that can be accessed through biocatalysis.

Cyclopropanation reactions by a class I unspecific peroxygenase

Non-natural biotransformations, such as alkene cyclopropanation through carbene insertion, have been demonstrated for the hemoproteins cytochrome P450 and myoglobin, but have not been investigated for unspecific peroxygenases (UPOs). Here we demonstrate that the diastereo- and enantioselective cyclopropanation of styrenes with ethyldiazoacetate can be performed by the class I enzyme artUPO.

Engineering catalytically promiscuous enzymes to serve new functions

Catalytic promiscuity in enzymes refers to their ability to catalyze multiple chemically distinct reactions in addition to their native activity. The increasing discovery of additional enzymes exhibiting catalytic promiscuity has underscored the significance of this trait in nature. The catalytic promiscuity of enzymes offers new avenues for functional redesign. Through protein engineering, existing enzymes can be modified to expand their natural catalytic boundaries. Furthermore, de novo designed artificial enzymes can achieve novel enzymatic reactions, broadening the scope of enzyme-catalyzed applications. Given that catalytic promiscuity plays a fundamental role in enzyme evolution, comprehensive research on its origins and influencing factors is essential. In this review, we comprehensively examine the factors influencing catalytic promiscuity, including variations in substrate binding modes in pre-reaction states, the instability of key high-energy intermediates, and the roles of critical residues in catalytic mechanisms. Moreover, altering the enzyme's catalytic environment can also induce novel types of catalytic reactions, such as light-induced promiscuous reactions catalyzed by cofactor-dependent oxidoreductase enzymes. Additionally, we summarize the current protein engineering technologies and strategies aimed at enhancing the activity and stereoselectivity of target enzymes to meet industrial requirements.

Bridging chemistry and biology for light-driven new-to-nature enantioselective photoenzymatic catalysis

Merging enzymes with light-driven photocatalysis has given rise to the burgeoning field of photoenzymatic catalysis. This approach combines the high reactivity from photoexcitation with the exceptional selectivity of biocatalysis, providing exciting opportunities to tackle challenges in enantioselective radical reactions and to access new-to-nature enzyme reactivities. This tutorial review aims to provide a comprehensive introduction to this interdisciplinary topic, catering to the growing interest from communities in asymmetric catalysis, photocatalysis, radical chemistry, enzyme engineering, and synthetic biology. We summarize the fundamental principles of utilizing light to power enzymatic reactions and different strategies exploring enantioselective photoenzymatic systems, including natural cofactor-based photoenzymatic catalysis, photocatalyst/enzyme synergistic catalysis, synthetic cofactor-based artificial photoenzymes, and cofactor-free photoenzymatic catalysis. We also discuss the challenges and prospects of enantioselective photoenzymatic catalysis in advancing sustainable asymmetric synthesis.

Chiral iron porphyrin (+)-D4-(por)FeCl catalyzes highly enantioselective cyclopropanation of alkenes using in situ generated diazoacetonitrile with up to 35 000 product turnover

Transition metal-catalyzed asymmetric cyclopropanation of alkenes is an important strategy to construct chiral cyclopropane skeletons of pharmaceutical interest, but highly enantioselective and practical carbene transfer reactions based on Earth abundant and bio-compatible metals are still a difficult challenge. In this work, we use a chiral iron porphyrin (+)-D4-(por)FeCl catalyst and in situ generated α-diazoacetonitrile for highly enantioselective cyclopropanation of arylalkene. This reaction is applicable to a wide range of arylalkenes (44 examples) with yield up to 99%, diastereomeric ratio (dr) up to 93 : 7, and enantiomeric excess (ee) values up to 98%. Importantly, for the cyclopropanation reaction of 3,4-difluorostyrene (1.40 g, 10.0 mmol) with α-diazoacetonitrile in the presence of 0.002 mol% of (+)-D4-(por)FeCl as a catalyst, the turnover number and enantioselectivity of the cyclopropyl nitrile product reached 31 000 and 88% ee, respectively. Using cyclopropyl nitriles as a starting material, downstream functionalization derivatives including cyclopropyl carboxylic acids, cyclopropylamines, and cyclopropylmethanamines can be produced as key intermediates for the preparation of a series of bioactive or drug-like molecules. In addition, the chiral Fe(ii)porphyrin-cyanocarbene intermediate [(-)-D4-(por)FeII(:CHCN)], which is directly responsible for the carbene transfer reaction, has been characterized by 1H NMR, HR ESI-MS, UV-vis and ATR-FTIR spectroscopy.

NHC-Mediated Radical Acylation Catalyzed by Thiamine- and Flavin-Dependent Enzymes

Cross-coupling reactions between short-lifetime radicals are challenging reactions in organic chemistry. Here, we report the development of an N-heterocyclic carbene (NHC)-mediated radical coupling reaction based on the catalytic machinery of thiamine- and flavin-dependent enzymes. Through a series of enzyme screenings, we found that acetolactate synthase from Thermobispora bispora (TbALS) and its engineered variants exhibit promising catalytic activity toward abiotic radical acylation reactions of α-bromo carbonyl compounds. Notably, the TbALS variant has higher catalytic activity for small nonaromatic substrates despite forming less stable radical intermediates. Furthermore, the catalytic system of TbALS can be applied to photocatalytic reactions utilizing the photoredox properties of FAD. Nonbenzylic alkyl radicals generated from N-acyloxyphthalimides are efficiently converted into the corresponding dialkyl ketones under irradiation of a blue LED. These findings highlight the utility of thiamine- and flavin-dependent enzymes for achieving selective cross-coupling reactions of short-lifetime radicals.

Merging Electrosynthesis and Biocatalysis to Access Sulfur-Based Chiral α-Fluorinated Carboxylic Acids

We describe a sustainable process to synthesize chiral sulfur-based organofluorine compounds by integrating electrosynthesis and biocatalysis within a single vessel while using water as a solvent. In this context, differently substituted 2-fluoro-3-mercaptopropionic acids have been synthesized in good isolated yields using thiophenols and fluorine-containing α,β-unsaturated alkenes. In addition, molecular docking and control experiments were carried out that suggest the formation of radical species during the electrolysis and participation of the lipase active site during the biocatalysis. The scalability and applicability of the developed protocol have been illustrated through the synthesis of a key intermediate of the MMP-3 inhibitor and by performing a gram-scale reaction. Further, the compatibility of the lipase enzyme with electricity highlights the promising potential of enzymatic electrosynthesis in advancing environmentally friendly organic transformations.

Light-Driven Deracemization by a Designed Photoenzyme

The creation of enzymes with abiological abilities offers exciting opportunities to access new-to-nature biocatalysis beyond that found in nature. Here, we repurpose a novel protein scaffold, CTB10, as an artificial photoenzyme through genetic code expansion. It enables catalytic deracemization of cyclopropane, a process that remains inaccessible to traditional biocatalysis due to its thermodynamically unfavorable nature. Following structural optimization through directed evolution, a broad substrate scope with high enantioselectivities is achieved. Furthermore, the crystal structure of the CTB10-based photoenzyme-substrate complex well demonstrates how the catalytic chiral cavity is sculpted to promote efficient and selective light-enabled deracemization. Therefore, this study unlocks the potential for achieving challenging deracemization through biocatalysis.

C 4-Symmetric Bowl-Shaped Diruthenium Tetracarboxylate Catalysts for Enantioselective C-H Functionalization Using Donor/Acceptor Carbenes

Cationic diruthenium (II,III) tetracarboxylate catalysts have been shown to catalyze selective intermolecular C-H functionalization reactions using donor/acceptor carbenes in high yield and with high levels of enantioselectivity. The diruthenium catalysts were compared to the analogous dirhodium (II,II) tetracarboxylate and showed similar levels of enantioselectivity for most reactions. A distinctive feature of the diruthenium catalysts is a greater preference for C-H functionalization over cyclopropanation compared to the corresponding dirhodium catalysts. Also, the diruthenium catalysts have a greater preference for sterically more accessible sites compared with their dirhodium counterparts. These studies show that the diruthenium catalysts are generally effective catalysts for enantioselective intermolecular C-H functionalization, but further optimization would be needed for them to match the dirhodium catalysts in terms of functional group compatibility, turnover frequency, and turnover numbers.

Catalytic Enantioselective Smiles Rearrangement Enabled by the Directed Evolution of P450 Radical Aryl Migratases

Despite its synthetic potential, catalytic enantioselective Smiles rearrangement has remained elusive. Through the directed evolution of P450 radical aryl migratases (P450Smiles's), we describe the first example of catalytic enantioselective Smiles rearrangement. A range of racemic N-arylsulfonyl-α-chloroamides could be transformed by P450Smiles in an enantioconvergent manner, affording acyclic amide products possessing an all-carbon quaternary stereocenter with excellent chemo- and enantioselectivity. Both electron-rich and electron-deficient substituents were compatible with the migrating aryl group, demonstrating this P450-catalyzed Smiles rearrangement is insensitive to the electronic properties of the migrating group. Importantly, our evolved P450 variants were capable of overriding the innate cyclization activity of the N-alkyl amidyl radical intermediate, allowing the chemoselective reductive formation of acyclic products. Classical molecular dynamics (MD) simulations revealed this unusual enzyme-controlled chemoselectivity stems from the restricted conformation of the amidyl radical within the enzyme active site, disfavoring the cyclization pathway. This new-to-nature biocatalytic asymmetric Smiles rearrangement showcases the synthetic potential of enzymatic chemo- and enantioselectivity control over highly reactive radical intermediates eluding small-molecule catalysts.

Iridium porphyrin-catalysed asymmetric carbene insertion into primary N-adjacent C-H bonds with TON over 1000000

Selective functionalization of ubiquitous C-H bonds in organic molecules provides a straightforward and efficient approach to construct complex molecules with fewer synthetic steps and high atom economy, thus promoting more sustainable and economical chemical synthesis. A formidable challenge in the field is to increase the turnover numbers (TONs) for catalytic C-H functionalization reactions reported in the literature (generally <10,000) to reasonably high levels to reduce the cost of the reaction. Another challenge is the selective functionalization of less reactive primary C(sp3)-H bonds compared to other types of more reactive C-H bonds. We now demonstrate an efficient iridium porphyrin-catalysed asymmetric carbene insertion into primary N-adjacent C(sp3)-H bond of N-methyl indoline and N-methyl aniline derivatives. Using chiral iridium porphyrin as a catalyst, chiral β-amino acid derivatives have been obtained with very high yields and excellent ee values (up to 99%), and TONs as high as 84,000 to 1,380,000. The reaction can be readily performed on a 100 g scale while retaining its high efficiency and selectivity. We also show that this iridium catalysis can efficiently access oligomers and polymers of β-amino acid derivatives via stepwise C-H insertion, demonstrating its potential applications in materials science via C-H bond functionalization reactions.

An Enzymatic Platform for Asymmetric Synthesis of Si-Stereogenic Silanols

Chiral silanols are important synthetic targets and have garnered increasing attention in the materials and pharmaceutical industries over recent decades. A promising approach for their efficient synthesis is asymmetric silane oxidation. While chemists have developed several transition-metal-catalyzed systems for asymmetric hydrolytic oxidation of silanes, no biocatalytic methods have been available for enantioselective synthesis of Si-stereogenic compounds, including chiral silanols. Here, we present an enzymatic platform for the asymmetric aerobic mono-oxidation of dihydrosilanes using an engineered P450BM3 enzyme. Through six iterative rounds of directed evolution, we identified the optimal evolved variant, ASOx-6, which exhibits a 54-fold improvement in kcat/KM compared with the wild-type enzyme. Moreover, a variety of aryl-alkyl substituted dihydrosilanes are accepted by ASOx-6, including those bearing heteroaromatic rings. Finally, mechanistic insights obtained from kinetic isotope experiments and computational studies further elucidate the nature of this biocatalytic transformation.

Improving the Long-term Enantioselectivity of a Silicon-Carbon Bond-Forming Enzyme

Enantioselectivity is a key advantage of enzymatic catalysis. Understanding the most important factors influencing enantioselectivity necessitates thorough investigation for each specific enzyme. In this study, we explore various approaches to optimize reaction conditions for organosilicon production using an immobilized Cytochrome C recently tailored via directed evolution. Over extended reactions, this enzyme experiences a loss of enantioselectivity. Mass spectrometry (MS) revealed covalent modifications on the enzyme, but mutating the respective amino acids did not restore enantioselectivity. Nuclear magnetic resonance (NMR), along with a detailed comparison of the influence of reaction components such as cosolvents and reducing agents, indicated significant conformational changes in the presence of the diazo ester substrate. Additionally, we identified sodium ascorbate as a suitable and milder reducing agent compared to the previously used sodium dithionite, ensuring anaerobic conditions for silicon-carbon bond formation. Ultimately, maintaining a high enzyme-to-substrate ratio in the reaction was found to be crucial for achieving high enantiomeric purity of the organosilicon product over four days in sequential, repetitive batch reactions, thus improving the previously established reaction system. The methods and findings presented here are particularly valuable for addressing enantioselectivity issues in other enzymes that operate with diazo compounds as the substrates in carbene-transfer reactions.

Advances in the Synthesis of Cyclopropylamines

Cyclopropylamines are an important subclass of substituted cyclopropanes that combine the unique electronic and steric properties of cyclopropanes with the presence of a donor nitrogen atom. In addition to their presence in a diverse array of biologically active compounds, cyclopropylamines are utilized as important synthetic intermediates, particularly in ring-opening or cycloaddition reactions. Consequently, the synthesis of these compounds has constituted a significant research topic, as evidenced by the abundant published synthetic methods. In addition to the widely used Curtius rearrangement, classical cyclopropanation methods have been adapted to integrate a nitrogen function (Simmons-Smith reaction, metal-catalyzed reaction of diazo compounds on olefins, Michael-initiated ring-closure reactions) with significant advances in enantioselective synthesis. More recently, specific methods have been developed for the preparation of the aminocyclopropane moiety (Kulinkovich reactions applied to amides and nitriles, addition to cyclopropenes, metal-catalyzed reactions involving C-H functionalization, ...). The topic of this review is to present the different methods for the preparation of cyclopropylamine derivatives, with the aim of covering the methodological advances as best as possible, highlighting their scope, their stereochemical aspects and future trends.

The role and mechanisms of canonical and non-canonical tailoring enzymes in bacterial terpenoid biosynthesis

Covering: up to April 2024Terpenoids represent the largest and structurally most diverse class of natural products. According to textbook knowledge, this diversity arises from a two-step biosynthetic process: first, terpene cyclases generate a vast array of mono- and polycyclic hydrocarbon scaffolds with multiple stereocenters from a limited set of achiral precursors, a process extensively studied over the past two decades. Subsequently, tailoring enzymes further modify these complex scaffolds through regio- and stereocontrolled oxidation and other functionalization reactions, a topic of increasing interest in recent years. The resulting highly functionalized terpenoids exhibit a broad spectrum of unique biological activities, making them promising candidates for drug development. Recent advances in genome sequencing technologies along with the development and application of sophisticated genome mining tools have revealed bacteria as a largely untapped resource for the discovery of complex terpenoids. Functional characterization of a limited number of bacterial terpenoid biosynthetic pathways, combined with in-depth mechanistic studies of key enzymes, has begun to reveal the versatility of bacterial enzymatic processes involved in terpenoid modification. In this review, we examine the various tailoring reactions leading to complex bacterial terpenoids. We first discuss canonical terpene-modifying enzymes, that catalyze the functionalization of unactivated C-H bonds, incorporation of diverse functional groups, and oxidative and non-oxidative rearrangements. We then explore non-canonical terpene-modifying enzymes that facilitate oxidative rearrangement, cyclization, isomerization, and dimerization reactions. The increasing number of characterized tailoring enzymes that participate in terpene hydrocarbon scaffold fomation, rather than merely decorating pre-formed scaffolds suggests that a re-evaluation of the traditional two-phase model for terpenoid biosynthesis might be warranted. Finally, we address the potential and challenges of mining bacterial genomes to identify terpene biosynthetic gene clusters and expand the bacterial terpene biosynthetic and chemical space.

Iron-catalysed stereoselective NH transfer enables dynamic kinetic resolution of sulfoxides

Transition metal-catalysed asymmetric nitrene transfer provides a powerful means to access various bioactive N-containing compounds as single enantiomers. However, enantioselective NH transfer that allows concise assembly of unprotected enantioenriched amines remains an enduring challenge. We report here an iron-catalysed stereoselective NH imidation of sulfoxide, which is integrated with photocatalytic racemisation of sulfoxide, enabling a dynamic kinetic resolution (DKR) strategy for direct and asymmetric synthesis of NH-sulfoximines. This approach is distinct from the existing methods by avoiding protecting group manipulations and/or the use of chiral substrates. Computational studies on the NH imidation reaction suggest the involvement of an iron-aminyl radical intermediate, and its reaction with sulfoxide proceeds through a synchronous nucleophilic addition of sulfoxide to nitrogen center and ligand-to-metal single electron transfer process to form the N-S bond. In addition, the stereoselectivity is primarily dictated by the difference in dispersion interactions of the transition states.

Automated Flow Synthesis of Artificial Heme Enzymes for Enantiodivergent Biocatalysis

The remarkable efficiency with which enzymes catalyze small-molecule reactions has driven their widespread application in organic chemistry. Here, we employ automated fast-flow solid-phase synthesis to access catalytically active full-length enzymes without restrictions on the number and structure of noncanonical amino acids incorporated. We demonstrate the total syntheses of iron-dependent Bacillus subtilis myoglobin (BsMb) and sperm whale myoglobin (SwMb). The synthetic enzymes displayed excellent enantioselectivity and yield in carbene transfer reactions. Absolute control over enantioselectivity in styrene cyclopropanation was achieved using synthetic L- and D-BsMb mutants, which delivered each enantiomer of cyclopropane product in identical and opposite enantiomeric enrichment. BsMb mutants outfitted with noncanonical amino acids were used to facilitate detailed structure-activity relationship studies, revealing a previously unrecognized hydrogen-bonding interaction as the primary driver of enantioselectivity in styrene cyclopropanation. We anticipate that our approach will advance biocatalysis by providing reliable and rapid access to fully synthetic enzymes possessing noncanonical amino acids.

Directed Evolution and Unusual Protonation Mechanism of Pyridoxal Radical C-C Coupling Enzymes for the Enantiodivergent Photobiocatalytic Synthesis of Noncanonical Amino Acids

Visible light-driven pyridoxal radical biocatalysis has emerged as a new strategy for the stereoselective synthesis of valuable noncanonical amino acids in a protecting-group-free fashion. In our previously developed dehydroxylative C-C coupling using engineered PLP-dependent tryptophan synthases, an enzyme-controlled unusual α-stereochemistry reversal and pH-controlled enantiopreference were observed. Herein, through high-throughput photobiocatalysis, we evolved a set of stereochemically complementary PLP radical enzymes, allowing the synthesis of both l- and d-amino acids with enhanced enantiocontrol across a broad pH window. These newly engineered l- and d-amino acid synthases permitted the use of a broad range of organoboron substrates, including boronates, trifluoroborates, and boronic acids, with excellent efficiency. Mechanistic studies unveiled unexpected PLP racemase activity with our earlier PLP enzyme variants. This promiscuous racemase activity was abolished in our evolved amino acid synthases, shedding light on the origin of enhanced enantiocontrol. Further mechanistic investigations suggest a switch of proton donor to account for the stereoinvertive formation of d-amino acids, highlighting an unusual stereoinversion mechanism that is rare in conventional two-electron PLP enzymology.

Copper-dependent halogenase catalyses unactivated C-H bond functionalization

Carbon-hydrogen (C-H) bonds are the foundation of essentially every organic molecule, making them an ideal place to do chemical synthesis. The key challenge is achieving selectivity for one particular C(sp3)-H bond1-3. In recent years, metalloenzymes have been found to perform C(sp3)-H bond functionalization4,5. Despite substantial progresses in the past two decades6,7, enzymatic halogenation and pseudohalogenation of unactivated C(sp3)-H-providing a functional handle for further modification-have been achieved with only non-haem iron/α-ketoglutarate-dependent halogenases, and are therefore limited by the chemistry possible with these enzymes8. Here we report the discovery and characterization of a previously unknown halogenase ApnU, part of a protein family containing domain of unknown function 3328 (DUF3328). ApnU uses copper in its active site to catalyse iterative chlorinations on multiple unactivated C(sp3)-H bonds. By taking advantage of the softer copper centre, we demonstrate that ApnU can catalyse unprecedented enzymatic C(sp3)-H bond functionalization such as iodination and thiocyanation. Using biochemical characterization and proteomics analysis, we identified the functional oligomeric state of ApnU as a covalently linked homodimer, which contains three essential pairs-one interchain and two intrachain-of disulfide bonds. The metal-coordination active site in ApnU consists of binuclear type II copper centres, as revealed by electron paramagnetic resonance spectroscopy. This discovery expands the enzymatic capability of C(sp3)-H halogenases and provides a foundational understanding of this family of binuclear copper-dependent oxidative enzymes.

Biocatalytic enantioselective formation and ring-opening of oxetanes

Although biocatalysis offers complementary or alternative approaches to traditional synthetic methods, the limited range of available enzymatic reactions currently poses challenges in synthesizing a diverse array of desired compounds. Consequently, there is a significant demand for developing novel biocatalytic processes to enable reactions that were previously unattainable. Herein, we report the discovery and subsequent protein engineering of a unique halohydrin dehalogenase to develop a biocatalytic platform for enantioselective formation and ring-opening of oxetanes. This biocatalytic platform, exhibiting high efficiency, excellent enantioselectivity, and broad scopes, facilitates the preparative-scale synthesis of chiral oxetanes and a variety of chiral γ-substituted alcohols. Additionally, both the enantioselective oxetane formation and ring-opening processes are proven scalable for large-scale transformations at high substrate concentrations, and can be integrated efficiently in a one-pot, one-catalyst cascade system. This work expands the enzymatic toolbox for non-natural reactions and will promote further exploration of the catalytic repertoire of halohydrin dehalogenases in synthetic and pharmaceutical chemistry.

An Open-Shell FeIV Nitrido

We report the photogeneration and characterization of an open-shell, terminal iron nitrido (EmL)Fe(N) using a sterically encumbered dipyrrin ligand environment. The Fe-N distance in the solid-state, zero-field 57Fe Mössbauer spectrum, and computational analysis are consistent with a triplet electronic ground state of the iron nitrido. Notably, the attenuation of Fe-N multiple bond character through occupying π*Fe-N enables (i) primary C(sp3)-H amination, (ii) H2 cleavage, (iii) aromatic C-C cleavage, and (iv) photocatalytic N-atom transfer reactivity. These modes of reactivity have not previously been observed in low-spin Fe(N) analogues.

Production of Phenyldiazene Derivatives Using the Biosynthetic Pathway of an Aromatic Diazo Group-Containing Natural Product from an Actinomycete

The diazo group is an important functional group in organic synthesis because it confers high reactivity to the compounds and has been applied in various chemical reactions, such as the Sandmeyer reaction, Wolff rearrangement, cyclopropanation, and C-N bond formation with active methylene compounds. Previously, we revealed that 3-diazoavenalumic acid (3-DAA), which is potentially produced by several actinomycete species and contains an aromatic diazo group, is a biosynthetic intermediate of avenalumic acid. In this study, we aimed to construct a production system for phenyldiazene derivatives by adding several active methylene compounds to the culture of a 3-DAA-producing recombinant actinomycete. First, acetoacetanilide and its derivatives, which have an active methylene and are raw materials for arylide yellow dyes, were individually added to the culture of a 3-DAA-producing actinomycete. When their metabolites were analyzed, each expected compound with a phenyldiazenyl moiety was detected in the culture extract. Moreover, we established a one-pot in vitro enzymatic production system for the same phenyldiazene derivatives using a highly reactive diazotase, CmaA6. These results showed that the diazo group of natural products is an attractive tool for expanding the structural diversity of natural products both in vivo and in vitro.

Modifications of Prenyl Side Chains in Natural Product Biosynthesis

In recent years, there has been a growing interest in understanding the enzymatic machinery responsible for the modifications of prenyl side chains and elucidating their roles in natural product biosynthesis. This interest stems from the pivotal role such modifications play in shaping the structural and functional diversity of natural products, as well as from their potential applications to synthetic biology and drug discovery. In addition to contributing to the diversity and complexity of natural products, unique modifications of prenyl side chains are represented by several novel biosynthetic mechanisms. Representative unique examples of epoxidation, dehydrogenation, oxidation of methyl groups to carboxyl groups, unusual C-C bond cleavage and oxidative cyclization are summarized and discussed. By revealing the intriguing chemistry and enzymology behind these transformations, this comprehensive and comparative review will guide future efforts in the discovery, characterization and application of modifications of prenyl side chains in natural product biosynthesis.

Directed evolution and metabolic engineering generate an Escherichia coli cell factory for de novo production of 4-hydroxymandelate

4-hydroxymandelate is a high-value aromatic compound used in the medicine, cosmetics, food, and chemical industry. However, existing natural extraction and chemical synthesis methods are costly and lead to environmental pollution. This study employed metabolic engineering and directed evolution strategies for de novo 4-hydroxymandelate biosynthesis. Two key challenges were addressed: insufficient precursor supply and limited activity of crucial enzymes. Through gene overexpression and multi-level gene interference using CRISPRi, An Escherichia coli chassis capable of producing the key precursor 4-hydroxyphenylpyruvate and the titer reached 5.05 mM (0.91 g/L). A mutant clone was obtained, HmaSV152G, which showed a 5.13-fold improvement in the catalytic rate. During fermentation, a high production of 194.87 mM (32.768 g/L) 4-hydroxymandelate was achieved in 76 h with a batch supply of glucose in a 5-L bioreactor. This study demonstrated the great potential of biosensors in protein engineering and provides a reference for large-scale production of other high-value aromatic compounds.

A metalloenzyme platform for catalytic asymmetric radical dearomatization

Catalytic asymmetric dearomatization represents a powerful means to convert flat aromatic compounds into stereochemically well-defined three-dimensional molecular scaffolds. Using new-to-nature metalloredox biocatalysis, we describe an enzymatic strategy for catalytic asymmetric dearomatization via a challenging radical mechanism that has eluded small-molecule catalysts. Enabled by directed evolution, new-to-nature radical dearomatases P450rad1-P450rad5 facilitated asymmetric dearomatization of a broad spectrum of aromatic substrates, including indoles, pyrroles and phenols, allowing both enantioconvergent and enantiodivergent radical dearomatization reactions to be accomplished with excellent enzymatic control. Computational studies revealed the importance of additional hydrogen bonding interactions between the engineered metalloenzyme and the reactive intermediate in enhancing enzymatic activity and enantiocontrol. Furthermore, designer non-ionic surfactants were found to significantly accelerate this biotransformation, providing an alternative means to promote otherwise sluggish new-to-nature biotransformations. Together, this evolvable metalloenzyme platform opens up new avenues to advance challenging catalytic asymmetric dearomatization processes involving free radical intermediates.

Unmasking the reverse catalytic activity of 'ene'-reductases for asymmetric carbonyl desaturation

Carbonyl desaturation is a fundamental reaction widely practised in organic synthesis. While numerous methods have been developed to expand the scope of this important transformation, most of them necessitate multi-step protocols or suffer from the use of high loadings of metal or strong oxidizing conditions. Moreover, approaches that can achieve precise stereochemical control of the desaturation process are extremely rare. Here we report a biocatalytic platform for desymmetrizing desaturation of cyclohexanones to generate diverse cyclohexenones bearing a remote quaternary stereogenic centre, by reengineering 'ene'-reductases to efficiently mediate dehydrogenation, the reverse process of their native activity. This 'ene'-reductase-based desaturation system operates under mild conditions with air as the terminal oxidant, tolerates oxidation-sensitive or metal-incompatible functional groups and, more importantly, exhibits unparalleled stereoselectivity compared with those achieved with small-molecule catalysts. Mechanistic investigations suggest that the reaction proceeded through α-deprotonation followed by a rate-determining β-hydride transfer.

Repurposing myoglobin into a carbene transferase for a [2,3]-sigmatropic Sommelet-Hauser rearrangement

New-to-Nature biocatalysis has emerged as a promising tool in organic synthesis thanks to progress in protein engineering. Notably, hemeproteins have been evolved into robust catalysts for carbene and nitrene transfers and related sigmatropic rearrangements. In this work, we report the first example of a [2,3]-sigmatropic Sommelet-Hauser rearrangement initiated by a carbene transfer of the sperm whale myoglobin mutant L29S,H64V,V68F that was previously reported to catalyze the mechanistically similar [2,3]-sigmatropic Doyle-Kirmse rearrangement. This repurposed heme enzyme catalyzes the Sommelet-Hauser rearrangement between ethyl diazoacetate and benzyl thioethers bearing strong electron-withdrawing substituents with good yields and enantiomeric excess. Optimized catalytic conditions in the absence of any reductant led to an increased asymmetric induction with up to 59% enantiomeric excess. This myoglobin mutant is therefore one of the few catalysts for the asymmetric Sommelet-Hauser rearrangement. This work broadens the scope of abiological reactions catalyzed by iron-carbene transferases with a new example of asymmetric sigmatropic rearrangement.

Biocatalytic Synthesis of α-Amino Esters via Nitrene C-H Insertion

α-Amino esters are precursors to noncanonical amino acids used in developing small-molecule therapeutics, biologics, and tools in chemical biology. α-C-H amination of abundant and inexpensive carboxylic acid esters through nitrene transfer presents a direct approach to α-amino esters. Methods for nitrene-mediated amination of the protic α-C-H bonds in carboxylic acid esters, however, are underdeveloped. This gap arises because hydrogen atom abstraction (HAA) of protic C-H bonds by electrophilic metal-nitrenoids is slow: metal-nitrenoids preferentially react with polarity-matched, hydridic C-H bonds, even when weaker protic C-H bonds are present. This study describes the discovery and evolution of highly stable protoglobin nitrene transferases that catalyze the enantioselective intermolecular amination of the α-C-H bonds in carboxylic acid esters. We developed a high-throughput assay to evaluate the activity and enantioselectivity of mutant enzymes together with their sequences using the Every Variant Sequencing (evSeq) method. The assay enabled the identification of enantiodivergent enzymes that function at ambient conditions in Escherichia coli whole cells and whose activities can be enhanced by directed evolution for the amination of a range of substrates.

Design and Evolution of an Artificial Friedel-Crafts Alkylation Enzyme Featuring an Organoboronic Acid Residue

Creating artificial enzymes by the genetic incorporation of noncanonical amino acids with catalytic side chains would expand the enzyme chemistries that have not been discovered in nature. Here, we report the design of an artificial enzyme that uses p-boronophenylalanine as the catalytic residue. The artificial enzyme catalyzes Michael-type Friedel-Crafts alkylation through covalent activation. The designer enzyme was further engineered to afford high yields with excellent enantioselectivities. We next developed a practical method for preparative-scale reactions by whole-cell catalysis. This enzymatic C-C bond formation reaction was combined with palladium-catalyzed dearomative arylation to achieve the efficient synthesis of spiroindolenine compounds.

Biocatalytic asymmetric aldol addition into unactivated ketones

Enzymes are renowned for their catalytic efficiency and selectivity, but many classical transformations in organic synthesis have no biocatalytic counterpart. Aldolases are prodigious C-C bond-forming enzymes, but their reactivity has only been extended past activated carbonyl electrophiles in special cases. To probe the mechanistic origins of this limitation, we use a pair of aldolases whose activity is dependent on pyridoxal phosphate. Our results reveal how aldolases are limited by kinetically favourable proton transfer with solvent, which undermines aldol addition into ketones. We show how a transaldolase can circumvent this limitation, enabling efficient addition into unactivated ketones. The resulting products are highly sought non-canonical amino acids with side chains that contain chiral tertiary alcohols. Mechanistic analysis reveals that transaldolase activity is an intrinsic feature of pyridoxal phosphate chemistry and identifies principles for extending aldolase catalysis beyond its previous limits to enable convergent, enantioselective C-C bond formation from simple starting materials.

Fabrication of bio-abiotic hybrid living hydrogel for bifunctional electrochemical conversion

Design and intelligent use renewable natural bioenergy is an important challenge. Electric microorganism-based materials are being serve as an important part of bioenergy devices for energy release and collection, calling for suitable skeleton materials to anchor live microbes. Herein we verified the feasibility of constructing bio-abiotic hybrid living materials based on the combination of gelatin, Li-ions and exoelectrogenic bacteria Shewanella oneidensis manganese-reducing-1 (MR-1). The gelatin-based mesh contains abundant pores, allowing microbes to dock and small molecules to diffuse. The hybrid materials hold plentiful electronegative groups, which effectively anchor Li-ions and facilitate their transition. Moreover, the electrochemical characteristics of the materials can be modulated through changing the ratios of gelatin, bacteria and Li-ions. Based on the gelatin-Li-ion-microorganism hybrid materials, a bifunctional device was fabricated, which could play dual roles alternatively, generation of electricity as a microbial fuel cell and energy storage as a pseudocapacitor. The capacitance and the maximum voltage output of the device reaches 68 F g-1 and 0.67 V, respectively. This system is a new platform and fresh start to fabricate bio-abiotic living materials for microbial electron storage and transfer. We expect the setup will extend to other living systems and devices for synthetic biological energy conversion.

Spiers Memorial Lecture: Engineering biocatalysts

Enzymes are being engineered to catalyze chemical reactions for many practical applications in chemistry and biotechnology. The approaches used are surveyed in this short review, emphasizing methods for accessing reactivities not expressed by native protein scaffolds. The successful generation of completely de novo enzymes that rival the rates and selectivities of their natural counterparts highlights the potential role that designer enzymes may play in the coming years in research, industry, and medicine. Some challenges that need to be addressed to realize this ambitious dream are considered together with possible solutions.

C-H functionalization reactions catalyzed by artificial metalloenzymes

CH functionalization, a promising frontier in modern organic chemistry, facilitates the direct conversion of inert CH bonds into many valuable functional groups. Despite its merits, traditional homogeneous catalysis, often faces challenges in efficiency, selectivity, and sustainability towards this transformation. In this context, artificial metalloenzymes (ArMs), resulting from the incorporation of a catalytically-competent metal cofactor within an evolvable protein scaffold, bridges the gap between the efficiency of enzymatic transformations and the versatility of transition metal catalysis. Accordingly, ArMs have emerged as attractive tools for various challenging catalytic transformations. Additionally, the coming of age of directed evolution has unlocked unprecedented avenues for optimizing enzymatic catalysis. Taking advantage of their genetically-encoded protein scaffold, ArMs have been evolved to catalyze various CH functionalization reactions. This review delves into the recent developments of ArM-catalyzed CH functionalization reactions, highlighting the benefits of engineering the second coordination sphere around a metal cofactor within a host protein.

Computational design of myoglobin-based carbene transferases for monoterpene derivatization

Carbene transfer reactions have emerged as pivotal methodologies for the synthesis of complex molecular architectures. Heme protein-catalyzed carbene transfer reactions have shown promising results on model compounds. However, their limited substrate scope has hindered their application in natural product functionalization. Building upon the foundation of previously published work on a carbene transferase-myoglobin variant, this study employs computer-aided protein engineering to design myoglobin variants, using either docking or the deep learning-based LigandMPNN method. These variants were utilized as catalysts in carbene transfer reactions with a selection of monoterpene substrates featuring C-C double bonds, leading to seven target products. This cost-effective methodology broadens the substrate scope for heme protein-catalyzed reactions, thereby opening novel pathways for research in heme protein functionalities and offering fresh perspectives in the synthesis of bioactive molecules.

Intermolecular Enantioselective Amination Reactions Mediated by Visible Light and a Chiral Iron Porphyrin Complex

In the presence of 1 mol % of a chiral iron porphyrin catalyst, various 3-arylmethyl-substituted 2-quinolones and 2-pyridones underwent an enantioselective amination reaction (20 examples; 93-99 % ee). The substrates were used as the limiting reagents, and fluorinated aryl azides (1.5 equivalents) served as nitrene precursors. The reaction is triggered by visible light which allows a facile dediazotation at ambient temperature. The selectivity of the reaction is governed by a two-point hydrogen bond interaction between the ligand of the iron catalyst and the substrate. Hydrogen bonding directs the amination to a specific hydrogen atom within the substrate that is displaced by the nitrogen substituent either in a concerted fashion or by a rebound mechanism.

Design and Evolution of an Enzyme for the Asymmetric Michael Addition of Cyclic Ketones to Nitroolefins by Enamine Catalysis

Consistent introduction of novel enzymes is required for developing efficient biocatalysts for challenging biotransformations. Absorbing catalytic modes from organocatalysis may be fruitful for designing new-to-nature enzymes with novel functions. Herein we report a newly designed artificial enzyme harboring a catalytic pyrrolidine residue that catalyzes the asymmetric Michael addition of cyclic ketones to nitroolefins through enamine activation with high efficiency. Diverse chiral γ-nitro cyclic ketones with two stereocenters were efficiently prepared with excellent stereoselectivity (up to 97 % e.e., >20 : 1 d.r.) and good yield (up to 86 %). This work provides an efficient biocatalytic strategy for cyclic ketone functionalization, and highlights the usefulness of artificial enzymes for extending biocatalysis to further non-natural reactions.

Triumphs and Challenges of Natural Product Discovery in the Postgenomic Era

Natural products have played significant roles as medicine and food throughout human history. Here, we first provide a brief historical overview of natural products, their classification and biosynthetic origins, and the microbiological and genetic methods used for their discovery. We also describe and discuss the technologies that revolutionized the field, which transitioned from classic genetics to genome-centric discovery approximately two decades ago. We then highlight the most recent advancements and approaches in the current postgenomic era, in which genome mining is a standard operation and high-throughput analytical methods allow parallel discovery of genes and molecules at an unprecedented pace. Finally, we discuss the new challenges faced by the field of natural products and the future of systematic heterologous expression and strain-independent discovery, which promises to deliver more molecules in vials than ever before.

Machine learning-guided co-optimization of fitness and diversity facilitates combinatorial library design in enzyme engineering

The effective design of combinatorial libraries to balance fitness and diversity facilitates the engineering of useful enzyme functions, particularly those that are poorly characterized or unknown in biology. We introduce MODIFY, a machine learning (ML) algorithm that learns from natural protein sequences to infer evolutionarily plausible mutations and predict enzyme fitness. MODIFY co-optimizes predicted fitness and sequence diversity of starting libraries, prioritizing high-fitness variants while ensuring broad sequence coverage. In silico evaluation shows that MODIFY outperforms state-of-the-art unsupervised methods in zero-shot fitness prediction and enables ML-guided directed evolution with enhanced efficiency. Using MODIFY, we engineer generalist biocatalysts derived from a thermostable cytochrome c to achieve enantioselective C-B and C-Si bond formation via a new-to-nature carbene transfer mechanism, leading to biocatalysts six mutations away from previously developed enzymes while exhibiting superior or comparable activities. These results demonstrate MODIFY's potential in solving challenging enzyme engineering problems beyond the reach of classic directed evolution.

Cytochrome P450 Enzyme Design by Constraining the Catalytic Pocket in a Diffusion Model

Although cytochrome P450 enzymes are the most versatile biocatalysts in nature, there is insufficient comprehension of the molecular mechanism underlying their functional innovation process. Here, by combining ancestral sequence reconstruction, reverse mutation assay, and progressive forward accumulation, we identified 5 founder residues in the catalytic pocket of flavone 6-hydroxylase (F6H) and proposed a "3-point fixation" model to elucidate the functional innovation mechanisms of P450s in nature. According to this design principle of catalytic pocket, we further developed a de novo diffusion model (P450Diffusion) to generate artificial P450s. Ultimately, among the 17 non-natural P450s we generated, 10 designs exhibited significant F6H activity and 6 exhibited a 1.3- to 3.5-fold increase in catalytic capacity compared to the natural CYP706X1. This work not only explores the design principle of catalytic pockets of P450s, but also provides an insight into the artificial design of P450 enzymes with desired functions.

Engineering the next-generation synthetic cell factory driven by protein engineering

Synthetic cell factory offers substantial advantages in economically efficient production of biofuels, chemicals, and pharmaceutical compounds. However, to create a high-performance synthetic cell factory, precise regulation of cellular material and energy flux is essential. In this context, protein components including enzymes, transcription factor-based biosensors and transporters play pivotal roles. Protein engineering aims to create novel protein variants with desired properties by modifying or designing protein sequences. This review focuses on summarizing the latest advancements of protein engineering in optimizing various aspects of synthetic cell factory, including: enhancing enzyme activity to eliminate production bottlenecks, altering enzyme selectivity to steer metabolic pathways towards desired products, modifying enzyme promiscuity to explore innovative routes, and improving the efficiency of transporters. Furthermore, the utilization of protein engineering to modify protein-based biosensors accelerates evolutionary process and optimizes the regulation of metabolic pathways. The remaining challenges and future opportunities in this field are also discussed.

A New Age of Biocatalysis Enabled by Generic Activation Modes

Biocatalysis is currently undergoing a profound transformation. The field moves from relying on nature's chemical logic to a discipline that exploits generic activation modes, allowing for novel biocatalytic reactions and, in many instances, entirely new chemistry. Generic activation modes enable a wide range of reaction types and played a pivotal role in advancing the fields of organo- and photocatalysis. This perspective aims to summarize the principal activation modes harnessed in enzymes to develop new biocatalysts. Although extensively researched in the past, the highlighted activation modes, when applied within enzyme active sites, facilitate chemical transformations that have largely eluded efficient and selective catalysis. This advance is attributed to multiple tunable interactions in the substrate binding pocket that precisely control competing reaction pathways and transition states. We will highlight cases of new synthetic methodologies achieved by engineered enzymes and will provide insights into potential future developments in this rapidly evolving field.

Merging DNA Repair with Bioorthogonal Conjugation Enables Accessible and Versatile Asymmetric DNA Catalysis

Optimizing catalysts through high-throughput screening for asymmetric catalysis is challenging due to the difficulty associated with assembling a library of catalyst analogues in a timely fashion. Here, we repurpose DNA excision repair and integrate it with bioorthogonal conjugation to construct a diverse array of DNA hybrid catalysts for highly accessible and high-throughput asymmetric DNA catalysis, enabling a dramatically expedited catalyst optimization process, superior reactivity and selectivity, as well as the first atroposelective DNA catalysis. The bioorthogonality of this conjugation strategy ensures exceptional tolerance toward diverse functional groups, thereby facilitating the facile construction of 44 DNA hybrid catalysts bearing various unprotected functional groups. This unique feature holds the potential to enable catalytic modalities in asymmetric DNA catalysis that were previously deemed unattainable.

Iron(II) Phthalocyanine-Catalyzed Homodimerization and Tandem Diamination of Diazo Compounds with Primary Amines: Access to Construct Substituted 2,3-Diaminosuccinonitriles in One-Pot

We herein first report the homodimerization and tandem diamination of diazo compounds with primary amines catalyzed by the iron(II) phthalocyanine (PcFe(II)), which can construct one C-C bond and two C-N bonds within 20 min in one-pot. Compared to the traditional metal-catalyzed N-H insertion reaction between amines with diazo reagents, the developed reaction almost does not generate the N-H insertion product, but the homodimerization/tandem diamination product. The proposed mechanism studies indicate that primary amines play a crucial role in the homocoupling of diazo compounds via dimerization of iron(III)-acetonitrile radical generated from the reaction between diazoacetonitrile with PcFe(II) coordinated by bis(amines); the β-hydride elimination is involved, and then, the attack of primary amines toward the carbon atoms on the formed C-C bond is followed. Moreover, this novel reaction can be used to effectively prepare substituted 2,3-diaminosuccinonitriles with high yields and even up to >99:1 d.r., encouragingly these products contain both 1,2-diamines and succinonitrile motifs, which are two classes of important organic compounds with significant applications in many yields. This reaction is also suitable for the gram-scale preparation of 2,3-bis(phenylamino)succinonitrile (2a) with a yield of 84%. Therefore, the developed reaction represents a new type of transformation.

Alkoxy Diazomethylation of Alkenes by Photoredox-Catalyzed Oxidative Radical-Polar Crossover

Herein, we present the discovery and development of the first photoredox-catalyzed alkoxy diazomethylation of alkenes with hypervalent iodine reagents and alcohols. This multicomponent process represents a new disconnection approach to diazo compounds and is featured by a broad scope, mild reaction conditions, and excellent selectivity. Key to the process was the generation of diazomethyl radicals, which engaged alkenes and alcohols in an inter- and intramolecular fashion by a photoredox-catalyzed oxidative radical-polar crossover leading to unexplored β-alkoxydiazo compounds. The synthetic utility of such diazo compounds was demonstrated with a series of transformations involving C-H, N-H, and O-H insertions as well as in the construction of complex sp3-rich heterocycles.

Recent developments in the enzymatic modifications of steroid scaffolds

Steroids are an important family of bioactive compounds. Steroid drugs are renowned for their multifaceted pharmacological activities and are the second-largest category in the global pharmaceutical market. Recent developments in biocatalysis and biosynthesis have led to the increased use of enzymes to enhance the selectivity, efficiency, and sustainability for diverse modifications of steroids. This review discusses the advancements achieved over the past five years in the enzymatic modifications of steroid scaffolds, focusing on enzymatic hydroxylation, reduction, dehydrogenation, cascade reactions, and other modifications for future research on the synthesis of novel steroid compounds and related drugs, and new therapeutic possibilities.

Biocatalytic, Enantioenriched Primary Amination of Tertiary C-H Bonds

Intermolecular functionalization of tertiary C-H bonds to construct fully substituted stereogenic carbon centers represents a formidable challenge: without the assistance of directing groups, state-of-the-art catalysts struggle to introduce chirality to racemic tertiary sp 3 -carbon centers. Direct asymmetric functionalization of such centers is a worthy reactivity and selectivity goal for modern biocatalysis. Here we present an engineered nitrene transferase (P411-TEA-5274), derived from a bacterial cytochrome P450, that is capable of aminating tertiary C-H bonds to provide chiral α-tertiary primary amines with high efficiency (up to 2300 total turnovers) and selectivity (up to >99% enantiomeric excess (e.e.)). The construction of fully substituted stereocenters with methyl and ethyl groups underscores the enzyme's remarkable selectivity. A comprehensive substrate scope study demonstrates the biocatalyst's compatibility with diverse functional groups and tertiary C-H bonds. Mechanistic studies elucidate how active-site residues distinguish between the enantiomers and enable the enzyme to perform this transformation with excellent enantioselectivity.

Choose Your Own Adventure: A Comprehensive Database of Reactions Catalyzed by Cytochrome P450 BM3 Variants

Cytochrome P450 BM3 monooxygenase is the topic of extensive research as many researchers have evolved this enzyme to generate a variety of products. However, the abundance of information on increasingly diversified variants of P450 BM3 that catalyze a broad array of chemistry is not in a format that enables easy extraction and interpretation. We present a database that categorizes variants by their catalyzed reactions and includes details about substrates to provide reaction context. This database of >1500 P450 BM3 variants is downloadable and machine-readable and includes instructions to maximize ease of gathering information. The database allows rapid identification of commonly reported substitutions, aiding researchers who are unfamiliar with the enzyme in identifying starting points for enzyme engineering. For those actively engaged in engineering P450 BM3, the database, along with this review, provides a powerful and user-friendly platform to understand, predict, and identify the attributes of P450 BM3 variants, encouraging the further engineering of this enzyme.

A Comparative Study of Cationic Copper(I) Reagents Supported by Bipodal Tetramethylguanidinyl-Containing Ligands as Nitrene-Transfer Catalysts

The bipodal compounds [(TMG2biphenN-R)CuI-NCMe](PF6) (R = Me, Ar (4-CF3Ph-)) and [(TMG2biphenN-Me)CuI-I] have been synthesized with ligands that feature a diarylmethyl- and triaryl-amine framework and superbasic tetramethylguanidinyl residues (TMG). The cationic Cu(I) sites mediate catalytic nitrene-transfer reactions between the imidoiodinane PhI = NTs (Ts = tosyl) and a panel of styrenes in MeCN, to afford aziridines, demonstrating comparable reactivity profiles. The copper reagents have been further explored to execute C-H amination reactions with a variety of aliphatic and aromatic hydrocarbons and two distinct nitrene sources PhI = NTs and PhI = NTces (Tces = 2,2,2-trichloroethylsulfamate) in benzene/HFIP (10:2 v/v). Good yields have been obtained for sec-benzylic and tert-C-H bonds of various substrates, especially with the more electron-deficient catalyst [(TMG2biphenN-Ar)CuI-NCMe](PF6). In conjunction with earlier studies, the order of reactivity of these bipodal cationic reagents as a function of the metal employed is established as Cu > Fe > Co ≥ Mn. However, as opposed to the base-metal analogues, the bipodal Cu reagents are less reactive than a similar tripodal Cu catalyst. The observed fluorophilicity of the bipodal Cu compounds may provide a deactivation pathway.

Engineering substrate specificity of quinone-dependent dehydrogenases for efficient oxidation of deoxynivalenol to 3-keto-deoxynivalenol

The oxidative reaction of Fusarium mycotoxin deoxynivalenol (DON) using the dehydrogenase is a desirable strategy and environmentally friendly to mitigate its toxicity. However, a critical issue for these dehydrogenases shows widespread substrate promiscuity. In this study, we conducted pocket reshaping of Devosia strain A6-243 pyrroloquinoline quinone (PQQ)-dependent dehydrogenase (DADH) on the basis of protein structure and kinetic analysis of substrate libraries to improve preference for particular substrate DON (10a). The variant presented an increased preference for substrate 10a and enhanced catalytic efficiency. A 4.7-fold increase in preference for substrate 10a was observed. Kinetic profiling and molecular dynamics (MD) simulations provided insights into the enhanced substrate specificity and activity. Moreover, the variant exhibited stronger conversion of substrate 10a to 3-keto-DON compared to the wild DADH. Overall, this study provides a feasible protocol for the redesign of PQQ-dependent dehydrogenases with favourable substrate specificity and catalytic activity, which is desperately needed for DON antidote development.

Stereodivergent Protein Engineering of Fatty Acid Photodecarboxylase for Light-Driven Kinetic Resolution of Sec-Alcohol Oxalates

Stereodivergent engineering of one enzyme to create stereocomplementary variants for synthesizing optically pure molecules with tailor-made (R) or (S) configurations on an optional basis is highly desirable and challenging. This study aimed to engineer fatty acid photodecarboxylase from Chlorella variabilis (CvFAP) using the focused rational iterative site-specific mutagenesis (FRISM) strategy to obtain two highly stereocomplementary variants with excellent selectivity (both giving products with up to 99 % e.e.). These variants were used for the CvFAP-catalyzed light-driven kinetic resolution of oxalates or oxamic acids prepared from the corresponding sec-alcohols or amines, providing a new biotransformation process for preparing chiral sec-alcohols and amines. Molecular dynamics simulation, kinetic data and transient spectra revealed the source of selectivity. This study represents the first example of the kinetic resolution of sec-alcohols or amines catalyzed by a pair of stereocomplementary CvFAPs.