Stereoselective amino acid synthesis by synergistic photoredox-pyridoxal radical biocatalysis

Catalyzing the future: recent advances in chemical synthesis using enzymes

Biocatalysis has the potential to address the need for more sustainable organic synthesis routes. Protein engineering can tune enzymes to perform in cascade reactions and for efficient synthesis of enantiomerically enriched compounds, using both natural and new-to-nature reaction pathways. This review highlights recent achievements in biocatalysis, especially the development of novel enzymatic syntheses to access versatile small molecule intermediates and complex biomolecules. Biocatalytic strategies for the degradation of persistent pollutants and approaches for biomass valorization are also discussed. The transition of chemical synthesis to a greener future will be accelerated by implementing enzymes and engineering them for high performance and new activities.

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.

Amyloid-Driven Allostery

The fields of allostery and amyloid-related pathologies, such as Parkinson's disease (PD), have been extensively explored individually, but less is known about how amyloids control allostery. Recent advancements have revealed that amyloids can drive allosteric effects in both intrinsically disordered proteins, such as alpha-synuclein (αS), and multi-domain signaling proteins, such as protein kinase A (PKA). Amyloid-driven allostery plays a central role in explaining the mechanisms of gain-of-pathological-function mutations in αS (e.g. E46K, which causes early PD onset) and loss-of-physiological-function mutations in PKA (e.g. A211D, which predisposes to tumors). This review highlights allosteric effects of disease-related mutations and how they can cause exposure of amyloidogenic regions, leading to amyloids that are either toxic or cause aberrant signaling. We also discuss multiple potential modulators of these allosteric effects, such as MgATP and kinase substrates, opening future opportunities to improve current pharmacological interventions against αS and PKA-related pathologies. Overall, we show that amyloid-driven allosteric models are useful to explain the mechanisms underlying disease-related mutations.

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.

Nature-Inspired Radical Pyridoxal-Mediated C-C Bond Formation

Pyridoxal-5'-phosphate (PLP) and derivatives of this cofactor enable a plethora of reactions in both enzyme-mediated and free-in-solution transformations. With few exceptions in each category, such chemistry has predominantly involved two-electron processes. This sometimes poses a significant challenge for using PLP to build tetrasubstituted carbon centers, especially when the reaction is reversible. The ability to access radical pathways is paramount to broadening the scope of reactions catalyzed by this coenzyme. In this study, we demonstrate the ability to access a radical PLP-based intermediate and engage this radical intermediate in a number of C-C bond-forming reactions. By selection of an appropriate oxidant, single-electron oxidation of the quinonoid intermediate can be achieved, which can subsequently be applied to C-C bond-forming reactions. Through this radical reaction pathway, we synthesized a series of α-tertiary amino acids and esters to investigate the substrate scope and identify nonproductive reaction pathways. Beyond the amino acid model system, we demonstrate that other classes of amine substrates can be applied in this reaction and that a range of small molecule reagents can serve as coupling partners to the semiquinone radical. We anticipate that this versatile semiquinone radical species will be central to the development of a range of novel reactions.

Threonine Aldolase-Catalyzed Enantioselective α-Alkylation of Amino Acids through Unconventional Photoinduced Radical Initiation

Visible light-driven pyridoxal radical biocatalysis has emerged as a promising strategy for the stereoselective synthesis of valuable noncanonical amino acids (ncAAs). Previously, the use of well-tailored photoredox catalysts represented the key to enable efficient pyridoxal phosphate (PLP) enzyme-catalyzed radical reactions. Here, we report a PLP-dependent threonine aldolase-catalyzed asymmetric α-C-H alkylation of abundant amino acids using Katritzky pyridinium salts as alkylating agents. The use of engineered threonine aldolases allowed for this redox-neutral radical alkylation to proceed efficiently, giving rise to challenging α-trisubstituted and -tetrasubstituted ncAA products in a protecting-group-free fashion with excellent enantiocontrol. Mechanistically, this enantioselective α-alkylation capitalizes on the unique reactivity of the persistent enzymatic quinonoid intermediate derived from the PLP cofactor and the amino acid substrate to allow for novel radical C-C coupling. Surprisingly, this photobiocatalytic process does not require the use of well-established photoredox catalysts and operates through an unconventional photoinduced radical generation involving a PLP-derived aldimine. The ability to develop photobiocatalytic reactions without relying on classic photocatalysts or photoenzymes opens up new avenues for advancing stereoselective intermolecular radical reactions that are not known in either organic chemistry or enzymology.

Stereodivergent photobiocatalytic radical cyclization through the repurposing and directed evolution of fatty acid photodecarboxylases

Despite their intriguing photophysical and photochemical activities, naturally occurring photoenzymes have not yet been repurposed for new-to-nature activities. Here we engineered fatty acid photodecarboxylases to catalyse unnatural photoredox radical C-C bond formation by leveraging the strongly oxidizing excited-state flavoquinone cofactor. Through genome mining, rational engineering and directed evolution, we developed a panel of radical photocyclases to facilitate decarboxylative radical cyclization with excellent chemo-, enantio- and diastereoselectivities. Our high-throughput experimental workflow allowed for the directed evolution of fatty acid photodecarboxylases. An orthogonal set of radical photocyclases was engineered to access all four possible stereoisomers of the stereochemical dyad, affording fully diastereo- and enantiodivergent biotransformations in asymmetric radical biocatalysis. Molecular dynamics simulations show that our evolved radical photocyclases allow near-attack conformations to be easily accessed, enabling chemoselective radical cyclization. The development of stereoselective radical photocyclases provides unnatural C-C-bond-forming activities in natural photoenzyme families, which can be used to tame the stereochemistry of free-radical-mediated reactions.

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.

Asymmetric photoenzymatic incorporation of fluorinated motifs into olefins

Enzymes capable of assimilating fluorinated feedstocks are scarce. This situation poses a challenge for the biosynthesis of fluorinated compounds used in pharmaceuticals, agrochemicals, and materials. We developed a photoenzymatic hydrofluoroalkylation that integrates fluorinated motifs into olefins. The photoinduced promiscuity of flavin-dependent ene-reductases enables the generation of carbon-centered radicals from iodinated fluoroalkanes, which are directed by the photoenzyme to engage enantioselectively with olefins. This approach facilitates stereocontrol through interaction between a singular fluorinated unit and the enzyme, securing high enantioselectivity at β, γ, or δ positions of fluorinated groups through enzymatic hydrogen atom transfer-a process that is notably challenging with conventional chemocatalysis. This work advances enzymatic strategies for integrating fluorinated chemical feedstocks and opens avenues for asymmetric synthesis of fluorinated compounds.

Chemogenetic Evolution of Diversified Photoenzymes for Enantioselective [2 + 2] Cycloadditions in Whole Cells

Artificial photoenzymes with novel catalytic modes not found in nature are in high demand; yet, they also present significant challenges in the field of biocatalysis. In this study, a chemogenetic modification strategy is developed to facilitate the rapid diversification of photoenzymes. This strategy integrates site-specific chemical conjugation of various artificial photosensitizers into natural protein cavities and the iterative mutagenesis in cell lysates. Through rounds of directed evolution, prominent visible-light-activatable photoenzyme variants were developed, featuring a thioxanthone chromophore. They successfully enabled the enantioselective [2 + 2] photocycloaddition of 2-carboxamide indoles, a class of UV-sensitive substrates that are traditionally challenging for known photoenzymes. Furthermore, the versatility of this photoenzyme is demonstrated in enantioselective whole-cell photobiocatalysis, enabling the efficient synthesis of enantioenriched cyclobutane-fused indoline tetracycles. These findings significantly expand the photophysical properties of artificial photoenzymes, a critical factor in enhancing their potential for harnessing excited-state reactivity in stereoselective transformations.

Merging High-Pressure Syngas Metal Catalysis and Biocatalysis in Tandem One-Pot Processes for the Synthesis of Nonchiral and Chiral Alcohols from Alkenes in Water

While simultaneously proceeding reactions are among the most fascinating features of biosynthesis, this concept of tandem processes also offers high potential in the chemical industry in terms of less waste production and improved process efficiency and sustainability. Although examples of one-pot chemoenzymatic syntheses exist, the combination of completely different reaction types is rare. Herein, we demonstrate that extreme "antipodes" of the "worlds of catalysis", such as syngas-based high-pressure hydroformylation and biocatalyzed reduction, can be combined within a tandem-type one-pot process in water. No significant deactivation was found for either the biocatalyst or the chemocatalyst. A proof-of-concept for the one-pot process starting from 1-octene was established with >99 % conversion and 80 % isolated yield of the desired alcohol isomers. All necessary components for hydroformylation and biocatalysis were added to the reactor from the beginning. This concept has been extended to the enantioselective synthesis of chiral products by conducting the hydroformylation of styrene and an enzymatic dynamic kinetic resolution in a tandem mode, leading to an excellent conversion of >99 % and an enantiomeric ratio of 91 : 9 for (S)-2-phenylpropanol. The overall process runs in water under mild and energy-saving conditions, without any need for intermediate isolation.

Expanding chemistry through in vitro and in vivo biocatalysis

Living systems contain a vast network of metabolic reactions, providing a wealth of enzymes and cells as potential biocatalysts for chemical processes. The properties of protein and cell biocatalysts-high selectivity, the ability to control reaction sequence and operation in environmentally benign conditions-offer approaches to produce molecules at high efficiency while lowering the cost and environmental impact of industrial chemistry. Furthermore, biocatalysis offers the opportunity to generate chemical structures and functions that may be inaccessible to chemical synthesis. Here we consider developments in enzymes, biosynthetic pathways and cellular engineering that enable their use in catalysis for new chemistry and beyond.

Repurposing Visible-Light-Excited Ene-Reductases for Diastereo- and Enantioselective Lactones Synthesis

Repurposing enzymes to catalyze non-natural asymmetric transformations that are difficult to achieve using traditional chemical methods is of significant importance. Although radical C-O bond formation has emerged as a powerful approach for constructing oxygen-containing compounds, controlling the stereochemistry poses a great challenge. Here we present the development of a dual bio-/photo-catalytic system comprising an ene-reductase and an organic dye for achieving stereoselective lactonizations. By integrating directed evolution and photoinduced single electron oxidation, we repurposed engineered ene-reductases to steer non-natural radical C-O formations (one C-O bond for hydrolactonizations and lactonization-alkylations while two C-O bonds for lactonization-oxygenations). This dual catalysis gave a new approach to a diverse array of enantioenhanced 5- and 6-membered lactones with vicinal stereocenters, part of which bears a quaternary stereocenter (up to 99 % enantiomeric excess, up to 12.9 : 1 diastereomeric ratio). Detailed mechanistic studies, including computational simulations, uncovered the synergistic effect of the enzyme and the externally added organophotoredox catalyst Rh6G.

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.

Mechanism-Guided Computational Design Drives meso-Diaminopimelate Dehydrogenase to Efficient Synthesis of Aromatic d-amino Acids

Aromatic d-amino acids (d-AAs) play a pivotal role as important chiral building blocks and key intermediates in fine chemical and drug synthesis. Meso-diaminopimelate dehydrogenase (DAPDH) serves as an excellent biocatalyst in the synthesis of d-AAs and their derivatives. However, its strict substrate specificity and the lack of efficient engineering methods have hindered its widespread application. Therefore, this study aims to elucidate the catalytic mechanism underlying DAPDH from Proteus vulgaris (PvDAPDH) through the examination of its crystallographic structure, computational simulations of potential energies and molecular dynamics simulations, and site-directed mutagenesis. Mechanism-guided computational design showed that the optimal mutant PvDAPDH-M3 increased specific activity and catalytic efficiency (kcat/Km) for aromatic keto acids up to 124-fold and 92.4-fold, respectively, compared to that of the wild type. Additionally, it expanded the substrate scope to 10 aromatic keto acid substrates. Finally, six high-value-added aromatic d-AAs and their derivatives were synthesized using a one-pot three-enzyme cascade reaction, exhibiting a good conversion rate ranging from 32 to 84% and excellent stereoselectivity (enantiomeric excess >99%). These findings provide a potential synthetic pathway for the green industrial production of aromatic d-AAs.

New advances in protein engineering for industrial applications: Key takeaways

Recent advancements in protein/enzyme engineering have enabled the production of a diverse array of high-value compounds in microbial systems with the potential for industrial applications. The goal of this review is to articulate some of the most recent protein engineering advances in bacteria, yeast, and other microbial systems to produce valuable substances. These high-value substances include α-farnesene, vitamin B12, fumaric acid, linalool, glucaric acid, carminic acid, mycosporine-like amino acids, patchoulol, orcinol glucoside, d-lactic acid, keratinase, α-glucanotransferases, β-glucosidase, seleno-methylselenocysteine, fatty acids, high-efficiency β-glucosidase enzymes, cellulase, β-carotene, physcion, and glucoamylase. Additionally, recent advances in enzyme engineering for enhancing thermostability will be discussed. These findings have the potential to revolutionize various industries, including biotechnology, food, pharmaceuticals, and biofuels.

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

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

Synergistic Photoenzymatic Catalysis Enables Synthesis of a-Tertiary Amino Acids Using Threonine Aldolases

a-Tertiary amino acids are essential components of drugs and agrochemicals, yet traditional syntheses are step-intensive and provide access to a limited range of structures with varying levels of enantioselectivity. Here, we report the α-alkylation of unprotected alanine and glycine by pyridinium salts using pyridoxal (PLP)-dependent threonine aldolases with a Rose Bengal photoredox catalyst. The strategy efficiently prepares various a-tertiary amino acids in a single chemical step as a single enantiomer. UV-vis spectroscopy studies reveal a ternary interaction between the pyridinium salt, protein, and photocatalyst, which we hypothesize is responsible for localizing radical formation to the active site. This method highlights the opportunity for combining photoredox catalysts with enzymes to reveal new catalytic functions for known enzymes.

Modular Access to Chiral Amines via Imine Reductase-Based Photoenzymatic Catalysis

The development of catalysts serves as the cornerstone of innovation in synthesis, as exemplified by the recent discovery of photoenzymes. However, the repertoire of naturally occurring enzymes repurposed by direct light excitation to catalyze new-to-nature photobiotransformations is currently limited to flavoproteins and keto-reductases. Herein, we shed light on imine reductases (IREDs) that catalyze the remote C(sp3)-C(sp3) bond formation, providing a previously elusive radical hydroalkylation of enamides for accessing chiral amines (45 examples with up to 99% enantiomeric excess). Beyond their natural function in catalyzing two-electron reductive amination reactions, upon direct visible-light excitation or in synergy with a synthetic photoredox catalyst, IREDs are repurposed to tune the non-natural photoinduced single-electron radical processes. By conducting wet mechanistic experiments and computational simulations, we unravel how engineered IREDs direct radical intermediates toward the productive and enantioselective pathway. This work represents a promising paradigm for harnessing nature's catalysts for new-to-nature asymmetric transformations that remain challenging through traditional chemocatalytic methods.

Recent advances in catalytic asymmetric synthesis

Asymmetric catalysis stands at the forefront of modern chemistry, serving as a cornerstone for the efficient creation of enantiopure chiral molecules characterized by their high selectivity. In this review, we delve into the realm of asymmetric catalytic reactions, which spans various methodologies, each contributing to the broader landscape of the enantioselective synthesis of chiral molecules. Transition metals play a central role as catalysts for a wide range of transformations with chiral ligands such as phosphines, N-heterocyclic carbenes (NHCs), etc., facilitating the formation of chiral C-C and C-X bonds, enabling precise control over stereochemistry. Enantioselective photocatalytic reactions leverage the power of light as a driving force for the synthesis of chiral molecules. Asymmetric electrocatalysis has emerged as a sustainable approach, being both atom-efficient and environmentally friendly, while offering a versatile toolkit for enantioselective reductions and oxidations. Biocatalysis relies on nature's most efficient catalysts, i.e., enzymes, to provide exquisite selectivity, as well as a high tolerance for diverse functional groups under mild conditions. Thus, enzymatic optical resolution, kinetic resolution and dynamic kinetic resolution have revolutionized the production of enantiopure compounds. Enantioselective organocatalysis uses metal-free organocatalysts, consisting of modular chiral phosphorus, sulfur and nitrogen components, facilitating remarkably efficient and diverse enantioselective transformations. Additionally, unlocking traditionally unreactive C-H bonds through selective functionalization has expanded the arsenal of catalytic asymmetric synthesis, enabling the efficient and atom-economical construction of enantiopure chiral molecules. Incorporating flow chemistry into asymmetric catalysis has been transformative, as continuous flow systems provide precise control over reaction conditions, enhancing the efficiency and facilitating optimization. Researchers are increasingly adopting hybrid approaches that combine multiple strategies synergistically to tackle complex synthetic challenges. This convergence holds great promise, propelling the field of asymmetric catalysis forward and facilitating the efficient construction of complex molecules in enantiopure form. As these methodologies evolve and complement one another, they push the boundaries of what can be accomplished in catalytic asymmetric synthesis, leading to the discovery of novel, highly selective transformations which may lead to groundbreaking applications across various industries.

From Ground-State to Excited-State Activation Modes: Flavin-Dependent "Ene"-Reductases Catalyzed Non-natural Radical Reactions

Enzymes are desired catalysts for chemical synthesis, because they can be engineered to provide unparalleled levels of efficiency and selectivity. Yet, despite the astonishing array of reactions catalyzed by natural enzymes, many reactivity patterns found in small molecule catalysts have no counterpart in the living world. With a detailed understanding of the mechanisms utilized by small molecule catalysts, we can identify existing enzymes with the potential to catalyze reactions that are currently unknown in nature. Over the past eight years, our group has demonstrated that flavin-dependent "ene"-reductases (EREDs) can catalyze various radical-mediated reactions with unparalleled levels of selectivity, solving long-standing challenges in asymmetric synthesis.This Account presents our development of EREDs as general catalysts for asymmetric radical reactions. While we have developed multiple mechanisms for generating radicals within protein active sites, this account will focus on examples where flavin mononucleotide hydroquinone (FMNhq) serves as an electron transfer radical initiator. While our initial mechanistic hypotheses were rooted in electron-transfer-based radical initiation mechanisms commonly used by synthetic organic chemists, we ultimately uncovered emergent mechanisms of radical initiation that are unique to the protein active site. We will begin by covering intramolecular reactions and discussing how the protein activates the substrate for reduction by altering the redox-potential of alkyl halides and templating the charge transfer complex between the substrate and flavin-cofactor. Protein engineering has been used to modify the fundamental photophysics of these reactions, highlighting the opportunity to tune these systems further by using directed evolution. This section highlights the range of coupling partners and radical termination mechanisms available to intramolecular reactions.The next section will focus on intermolecular reactions and the role of enzyme-templated ternary charge transfer complexes among the cofactor, alkyl halide, and coupling partner in gating electron transfer to ensure that it only occurs when both substrates are bound within the protein active site. We will highlight the synthetic applications available to this activation mode, including olefin hydroalkylation, carbohydroxylation, arene functionalization, and nitronate alkylation. This section also discusses how the protein can favor mechanistic steps that are elusive in solution for the asymmetric reductive coupling of alkyl halides and nitroalkanes. We are aware of several recent EREDs-catalyzed photoenzymatic transformations from other groups. We will discuss results from these papers in the context of understanding the nuances of radical initiation with various substrates.These biocatalytic asymmetric radical reactions often complement the state-of-the-art small-molecule-catalyzed reactions, making EREDs a valuable addition to a chemist's synthetic toolbox. Moreover, the underlying principles studied with these systems are potentially operative with other cofactor-dependent proteins, opening the door to different types of enzyme-catalyzed radical reactions. We anticipate that this Account will serve as a guide and inspire broad interest in repurposing existing enzymes to access new transformations.

Evaluation of Organic Hydride/Acid Pairs as a Type of Thermodynamic-Potential-Regulated Multisite Proton-Coupled Electron Transfer Reagents

Organic hydride/acid pairs have been reported as multisite proton-coupled electron transfer (MS-PCET) reagents in reductive MS-PCET reactions recently. Since the key step for an organic hydride/acid pair acting as an MS-PCET reagent is a chemical process of the organic hydride/acid pair releasing a formal hydrogen atom, the bond dissociation free energy of the organic hydride/acid pair releasing a formal hydrogen atom is a valuable thermodynamic parameter for objectively evaluating the thermodynamic potential for an organic hydride/acid pair to act as an MS-PCET reagent. Now, organic hydride/acid pairs of 216 organic hydrides have been demonstrated to be a potential type of thermodynamically potential-regulated MS-PCET reagent. Without a doubt, organic hydride/acid pairs reflect the change of N-substituted organic hydrides from simple hydride reductants to thermodynamically-regulated MS-PCET reagents, which could significantly expand the availability of novel MS-PCET reagents.

Stereoselective amino acid synthesis by photobiocatalytic oxidative coupling

Photobiocatalysis-where light is used to expand the reactivity of an enzyme-has recently emerged as a powerful strategy to develop chemistries that are new to nature. These systems have shown potential in asymmetric radical reactions that have long eluded small-molecule catalysts1. So far, unnatural photobiocatalytic reactions are limited to overall reductive and redox-neutral processes2-9. Here we report photobiocatalytic asymmetric sp3-sp3 oxidative cross-coupling between organoboron reagents and amino acids. This reaction requires the cooperative use of engineered pyridoxal biocatalysts, photoredox catalysts and an oxidizing agent. We repurpose a family of pyridoxal-5'-phosphate-dependent enzymes, threonine aldolases10-12, for the α-C-H functionalization of glycine and α-branched amino acid substrates by a radical mechanism, giving rise to a range of α-tri- and tetrasubstituted non-canonical amino acids 13-15 possessing up to two contiguous stereocentres. Directed evolution of pyridoxal radical enzymes allowed primary and secondary radical precursors, including benzyl, allyl and alkylboron reagents, to be coupled in an enantio- and diastereocontrolled fashion. Cooperative photoredox-pyridoxal biocatalysis provides a platform for sp3-sp3 oxidative coupling16, permitting the stereoselective, intermolecular free-radical transformations that are unknown to chemistry or biology.

Biocatalytic Steroidal 9α-Hydroxylation and Fragmentation Enable the Concise Chemoenzymatic Synthesis of 9,10-Secosteroids

9,10-Secosteroids are an important group of marine steroids with diverse biological activities. Herein, we report a chemoenzymatic strategy for the concise, modular, and scalable synthesis of ten naturally occurring 9,10-secosteroids from readily available steroids in three to eight steps. The key feature lies in utilizing a Rieske oxygenase-like 3-ketosteroid 9α-hydroxylase (KSH) as the biocatalyst to achieve efficient C9-C10 bond cleavage and A-ring aromatization of tetracyclic steroids through 9α-hydroxylation and fragmentation. With synthesized 9,10-secosteroides, structure-activity relationship was evaluated based on bioassays in terms of previously unexplored anti-infective activity. This study provides experimental evidence to support the hypothesis that the biosynthetic pathway through which 9,10-secosteroids are formed in nature shares a similar 9α-hydroxylation and fragmentation cascade. In addition to the development of a biomimetic approach for 9,10-secosteroid synthesis, this study highlights the great potential of chemoenzymatic strategies in chemical synthesis.

Molecular and Structural Basis for Cγ-C Bond Formation by PLP-Dependent Enzyme Fub7

Pyridoxal 5'-phosphate (PLP)-dependent enzymes that catalyze γ-replacement reactions are prevalent, yet their utilization of carbon nucleophile substrates is rare. The recent discovery of two PLP-dependent enzymes, CndF and Fub7, has unveiled unique C-C bond forming capabilities, enabling the biocatalytic synthesis of alkyl- substituted pipecolic acids from O-acetyl-L-homoserine and β-keto acid or aldehyde derived enolates. This breakthrough presents fresh avenues for the biosynthesis of pipecolic acid derivatives. However, the catalytic mechanisms of these enzymes remain elusive, and a dearth of structural information hampers their extensive application. Here, we have broadened the catalytic scope of Fub7 by employing ketone-derived enolates as carbon nucleophiles, revealing Fub7's capacity for substrate-dependent regioselective α-alkylation of unsymmetrical ketones. Through an integrated approach combining X-ray crystallography, spectroscopy, mutagenesis, and computational docking studies, we offer a detailed mechanistic insight into Fub7 catalysis. Our findings elucidate the structural basis for its substrate specificity, stereoselectivity, and regioselectivity. Our work sets the stage ready for subsequent protein engineering effort aimed at expanding the synthetic utility of Fub7, potentially unlocking novel methods to access a broader array of noncanonical amino acids.

Hydrophobic Trinuclear Copper Cluster-Containing Organic Framework for Synergetic Electrocatalytic Synthesis of Amino Acids

Electrocatalytic synthesis of amino acids provides a promising green and efficient pathway to manufacture the basic substances of life. Herein, reaction of 2,5-perfluroalkyl-terepthalohydrazide and tris(4-µ2 -O-carboxaldehyde-pyrazolato-N, N')-tricopper affords a crystalline trinuclear copper cluster-containing organic framework, named F-Cu3 -OF. Incorporation of abundant hydrophobic perfluroalkyl groups inside the channels of F-Cu3 -OF is revealed to successfully suppress the hydrogen evolution reaction via preventing H+ cation with large polarity from the framework of F-Cu3 -OF and in turn increasing the adsorption of other substrates with relatively small polarity like NO3 - and keto acids on the active sites. The copper atoms with short distance in the trinuclear copper clusters of F-Cu3 -OF enable simultaneous activization of NO3 - and keto acids, facilitating the following synergistic and efficient C─N coupling on the basis of in situ spectroscopic investigations together with theoretical calculation. Combination of these effects leads to efficient electroproduction of various amino acids including glycine, alanine, leucine, valine, and phenylalanine from NO3 - and keto acids with a Faraday efficiency of 42%-71% and a yield of 187-957 µmol cm-2 h-1 , representing the thus far best performance. This work shall be helpful for developing economical, eco-friendly, and high-efficiency strategy for the production of amino acids and other life substances.

One-pot chemoenzymatic syntheses of non-canonical amino acids

Despite their prevalent use in drug discovery and protein biochemistry, non-canonical amino acids are still challenging to synthesize through purely chemical means. In recent years, biocatalysis has emerged as a transformative paradigm for small-molecule synthesis. One strategy to further empower biocatalysis is to use it in combination with modern chemical reactions and take advantage of the strengths of each method to enable access to challenging structural motifs that were previously unattainable using each method alone. In this Mini-Review, we highlight several recent case studies that feature the synergistic use of chemical and enzymatic transformations in one pot to synthesize novel non-canonical amino acids.

ONE-SENTENCE SUMMARY

This Mini-Review highlights several recent case studies that feature the synergistic use of chemical and enzymatic transformations in one pot to synthesize novel non-canonical amino acids.

From nature to industry: Harnessing enzymes for biocatalysis

Biocatalysis harnesses enzymes to make valuable products. This green technology is used in countless applications from bench scale to industrial production and allows practitioners to access complex organic molecules, often with fewer synthetic steps and reduced waste. The last decade has seen an explosion in the development of experimental and computational tools to tailor enzymatic properties, equipping enzyme engineers with the ability to create biocatalysts that perform reactions not present in nature. By using (chemo)-enzymatic synthesis routes or orchestrating intricate enzyme cascades, scientists can synthesize elaborate targets ranging from DNA and complex pharmaceuticals to starch made in vitro from CO2-derived methanol. In addition, new chemistries have emerged through the combination of biocatalysis with transition metal catalysis, photocatalysis, and electrocatalysis. This review highlights recent key developments, identifies current limitations, and provides a future prospect for this rapidly developing technology.

Predicting pathogenic protein variants

Machine-learning algorithm uses structure prediction to spot disease-causing mutations.

Recent Advances in the Enantioselective Radical Reactions

The review covers research published since 2017 and is focused on enantioselective synthesis using radical reactions. It describes recent approaches to the asymmetric synthesis of chiral molecules based on the application of the metal catalysis, dual metal and organocatalysis and finally, pure organocatalysis including enzyme catalysis. This review focuses on the synthetic aspects of the methodology and tries to show which compounds can be obtained in enantiomerically enriched forms.