Preparative scale Achmatowicz and aza-Achmatowicz rearrangements catalyzed by Agrocybe aegerita unspecific peroxygenase

The unspecific peroxygenase (UPO) from Agrocybe aegerita (rAaeUPO-PaDa-I-H) is an effective and practical biocatalyst for the oxidative expansion of furfuryl alcohols/amines on a preparative scale, using the Achmatowicz and aza-Achmatowicz reaction. The high activity and stability of the enzyme, which can be produced on a large scale as an air-stable lyophilised powder, renders it a versatile and scalable biocatalyst for the preparation of synthetically valuable 6-hydroxypyranones and dihydropiperidinones. In several cases, the biotransformation out-performed the analogous chemo-catalysed process, and operates under milder and greener reaction conditions.

Unspecific Peroxygenase (UPO) can be Tuned for Oxygenation or Halogenation Activity by Controlling the Reaction pH

Unspecific Peroxygenases (UPOs) are increasingly significant enzymes for selective oxygenations as they are stable, highly active and catalyze their reactions at the expense of only hydrogen peroxide as the oxidant. Their structural similarity to chloroperoxidase (CPO) means that UPOs can also catalyze halogenation reactions based upon the generation of hypohalous acids from halide and H2O2. Here we show that the halogenation and oxygenation modes of a UPO can be stimulated at different pH values. Using simple aromatic compounds such as thymol, we show that, at a pH of 3.0 and 6.0, either brominated or oxygenated products respectively are produced. Preparative 100 mg scale transformations of substrates were performed with 60-72 % isolated yields of brominated products obtained. A one-pot bromination-oxygenation cascade reaction on 4-ethylanisole, in which the pH was adjusted from 3.0 to 6.0 at the halfway stage, yielded sequentially brominated and oxygenated products 1-(3-bromo-4-methoxyphenyl)ethyl alcohol and 3-bromo-4-methoxy acetophenone with 82 % combined conversion. These results identify UPOs as an unusual example of a biocatalyst that is tunable for entirely different chemical reactions, dependent upon the reaction conditions.

Controlled Delivery of H2O2: A Three-Enzyme Cascade Flow Reactor for Peroxidase-Catalyzed Reactions

Peroxidases are promising catalysts for oxidation reactions, yet their practical utility has been hindered by the fact that they require hydrogen peroxide (H2O2), which at high concentrations can cause deactivation of enzymes. Practical processes involving the use of peroxidases require the frequent addition of low concentrations of H2O2. In situ generation of H2O2 can be achieved using oxidase-type enzymes. In this study, a three-enzyme cascade system comprised of a H2O2 generator (glucose oxidase (GOx)), H2O2-dependent enzymes (chloroperoxidase (CPO) or horseradish peroxidase (HRP)), and a H2O2 scavenger (catalase (CAT)) was deployed in a flow reactor. Immobilization of the enzymes on a graphite rod was achieved through electrochemically driven physical adsorption, followed by cross-linking with glutaraldehyde. Modeling studies indicated that the flow in the reactor was laminar (Reynolds number, R e < 2000) and was nearly fully developed at the midplane of the annular reactor. Immobilized CAT and GOx displayed good stability, retaining 79% and 84% of their initial activity, respectively, after three cycles of operation. Conversely, immobilized CPO exhibited a considerable reduction in activity after one use, retaining only 30% of its initial activity. The GOx-CAT-GRE system enabled controlled delivery of H2O2 in a more stable manner with a 4-fold enhancement in the oxidation of indole compared to the direct addition of H2O2. Using CPO in solution coupled with GOx-CAT-GRE yields of 90% for the oxidation of indole to 2-oxyindole and of 93% and 91% for the chlorination of thymol and carvacrol, respectively.

Modularization of Immobilized Multienzyme Cascades for Continuous-Flow Enantioselective C-H Amination

Multienzyme cascades (MECs) have gained much attention in synthetic chemistry but remain far from being a reliable synthetic tool. Here we report a four-enzyme cascade comprising a cofactor-independent and a cofactor self-sustaining bienzymatic modules for the enantioselective benzylic C-H amination of arylalkanes, a challenging transformation from bulk chemicals to high value-added chiral amines. The two modules were subsequently optimized by enzyme co-immobilization with microenvironmental tuning, and finally integrated in a gas-liquid segmented flow system, resulting in simultaneous improvements in enzyme performance, mass transfer, system compatibility, and productivity. The flow system enabled continuous C-H amination of arylalkanes (up to 100 mM) utilizing the sole cofactor NADH (0.5 mM) in >90 % conversion, achieving a high space-time yield (STY) of 3.6 g ⋅ L-1 ⋅ h-1, which is a 90-fold increase over the highest value previously reported.

Engineering carboxylic acid reductases and unspecific peroxygenases for flavor and fragrance biosynthesis

Emerging consumer demand for safer, more sustainable flavors and fragrances has created new challenges for the industry. Enzymatic syntheses represent a promising green production route, but the broad application requires engineering advancements for expanded diversity, improved selectivity, and enhanced stability to be cost-competitive with current methods. This review discusses recent advances and future outlooks for enzyme engineering in this field. We focus on carboxylic acid reductases (CARs) and unspecific peroxygenases (UPOs) that enable selective productions of complex flavor and fragrance molecules. Both enzyme types consist of natural variants with attractive characteristics for biocatalytic applications. Applying protein engineering methods, including rational design and directed evolution in concert with computational modeling, present excellent examples for property improvements to unleash the full potential of enzymes in the biosynthesis of value-added chemicals.

Reaction engineering blocks ether cleavage for synthesizing chiral cyclic hemiacetals catalyzed by unspecific peroxygenase

Hemiacetal compounds are valuable building blocks in synthetic chemistry, but their enzymatic synthesis is limited and often hindered by the instability of hemiacetals in aqueous environments. Here, we show that this challenge can be addressed through reaction engineering by using immobilized peroxygenase from Agrocybe aegerita (AaeUPO) under neat reaction conditions, which allows for the selective C-H bond oxyfunctionalization of environmentally significant cyclic ethers to cyclic hemiacetals. A wide range of chiral cyclic hemiacetal products are prepared in >99% enantiomeric excess and 95170 turnover numbers of AaeUPO. Furthermore, by changing the reaction medium from pure organic solvent to alkaline aqueous conditions, cyclic hemiacetals are in situ transformed into lactones. Lactams are obtained under the applied conditions, albeit with low enzyme activity. These findings showcase the synthetic potential of AaeUPO and offer a practical enzymatic approach to produce chiral cyclic hemiacetals through C-H oxyfunctionalization under mild conditions.

Electrochemical H2O2 - stat mode as reaction concept to improve the process performance of an unspecific peroxygenase

The electroenzymatic hydroxylation of 4-ethylbenzoic acid catalyzed by the recombinant unspecific peroxygenase from the fungus Agrocybe aegerita (rAaeUPO) was performed in a gas diffusion electrode (GDE)-based system. Enzyme stability and productivity are significantly affected by the way the co-substrate hydrogen peroxide (H2O2) is supplied. In this study, two in-situ electrogeneration modes of H2O2 were established and compared. Experiments under galvanostatic conditions (constant productivity of H2O2) were conducted at current densities spanning from 0.8 mA cm-2 to 6.4 mA cm-2. For comparison, experiments under H2O2-stat mode (constant H2O2 concentration) were performed. Here, four H2O2 concentrations between 0.06 mM and 0.28 mM were tested. A maximum H2O2 productivity of 5.5 µM min-1 cm-2 and productivity of 10.5 g L-1 d-1 were achieved under the galvanostatic condition at 6.4 mA cm-2. Meanwhile, the highest total turnover number (TTN) of 710,000 mol mol-1 and turnover frequency (TOF) of 87.5 s-1 were obtained under the H2O2-stat mode at concentration limits of 0.15 mM and 0.28 mM, respectively. The most favorable outcome in terms of maximum achievable TTN, TOF and productivity was found under the H2O2-stat mode at concentration limit of 0.2 mM. Here, a TTN of 655,000 mol mol-1, a TOF of 80.3 s-1 and a productivity of 6.1 g L-1 d-1 were achieved. The electrochemical H2O2-stat mode not only offers a promising alternative reaction concept to the well-established galvanostatic mode but also enhances the process performance of unspecific peroxygenases.

Analysis of critical residues for peroxygenation and improved peroxygenase activity via in situ H2O2 generation in CYP105D18

Limited numbers of CYPs have been reported to work naturally as peroxygenases. The peroxide shunt pathway can be efficiently used as an alternative for the NAD(P)H and reductase systems, particularly in high hydrogen peroxide (H2O2) resistance CYPs. We reported the structural and biochemical features of CYP105D18 peroxygenase for its high H2O2 tolerance capacity. Q348 was a crucial residue for the stability of CYP105D18 during the exposure to H2O2. In addition, the role of the hydrophilic amino acid T239 from the I helix for peroxygenation and regiospecificity toward testosterone was investigated. Interestingly, T239E differs in product formation from wild type, catalyzing testosterone to androstenedione in the presence of H2O2. The other variant, T239A, worked with the Pdx/Pdr system and was unable to catalyze testosterone conversion in the presence of H2O2, suggesting the transformation of peroxygenase into monooxygenase. CYP105D18 supported the alternative method of H2O2 used for the catalysis of testosterone. The use of the same concentration of urea hydrogen peroxide adducts in place of direct H2O2 was more efficient for 2β-hydroxytestosterone conversion. Furthermore, in situ H2O2 generation using GOx/glucose system enhanced the catalytic efficiency (kcat/Km) for wild type and F184A by 1.3- and 1.9-fold, respectively, compared to direct use of H2O2 The engineering of CYP105D18, its improved peroxygenase activity, and alteration in the product oxidation facilitate CYP105D18 as a potential candidate for biotechnological applications.

Enzymatic Oxy- and Amino-Functionalization in Biocatalytic Cascade Synthesis: Recent Advances and Future Perspectives

Biocatalytic cascades are a powerful tool for building complex molecules containing oxygen and nitrogen functionalities. Moreover, the combination of multiple enzymes in one pot offers the possibility to minimize downstream processing and waste production. In this review, we illustrate various recent efforts in the development of multi-step syntheses involving C-O and C-N bond-forming enzymes to produce high value-added compounds, such as pharmaceuticals and polymer precursors. Both in vitro and in vivo examples are discussed, revealing the respective advantages and drawbacks. The use of engineered enzymes to boost the cascades outcome is also addressed and current co-substrate and cofactor recycling strategies are presented, highlighting the importance of atom economy. Finally, tools to overcome current challenges for multi-enzymatic oxy- and amino-functionalization reactions are discussed, including flow systems with immobilized biocatalysts and cascades in confined nanomaterials.

Effect of Nitrogen, Air, and Oxygen on the Kinetic Stability of NAD(P)H Oxidase Exposed to a Gas-Liquid Interface

Biocatalytic oxidation is an interesting prospect for the selective synthesis of active pharmaceutical intermediates. Bubbling air or oxygen is considered as an efficient method to increase the gas-liquid interface and thereby enhance oxygen transfer. However, the enzyme is deactivated in this process and needs to be further studied and understood to accelerate the implementation of oxidative biocatalysis in larger production processes. This paper reports data on the stability of NAD(P)H oxidase (NOX) when exposed to different gas-liquid interfaces introduced by N2 (0% oxygen), air (21% oxygen), and O2 (100% oxygen) in a bubble column. A pH increase was observed during gas bubbling, with the highest increase occurring under air bubbling from 6.28 to 7.40 after 60 h at a gas flow rate of 0.15 L min-1. The kinetic stability of NOX was studied under N2, air, and O2 bubbling by measuring the residual activity, the deactivation constants (kd1) were 0.2972, 0.0244, and 0.0346 with the corresponding half-lives of 2.2, 28.6, and 20.2 h, respectively. A decrease in protein concentration of the NOX solution was also observed and was attributed to likely enzyme aggregation at the gas-liquid interface. Most aggregation occurred at the air-water interface and decreased greatly from 100 to 14.16% after 60 h of bubbling air. Furthermore, the effect of the gas-liquid interface and the dissolved gas on the NOX deactivation process was also studied by bubbling N2 and O2 alternately. It was found that the N2-water interface and O2-water interface both had minor effects on the protein concentration decrease compared with the air-water interface, whilst the dissolved N2 in water caused serious deactivation of NOX. This was attributed not only to the NOX unfolding and aggregation at the interface but also to the N2 occupying the oxygen channel of the enzyme and the resultant inaccessibility of dissolved O2 to the active site of NOX. These results shed light on the enzyme deactivation process and might further inspire bioreactor operation and enzyme engineering to improve biocatalyst performance.

Photobiocatalytic Strategies for Organic Synthesis

Biocatalysis has revolutionized chemical synthesis, providing sustainable methods for preparing various organic molecules. In enzyme-mediated organic synthesis, most reactions involve molecules operating from their ground states. Over the past 25 years, there has been an increased interest in enzymatic processes that utilize electronically excited states accessed through photoexcitation. These photobiocatalytic processes involve a diverse array of reaction mechanisms that are complementary to one another. This comprehensive review will describe the state-of-the-art strategies in photobiocatalysis for organic synthesis until December 2022. Apart from reviewing the relevant literature, a central goal of this review is to delineate the mechanistic differences between the general strategies employed in the field. We will organize this review based on the relationship between the photochemical step and the enzymatic transformations. The review will include mechanistic studies, substrate scopes, and protein optimization strategies. By clearly defining mechanistically-distinct strategies in photobiocatalytic chemistry, we hope to illuminate future synthetic opportunities in the area.

CYP102A1 peroxygenase catalyzed reaction via in situ H2O2 generation

CYP102A1 is a promiscuous bacterial cytochrome P450 (CYP or P450) known for its diverse substrates and comparable activity with human P450 enzymes. The development of CYP102A1 peroxygenase activity can contribute significantly to human drug development and drug metabolite production. Peroxygenase has recently emerged as an alternative to a dependency of P450 on NADPH-P450 reductase and NADPH cofactor and gives more opportunity for practical application. However, the H₂O₂ dependency also leads to challenges regarding its practical application, in which the excessive H₂O₂ concentration causes the activation of the peroxygenases. Therefore, we need the optimization of H₂O₂ production to minimize oxidative inactivation. In this study, we report the CYP102A1 peroxygenase-catalyzed atorvastatin hydroxylation reaction with an enzymatic H₂O₂ generation using glucose oxidase. Random mutagenesis at the CYP102A1 heme domain was used to generate mutant libraries with high throughput screening of highly active mutants, which can pair with the in situ H₂O₂ generation. The setup of the CYP102A1 peroxygenase reaction was also possible for other statin drugs and could be developed to produce drug metabolites. We also found a relationship between enzyme inactivation and product formation during the catalytic reaction, supported by enzymatic in situ H₂O₂ supply. It can be suggested that the low product formation is due to enzyme inactivation.

Immobilization of Enzymes on Cyclodextrin-Anchored Dehiscent Mesoporous TiO2 for Efficient Photoenzymatic Hydroxylation

A three-in-one heterogeneous catalyst (UPO@dTiO₂-CD) was fabricated by grafting cyclodextrins (CDs) on the dehiscent TiO₂ (dTiO₂) surface and subsequently immobilizing unspecific peroxygenase (rAaeUPO), which exhibited double enhanced electron/mass transfer in photo-enzymatic enantioselective hydroxylation of the C-H bond. The tunable anatase/rutile phase ratio and dehiscent mesoporous architectures of dTiO₂ and the electron donor feature and hydrophobic inner cavity of the CDs are independently responsible for accelerating both electron and mass transfer. The coordination of the photocatalytic and enzymatic steps was achieved by structural and compositional regulation. The optimized UPO@dTiO₂-CD not only displayed high catalytic efficiency (turnover number and turnover frequency of rAaeUPO up to >65,000 and 91 min^-1, respectively) but also exhibited high stability and reusability.

Advances in the One-Step Approach of Polymeric Materials Using Enzymatic Techniques

The formulation in which biochemical enzymes are administered in polymer science plays a key role in retaining their catalytic activity. The one-step synthesis of polymers with highly sequence-controlled enzymes is a strategy employed to provide enzymes with higher catalytic activity and thermostability in material sustainability. Enzyme-catalyzed chain growth polymerization reactions using activated monomers, protein-polymer complexation techniques, covalent and non-covalent interaction, and electrostatic interactions can provide means to develop formulations that maintain the stability of the enzyme during complex material processes. Multifarious applications of catalytic enzymes are usually attributed to their efficiency, pH, and temperature, thus, progressing with a critical structure-controlled synthesis of polymer materials. Due to the obvious economics of manufacturing and environmental sustainability, the green synthesis of enzyme-catalyzed materials has attracted significant interest. Several enzymes from microorganisms and plants via enzyme-mediated material synthesis have provided a viable alternative for the appropriate synthesis of polymers, effectively utilizing the one-step approach. This review analyzes more and deeper strategies and material technologies widely used in multi-enzyme cascade platforms for engineering polymer materials, as well as their potential industrial applications, to provide an update on current trends and gaps in the one-step synthesis of materials using catalytic enzymes.

Preparative-Scale Biocatalytic Oxygenation of N-Heterocycles with a Lyophilized Peroxygenase Catalyst

A lyophilized preparation of an unspecific peroxygenase variant from Agrocybe aegerita (rAaeUPO-PaDa-I-H) is a highly effective catalyst for the oxygenation of a diverse range of N-heterocyclic compounds. Scalable biocatalytic oxygenations (27 preparative examples, ca. 100 mg scale) have been developed across a wide range of substrates, including alkyl pyridines, bicyclic N-heterocycles and indoles. H₂ O₂ is the only stoichiometric oxidant needed, without auxiliary electron transport proteins, which is key to the practicality of the method. Reaction outcomes can be altered depending on whether hydrogen peroxide was delivered by syringe pump or through in situ generation using an alcohol oxidase from Pichia pastoris (PpAOX) and methanol as a co-substrate. Good synthetic yields (up to 84 %), regioselectivity and enantioselectivity (up to 99 % ee) were observed in some cases, highlighting the promise of UPOs as practical, versatile and scalable oxygenation biocatalysts.

Electrochemical immobilization of glucose oxidase for the controlled production of H2O2 in a biocatalytic flow reactor

Electrochemical methods can be used to selectively modify the surfaces of electrodes, enabling the immobilisation of enzymes on defined areas on the surfaces of electrodes. Such selective immobilisation methods can be used to pattern catalysts on surfaces in a controlled manner. Using this approach, the selective patterning of the enzyme glucose oxidase on the electrodes was used to develop a flow reactor for the controlled delivery of the oxidant H₂O₂. GOx was immobilised on a glassy carbon electrode using polypyrrole, silica films, and diazonium linkers. The rate of production of H₂O₂ and the stability of the response was dependent on the immobilisation method. GOx encapsulated in polypyrrole was selected as the optimal method of immobilisation, with a rate of production of 91±11 μM h⁻¹ for 4 hours of continuous operation. The enzyme was subsequently immobilised on carbon rod electrodes (surface area of 5.76 cm²) using a polypyrrole/Nafion® film and incorporated into a flow reactor. The rate of production of H₂O₂ was 602±57 μM h⁻¹, with 100 % retention of activity after 7 h of continuous operation, demonstrating that such a system can be used to prepare H₂O₂ at continuous and stable rate for use in downstream oxidation reactions.

Oxygenating Biocatalysts for Hydroxyl Functionalisation in Drug Discovery and Development

C-H oxyfunctionalisation remains a distinct challenge for synthetic organic chemists. Oxygenases and peroxygenases (grouped here as "oxygenating biocatalysts") catalyse the oxidation of a substrate with molecular oxygen or hydrogen peroxide as oxidant. The application of oxygenating biocatalysts in organic synthesis has dramatically increased over the last decade, producing complex compounds with potential uses in the pharmaceutical industry. This review will focus on hydroxyl functionalisation using oxygenating biocatalysts as a tool for drug discovery and development. Established oxygenating biocatalysts, such as cytochrome P450s and flavin-dependent monooxygenases, have widely been adopted for this purpose, but can suffer from low activity, instability or limited substrate scope. Therefore, emerging oxygenating biocatalysts which offer an alternative will also be covered, as well as considering the ways in which these hydroxylation biotransformations can be applied in drug discovery and development, such as late-stage functionalisation (LSF) and in biocatalytic cascades.

Sustainable Approach for Peroxygenase-Catalyzed Oxidation Reactions Using Hydrogen Peroxide Generated from Spent Coffee Grounds and Tea Leaf Residues

Peroxygenases are promising catalysts for use in the oxidation of chemicals as they catalyze the direct oxidation of a variety of compounds under ambient conditions using hydrogen peroxide (H₂O₂) as an oxidant. Although the use of peroxygenases provides a simple method for oxidation of chemicals, the anthraquinone process currently used to produce H₂O₂ requires significant energy input and generates considerable waste, which negatively affects process sustainability and production costs. Thus, generating H₂O₂ for peroxygenases on site using an environmentally benign method would be advantageous. Here, we utilized spent coffee grounds (SCGs) and tea leaf residues (TLRs) for the production of H₂O₂. These waste biomass products reacted with molecular oxygen and effectively generated H₂O₂ in sodium phosphate buffer. The resulting H₂O₂ was utilized by the bacterial P450 peroxygenase, CYP152A1. Both SCG-derived and TLR-derived H₂O₂ promoted the CYP152A1-catalyzed oxidation of 4-methoxy-1-naphthol to Russig's blue as a model reaction. In addition, when CYP152A1 was incubated with styrene, the SCG and TLR solutions enabled the synthesis of styrene oxide and phenylacetaldehyde. This new approach using waste biomass provides a simple, cost-effective, and sustainable oxidation method that should be readily applicable to other peroxygenases for the synthesis of a variety of valuable chemicals.

Valorization of Small Alkanes by Biocatalytic Oxyfunctionalization

The oxidation of alkanes into valuable chemical products is a vital reaction in organic synthesis. This reaction, however, is challenging, owing to the inertness of C-H bonds. Transition metal catalysts for C-H functionalization are frequently explored. Despite chemical alternatives, nature has also evolved powerful oxidative enzymes (e. g., methane monooxygenases, cytochrome P450 oxygenases, peroxygenases) that are capable of transforming C-H bonds under very mild conditions, with only the use of molecular oxygen or hydrogen peroxide as electron acceptors. Although progress in alkane oxidation has been reviewed extensively, little attention has been paid to small alkane oxidation. The latter holds great potential for the manufacture of chemicals. This Minireview provides a concise overview of the most relevant enzyme classes capable of small alkanes (C_<6 ) oxyfunctionalization, describes the essentials of the catalytic mechanisms, and critically outlines the current state-of-the-art in preparative applications.

A Unique P450 Peroxygenase System Facilitated by a Dual-Functional Small Molecule: Concept, Application, and Perspective

Cytochrome P450 monooxygenases (P450s) are promising versatile oxidative biocatalysts. However, the practical use of P450s in vitro is limited by their dependence on the co-enzyme NAD(P)H and the complex electron transport system. Using H₂O₂ simplifies the catalytic cycle of P450s; however, most P450s are inactive in the presence of H₂O₂. By mimicking the molecular structure and catalytic mechanism of natural peroxygenases and peroxidases, an artificial P450 peroxygenase system has been designed with the assistance of a dual-functional small molecule (DFSM). DFSMs, such as N-(ω-imidazolyl fatty acyl)-l-amino acids, use an acyl amino acid as an anchoring group to bind the enzyme, and the imidazolyl group at the other end functions as a general acid-base catalyst in the activation of H₂O₂. In combination with protein engineering, the DFSM-facilitated P450 peroxygenase system has been used in various oxidation reactions of non-native substrates, such as alkene epoxidation, thioanisole sulfoxidation, and alkanes and aromatic hydroxylation, which showed unique activities and selectivity. Moreover, the DFSM-facilitated P450 peroxygenase system can switch to the peroxidase mode by mechanism-guided protein engineering. In this short review, the design, mechanism, evolution, application, and perspective of these novel non-natural P450 peroxygenases for the oxidation of non-native substrates are discussed.

Continuous oxyfunctionalizations catalyzed by unspecific peroxygenase

Unspecific peroxygenase (UPO) has been shown to be a promising biocatalyst for oxyfunctionalization of a broad range of substrates with hydrogen peroxide (H2O2) as the cosubstrate.

Recent developments in the use of peroxygenases - Exploring their high potential in selective oxyfunctionalisations

Peroxygenases are an emerging new class of enzymes allowing selective oxyfunctionalisation reactions in a cofactor-independent way different from well-known P450 monooxygenases. Herein, we focused on recent developments from organic synthesis, molecular biotechnology and reaction engineering viewpoints that are devoted to bring these enzymes in industrial applications. This covers natural diversity from different sources, protein engineering strategies for expression, substrate scope, activity and selectivity, stabilisation of enzymes via immobilisation, and the use of peroxygenases in low water media. We believe that peroxygenases have much to offer for selective oxyfunctionalisations and we have much to study to explore the full potential of these versatile biocatalysts in organic synthesis.

Hemoprotein Catalyzed Oxygenations: P450s, UPOs, and Progress toward Scalable Reactions

The selective oxygenation of nonactivated carbon atoms is an ongoing synthetic challenge, and biocatalysts, particularly hemoprotein oxygenases, continue to be investigated for their potential, given both their sustainable chemistry credentials and also their superior selectivity. However, issues of stability, activity, and complex reaction requirements often render these biocatalytic oxygenations problematic with respect to scalable industrial processes. A continuing focus on Cytochromes P450 (P450s), which require a reduced nicotinamide cofactor and redox protein partners for electron transport, has now led to better catalysts and processes with a greater understanding of process requirements and limitations for both in vitro and whole-cell systems. However, the discovery and development of unspecific peroxygenases (UPOs) has also recently provided valuable complementary technology to P450-catalyzed reactions. UPOs need only hydrogen peroxide to effect oxygenations but are hampered by their sensitivity to peroxide and also by limited selectivity. In this Perspective, we survey recent developments in the engineering of proteins, cells, and processes for oxygenations by these two groups of hemoproteins and evaluate their potential and relative merits for scalable reactions.

Identification and Expression of New Unspecific Peroxygenases - Recent Advances, Challenges and Opportunities

In 2004, the fungal heme-thiolate enzyme subfamily of unspecific peroxygenases (UPOs) was first described in the basidiomycete Agrocybe aegerita. As UPOs naturally catalyze a broad range of oxidative transformations by using hydrogen peroxide as electron acceptor and thus possess a great application potential, they have been extensively studied in recent years. However, despite their versatility to catalyze challenging selective oxyfunctionalizations, the availability of UPOs for potential biotechnological applications is restricted. Particularly limiting are the identification of novel natural biocatalysts, their production, and the description of their properties. It is hence of great interest to further characterize the enzyme subfamily as well as to identify promising new candidates. Therefore, this review provides an overview of the state of the art in identification, expression, and screening approaches of fungal UPOs, challenges associated with current protein production and screening strategies, as well as potential solutions and opportunities.

Improving the Heterologous Production of Fungal Peroxygenases through an Episomal Pichia pastoris Promoter and Signal Peptide Shuffling System

Fungal peroxygenases (UPOs) have emerged as oxyfunctionalization catalysts of tremendous interest in recent years. However, their widespread use in the field of biocatalysis is still hampered by their challenging heterologous production, substantially limiting the panel of accessible enzymes for investigation and enzyme engineering. Building upon previous work on UPO production in yeast, we have developed a combined promoter and signal peptide shuffling system for episomal high throughput UPO production in the industrially relevant, methylotrophic yeast Pichia pastoris. Eleven endogenous and orthologous promoters were shuffled with a diverse set of 17 signal peptides. Three previously described UPOs were selected as first test set, leading to the identification of beneficial promoter/signal peptide combinations for protein production. We applied the system then successfully to produce two novel UPOs: MfeUPO from Myceliophthora fergusii and MhiUPO from Myceliophthora hinnulea. To demonstrate the feasibility of the developed system to other enzyme classes, it was applied for the industrially relevant lipase CalB and the laccase Mrl2. In total, approximately 3200 transformants of eight diverse enzymes were screened and the best promoter/signal peptide combinations studied at various cofeeding, derepression, and induction conditions. High volumetric production titers were achieved by subsequent creation of stable integration lines and harnessing orthologous promoters from Hansenula polymorpha. In most cases promising yields were also achieved without the addition of methanol under derepressed conditions. To foster the use of the episomal high throughput promoter/signal peptide Pichia pastoris system, we made all plasmids available through Addgene.

Accessing Chemo- and Regioselective Benzylic and Aromatic Oxidations by Protein Engineering of an Unspecific Peroxygenase

Unspecific peroxygenases (UPOs) enable oxyfunctionalizations of a broad substrate range with unparalleled activities. Tailoring these enzymes for chemo- and regioselective transformations represents a grand challenge due to the difficulties in their heterologous productions. Herein, we performed protein engineering in Saccharomyces cerevisiae using the MthUPO from Myceliophthora thermophila. More than 5300 transformants were screened. This protein engineering led to a significant reshaping of the active site as elucidated by computational modelling. The reshaping was responsible for the increased oxyfunctionalization activity, with improved k_cat/K_m values of up to 16.5-fold for the model substrate 5-nitro-1,3-benzodioxole. Moreover, variants were identified with high chemo- and regioselectivities in the oxyfunctionalization of aromatic and benzylic carbons, respectively. The benzylic hydroxylation was demonstrated to perform with enantioselectivities of up to 95% ee. The proposed evolutionary protocol and rationalization of the enhanced activities and selectivities acquired by MthUPO variants represent a step forward toward the use and implementation of UPOs in biocatalytic synthetic pathways of industrial interest.

A modular two yeast species secretion system for the production and preparative application of unspecific peroxygenases

Fungal unspecific peroxygenases (UPOs) represent an enzyme class catalysing versatile oxyfunctionalisation reactions on a broad substrate scope. They are occurring as secreted, glycosylated proteins bearing a haem-thiolate active site and rely on hydrogen peroxide as the oxygen source. However, their heterologous production in a fast-growing organism suitable for high throughput screening has only succeeded once-enabled by an intensive directed evolution campaign. We developed and applied a modular Golden Gate-based secretion system, allowing the first production of four active UPOs in yeast, their one-step purification and application in an enantioselective conversion on a preparative scale. The Golden Gate setup was designed to be universally applicable and consists of the three module types: i) signal peptides for secretion, ii) UPO genes, and iii) protein tags for purification and split-GFP detection. The modular episomal system is suitable for use in Saccharomyces cerevisiae and was transferred to episomal and chromosomally integrated expression cassettes in Pichia pastoris. Shake flask productions in Pichia pastoris yielded up to 24 mg/L secreted UPO enzyme, which was employed for the preparative scale conversion of a phenethylamine derivative reaching 98.6 % ee. Our results demonstrate a rapid, modular yeast secretion workflow of UPOs yielding preparative scale enantioselective biotransformations.

Chromoselective Photocatalysis Enables Stereocomplementary Biocatalytic Pathways*

Controlling the selectivity of a chemical reaction with external stimuli is common in thermal processes, but rare in visible-light photocatalysis. Here we show that the redox potential of a carbon nitride photocatalyst (CN-OA-m) can be tuned by changing the irradiation wavelength to generate electron holes with different oxidation potentials. This tuning was the key to realizing photo-chemo-enzymatic cascades that give either the (S)- or the (R)-enantiomer of phenylethanol. In combination with an unspecific peroxygenase from Agrocybe aegerita, green light irradiation of CN-OA-m led to the enantioselective hydroxylation of ethylbenzene to (R)-1-phenylethanol (99 % ee). In contrast, blue light irradiation triggered the photocatalytic oxidation of ethylbenzene to acetophenone, which in turn was enantioselectively reduced with an alcohol dehydrogenase from Rhodococcus ruber to form (S)-1-phenylethanol (93 % ee).

Chromoselective Photocatalysis Enables Stereocomplementary Biocatalytic Pathways

Controlling the selectivity of a chemical reaction with external stimuli is common in thermal processes, but rare in visible-light photocatalysis. Here we show that the redox potential of a carbon nitride photocatalyst (CN-OA-m) can be tuned by changing the irradiation wavelength to generate electron holes with different oxidation potentials. This tuning was the key to realizing photo-chemo-enzymatic cascades that give either the (S)- or the (R)-enantiomer of phenylethanol. In combination with an unspecific peroxygenase from Agrocybe aegerita, green light irradiation of CN-OA-m led to the enantioselective hydroxylation of ethylbenzene to (R)-1-phenylethanol (99 % ee). In contrast, blue light irradiation triggered the photocatalytic oxidation of ethylbenzene to acetophenone, which in turn was enantioselectively reduced with an alcohol dehydrogenase from Rhodococcus ruber to form (S)-1-phenylethanol (93 % ee).

Photo-biocatalytic Cascades: Combining Chemical and Enzymatic Transformations Fueled by Light

In the field of green chemistry, light - an attractive natural agent - has received particular attention for driving biocatalytic reactions. Moreover, the implementation of light to drive (chemo)enzymatic cascade reactions opens up a golden window of opportunities. However, there are limitations to many current examples, mostly associated with incompatibility between the enzyme and the photocatalyst. Additionally, the formation of reactive radicals upon illumination and the loss of catalytic activities in the presence of required additives are common observations. As outlined in this review, the main question is how to overcome current challenges to the exploitation of light to drive (chemo)enzymatic transformations. First, we highlight general concepts in photo-biocatalysis, then give various examples of photo-chemoenzymatic (PCE) cascades, further summarize current synthetic examples of PCE cascades and discuss strategies to address the limitations.

An Integrated Cofactor/Co-Product Recycling Cascade for the Biosynthesis of Nylon Monomers from Cycloalkylamines

We report a highly atom-efficient integrated cofactor/co-product recycling cascade employing cycloalkylamines as multifaceted starting materials for the synthesis of nylon building blocks. Reactions using E. coli whole cells as well as purified enzymes produced excellent conversions ranging from >80 and 95 % into desired ω-amino acids, respectively with varying substrate concentrations. The applicability of this tandem biocatalytic cascade was demonstrated to produce the corresponding lactams by employing engineered biocatalysts. For instance, ϵ-caprolactam, a valuable polymer building block was synthesized with 75 % conversion from 10 mM cyclohexylamine by employing whole-cell biocatalysts. This cascade could be an alternative for bio-based production of ω-amino acids and corresponding lactam compounds.

Advances in enzymatic oxyfunctionalization of aliphatic compounds

Selective oxyfunctionalizations of aliphatic compounds are difficult chemical reactions, where enzymes can play an important role due to their stereo- and regio-selectivity and operation under mild reaction conditions. P450 monooxygenases are well-known biocatalysts that mediate oxyfunctionalization reactions in different living organisms (from bacteria to humans). Unspecific peroxygenases (UPOs), discovered in fungi, have arisen as "dream biocatalysts" of great biotechnological interest because they catalyze the oxyfunctionalization of aliphatic and aromatic compounds, avoiding the necessity of expensive cofactors and regeneration systems, and only depending on H₂O₂ for their catalysis. Here, we summarize recent advances in aliphatic oxyfunctionalization reactions by UPOs, as well as the molecular determinants of the enzyme structures responsible for their activities, emphasizing the differences found between well-known P450s and the novel fungal peroxygenases.

Decoupling hydrogen production from water oxidation by integrating a triphase interfacial bioelectrochemical cascade reaction

Water electrolysis to produce H₂ is a promising strategy for generating a renewable fuel. However, the sluggish-kinetics and low value-added anodic oxygen evolution reaction (OER) restricts the overall energy conversion efficiency. Herein we report a strategy of boosting H₂ production at low voltages by replacing OER with a bioelectrochemical cascade reaction at a triphase bioanode. In the presence of oxygen, oxidase enzymes can convert biomass into valuable products, and concurrently generate H₂O₂ that can be further electrooxidized at the bioanode. Benefiting from the efficient oxidase kinetics at an oxygen-rich triphase bioanode and the more favorable thermodynamics of H₂O₂ oxidation than that of OER, the cell voltage and energy consumption are reduced by ~0.70 V and ~36%, respectively, relative to regular water electrolysis. This leads to an efficient H₂ production at the cathode and valuable product generation at the bioanode. Integration of a bioelectrochemical cascade into the water splitting process provides an energy-efficient and promising pathway for achieving a renewable fuel.

In situ H2O2 generation methods in the context of enzyme biocatalysis

Hydrogen peroxide is a versatile oxidant that has use in medical and biotechnology industries. Many enzymes require this oxidant as a reaction mediator in order to undergo their oxygenation chemistries. While there is a reliable method for generating hydrogen peroxide via an anthraquinone cycle, there are several advantages for generating hydrogen in situ. As highlighted in this review, this is particularly beneficial in the case of biocatalysts that require hydrogen peroxide as a reaction mediator because the exogenous addition of hydrogen peroxide can damage their reactive heme centers and render them inactive. In addition, generation of hydrogen peroxide in situ does not dilute the reaction mixture and cause solution parameters to change. The environment would also benefit from a hydrogen peroxide synthesis cycle that does not rely on nonrenewable chemicals obtained from fossil fuels. Generation of hydrogen peroxide in situ for biocatalysis using enzymes, bioelectrocatalyis, photocatalysis, and cold temperature plasmas are addressed. Particular emphasis is given to reaction processes that support high total turnover numbers (TTNs) of the hydrogen peroxide-requiring enzymes. Discussion of innovations in the use of hydrogen peroxide-producing enzyme cascades for antimicrobial activity, wastewater effluent treatment, and biosensors are also included.

Biocatalysis: Enzymatic Synthesis for Industrial Applications

Biocatalysis has found numerous applications in various fields as an alternative to chemical catalysis. The use of enzymes in organic synthesis, especially to make chiral compounds for pharmaceuticals as well for the flavors and fragrance industry, are the most prominent examples. In addition, biocatalysts are used on a large scale to make specialty and even bulk chemicals. This review intends to give illustrative examples in this field with a special focus on scalable chemical production using enzymes. It also discusses the opportunities and limitations of enzymatic syntheses using distinct examples and provides an outlook on emerging enzyme classes.

Simultaneous screening of multiple substrates with an unspecific peroxygenase enabled modified alkane and alkene oxyfunctionalisations

Protein engineering of an unspecific peroxygenase (UPO) was performed with three substrates and six products in parallel by a high throughput GC-MS setup. Modified chemo- and regioselective variants were identified for aliphatic substrates.

Nuclear Waste and Biocatalysis: A Sustainable Liaison?

It is well-known that energy-rich radiation induces water splitting, eventually yielding hydrogen peroxide. Synthetic applications, however, are scarce and to the best of our knowledge, the combination of radioactivity with enzyme-catalysis has not been considered yet. Peroxygenases utilize H₂O₂ as an oxidant to promote highly selective oxyfunctionalization reactions but are also irreversibly inactivated in the presence of too high H₂O₂ concentrations. Therefore, there is a need for efficient in situ H₂O₂ generation methods. Here, we show that radiolytic water splitting can be used to promote specific biocatalytic oxyfunctionalization reactions. Parameters influencing the efficiency of the reaction and current limitations are shown. Particularly, oxidative inactivation of the biocatalyst by hydroxyl radicals influences the robustness of the overall reaction. Radical scavengers can alleviate this issue, but eventually, physical separation of the enzymes from the ionizing radiation will be necessary to achieve robust reaction schemes. We demonstrate that nuclear waste can also be used to drive selective, peroxygenase-catalyzed oxyfunctionalization reactions, challenging our view on nuclear waste in terms of sustainability.

Modeling and simulation-based design of electroenzymatic batch processes catalyzed by unspecific peroxygenase from A. aegerita

Unspecific peroxygenases have attracted interest due to their ability to catalyze the oxygenation of various types of C-H bonds using only hydrogen peroxide as a cosubstrate. Due to the instability of these enzymes at even low hydrogen peroxide concentrations, careful fed-batch addition of the cosubstrate or ideally in situ production is required. While various approaches for hydrogen peroxide addition have been qualitatively assessed, only limited kinetic data concerning enzyme inactivation and peroxide accumulation has been reported so far. To obtain quantitative insights into the kinetics of such a process, a detailed data set for a peroxygenase-catalyzed benzylic hydroxylation coupled with electrochemical hydrogen peroxide production is presented. Based on this data set, we set out to model such an electroenzymatic process. For this, initial velocity data for the benzylic hydroxylation is collected and an extended Ping-Pong-Bi-Bi type rate equation is established, which sufficiently describes the enzyme kinetic. Moreover, we propose an empirical inactivation term based on the collected data set. Finally, we show that the full model does not only describe the process with sufficient accuracy, but can also be used predictively to control hydrogen peroxide feeding rates To limit the concentration of this critical cosubstrate in the system.

Biocatalytic Oxidation of Alcohols

Enzymatic methods for the oxidation of alcohols are critically reviewed. Dehydrogenases and oxidases are the most prominent biocatalysts, enabling the selective oxidation of primary alcohols into aldehydes or acids. In the case of secondary alcohols, region and/or enantioselective oxidation is possible. In this contribution, we outline the current state-of-the-art and discuss current limitations and promising solutions.

Water-Soluble Anthraquinone Photocatalysts Enable Methanol-Driven Enzymatic Halogenation and Hydroxylation Reactions

Peroxyzymes simply use H₂O₂ as a cosubstrate to oxidize a broad range of inert C–H bonds. The lability of many peroxyzymes against H₂O₂ can be addressed by a controlled supply of H₂O₂, ideally in situ. Here, we report a simple, robust, and water-soluble anthraquinone sulfonate (SAS) as a promising organophotocatalyst to drive both haloperoxidase-catalyzed halogenation and peroxygenase-catalyzed oxyfunctionalization reactions. Simple alcohols, methanol in particular, can be used both as a cosolvent and an electron donor for H₂O₂ generation. Very promising turnover numbers for the biocatalysts of up to 318 000 have been achieved.