Development of Continuous Flow Systems to Access Secondary Amines Through Previously Incompatible Biocatalytic Cascades*

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.

Co-immobilization of amine dehydrogenase and glucose dehydrogenase for the biosynthesis of (S)-2-aminobutan-1-ol in continuous flow

Reductive amination by amine dehydrogenases is a green and sustainable process that produces only water as the by-product. In this study, a continuous flow process was designed utilizing a packed bed reactor filled with co-immobilized amine dehydrogenase wh84 and glucose dehydrogenase for the highly efficient biocatalytic synthesis of chiral amino alcohols. The immobilized amine dehydrogenase wh84 exhibited better thermo-, pH and solvent stability with high activity recovery. (S)-2-aminobutan-1-ol was produced in up to 99% conversion and 99% ee in the continuous flow processes, and the space-time yields were up to 124.5 g L-1 d-1. The continuous reactions were also extended to 48 h affording up to 91.8% average conversions. This study showcased the important potential to sustainable production of chiral amino alcohols in continuous flow processes.

Substrate Promiscuity of Thermoplasma acidophilum Malic Enzyme for CO2 Fixation Reaction

CO2 fixation technology has gained attention as a method to effectively utilize the abundant CO2 in the atmosphere by converting it into useful chemicals. However, since CO2 is a highly stable molecule, many of the currently developed methods for chemical CO2 fixation require harsh conditions and reactive reagents. The establishment of efficient and sustainable processes is eagerly awaited. In this study, we investigated a biocatalytic process and achieved a carboxylation reaction under mild conditions (37 °C, 0.1 MPa CO2) using a biocatalyst, Thermoplasma acidophilum NADP+-malic enzyme (TaME), and gaseous CO2 by coupling enzymatic coenzyme regeneration. We also demonstrated for the first time that the carboxylation reaction by ME proceeds not only with pyruvate, a natural substrate, but also with 2-ketoglutarate.

Catch-and-Release: The Assembly, Immobilization, and Recycling of Redox-Reversible Artificial Metalloenzymes

Technologies to improve the applicability of artificial metalloenzymes (ArMs) are gaining considerable interest; one such approach is the immobilization of these biohybrid catalysts on support materials to enhance stability and enable their retention, recovery, and reuse. Here, we describe the immobilization of polyhistidine-tagged ArMs that allow the redox-controlled replacement of catalytic cofactors that have lost activity, e.g., due to poisoning or decomposition, on immobilized metal affinity chromatography resins. By using periplasmic siderophore-binding protein scaffolds that originate from thermophilic bacteria (GstCeuE and PthCeuE) in combination with a siderophore-linked imine reduction catalyst, reaction rates were achieved that are about 3.5 times faster than those previously obtained with CjCeuE, the analogous protein of Campylobacter jejuni. Upon immobilization, the GstCeuE-derived ArM showed a decrease in turnover frequency in the reduction of dehydrosalsolidine by 3.4-fold, while retaining enantioselectivity (36%) and showing improved stability that allowed repeat recovery and recycling cycles. Catalytic activity was preserved over the initial four cycles. In subsequent cycles, a gradual reduction of activity was evident. Once the initial activity decreased to around 40% of the initial activity (23rd recycling cycle), the redox-triggered artificial cofactor release permitted the subsequent recharging of the immobilized protein scaffold with fresh, active cofactor, thereby restoring the initial catalytic activity of the immobilized ArM and allowing its reuse for several more cycles. Furthermore, the ArM could be assembled directly from protein present in crude cell extracts, avoiding time-consuming and costly protein purification steps. Overall, this study demonstrates that the immobilization of redox-reversible ArMs facilitates their "catch-and-release" assembly and disassembly and the recycling of their components, improving their potential commercial viability and environmental footprint.

Development of a green, concise synthesis of nicotinamide derivatives catalysed by Novozym® 435 from Candida antarctica in sustainable continuous-flow microreactors

An increasing number of studies have shown that many nicotinamide derivatives exhibited extensive biological activities, such as anti-inflammatory and antitumor activity. In this paper, a green, concise synthesis of nicotinamide derivatives in sustainable continuous-flow microreactors catalysed by Novozym® 435 from Candida antarctica has been developed. Application of an easily obtainable and reusable lipase in the synthesis of nicotinamide derivatives from methyl nicotinate and amines/benzylamines reacted for 35 min at 50 °C led to high product yields (81.6-88.5%). Environmentally friendly tert-amyl alcohol was applied as a reaction medium. Substantially shorter reaction times as well as a significant increase in the product yield were obtained as compared to the batch process. This innovative approach provides a promising green, efficient and rapid synthesis strategy for pharmaceutical synthesis and further activity research of novel nicotinamide derivatives.

Biocatalytic reductive aminations with NAD(P)H-dependent enzymes: enzyme discovery, engineering and synthetic applications

Chiral amines are pivotal building blocks for the pharmaceutical industry. Asymmetric reductive amination is one of the most efficient and atom economic methodologies for the synthesis of optically active amines. Among the various strategies available, NAD(P)H-dependent amine dehydrogenases (AmDHs) and imine reductases (IREDs) are robust enzymes that are available from various sources and capable of utilizing a broad range of substrates with high activities and stereoselectivities. AmDHs and IREDs operate via similar mechanisms, both involving a carbinolamine intermediate followed by hydride transfer from the co-factor. In addition, both groups catalyze the formation of primary and secondary amines utilizing both organic and inorganic amine donors. In this review, we discuss advances in developing AmDHs and IREDs as biocatalysts and focus on evolutionary history, substrate scope and applications of the enzymes to provide an outlook on emerging industrial biotechnologies of chiral amine production.

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.

Organic porous heterogeneous composite with antagonistic catalytic sites as a cascade catalyst for continuous flow reaction

One-pot cascade catalytic reactions easily allow the circumvention of pitfalls of traditional catalytic reactions, such as multi-step syntheses, longer duration, waste generation, and high operational cost. Despite advances in this area, the facile assimilation of chemically antagonistic bifunctional sites in close proximity inside a well-defined scaffold via a process of rational structural design still remains a challenge. Herein, we report the successful fusion of incompatible acid-base active sites in an ionic porous organic polymer (iPOP), 120-MI@OH, via a simple ion-exchange strategy. The fabricated polymer catalyst, 120-MI@OH, performed exceedingly well as a cascade acid-base catalyst in a deacetylation-Knoevenagel condensation reaction under mild and eco-friendly continuous flow conditions. In addition, the abundance of spatially isolated distinct acidic (imidazolium cations) and basic (hydroxide anions) catalytic sites give 120-MI@OH its excellent solid acid and base catalytic properties. To demonstrate the practical relevance of 120-MI@OH, stable millimeter-sized spherical composite polymer bead microstructures were synthesized and utilized in one-pot cascade catalysis under continuous flow, thus illustrating promising catalytic activity. Additionally, the heterogeneous polymer catalyst displayed good recyclability, scalability, as well as ease of fabrication. The superior catalytic activity of 120-MI@OH can be rationalized by its unique structure that reconciles close proximity of antagonistic catalytic sites that are sufficiently isolated in space.

Biocatalytic Cascades toward Iminosugar Scaffolds Reveal Promiscuous Activity of Shikimate Dehydrogenases

Iminosugar scaffolds are highly sought-after pharmaceutical targets, but their chemical synthesis is lengthy and can suffer from poor scalability and purification. Here we report protecting-group-free chemoenzymatic and biocatalytic cascades to synthesize iminosugars from sugar-derived aminopolyols in two steps. Using galactose oxidase variant F₂ followed by a chemical or enzymatic reduction provided an efficient one-pot route to these targets, with product formation >70%. Key to success of this strategy was the application of genome mining, which identified bacterial shikimate dehydrogenases as promiscuous iminosugar reductases. The cell-free protocols allowed for isolation of highly polar iminosugar products from biotransformations in a single step through development of a gradient-elution cation exchange purification. The two-step pathway provides a short synthetic route that can be used as a cell-free platform for broader iminosugar synthesis.

Mimicking Natural Metabolisms: Cell-Free Flow Preparation of Dopamine

The biosynthesis of dopamine (DA) from L-tyrosine as starting material is an excellent yet challenging strategy. Here we developed a versatile, multi-enzymatic platform for the biocatalytic preparation of DA in a continuous mode with excellent conversion (90 %) and reaction time (45 min). The system exploits the immobilization of a decarboxylase from Bacillus pumilis (Fdc) and a tyrosinase from Agaricus bisporus (Tyr), which were combined to mimic the in-vivo synthesis of DA (both primary and secondary metabolisms) giving rise to an efficient strategy with a considerable reduction of process associated costs and environmental impact. To enhance the system automation, an in-line purification via catch-and-release procedure was added.

Continuous Flow Biocatalytic Reductive Amination by Co-Entrapping Dehydrogenases with Agarose Gel in a 3D-Printed Mould Reactor

Herein, we show how the merge of biocatalysis with flow chemistry aided by 3D-printing technologies can facilitate organic synthesis. This concept was exemplified for the reductive amination of benzaldehyde catalysed by co-immobilised amine dehydrogenase and formate dehydrogenase in a continuous flow micro-reactor. For this purpose, we investigated enzyme co-immobilisation by covalent binding, or ion-affinity binding, or entrapment. Entrapment in an agarose hydrogel turned out to be the most promising solution for this biocatalytic reaction. Therefore, we developed a scalable and customisable approach whereby an agarose hydrogel containing the co-entrapped dehydrogenases was cast in a 3D-printed mould. The reactor was applied to the reductive amination of benzaldehyde in continuous flow over 120 h and afforded 47 % analytical yield and a space-time yield of 7.4 g L day-1 using 0.03 mol% biocatalysts loading. This work also exemplifies how rapid prototyping of enzymatic reactions in flow can be achieved through 3D-printing technology.

Strategies for Transferring Photobiocatalysis to Continuous Flow Exemplified by Photodecarboxylation of Fatty Acids

The challenges of light-dependent biocatalytic transformations of lipophilic substrates in aqueous media are manifold. For instance, photolability of the catalyst as well as insufficient light penetration into the reaction vessel may be further exacerbated by a heterogeneously dispersed substrate. Light penetration may be addressed by performing the reaction in continuous flow, which allows two modes of applying the catalyst: (i) heterogeneously, immobilized on a carrier, which requires light-permeable supports, or (ii) homogeneously, dissolved in the reaction mixture. Taking the light-dependent photodecarboxylation of palmitic acid catalyzed by fatty-acid photodecarboxylase from Chlorella variabilis (CvFAP) as a showcase, strategies for the transfer of a photoenzyme-catalyzed reaction into continuous flow were identified. A range of different supports were evaluated for the immobilization of CvFAP, whereby Eupergit C250 L was the carrier of choice. As the photostability of the catalyst was a limiting factor, a homogeneous system was preferred instead of employing the heterogenized enzyme. This implied that photolabile enzymes may preferably be applied in solution if repair mechanisms cannot be provided. Furthermore, when comparing different wavelengths and light intensities, extinction coefficients may be considered to ensure comparable absorption at each wavelength. Employing homogeneous conditions in the CvFAP-catalyzed photodecarboxylation of palmitic acid afforded a space-time yield unsurpassed by any reported batch process (5.7 g·L-1·h-1, 26.9 mmol·L-1·h-1) for this reaction, demonstrating the advantage of continuous flow in attaining higher productivity of photobiocatalytic processes.

Cofactor and Process Engineering for Nicotinamide Recycling and Retention in Intensified Biocatalysis

There is currently considerable interest in the intensification of biocatalytic processes to reduce the cost of goods for biocatalytically produced chemicals, including pharmaceuticals and advanced pharmaceutical intermediates. Continuous-flow biocatalysis shows considerable promise as a method for process intensification; however, the reliance of some reactions on the use of diffusible cofactors (such as the nicotinamide cofactors) has proven to be a technical barrier for key enzyme classes. This minireview covers attempts to overcome this limitation, including the cofactor recapture and recycling retention of chemically modified cofactors. For the latter, we also consider the state of science for cofactor modification, a field reinvigorated by the current interest in continuous-flow biocatalysis.

Biomacromolecule-Assisted Screening for Reaction Discovery and Catalyst Optimization

Reaction discovery and catalyst screening lie at the heart of synthetic organic chemistry. While there are efforts at de novo catalyst design using computation/artificial intelligence, at its core, synthetic chemistry is an experimental science. This review overviews biomacromolecule-assisted screening methods and the follow-on elaboration of chemistry so discovered. All three types of biomacromolecules discussed─enzymes, antibodies, and nucleic acids─have been used as "sensors" to provide a readout on product chirality exploiting their native chirality. Enzymatic sensing methods yield both UV-spectrophotometric and visible, colorimetric readouts. Antibody sensors provide direct fluorescent readout upon analyte binding in some cases or provide for cat-ELISA (Enzyme-Linked ImmunoSorbent Assay)-type readouts. DNA biomacromolecule-assisted screening allows for templation to facilitate reaction discovery, driving bimolecular reactions into a pseudo-unimolecular format. In addition, the ability to use DNA-encoded libraries permits the barcoding of reactants. All three types of biomacromolecule-based screens afford high sensitivity and selectivity. Among the chemical transformations discovered by enzymatic screening methods are the first Ni(0)-mediated asymmetric allylic amination and a new thiocyanopalladation/carbocyclization transformation in which both C-SCN and C-C bonds are fashioned sequentially. Cat-ELISA screening has identified new classes of sydnone-alkyne cycloadditions, and DNA-encoded screening has been exploited to uncover interesting oxidative Pd-mediated amido-alkyne/alkene coupling reactions.

Efficient synthesis 1,4-cyclohexanedicarboxaldehyde by an engineered alcohol oxidase

In this study, we selected and engineered a flavin adenine dinucleotide (FAD)-dependent alcohol oxidase (AOX) to produce 1,4-cyclohexanedicarboxaldehyde (CHDA), an initial raw material for spiral compounds, from 1,4-cyclohexanedimethanol (CHDM). First, the structure of alcohol oxidase from Arthrobacter cholorphenolicus (AcCO) was analyzed, and the mechanism of AcCO-catalyzed primary alcohol oxidation was elucidated, demonstrating that the energy barrier of the hydride (H-) transfer (13.4 kcal·mol-1 and 20.4 kcal·mol-1) decreases the catalytic efficiency of the primary alcohol oxidation reaction. Therefore, we designed a protein engineering strategy to adjust the catalytically active conformation to shorten the distance of hydride (H-) transfer and further decreased the core energy barrier. Following this strategy, variant W4 (S101A/H351V/N378S/Q329N) was obtained with 112.5-fold increased catalytic efficiency to produce CHDA compared to that of the wild-type strain. The 3 L scale preparation of CHDA reached a titer up to 29.6 g·L-1 with a 42.2% yield by an Escherichia coli whole-cell catalyst, which demonstrates the potential of this system for industrial application.

Monitoring and control of the release of soluble O2 from H2 O2 inside porous enzyme carrier for O2 supply to an immobilized D-amino acid oxidase

While O₂ substrate for bio‐transformations in bulk liquid is routinely provided from entrained air or O₂ gas, tailored solutions of O₂ supply are required when the bio‐catalysis happens spatially confined to the microstructure of a solid support. Release of soluble O₂ from H₂O₂ by catalase is promising, but spatiotemporal control of the process is challenging to achieve. Here, we show monitoring and control by optical sensing within a porous carrier of the soluble O₂ formed by an immobilized catalase upon feeding of H₂O₂. The internally released O₂ is used to drive the reaction of d‐amino acid oxidase (oxidation of d‐methionine) that is co‐immobilized with the catalase in the same carrier. The H₂O₂ is supplied in portions at properly timed intervals, or continuously at controlled flow rate, to balance the O₂ production and consumption inside the carrier so as to maintain the internal O₂ concentration in the range of 100–500 µM. Thus, enzyme inactivation by excess H₂O₂ is prevented and gas formation from the released O₂ is avoided at the same time. The reaction rate of the co‐immobilized enzyme preparation is shown to depend linearly on the internal O₂ concentration up to the air‐saturated level. Conversions at a 200 ml scale using varied H₂O₂ feed rate (0.04–0.18 mmol/min) give the equivalent production rate from d‐methionine (200 mM) and achieve rate enhancement by ∼1.55‐fold compared to the same oxidase reaction under bubble aeration. Collectively, these results show an integrated strategy of biomolecular engineering for tightly controlled supply of O₂ substrate from H₂O₂ into carrier‐immobilized enzymes. By addressing limitations of O₂ supply via gas‐liquid transfer, especially at the microscale, this can be generally useful to develop specialized process strategies for O₂‐dependent biocatalytic reactions.

New Trends and Future Opportunities in the Enzymatic Formation of C-C, C-N, and C-O bonds

Organic chemistry provides society with fundamental products we use daily. Concerns about the impact that the chemical industry has over the environment is propelling major changes in the way we manufacture chemicals. Biocatalysis offers an alternative to other synthetic approaches as it employs enzymes, Nature's catalysts, to carry out chemical transformations. Enzymes are biodegradable, come from renewable sources, operate under mild reaction conditions, and display high selectivities in the processes they catalyse. As a highly multidisciplinary field, biocatalysis benefits from advances in different areas, and developments in the fields of molecular biology, bioinformatics, and chemical engineering have accelerated the extension of the range of available transformations (E. L. Bell et al., Nat. Rev. Meth. Prim. 2021, 1, 1-21). Recently, we surveyed advances in the expansion of the scope of biocatalysis via enzyme discovery and protein engineering (J. R. Marshall et al., Tetrahedron 2021, 82, 131926). Herein, we focus on novel enzymes currently available to the broad synthetic community for the construction of new C-C, C-N and C-O bonds, with the purpose of providing the non-specialist with new and alternative tools for chiral and sustainable chemical synthesis.

Reaching New Biocatalytic Reactivity Using Continuous Flow Reactors

The use of flow reactors in biocatalysis has increased significantly in recent years. Chemists have begun to design flow systems that even allow new biocatalytic reactions to take place. This concept article will focus on the design of flow systems that have allowed enzymes to go beyond their limits in batch. The case is made for moving towards fully continuous systems. With flow chemistry increasingly seen as an enabling technology for automated synthesis, and with advancements in AI-assisted enzyme design, there is a real possibility to fully automate the development and implementation of a continuous biocatalytic processes. This will lead to significantly improved enzyme processes for synthesis.

Reductive aminations by imine reductases: from milligrams to tons

The synthesis of secondary and tertiary amines through the reductive amination of carbonyl compounds is one of the most significant reactions in synthetic chemistry. Asymmetric reductive amination for the formation of chiral amines, which are required for the synthesis of pharmaceuticals and other bioactive molecules, is often achieved through transition metal catalysis, but biocatalytic methods of chiral amine production have also been a focus of interest owing to their selectivity and sustainability. The discovery of asymmetric reductive amination by imine reductase (IRED) and reductive aminase (RedAm) enzymes has served as the starting point for a new industrial approach to the production of chiral amines, leading from laboratory-scale milligram transformations to ton-scale reactions that are now described in the public domain. In this perspective we trace the development of the IRED-catalyzed reductive amination reaction from its discovery to its industrial application on kg to ton scale. In addition to surveying examples of the synthetic chemistry that has been achieved with the enzymes, the contribution of structure and protein engineering to the understanding of IRED-catalyzed reductive amination is described, and the consequent benefits for activity, selectivity and stability in the design of process suitable catalysts.

IRED-catalyzed reductive aminations have progressed from mg to ton scale, through advances in enzyme discovery, protein engineering and process biocatalysis.

Oxidase enzymes as sustainable oxidation catalysts

Oxidation is one of the most important processes used by the chemical industry. However, many of the methods that are used pose significant sustainability and environmental issues. Biocatalytic oxidation offers an alternative to these methods, with a now significant enzymatic oxidation toolbox on offer to chemists. Oxidases are one of these options, and as they only depend on molecular oxygen as a terminal oxidant offer perfect atom economy alongside the selectivity benefits afforded by enzymes. This review will focus on examples of oxidase biocatalysts that have been used for the sustainable production of important molecules and highlight some important processes that have been significantly improved through the use of oxidases. It will also consider emerging classes of oxidases, and how they might fit in a future biorefinery approach for the sustainable production of important chemicals.

Shortening Synthetic Routes to Small Molecule Active Pharmaceutical Ingredients Employing Biocatalytic Methods

Biocatalysis, using enzymes for organic synthesis, has emerged as powerful tool for the synthesis of active pharmaceutical ingredients (APIs). The first industrial biocatalytic processes launched in the first half of the last century exploited whole-cell microorganisms where the specific enzyme at work was not known. In the meantime, novel molecular biology methods, such as efficient gene sequencing and synthesis, triggered breakthroughs in directed evolution for the rapid development of process-stable enzymes with broad substrate scope and good selectivities tailored for specific substrates. To date, enzymes are employed to enable shorter, more efficient, and more sustainable alternative routes toward (established) small molecule APIs, and are additionally used to perform standard reactions in API synthesis more efficiently. Herein, large-scale synthetic routes containing biocatalytic key steps toward >130 APIs of approved drugs and drug candidates are compared with the corresponding chemical protocols (if available) regarding the steps, reaction conditions, and scale. The review is structured according to the functional group formed in the reaction.

The role of biocatalysis in the asymmetric synthesis of alkaloids - an update

Alkaloids are a group of natural products with interesting pharmacological properties and a long history of medicinal application. Their complex molecular structures have fascinated chemists for decades, and their total synthesis still poses a considerable challenge. In a previous review, we have illustrated how biocatalysis can make valuable contributions to the asymmetric synthesis of alkaloids. The chemo-enzymatic strategies discussed therein have been further explored and improved in recent years, and advances in amine biocatalysis have vastly expanded the opportunities for incorporating enzymes into synthetic routes towards these important natural products. The present review summarises modern developments in chemo-enzymatic alkaloid synthesis since 2013, in which the biocatalytic transformations continue to take an increasingly 'central' role.

Development of a continuous-flow system with immobilized biocatalysts towards sustainable bioprocessing

Implementing immobilized biocatalysts in continuous-flow systems can enable a sustainable process through enhanced enzyme stability, better transport and process continuity as well as simplified recycle and downstream processing.