Flow Bioreactors as Complementary Tools for Biocatalytic Process Intensification

Self-Assembling All-Enzyme Hydrogels for Flow Biocatalysis

Continuous flow biocatalysis is an emerging field of industrial biotechnology that uses enzymes immobilized in flow channels for the production of value-added chemicals. We describe the construction of self-assembling all-enzyme hydrogels that are comprised of two tetrameric enzymes. The stereoselective dehydrogenase LbADH and the cofactor-regenerating glucose 1-dehydrogenase GDH were genetically fused with a SpyTag or SpyCatcher domain, respectively, to generate two complementary homo-tetrameric building blocks that polymerize under physiological conditions into porous hydrogels. Mounted in microfluidic reactors, the gels show excellent stereoselectivity with near quantitative conversion in the reduction of prochiral ketones along with high robustness under process and storage conditions. The gels function as compartment that retains intermediates thus enabling high total turnover numbers of the expensive cofactor NADP(H).

Strategic single point mutation yields a solvent- and salt-stable transaminase from Virgibacillus sp. in soluble form

A new transaminase (VbTA) was identified from the genome of the halotolerant marine bacterium Virgibacillus 21D. Following heterologous expression in Escherichia coli, it was located entirely in the insoluble fraction. After a single mutation, identified via sequence homology analyses, the VbTA T16F mutant was successfully expressed in soluble form and characterised. VbTA T16F showed high stability towards polar organic solvents and salt exposure, accepting mainly hydrophobic aromatic amine and carbonyl substrates. The 2.0 Å resolution crystal structure of VbTA T16F is here reported, and together with computational calculations, revealed that this mutation is crucial for correct dimerisation and thus correct folding, leading to soluble protein expression.

Designing continuous flow reaction of xylan hydrolysis for xylooligosaccharides production in packed-bed reactors using xylanase immobilized on methacrylic polymer-based supports

The present study focuses on the development and optimization of a packed-bed reactor (PBR) for continuous production of xylooligosaccharides (XOS) from xylan. For this purpose, three different methacrylic polymer-based supports (Relizyme R403/S, Purolite P8204F and Purolite P8215F) activated with glyoxyl groups were morphologically characterized and screened for the multipoint covalent immobilization of a xylanase. Based on its physical and mechanical properties, maximum protein loading and thermal stability, Relizyme R403/S was selected to set up a PRB for continuous production of XOS from corncob xylan. The specific productivity for XOS at 10 mL/min flow rate was 3277 g_XOS g_enzyme^-1 h^-1 with a PBR. This PBR conserved >90% of its initial activity after 120 h of continuous operation.

Demystifying the Flow: Biocatalytic Reaction Intensification in Microstructured Enzyme Reactors

Continuous (flow) reactors have drawn a wave of renewed interest in biocatalysis. Many studies find that the flow reactor offers enhanced conversion efficiency. What the reported reaction intensification actually consists in, however, often remains obscure. Here, a canonical microreactor design for heterogeneously catalyzed continuous biotransformations, featuring flow microchannels that contain the enzyme immobilized on their wall surface are examined. Glycosylations by sucrose phosphorylase are used to assess the potential for reaction intensification due to microscale effects. Key variables are identified, and their corresponding relationship equations, to describe, and optimize, the interplay between reaction characteristics, microchannel geometry and reactor operation. The maximum space-time-yield (STY_max) scales directly with the enzyme activity immobilized on the available wall surface. Timescale analysis, comparing the characteristic times of reaction (τ_reac ) and diffusion (τ_diff ) to the mean residence time (τ_res ), reveals operational conditions for optimum reactor output. Theoretical insight into determinants of microreactor performance is applied to biocatalytic syntheses of α-d-glucose 1-phosphate and α-glucosyl glycerol. Process boundaries for enzyme showing, respectively, high (80 U mg^-1 ) and low (4 U mg^-1 ) specific activities are thus established and options for process design revealed. Opportunities, and limitations, of the application of principles of microscale flow chemistry to biocatalytic transformations are made evident.

Continuous flow biocatalysis

The continuous flow synthesis of active pharmaceutical ingredients, value-added chemicals, and materials has grown tremendously over the past ten years. This revolution in chemical manufacturing has resulted from innovations in both new methodology and technology. This field, however, has been predominantly focused on synthetic organic chemistry, and the use of biocatalysts in continuous flow systems is only now becoming popular. Although immobilized enzymes and whole cells in batch systems are common, their continuous flow counterparts have grown rapidly over the past two years. With continuous flow systems offering improved mixing, mass transfer, thermal control, pressurized processing, decreased variation, automation, process analytical technology, and in-line purification, the combination of biocatalysis and flow chemistry opens powerful new process windows. This Review explores continuous flow biocatalysts with emphasis on new technology, enzymes, whole cells, co-factor recycling, and immobilization methods for the synthesis of pharmaceuticals, value-added chemicals, and materials.

Magnetic Microreactors with Immobilized Enzymes—From Assemblage to Contemporary Applications

Microfluidics, as the technology for continuous flow processing in microscale, is being increasingly elaborated on in enzyme biotechnology and biocatalysis. Enzymatic microreactors are a precious tool for the investigation of catalytic properties and optimization of reaction parameters in a thriving and high-yielding way. The utilization of magnetic forces in the overall microfluidic system has reinforced enzymatic processes, paving the way for novel applications in a variety of research fields. In this review, we hold a discussion on how different magnetic particles combined with the appropriate biocatalyst under the proper system configuration may constitute a powerful microsystem and provide a highly explorable scope.

Development and performance of a 3D-printable poly(ethylene glycol) diacrylate hydrogel suitable for enzyme entrapment and long-term biocatalytic applications

Physical entrapment of enzymes within a porous matrix is a fast and gentle process to immobilize biocatalysts to enable their recycling and long-term use. This study introduces the development of a biocompatible 3D-printing material suitable for enzyme entrapment, while having good rheological and UV-hardening properties. Three different viscosity-enhancing additives have been tested in combination with a poly(ethylene glycol) diacrylate-based hydrogel system. The addition of polyxanthan or hectorite clay particles results in hydrogels that degrade over hours or days, releasing entrapped compounds. In contrast, the addition of nanometer-sized silicate particles ensures processability while preventing disintegration of the hydrogel. Lattice structures with a total height of 6 mm consisting of 40 layers were 3D-printed with all materials and characterized by image analysis. Rheological measurements identified a shear stress window of 200 < τ < 500 Pa at shear rates of 25 s^-1 and 25°C for well-defined geometries with an extrusion-based printhead. Enzymes immobilized in these long-term stable hydrogel structures retained an effective activity of approximately 10% compared to the free enzyme in solution. It could be shown that the reduction of effective activity is not caused by a significant reduction of the intrinsic enzyme activity but by mass transfer limitations within the printed hydrogel structures.

Biocatalytic synthesis of the Green Note trans-2-hexenal in a continuous-flow microreactor

The biocatalytic preparation of trans-hex-2-enal from trans-hex-2-enol using a novel aryl alcohol oxidase from Pleurotus eryngii (PeAAOx) is reported. As O₂-dependent enzyme PeAAOx-dependent reactions are generally plagued by the poor solubility of O₂ in aqueous media and mass transfer limitations resulting in poor reaction rates. These limitations were efficiently overcome by conducting the reaction in a flow-reactor setup reaching unpreceded catalytic activities for the enzyme in terms of turnover frequency (up to 38 s⁻¹) and turnover numbers (more than 300000) pointing towards preparative usefulness of the proposed reaction scheme.

The Fourth Wave of Biocatalysis Emerges- The 13 th International Symposium on Biocatalysis and Biotransformations

Ride the wave! Biocatalysis uses nature's catalysts, enzymes and whole cell systems, for synthetic purposes. In a biotransformation, the biocatalyst transforms a well-defined substrate to the desired product, in contrast to the fermentation process, which produces the desired product from a complex mixture of nutrients. Biocatalysis has reached an industrially established level through several waves of technological evolution; participants of the BioTrans 2017 conference in Budapest could witness the newest wave of this technology.

Synergy of Microtechnology and Biotechnology: Microreactors as an Effective Tool for Biotransformation Processes§: §The paper was presented at European Biotechnology Congress, 25-27 May 2017, Dubrovnik, Croatia

Despite the fact that microreactors have been present for more than 40 years now and that their potential has been extensively exploited in chemical synthesis, analytics and screening, to date very few biocatalytic processes have been explored in microreactors. It is claimed that enzymatic microreactor technology is exactly in the same place where chemical microreactors were 15 years ago. However, general opinion is that the efforts devoted to the research of micro-enzymatic reactors will inaugurate a new breakthrough in bio-based processing. The aim of this review is to explore the synergy between microtechnology, mainly microreactors, and biotechnology, and to assess its potential, opportunities, challenges and future application in biotechnology.

Stabilization of multimeric sucrose synthase from Acidithiobacillus caldus via immobilization and post-immobilization techniques for synthesis of UDP-glucose

Sucrose synthases (SuSys) have been attracting great interest in recent years in industrial biocatalysis. They can be used for the cost-effective production of uridine 5'-diphosphate glucose (UDP-glucose) or its in situ recycling if coupled to glycosyltransferases on the production of glycosides in the food, pharmaceutical, nutraceutical, and cosmetic industry. In this study, the homotetrameric SuSy from Acidithiobacillus caldus (SuSyAc) was immobilized-stabilized on agarose beads activated with either (i) glyoxyl groups, (ii) cyanogen bromide groups, or (iii) heterogeneously activated with both glyoxyl and positively charged amino groups. The multipoint covalent immobilization of SuSyAc on glyoxyl agarose at pH 10.0 under optimized conditions provided a significant stabilization factor at reaction conditions (pH 5.0 and 45 °C). However, this strategy did not stabilize the enzyme quaternary structure. Thus, a post-immobilization technique using functionalized polymers, such as polyethyleneimine (PEI) and dextran-aldehyde (dexCHO), was applied to cross-link all enzyme subunits. The coating of the optimal SuSyAc immobilized glyoxyl agarose with a bilayer of 25 kDa PEI and 25 kDa dexCHO completely stabilized the quaternary structure of the enzyme. Accordingly, the combination of immobilization and post-immobilization techniques led to a biocatalyst 340-fold more stable than the non-cross-linked biocatalyst, preserving 60% of its initial activity. This biocatalyst produced 256 mM of UDP-glucose in a single batch, accumulating 1 M after five reaction cycles. Therefore, this immobilized enzyme can be of great interest as a biocatalyst to synthesize UDP-glucose.