Dissecting Physical and Biochemical Factors of Catalytic Effectiveness in Immobilized D -Amino Acid Oxidase by Real-Time Sensing of O2 Availability Inside Porous Carriers

Overcoming Biochemical Limitations of Galactose Oxidase through the Design of a Solid-Supported Self-Sufficient Biocatalyst

Galactose Oxidase (GalOx) has gained significant interest in biocatalysis due to its ability for selective oxidation beyond the natural oxidation of galactose, enabling the production of valuable derivatives. However, the practical application of GalOx has been hindered by the limited availability of active and stable biocatalysts, as well as the inherent biochemical limitations such as oxygen (O2 ) dependency and the need for activation. In this study, we addressed these challenges by immobilizing GalOx into agarose-based and Purolite supports to enhance its activity and stability. Additionally, we identified and quantified the oxygen supply limitation into solid catalysts by intraparticle oxygen sensing showing a trade-off between the amount of protein loaded onto the solid support and the catalytic effectiveness of the immobilized enzyme. Furthermore, we coimmobilized a heme-containing protein along with the enzyme to function as an activator. To evaluate the practical application of the immobilized GalOx, we conducted the oxidation of galactose in an instrumented aerated reactor. The results showcased the efficient performance of the immobilized enzyme in the 8 h reaction cycle. Notably, the GalOx immobilized into dextran sulfate-activated agarose exhibited improved stability, overcoming the need for a soluble activator supply, and demonstrated exceptional performance in galactose oxidation. These findings offer promising prospects for the utilization of GalOx in technical biocatalytic applications.

Is enzyme immobilization a mature discipline? Some critical considerations to capitalize on the benefits of immobilization

Enzyme immobilization has been developing since the 1960s and although many industrial biocatalytic processes use the technology to improve enzyme performance, still today we are far from full exploitation of the field. One clear reason is that many evaluate immobilization based on only a few experiments that are not always well-designed. In contrast to many other reviews on the subject, here we highlight the pitfalls of using incorrectly designed immobilization protocols and explain why in many cases sub-optimal results are obtained. We also describe solutions to overcome these challenges and come to the conclusion that recent developments in material science, bioprocess engineering and protein science continue to open new opportunities for the future. In this way, enzyme immobilization, far from being a mature discipline, remains as a subject of high interest and where intense research is still necessary to take full advantage of the possibilities.

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.

Framework of the kinetic analysis of O2-dependent oxidative biocatalysts for reaction intensification

A framework for kinetic modelling and evaluation of the reaction intensification of O2-dependent enzyme catalyzed reactions is built from measurements of consumption rates of the initially dissolved O2 in homogeneous liquid phase.

Design of the Enzyme-Carrier Interface to Overcome the O2 and NADH Mass Transfer Limitations of an Immobilized Flavin Oxidase

Understanding how the immobilization of enzymes on solid carriers affects their performance is paramount for the design of highly efficient heterogeneous biocatalysts. An efficient supply of substrates onto the solid phase is one of the main challenges to maximize the activity of the immobilized enzymes. Herein, we apply advanced single-particle analysis to decipher the optimal design of an immobilized NADH oxidase (NOX) whose activity depends both on O₂ and NADH concentrations. Carrier physicochemical properties and its functionality along with the enzyme distribution across the carrier were implemented as design variables to study the effects of the intraparticle concentration of substrates (O₂ and NADH) on the activity. Intraparticle O₂-sensing analysis revealed the superior performance of the enzyme immobilized at the outer surface in terms of effective supply of O₂. Furthermore, the co-immobilization of NADH and NOX within the tuned surface of porous microbeads increases the effective concentration of NADH in the surroundings of the enzyme. As a result, the optimal spatial organization of NOX and its confinement with NADH allow a 100% recovery of the activity of the soluble enzyme upon the immobilization process. By engineering these variables, we increase the NADH oxidation activity of the heterogeneous biocatalyst by up to 650% compared to NOX immobilized under suboptimal conditions. In conclusion, this work highlights the rational design and engineering of the enzyme-carrier interface to maximize the efficiency of heterogeneous biocatalysts.

The Microenvironment in Immobilized Enzymes: Methods of Characterization and Its Role in Determining Enzyme Performance

The liquid milieu in which enzymes operate when they are immobilized in solid materials can be quite different from the milieu in bulk solution. Important differences are in the substrate and product concentration but also in pH and ionic strength. The internal milieu for immobilized enzymes is affected by the chemical properties of the solid material and by the interplay of reaction and diffusion. Enzyme performance is influenced by the internal milieu in terms of catalytic rate (“activity”) and stability. Elucidation, through direct measurement of differences in the internal as compared to the bulk milieu is, therefore, fundamentally important in the mechanistic characterization of immobilized enzymes. The deepened understanding thus acquired is critical for the rational development of immobilized enzyme preparations with optimized properties. Herein we review approaches by opto-chemical sensing to determine the internal milieu of enzymes immobilized in porous particles. We describe analytical principles applied to immobilized enzymes and focus on the determination of pH and the O₂ concentration. We show measurements of pH and [O₂] with spatiotemporal resolution, using in operando analysis for immobilized preparations of industrially important enzymes. The effect of concentration gradients between solid particle and liquid bulk on enzyme performance is made evident and quantified. Besides its use in enzyme characterization, the method can be applied to the development of process control strategies.

Process intensification for O2 -dependent enzymatic transformations in continuous single-phase pressurized flow

Oxidative O₂ -dependent biotransformations are promising for chemical synthesis, but their development to an efficiency required in fine chemical manufacturing has proven difficult. General problem for process engineering of these systems is that thermodynamic and kinetic limitations on supplying O₂ to the enzymatic reaction combine to create a complex bottleneck on conversion efficiency. We show here that continuous-flow microreactor technology offers a comprehensive solution. It does so by expanding the process window to the medium pressure range (here, ≤34 bar) and thus enables biotransformations to be conducted in a single liquid phase at boosted concentrations of the dissolved O₂ (here, up to 43 mM). We take reactions of glucose oxidase and d-amino acid oxidase as exemplary cases to demonstrate that the pressurized microreactor presents a powerful engineering tool uniquely apt to overcome restrictions inherent to the individual O₂ -dependent transformation considered. Using soluble enzymes in liquid flow, we show reaction rate enhancement (up to six-fold) due to the effect of elevated O₂ concentrations on the oxidase kinetics. When additional catalase was used to recycle dissolved O₂ from the H₂ O₂ released in the oxidase reaction, product formation was doubled compared to the O₂ supplied, in the absence of transfer from a gas phase. A packed-bed reactor containing oxidase and catalase coimmobilized on porous beads was implemented to demonstrate catalyst recyclability and operational stability during continuous high-pressure conversion. Product concentrations of up to 80 mM were obtained at low residence times (1-4 min). Up to 360 reactor cycles were performed at constant product release and near-theoretical utilization of the O₂ supplied. Therefore, we show that the pressurized microreactor is practical embodiment of a general reaction-engineering concept for process intensification in enzymatic conversions requiring O₂ as the cosubstrate.

Oriented Coimmobilization of Oxidase and Catalase on Tailor-Made Ordered Mesoporous Silica

Mesoporous silica materials are promising carriers for enzyme immobilization in heterogeneous biocatalysis applications. By tailoring their pore structural framework, these materials are designable for appropriate enzyme binding capacity and internal diffusivity. To supply O₂ efficiently to solid-supported immobilized enzymes represents a core problem of heterogeneously catalyzed oxidative biotransformations. In this study, therefore, we synthesized and compared three internally well-ordered and two amorphous silica materials as enzyme carriers, each of those with pore sizes of ≥10 nm, to enable the coimmobilization of d-amino-acid oxidase (79 kDa) and catalase (217 kDa). Both enzymes were fused to the silica-binding module Z_basic2 to facilitate their selective and oriented immobilization directly from crude protein mixtures on native silica materials. Analyzing the effects of varied pore architecture and internal surface area on the performance of the immobilized bienzymatic system, we showed that a uniform pore structural framework was beneficial for enzyme loading (≥70 mg protein/g carrier), immobilization yield (≥90%), surface and pore volume filling without hindered adsorption, and catalytic effectiveness (≥60%) of the coimmobilizate. Using the best carrier LP-SBA-15, we obtained a solid oxidase-catalase preparation with an activity of 2000 μmol/(min g_material) that was recyclable and stable during oxidation of d-methionine. These results affirm a strategy of optimizing immobilized O₂-dependent enzymes via tunable internal structuring of the silica material used as carrier. They thus make a significant advance toward the molecular design of heterogeneous oxidation biocatalysts on mesoporous silica supports.

Polyethylenimine: a very useful ionic polymer in the design of immobilized enzyme biocatalysts

This review discusses the possible roles of polyethylenimine (PEI) in the design of improved immobilized biocatalysts from diverse perspectives.