Microfluidic reactor with immobilized enzyme-from construction to applications: A review

Immobilization of alkaline protease produced by Streptomyces rochei strain NAM-19 in solid state fermentation based on medium optimization using central composite design

This study evaluated Streptomyces rochei strain NAM-19 solid-state fermentation of agricultural wastes to produce alkaline protease. Alkaline protease production increased with flaxseed, rice bran, and cheese whey fermentation reaching 147 U/mL at 48 h. Statistical optimization of alkaline protease production was performed using the central composite design (CDD). Results of CDD and the optimization plot showed that 4.59 g/L flaxseed, 4.31 g/L rice bran, 4.17 mL cheese whey, and a vegetative inoculum size of 7.0% increased alkaline protease production by 27.2% reaching 186 U/mL. Using the 20-70% ammonium sulfate fractionation method, the optimally produced enzyme was partially purified to fivefold. The partially purified alkaline protease was then covalently immobilized on a biopolymer carrier, glutaraldehyde-polyethylene-imine-κ-carrageenan (GA-PEI-Carr), with 90% immobilization efficiency. Characterizations revealed that immobilization improved thermostability, reusability, optimum temperature, and sensitivity towards metal ions of the free enzyme. The optimal temperature for free and immobilized enzymes was 40 and 50 °C, respectively. Both enzymes had the same optimum pH of 10. Immobilization increased Km from 19.73 to 26.52 mM and Vmax from 56.7 to 62.5 mmol min-1L-1. The immobilized enzyme retained 35% of its initial activity at 70 °C, while the free enzyme retained only 5%. The immobilized enzyme kept 80% of its initial activity at the 20th cycle. After 7 weeks of storage, the free enzyme lost all its initial activity, whereas the immobilized enzyme retained 50%. The free and immobilized enzymes were able to hydrolyze gelatin, and azo-casein demonstrating different relative activity, 85, 80, 90 and 95%, respectively, compared to casein (100%).

Laccase-based catalytic microreactor for BPA biotransformation

A laccase-based catalytic reactor was developed into a polydimethylsiloxane (PDMS) microfluidic device, allowing the degradation of different concentrations of the emergent pollutant, Bisphenol-A (BPA), at a rate similar to free enzyme. Among the immobilizing agents used, 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) was capable of immobilizing a more significant amount of the laccase enzyme in comparison to glutaraldehyde (GA), and the passive method (2989, 1537, and 1905 U/mL, respectively). The immobilized enzyme inside the microfluidic device could degrade 55 ppm of BPA at a reaction rate of 0.5309 U/mL*min with a contaminant initial concentration of 100 ppm at room temperature. In conclusion, the design of a microfluidic device and the immobilization of the laccase enzyme successfully allowed a high capacity of BPA degradation.

Pros and Cons in Various Immobilization Techniques and Carriers for Enzymes

In recent years, enzyme immobilization technology has been developed, and studies on immobilized enzyme materials have become very prominent. With the immobilization technique, enzymes and compatible carrier materials are combined or enzyme crystals/aggregates are used in a carrier-free fashion, by physical, chemical, or biochemical methods. As a kind of biocatalyst, immobilized enzymes can catalyze certain chemical reactions with high selectivity and high efficiency under relatively mild reaction conditions and eliminate pollution to the environment. Considering the current status and applications of immobilized enzyme technology and materials emerging in the last 5 years, this mini-review introduces the advantages and disadvantages of various enzyme immobilization techniques with carriers as well as the pros and cons of different materials for immobilization. The future prospects of immobilization technology and carrier materials are outlined, aiming to provide a reference for further research and applications of sustainable technology.

Immobilised-enzyme microreactors for the identification and synthesis of conjugated drug metabolites

The study of naturally circulating drug metabolites has been a focus of interest, since these metabolites may have different therapeutic and toxicological effects compared to the parent drug. The synthesis of metabolites outside of the human body is vital in order to conduct studies into the pharmacological activities of drugs and bioactive compounds. Current synthesis methods require significant purification and separation efforts or do not provide sufficient quantities for use in pharmacology experiments. Thus, there is a need for simple methods yielding high conversions whilst bypassing the requirement for a separation. Here we have developed and optimised flow chemistry methods in glass microfluidic reactors utilising surface-immobilised enzymes for sulfonation (SULT1a1) and glucuronidation (UGT1a1). Conversion occurs in flow, the precursor and co-factor are pumped through the device, react with the immobilised enzymes and the product is then simply collected at the outlet with no separation from a complex biological matrix required. Conversion only occurred when both the correct co-factor and enzyme were present within the microfluidic system. Yields of 0.97 ± 0.26 μg were obtained from the conversion of resorufin into resorufin sulfate over 2 h with the SULT1a1 enzyme and 0.47 μg of resorufin glucuronide over 4 h for UGT1a1. This was demonstrated to be significantly more than static test tube reactions at 0.22 μg (SULT1a1) and 0.19 μg (UGT1a1) over 4 h. With scaling out and parallelising, useable quantities of hundreds of micrograms for use in pharmacology studies can be synthesised simply.

An integrated microfluidic-based biosensor using a magnetically controlled MNPs-enzyme microreactor to determine cholesterol in serum with fluorometric detection

This work provides a microfluidic-based biosensor to determine total cholesterol in serum based on integrating the reaction/detection zone of a microfluidic chip of a magnetically retained enzyme microreactor (MREµR) coupled with the remote fluorometric detection through a bifurcated fiber-optic bundle (BFOB) connected with a conventional spectrofluorometer. The method is based on developing the enzymatic hydrolysis and oxidation of cholesterol at microscale size using both enzymes (cholesterol esterase (ChE) and cholesterol oxidase (ChOx)) immobilized on magnetic nanoparticles (MNPs). The biocatalyst reactions were followed by monitoring the fluorescence decreasing by the naphtofluorescein (NF) oxidation in the presence of the previous H₂O₂ formed. This microfluidic biosensor supposes the physical integration of a minimal MREµR as a bioactive enzyme area and the focused BFOB connected with the spectrofluorometer detector. The MREµR was formed by a 1 mm length of magnetic retained 2:1 ChE-MNP/ChOx-MNP mixture. The dynamic range of the calibration graph was 0.005-10 mmol L^-1, expressed as total cholesterol concentration with a detection limit of 1.1 µmol L^-1 (r² = 0.9999, s_y/x = 0.03, n = 10, r = 3). The precision expressed as the relative standard deviation (RSD%) was between 1.3 and 2.1%. The microfluidic-based biosensors showed a sampling frequency estimated at 30 h^-1. The method was applied to determine cholesterol in serum samples with recovery values between 94.8 and 102%. The results of the cholesterol determination in serum were also tested by correlation with those obtained using the other two previous methods.

Non-covalent binding tags for batch and flow biocatalysis

Enzyme immobilization offers considerable advantage for biocatalysis in batch and continuous flow reactions. However, many currently available immobilization methods require that the surface of the carrier is chemically modified to allow site specific interactions with their cognate enzymes, which requires specific processing steps and incurs associated costs. Two carriers (cellulose and silica) were investigated here, initially using fluorescent proteins as models to study binding, followed by assessment of industrially relevant enzyme performance (transaminases and an imine reductase/glucose oxidoreductase fusion). Two previously described binding tags, the 17 amino acid long silica-binding peptide from the Bacillus cereus CotB protein and the cellulose binding domain from the Clostridium thermocellum, were fused to a range of proteins without impairing their heterologous expression. When fused to a fluorescent protein both tags conferred high avidity specific binding with their respective carriers (low nanomolar K_d values). The CotB peptide (CotB1p) induced protein aggregation in the transaminase and imine reductase/glucose oxidoreductase fusions when incubated with the silica carrier. The Clostridium thermocellum cellulose binding domain (CBDclos) allowed immobilization of all the proteins tested, but immobilization led to loss of enzymatic activity in the transaminases (< 2-fold) and imine reductase/glucose oxidoreductase fusion (> 80%). A transaminase-CBDclos fusion was then successfully used to demonstrate the application of the binding tag in repetitive batch and a continuous-flow reactor.

In-depth analysis of biocatalysts by microfluidics: An emerging source of data for machine learning

Nowadays, the vastly increasing demand for novel biotechnological products is supported by the continuous development of biocatalytic applications which provide sustainable green alternatives to chemical processes. The success of a biocatalytic application is critically dependent on how quickly we can identify and characterize enzyme variants fitting the conditions of industrial processes. While miniaturization and parallelization have dramatically increased the throughput of next-generation sequencing systems, the subsequent characterization of the obtained candidates is still a limiting process in identifying the desired biocatalysts. Only a few commercial microfluidic systems for enzyme analysis are currently available, and the transformation of numerous published prototypes into commercial platforms is still to be streamlined. This review presents the state-of-the-art, recent trends, and perspectives in applying microfluidic tools in the functional and structural analysis of biocatalysts. We discuss the advantages and disadvantages of available technologies, their reproducibility and robustness, and readiness for routine laboratory use. We also highlight the unexplored potential of microfluidics to leverage the power of machine learning for biocatalyst development.

Protease immobilization on a novel activated carrier alginate/dextrose beads: Improved stability and catalytic activity via covalent binding

Protease from Bacillus thuringiensis strain-MA8 was successfully immobilized onto activated Alginate/dextrose (Alg/dex) beads as a new carrier with immobilization yield 77.6 %. The carrier was characterized using Scanning electron microscopy and Fourier transforms infrared spectrophotometer at every step of the immobilization process. Immobilized protease showed an increase of 10 °C in the optimum temperature compared to the free enzyme. However, the optimum pH for both the free and the Alg/dex/protease was found to be 8. The lower activation energy and deactivation rate constant and the higher half-life time and D-value confirm that the new Alg/dex carrier is suitable for promoting enzyme stability. The raise in thermal stability is also shown by the increased deactivation energy of the Alg/dex/protease compared to its free form by 1.47-fold. Likewise, the enzyme immobilization enhancement of Alg/dex/protease was accompanied by a marked increase in enthalpy and Gibbs free energy. The negative entropy for both free and Alg/dex/protease indicates that the enzyme is more stable in thermal deactivation. The Km and Vmax for the Alg/dex/protease were 2.05 and 1.22-times greater than the free form. Furthermore, Alg/dex/protease displayed good reusability as it retained 92.7 and 52.4 % of its activity after 8 and 12 hydrolysis cycles.

Droplet Microfluidic Device for Chemoenzymatic Sensing

The rapid detection of pollutants in water can be performed with enzymatic probes, the catalytic light-emitting activity of which decreases in the presence of many types of pollutants. Herein, we present a microfluidic system for continuous chemoenzymatic biosensing that generates emulsion droplets containing two enzymes of the bacterial bioluminescent system (luciferase and NAD(P)H:FMN–oxidoreductase) with substrates required for the reaction. The developed chip generates “water-in-oil” emulsion droplets with a volume of 0.1 μL and a frequency of up to 12 drops per minute as well as provides the efficient mixing of reagents in droplets and their distancing. The bioluminescent signal from each individual droplet was measured by a photomultiplier tube with a signal-to-noise ratio of up to 3000/1. The intensity of the luminescence depended on the concentration of the copper sulfate with the limit of its detection of 5 μM. It was shown that bioluminescent enzymatic reactions could be carried out in droplet reactors in dispersed streams. The parameters and limitations required for the bioluminescent reaction to proceed were also studied. Hereby, chemoenzymatic sensing capabilities powered by a droplet microfluidics manipulation technique may serve as the basis for early-warning online water pollution systems.

Enzyme Immobilization and Co-Immobilization: Main Framework, Advances and Some Applications

Enzymes are outstanding (bio)catalysts, not solely on account of their ability to increase reaction rates by up to several orders of magnitude but also for the high degree of substrate specificity, regiospecificity and stereospecificity. The use and development of enzymes as robust biocatalysts is one of the main challenges in biotechnology. However, despite the high specificities and turnover of enzymes, there are also drawbacks. At the industrial level, these drawbacks are typically overcome by resorting to immobilized enzymes to enhance stability. Immobilization of biocatalysts allows their reuse, increases stability, facilitates process control, eases product recovery, and enhances product yield and quality. This is especially important for expensive enzymes, for those obtained in low fermentation yield and with relatively low activity. This review provides an integrated perspective on (multi)enzyme immobilization that abridges a critical evaluation of immobilization methods and carriers, biocatalyst metrics, impact of key carrier features on biocatalyst performance, trends towards miniaturization and detailed illustrative examples that are representative of biocatalytic applications promoting sustainability.

Microfluidic Immobilized Enzymatic Reactors for Proteomic Analyses—Recent Developments and Trends (2017–2021)

Given the strong interdisciplinary nature of microfluidic immobilized enzyme reactor (μ-IMER) technology, several branches of science contribute to its successful implementation. A combination of physical, chemical knowledge and engineering skills is often required. The development and application of μ-IMERs in the proteomic community are experiencing increasing importance due to their attractive features of enzyme reusability, shorter digestion times, the ability to handle minute volumes of sample and the prospect of on-line integration into analytical workflows. The aim of this review is to give an account of the current (2017–2021) trends regarding the preparation of microdevices, immobilization strategies, and IMER configurations. The different aspects of microfabrication (designs, fabrication technologies and detectors) and enzyme immobilization (empty and packed channels, and monolithic supports) are surveyed focusing on μ-IMERs developed for proteomic analysis. Based on the advantages and limitations of the published approaches and the different applications, a probable perspective is given.