Surface modification with highly-homogeneous porous silica layer for enzyme immobilization in capillary enzyme microreactors

Immobilized PAD4 enzyme on magnetic nanoparticles for screening natural inhibitors from traditional Chinese medicines

Dysregulation of peptidyl arginine deiminase 4 (PAD4) is involved in a variety of diseases including rheumatoid arthritis (RA) and Alzheimer's disease (AD), and it has emerged as potential and promising therapeutic target. However, no PAD4 inhibitor is ready for clinical use. Immobilized enzyme screening technology has gained increasing attention due to its low cost, reusability, easy separation from the reaction mixture, and resistance to changes in environmental conditions. In this study, PAD4 was immobilized on the magnetic nanoparticles (MNP) to prolong its activity stability, and a simple and rapid screening strategy of traditional Chinese medicine inhibitors based on immobilized PAD4 was established. The PAD4 enzyme was immobilized on magnetic nanoparticles (MNP) via Schiff base reaction using glutaraldehyde (GA) as crosslinking agent. Compared with free PAD4, the resulting MNP@GA@PAD4 exhibited an enhanced tolerance to temperature and storage stability, and its reusability was greatly improved with 66 % of initial enzyme activity after being recycled 10 times. The inhibitory activity of the immobilized PAD4 was assessed using two known PAD4 inhibitors GSK484 and BB-Cl-amidine. The semi-maximum inhibitory concentrations (IC50) of GSK484 and BB-Cl-amidine for MNP@GA@PAD4 were 1.00 and 0.97 μM, respectively, for free PAD4 were 0.64 and 0.85 μM, respectively. Finally, the MNP@GA@PAD4 was employed to rapid screen of natural PAD4 inhibitors from forty traditional Chinese medicines (TCMs). Under the same conditions, the controlled experiment was conducted with free PAD4. The screening results of TCMs inhibitors on MNP@GA@PAD4 and free PAD4 were similar, the alcohol extracts of Cinnamomi Cortex and Caryophylli Flos had significant inhibitory effects on PAD4 enzyme activity. The IC50 values of Cinnamomi Cortex extract for MNP@GA@PAD4 and free PAD4 were determined as 27 and 48 μg/mL, respectively. The IC50 values of Caryophylli Flos extracts for MNP@GA@PAD4 and free PAD4 were determined as 48 and 32 μg/mL, respectively. For the first time, this study proposed a method to immobilize PAD4 on magnetic materials, and developed a rapid, reusable and feasible strategy to screening natural PAD4 inhibitors from TCMs.

Trypsin Encapsulation in the Zeolitic Imidazolate Framework for Low-Molecular Weight Protein Analysis with High Selectivity and Efficiency

Low-molecular weight proteins (LWPs) are important sources of biological information in biomarkers, signaling molecules, and pathology. However, the separation and analysis of LWPs in complex biological samples are challenging, mainly due to their low abundance and the complex sample pretreatment procedure. Herein, trypsin modified by poly(acrylic acid) (PAA) was encapsulated by a zeolitic imidazolate framework (ZIF-L). Mesopores were formed on the ZIF-L with the introduction of PAA. An alternative strategy for separation and pretreatment of LWPs was developed based on the prepared ZIF-L-encapsulated trypsin with adjustable pore size. The mesoporous structure of the prepared materials selectively excluded high-molecular weight proteins from the reaction system, allowing LWPs to enter the pores and react with the internal trypsin, resulting in an improved separation efficiency. The hydrophobicity of the ZIF-L simplified the digestion process by inducing significant structural changes in substrate proteins. In addition, the enzymatic activity was significantly enhanced by the developed encapsulation method that maintained the enzyme conformation, allowed low mass transfer resistance, and possessed a high enzyme-to-substrate ratio. As a result, the ZIF-L-encapsulated trypsin can achieve highly selective separation, valid denaturation, and efficient digestion of LWPs in a short time by simply mixing with substrate proteins, greatly simplifying the separation and pretreatment process of the traditional hydrolysis method. The prepared materials and the developed strategy demonstrated an excellent size-selective assay performance in model protein mixtures, showing great potential in the application of proteomics analysis.

Cell-membrane-inspired polymers for constructing biointerfaces with efficient molecular recognition

Fabrication of devices that accurately recognize, detect, and separate target molecules from mixtures is a crucial aspect of biotechnology for applications in medical, pharmaceutical, and food sciences. This technology has also been recently applied in solving environmental and energy-related problems. In molecular recognition, biomolecules are typically complexed with a substrate, and specific molecules from a mixture are recognized, captured, and reacted. To increase sensitivity and efficiency, the activity of the biomolecules used for capture should be maintained, and non-specific reactions on the surface should be prevented. This review summarizes polymeric materials that are used for constructing biointerfaces. Precise molecular recognition occurring at the surface of cell membranes is fundamental to sustaining life; therefore, materials that mimic the structure and properties of this particular surface are emphasized in this article. The requirements for biointerfaces to eliminate nonspecific interactions of biomolecules are described. In particular, the major issue of protein adsorption on biointerfaces is discussed by focusing on the structure of water near the interface from a thermodynamic viewpoint; moreover, the structure of polymer molecules that control the water structure is considered. Methodologies enabling stable formation of these interfaces on material surfaces are also presented.

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.

Capillary-based fluorescence microsensor with polyoxometalates as nanozyme for rapid and ultrasensitive detection of artemisinin

A novel capillary-based fluorescence microsensor for artemisinin was developed with functional polyoxometalates (POMs) as nanozyme by a layer-by-layer self-assembly strategy. Vanadomolybdophosphoric heteropoly acid (H₅PMo₁₀V₂O₄₀, PMoV₂) and tungstophosphoric heteropoly acid (Na₅PW₁₁O₃₉Cu, PW₁₁Cu) with high peroxidase-like activity were synthesized and immobilized on capillary to catalyze artemisinin/thiamine reaction and generate the amplified fluorescence signal. The wide linear range up to 13.0 μM with the low limit of detection of 0.03 μM (S/N = 3) was achieved for the determination of artemisinin by using the proposed POMs-microsensor. The method has been successfully used to detect artemisinin in human plasma and antimalarial drugs with satisfactory accuracy. This work developed a novel capillary fluorescence microsensor with functional POMs as nanozyme, which can serve as a promising candidate in fluorescence microanalysis.

Capillary electrophoresis-UV analysis using silica-layer coated capillary for separation of seven phenolic acids and caffeine and its application to tea analysis

Abstract

This work presents an innovative silica-layer coated capillary with comparison study of the silica-layer coated capillary and the fused-silica capillary for the separation of seven phenolic acids viz. p-hydroxyphenylacetic acid (PHPA), p-coumaric acid (PCA), p-hydroxybenzoic acid (PHBA), caffeic acid (CFA), (3,4-dihydroxyphenyl)acetic acid (DHPA), gallic acid (GLA), and 2,3,4-trihydroxybenzoic acid (THBA), together with caffeine (CF), by capillary electro-chromatography (CEC) and micellar electrokinetic chromatography (MEKC), respectively. The running buffer was 25.0 mM borate at pH 9.0, with addition of 50.0 mM sodium dodecyl sulfate for the MEKC mode. The non-coated capillary could not separate all seven phenolic acids by CEC or MEKC. This was achieved using the coated capillary for both CEC and MEKC. The innovative coated capillary with CEC had plate number N ≥ 2.0 × 104 m−1 and resolution Rs ≥ 1.6 for all adjacent pairs of peaks. The capillary was also able to separate GLA and THBA which are structural isomers. Although MEKC mode provided comparable efficiency and selectivity, the reduced EOF of the coated capillary led to longer separation time. The linear calibration range of the seven phenolic acids and caffeine were different but the coefficients of determinations (r2) were all > 0.9965. The precisions of the relative migration times and peak area ratios of analyte to internal standard were 0.1–1.8% and 1.8–6.8%, respectively. There were no statistical differences in the efficiency of separation of the phenolic acids and caffeine for three coated capillaries. It was applied to the analysis of caffeine and phenolic acids in brewed tea using tyramine as the internal standard. The tea samples were diluted prior to analysis by CEC. The separation was less than 15 min. Caffeine, gallic acid and p-coumaric acid were detected and quantified. Caffeine and gallic acid contents were 10.8–15.0 and 2.6–4.8 mg g−1 dry tea leaves, respectively. p-Coumaric acid was detected in only one of the samples with a content of 0.4 mg g−1. Percent recoveries of spiked diluted samples were 90 ± 9 to 106 ± 13%, respectively.

Article highlights

Graphical abstract

Development of an In-Line Enzyme Reactor Integrated into a Capillary Electrophoresis System

The goal of this paper was to develop an in-line immobilized enzyme reactor (IMER) integrated into a capillary electrophoresis platform. In our research, we created the IMER by adsorbing trypsin onto the inner surface of a capillary in a short section. Enzyme immobilization was possible due to the electrostatic attraction between the oppositely charged fused silica capillary surface and trypsin. The reactor was formed by simply injecting and removing trypsin solution from the capillary inlet (~1–2 cms). We investigated the factors affecting the efficiency of the reactor. The main advantages of the proposed method are the fast, cheap, and easy formation of an IMER with in-line protein digestion capability. Human tear samples were used to test the efficiency of the digestion in the microreactor.

Magnetic particles for enzyme immobilization: A versatile support for ligand screening

Enzyme inhibitors represent a substantial fraction of all small molecules currently in clinical use. Therefore, the early stage of drug-discovery process and development efforts are focused on the identification of new enzyme inhibitors through screening assays. The use of immobilized enzymes on solid supports to probe ligand-enzyme interactions have been employed with success not only to identify and characterize but also to isolate new ligands from complex mixtures. Between the available solid supports, magnetic particles have emerged as a promising support for enzyme immobilization due to the high superficial area, easy separation from the reaction medium and versatility. Particularly, the ligand fishing assay has been employed as a very useful tool to rapidly isolate bioactive compounds from complex mixtures, and hence the use of magnetic particles for enzyme immobilization has been widespread. Thus, this review provides a critical overview of the screening assays using immobilized enzymes on magnetic particles between 2006 and 2021.

One-pot surface modification of magnetic nanoparticles using phase-transitioned lysozyme for robust immobilization of enzymes

Enzymes were one-pot immobilized between Fe₃O₄ nanoparticles and a phase-transitioned lysozyme film, providing a new strategy for enzyme immobilization.

A novel column modification approach for capillary gas chromatography: combination with a triptycene-based stationary phase achieves high separation performance and inertness

Integration of the novel column modification approach with a triptycene-based stationary phase achieves high-resolution performance and inertness towards acids/bases and isomers for capillary GC analysis.

Recent developments in microreactor technology for biocatalysis applications

Through the use of microfluidics technology, one can severely accelerate the development and optimization of biocatalytic processes. In this work, the authors present a comprehensive review of the recent advances in the field.

Enzymatic Bioreactors: An Electrochemical Perspective

Biocatalysts provide a number of advantages such as high selectivity, the ability to operate under mild reaction conditions and availability from renewable resources that are of interest in the development of bioreactors for applications in the pharmaceutical and other sectors. The use of oxidoreductases in biocatalytic reactors is primarily focused on the use of NAD(P)-dependent enzymes, with the recycling of the cofactor occurring via an additional enzymatic system. The use of electrochemically based systems has been limited. This review focuses on the development of electrochemically based biocatalytic reactors. The mechanisms of mediated and direct electron transfer together with methods of immobilising enzymes are briefly reviewed. The use of electrochemically based batch and flow reactors is reviewed in detail with a focus on recent developments in the use of high surface area electrodes, enzyme engineering and enzyme cascades. A future perspective on electrochemically based bioreactors is presented.

Continuous Flow Reactors from Microfluidic Compartmentalization of Enzymes within Inorganic Microparticles

Compartmentalization and selective transport of molecular species are key aspects of chemical transformations inside the cell. In an artificial setting, the immobilization of a wide range of enzymes onto surfaces is commonly used for controlling their functionality but such approaches can restrict their efficacy and expose them to degrading environmental conditions, thus reducing their activity. Here, we employ an approach based on droplet microfluidics to generate enzyme-containing microparticles that feature an inorganic silica shell that forms a semipermeable barrier. We show that this porous shell permits selective diffusion of the substrate and product while protecting the enzymes from degradation by proteinases and maintaining their functionality over multiple reaction cycles. We illustrate the power of this approach by synthesizing microparticles that can be employed to detect glucose levels through simultaneous encapsulation of two distinct enzymes that form a controlled reaction cascade. These results demonstrate a robust, accessible, and modular approach for the formation of microparticles containing active but protected enzymes for molecular sensing applications and potential novel diagnostic platforms.

Evaluation inhibitory activity of catechins on trypsin by capillary electrophoresis-based immobilized enzyme microreactor with chromogenic substrate

In this study, a capillary electrophoresis-based online immobilized enzyme microreactor was developed for evaluating the inhibitory activity of green tea catechins and tea polyphenol extracts on trypsin. The immobilized trypsin activity and other kinetic parameters were evaluated by measuring the peak area of the hydrolyzate of chromogenic substrate S-2765. The results indicated that the activity of the immobilized trypsin remained approximately 90.0% of the initial immobilized enzyme activity after 30 runs. The value of Michaelis-Menten constant (K_m ) was (0.47 ± 0.08) mM, and the half-maximal inhibitory concentration (IC₅₀ ) and inhibition constant (K_i ) of benzamidine were measured as 3.34 and 3.00 mM, respectively. Then, the inhibitory activity of four main catechins (epicatechin, epigallocatechin, epicatechin gallate, and epigallocatechin gallate) and three tea polyphenol extracts (green tea, white tea, and black tea) on trypsin were investigated. The results showed that four catechins and three tea polyphenol extracts had potential trypsin inhibitory activity. In addition, molecular docking results illustrated that epigallocatechin gallate, epicatechin gallate, epicatechin, and epigallocatechin were all located not only in the catalytic cavity, but also in the substrate-binding pocket of trypsin. These results indicated that the developed method is an effective tool for evaluating inhibitory activity of catechins on trypsin.

Covalent immobilization of trypsin on polyvinyl alcohol-coated magnetic nanoparticles activated with glutaraldehyde

Magnetic nanoparticles were coated with polyvinyl alcohol and activated with glutaraldehyde for trypsin immobilization. The prepared magnetic nanoparticles were characterized by transmission electron microscopy, fourier transform infrared spectroscopy, thermal gravimetric analysis, zeta potential meter and vibrating sample magnetometer. Free and immobilized trypsin showed optimum activity at pH 6.0, 30 °C and pH 7.0, 40 °C, respectively. Immobilized trypsin was more stable than the free enzyme at 40 °C. After immobilization, K_m of the immobilized trypsin increased, however, V_max value was almost the same with free trypsin. According to the results, the immobilized trypsin retained 50 % of its initial activity, whereas free trypsin retained 19 % of its initial activity after 12-days at 4 °C. Immobilized trypsin sustained 56 % of its initial activity after eight times of successive reuse. The performance of the immobilized trypsin was evaluated by digestion of cytochrome c. The peptide fragments in digest solution were determined by using MALDI-TOF mass spectrometry. Immobilized trypsin showed effective proteolytic activity in shorter time (15 min) than free trypsin (24 h). Hence, immobilized trypsin on the polyvinyl alcohol coated magnetic nanoparticles could be promising biocatalyst for large-scale proteomics studies and practical applications.

An l-glutaminase enzyme reactor based on porous bamboo sticks and its application in enzyme inhibitors screening

Inspired by the porous and fibrous structure of commercially available bamboo, herein we created an l-glutaminase enzyme reactor based on bamboo sticks. The enzyme was immobilized onto the bamboo sticks through a glutaraldehyde modification to achieve covalent bonding. The enzymatic hydrolysis efficiency of the prepared l-glutaminase@bamboo sticks based porous enzyme reactor was evaluated by chiral ligand exchange capillary electrochromatography using l-glutamine as the substrate. l-glutaminase@bamboo exhibited improved enzymatic hydrolysis performances, including high hydrolysis efficiency (maximum rate V_max: two fold higher than the free enzyme), prolonged stability (14 days) and good reusability. l-Glutaminase@bamboo sticks also expanded application capability in pharmaceutical industry in enzyme inhibitor screening. These excellent properties could be attributed to the micropores of bamboo sticks, which led to the fast enzymatic kinetics. The results suggest that the pores of bamboo sticks played an important role in the proposed enzyme reactor during the hydrolysis of l-glutamine and l-glutaminase inhibitor screening.

Capillary electrophoresis-integrated immobilized enzyme microreactor with graphene oxide as support: Immobilization of negatively charged L-lactate dehydrogenase via hydrophobic interactions

We report the first application of hydrophobic interaction between graphene oxide (GO) and negatively charged enzymes to fabricate CE-integrated immobilized enzyme microreactors (IMERs) by a simple and reliable immobilization procedure based on layer by layer assembly. L-lactate dehydrogenase (L-LDH), which is negatively charged during the enzymatic reaction, is selected as the model enzyme. Various spectroscopic techniques, including SEM, FTIR, and UV-vis are used to characterize the fabricated CE-IMERs, demonstrating the successful immobilization of enzymes on the negatively charged GO layer in the capillary surface. The IMER exhibits excellent repeatability with RSDs of inter-day and batch-to-batch less than 3.49 and 6.37%, respectively, and the activity of immobilized enzymes remains about 90% after five-day usage. The measured K_m values of pyruvate and NADH of the immobilized L-LDH are in good agreement with those obtained by free enzymes. The results demonstrate that the hydrophobic interactions and/or π-π stacking is significant between the GO backbone and the aromatic residues of L-LDH and favorable to fabrication of CE-integrated IMERs. Finally, the method is successfully applied to the determination of pyruvate in beer samples.

Capillary electrophoresis-immobilized enzyme microreactors for acetylcholinesterase assay with surface modification by highly-homogeneous microporous layer

We propose a new capillary electrophoresis (CE)-based open-tubular immobilized enzyme microreactor (OT-IMER) and its application in acetylcholinesterase (AChE) assays. The IMER is fabricated at the capillary inlet (reactor length of ∼1 cm) with the inner surface modified by a micropore-structured layer (thickness of ∼220 nm, pore size of ∼15-20 nm). The use of IMER accomplishes the enzymatic reaction and separation/detection of the products in the same capillary within 3 min. The feasibility of the proposed method is evaluated via online analysis of the activity and inhibition of AChE enzymes. Such method exhibits good reproducibility with relative standard deviation (RSD) of less than 4% for 20 runs, and the enzyme remains over 82% of the initial activity after usage of 7 days. The IMERs are successfully applied to detect the organophosphorus pesticide, paraoxon, in three types of vegetable juice samples with a limit of detection of as low as 61 ng mL^-1. Results show that the spiked samples are in the range of 89.6-105.9% with RSD less than 2.7%, thereby indicating its satisfactory level of accurate and reliable analysis of real samples by using the proposed method. Our study indicates that, with combination of advantages of both porous-layer capillary and CE OT-IMER, the proposed method is capable to enhance enzymatic reactions and to achieve rapid analysis with simple instrumentation and operation, thus would pave the way for extensive application of CE-based IMERs in a variety of bioanalysis.