Impact of the size effect on enzymatic electrochemical detection based on metal-organic frameworks

Biomimetic immobilization of α-glucosidase inspired by antibody-antigen specific recognition for catalytic preparation of 4-methylumbelliferone

Immobilization technology plays an important role in enhancing enzyme stability and environmental adaptability. Despite its rapid development, this technology still encounters many challenges such as enzyme leakage, difficulties in large-scale implementation, and limited reusability. Drawing inspiration from natural paired molecules, this study aimed to establish a method for immobilized α-glucosidase using artificial antibody-antigen interaction. The proposed method consists of three main parts: synthesis of artificial antibodies, synthesis of artificial antigens, and assembly of the artificial antibody-antigen complex. The critical step in this method involves selecting a pair of structurally similar compounds: catechol as a template for preparing artificial antibodies and protocatechualdehyde for modifying the enzyme to create the artificial antigens. By utilizing the same functional groups in these compounds, specific recognition of the antigen by the artificial antibody can be achieved, thereby immobilizing the enzymes. The results demonstrated that the immobilization amount, specific activity, and enzyme activity of the immobilized α-glucosidase were 25.09 ± 0.10 mg/g, 5.71 ± 0.17 U/mgprotein and 143.25 ± 1.71 U/gcarrier, respectively. The immobilized α-glucosidase not only exhibited excellent reusability but also demonstrated remarkable performance in catalyzing the hydrolysis of 4-methylumbelliferyl-α-D-glucopyranoside.

Enzyme-based electrochemical biosensor for antimonite detection in water

Antimonite (Sb^III) is a naturally occurring contaminant demanding on-site ultrasensitive detection. The enzyme-based electrochemical (EC) biosensors are promising, but the lack of specific Sb^III oxidizing enzymes hindered the past efforts. Herein, we modulated the specificity of arsenite oxidase AioAB toward Sb^III by regulating its spatial conformation from tight to loose using the metal-organic framework ZIF-8. The constructed EC biosensor, AioAB@ZIF-8, exhibited the substrate specificity toward Sb^III at 12.8 s^-1 μM^-1, an order of magnitude higher than that of As^III (1.1 s^-1 μM^-1). Relaxing AioAB structure in ZIF-8 was evidenced by the break of the S-S bond and the conversion of α helix to the random coil as suggested by Raman spectroscopy. Our AioAB@ZIF-8 EC sensor exhibited a dynamic linear range in 0.041-4.1 μM at a response time of 5 s, and the detection limit at 0.041 μM at a high sensitivity of 1894 nA μM^-1. The insights into tuning the specificity of an enzyme shed new light on biosensing metal(loid)s without specific proteins.

Confining enzymes in porous organic frameworks: from synthetic strategy and characterization to healthcare applications

Enzymes are a class of natural catalysts with high efficiency, specificity, and selectivity unmatched by their synthetic counterparts and dictate a myriad of reactions that constitute various cascades in living cells. The development of suitable supports is significant for the immobilization of structurally flexible enzymes, enabling biomimetic transformation in the extracellular environment. Accordingly, porous organic frameworks, including metal organic frameworks (MOFs), covalent organic frameworks (COFs) and hydrogen-bonded organic frameworks (HOFs), have emerged as ideal supports for the immobilization of enzymes because of their structural features including ultrahigh surface area, tailorable porosity, and versatile framework compositions. Specially, organic framework-encased enzymes have shown significant enhancement in stability and reusability, and their tailorable pore opening provides a gatekeeper-like effect for guest sieving, which is beneficial for mimicking intracellular biocatalysis processes. This immobilization technique brings new insight into the development of next-generation enzyme materials and shows huge potential in healthcare applications, such as biomarker diagnosis, biostorage, and cancer and antibacterial therapies. In this review, we describe the state-of-the-art strategies for the structural immobilization of enzymes using the well-explored MOFs and burgeoning COFs and HOFs as scaffolds, with special emphasis on how these porous framework-confined technologies can provide a favorable microenvironment for mimicking natural biocatalysis. Subsequently, advanced characterization techniques for enzyme conformation, the effect of the confined microenvironment on the activity of enzymes, and the emerging healthcare applications will be surveyed.

Glucose/ROS cascade-responsive ceria nanozymes for diabetic wound healing

Diabetic wounds have an extremely complex microenvironment of hyperglycemia, hypoxia and high reactive oxygen species (ROS). Therefore, the regulation and management of this microenvironment may provide a new and improved treatment method for chronic diabetic wound healing. Herein, a glucose/ROS cascade-responsive nanozyme (CHA@GOx) was developed for diabetic wound treatment based on Ce-driven coassembly by a special dual ligand (alendronic acid and 2-methylimidazole) and glucose oxidase (GOx). It possesses superoxide dismutase and catalase mimic activities, which effectively remove excess ROS. In particular, it can catalyze excessive hydrogen peroxide generated by the glucose oxidation reaction to produce oxygen, regulate the oxygen balance of the wound, and reduce the toxic side effects of GOx, thus achieving the purpose of synergistically repairing diabetic wounds. In vitro experiments show that CHA@GOx assists mouse fibroblast migration and promotes human umbilical vein endothelial cell tube formation. In vivo, it can induce angiogenesis, collagen deposition, and re-epithelialization during wound healing in diabetic mice. Taken together, this study indicates that the coassembly of multifunctional nanozymes has implications in diabetic wound healing.

Immobilization of Lipase from Thermomyces lanuginosus in Magnetic Macroporous ZIF-8 Improves Lipase Reusability in Biodiesel Preparation

In recent years, metal-organic frameworks (MOFs) have emerged as a promising support for immobilizing enzymes due to their high designability and structural diversity. Previous studies show that MOFs with single-crystal-ordered macroporous structures can effectively improve the accessibility of large-size enzyme and reduce the mass transfer resistance compared to conventional MOFs. In order to further enhance the reusability of lipase immobilized on macroporous MOFs, modification of MOFs through some magnetic particles could be an efficient approach. In this work, magnetic macroporous zeolitic imidazolate framework-8 (ZIF-8), referred to as m-M-ZIF-8 (with an average macropore size of about 140 nm), was synthesized and used for the immobilization of Thermomyces lanuginosus lipase (TLL). It was found that enzyme loading and the specific enzyme activity of the immobilized lipase were greatly enhanced through this magnetic modification. The enzyme loading of TLL@C-ZIF-8, T LL@M-ZIF-8, and TLL@m-M-ZIF-8 was 0.060, 0.074, and 0.076 mg/mg respectively. Besides, the activity of 93.5% was maintained after the immobilized lipase being repeatedly used for five batches, which was much higher than that of the immobilized lipase without magnetic modification, which was only 73.4%.

Recent Advances in Metal-Organic Framework-Based Electrochemical Biosensing Applications

In the face of complex environments, considerable effort has been made to accomplish sensitive, accurate and highly-effective detection of target analytes. Given the versatility of metal clusters and ligands, high porosity and large specific surface area, metal–organic framework (MOF) provides researchers with prospective solutions for the construction of biosensing platforms. Combined with the benefits of electrochemistry method such as fast response, low cost and simple operation, the untapped applications of MOF for biosensors are worthy to be exploited. Therefore, this review briefly summarizes the preparation methods of electroactive MOF, including synthesize with electroactive ligands/metal ions, functionalization of MOF with biomolecules and modification for MOF composites. Moreover, recent biosensing applications are highlighted in terms of small biomolecules, biomacromolecules, and pathogenic cells. We conclude with a discussion of future challenges and prospects in the field. It aims to offer researchers inspiration to address the issues appropriately in further investigations.

Influence of the Synthesis and Storage Conditions on the Activity of Candida antarctica Lipase B ZIF-8 Biocomposites

The biomimetic mineralization of zeolitic imidazolate framework-8 (ZIF-8) has been reported as a strategy for enzyme immobilization, enabling the heterogenization and protection of biomacromolecules. Here, we report the preparation of different Candida antarctica lipase B biocomposites (CALB@ZIF-8) formed by altering the concentrations of Zn2+ and 2-methylimidazole (2-mIM). The influence of synthetic conditions on the catalytic activity of the lipase CALB was examined by hydrolysis and transesterification assays in aqueous and organic media, respectively. We demonstrated that for both reactions, activity was retained for the biocomposites formed at low Zn2+/2-mIM ratios but notably almost entirely lost when the ligand concentration used to form the biocomposites was increased. Additionally, phosphate buffer could regenerate the activity of larger particles by degrading the crystal surfaces and releasing encapsulated CALB into solution. Transesterification reactions using CALB@ZIF-8 biocomposites were undertaken in 100% hexane, giving rise to enhanced CALB activity relative to the free enzyme. These observations highlight the fundamental importance of synthetic protocols and operating parameters for developing enzyme@MOF biocomposites with improved activity in challenging conditions.

Regulated Polyelectrolyte Nanogels for Enzyme Encapsulation and Activation

Polyelectrolyte (PE) nanogels consisting of cross-linked PE networks integrate the advanced features of both nanogels and PEs. The soft environment and abundant intrinsic charges are of special interest for enzyme immobilization. However, the crucial factors that regulate enzyme encapsulation and activation remain obscure to date. Herein, we synthesized cationic poly (dimethyl aminoethyl methacrylate), PDMAEMA, nanogels with well-defined size and cross-link degrees and fully investigated the effects of different control factors on lipase immobilization. We demonstrate that the cationic PDMAEMA nanogels indeed enable efficient and safe loading of anionic lipase without disturbing their structures. Strong charge interaction achieved by tuning pH and larger particle size are favorable for lipase loading, while the enhanced enzymatic activity demands nanogels with smaller size and a moderate cross-link degree. As such, PDMAEMA nanogels with a hydrodynamic radius of 35 nm and 30% cross-linker fraction display the optimal catalytic efficiency, which is fourfold of that of free lipase. Moreover, the immobilization endows enhanced enzymatic activity in a broad scope of pH, ionic strength, and temperature, demonstrating effective protection and activation of lipase by the designed nanogels. Our study validates the crucial controls of the size and structure of PE nanogels on enzyme encapsulation and activation, and the revealed findings shall be helpful for designing functional PE nanogels and boosting their applications for enzyme immobilization.

Antioxidative Composites Based on Multienzyme Systems Encapsulated in Metal-Organic Frameworks

Skin is exposed to ultraviolet radiation from the sun constantly, which may induce overproduction of reactive oxygen species (ROS) causing oxidative stress to cells and tissues. Enzymes and small molecules work together to maintain the redox homeostasis, among which superoxide dismutase (SOD) and catalase (CAT) are two kinds of most important antioxidants that suffer from the fragile nature of proteins. Moreover, the proportion of two enzymes used in products must be precisely controlled to reduce the damage caused by the toxic intermediate H₂O₂. Metal-organic frameworks (MOFs) are emerging as promising candidates for multiple enzyme encapsulation due to their high porosity, easy synthesis, and good biocompatibility. Herein, we developed enzyme-MOF composites, SC@ZIF-8, which exhibited an excellent antioxidative activity in vitro. Chemically protective cages formed by MOFs endow the encapsulated enzymes the long-term stability under unnatural conditions in cosmetic and biomedical materials. The pH-dependent protein release profile of SC@ZIF-8 facilitated the successful delivery of enzymes into the cytoplasm to scavenge toxic ROS. The nanocomposites protected human cells from paraquat-induced oxidative stress, paving a new path for the stable and efficient application of antioxidative enzymes in cosmetic and dermatological fields.