Modulating the Biofunctionality of Metal–Organic‐Framework‐Encapsulated Enzymes through Controllable Embedding Patterns

Polymeric nanoparticles—Promising carriers for cancer therapy

Polymeric nanoparticles (NPs) play an important role in controlled cancer drug delivery. Anticancer drugs can be conjugated or encapsulated by polymeric nanocarriers, which are known as polymeric nanomedicine. Polymeric nanomedicine has shown its potential in providing sustained release of drugs with reduced cytotoxicity and modified tumor retention, but until now, few delivery systems loading drugs have been able to meet clinical demands, so more efforts are needed. This research reviews the current state of the cancer drug-loading system by exhibiting a series of published articles that highlight the novelty and functions from a variety of different architectures including micelles, liposomes, dendrimers, polymersomes, hydrogels, and metal–organic frameworks. These may contribute to the development of useful polymeric NPs to achieve different therapeutic purposes.

Hierarchically encapsulating enzymes with multi-shelled metal-organic frameworks for tandem biocatalytic reactions

Biocatalytic transformations in living organisms, such as multi-enzyme catalytic cascades, proceed in different cellular membrane-compartmentalized organelles with high efficiency. Nevertheless, it remains challenging to mimicking biocatalytic cascade processes in natural systems. Herein, we demonstrate that multi-shelled metal-organic frameworks (MOFs) can be used as a hierarchical scaffold to spatially organize enzymes on nanoscale to enhance cascade catalytic efficiency. Encapsulating multi-enzymes with multi-shelled MOFs by epitaxial shell-by-shell overgrowth leads to 5.8~13.5-fold enhancements in catalytic efficiencies compared with free enzymes in solution. Importantly, multi-shelled MOFs can act as a multi-spatial-compartmental nanoreactor that allows physically compartmentalize multiple enzymes in a single MOF nanoparticle for operating incompatible tandem biocatalytic reaction in one pot. Additionally, we use nanoscale Fourier transform infrared (nano-FTIR) spectroscopy to resolve nanoscale heterogeneity of vibrational activity associated to enzymes encapsulated in multi-shelled MOFs. Furthermore, multi-shelled MOFs enable facile control of multi-enzyme positions according to specific tandem reaction routes, in which close positioning of enzyme-1-loaded and enzyme-2-loaded shells along the inner-to-outer shells could effectively facilitate mass transportation to promote efficient tandem biocatalytic reaction. This work is anticipated to shed new light on designing efficient multi-enzyme catalytic cascades to encourage applications in many chemical and pharmaceutical industrial processes.

Mimicking multi-enzyme catalytic cascades in natural systems with spatial organization in confined structures is gaining increasing attention in the emerging field of systems chemistry. Here, the authors demonstrate that multi-shelled metal-organic frameworks can be used as a hierarchical scaffold to spatially organize enzymes on nanoscale to enhance cascade catalytic efficiency.

Rapid Fabrication of Biocomposites by Encapsulating Enzymes into Zn-MOF-74 via a Mild Water-Based Approach

A zinc-based metal organic framework, Zn-MOF-74, which has a unique one-dimensional (1D) channel and nanoscale aperture size, was rapidly obtained in 10 min using a de novo mild water-based system at room temperature, which is an example of green and sustainable chemistry. First, catalase (CAT) enzyme was encapsulated into Zn-MOF-74 (denoted as CAT@Zn-MOF-74), and comparative assays of biocatalysis, size-selective protection, and framework-confined effects were investigated. Electron microscopy and powder X-ray diffraction were used for characterization, while electrophoresis and confocal microscopy confirmed the immobilization of CAT molecules inside the single hexagonal MOF crystals at loading of ∼15 wt %. Furthermore, the CAT@Zn-MOF-74 hybrid was exposed to a denaturing reagent (urea) and proteolytic conditions (proteinase K) to evaluate its efficacy. The encapsulated CAT maintained its catalytic activity in the decomposition of hydrogen peroxide (H2O2), even when exposed to 0.05 M urea and proteinase K, yielding an apparent observed rate constant (kobs) of 6.0 × 10-2 and 6.6 × 10-2 s-1, respectively. In contrast, free CAT exhibited sharply decreased activity under these conditions. Additionally, the bioactivity of CAT@Zn-MOF-74 for H2O2 decomposition was over three times better than that of the biocomposites based on zeolitic imidazolate framework 90 (ZIF-90) owing to the nanometer-scaled apertures, 1D channel, and less confinement effects in Zn-MOF-74 crystallites. To demonstrate the general applicability of this strategy, another enzyme, α-chymotrypsin (CHT), was also encapsulated in Zn-MOF-74 (denoted as CHT@Zn-MOF-74) for action against a substrate larger than H2O2. In particular, CHT@Zn-MOF-74 demonstrated a biological function in the hydrolysis of l-phenylalanine p-nitroanilide (HPNA), the activity of ZIF-90-encapsulated CHT was undetectable due to aperture size limitations. Thus, we not only present a rapid eco-friendly approach for Zn-MOF-74 synthesis but also demonstrate the broader feasibility of enzyme encapsulation in MOFs, which may help to meet the increasing demand for their industrial applications.

A Biocatalytic Cascade in an Ultrastable Mesoporous Hydrogen-Bonded Organic Framework for Point-of-Care Biosensing

Herein, we report the first example of using mesoporous hydrogen-bonded organic frameworks (MHOFs) as the protecting scaffold to organize a biocatalytic cascade. The confined microenvironment of MHOFs has robust and large transport channels, allowing the efficient transport of a wide range of biocatalytic substrates. This new MHOF-confined cascade system shows superior activity, extended scope of catalytic substrates, and ultrahigh stability that enables the operation of complex chemical transformations in a porous carrier. In addition, the advantages of MHOF-confined cascades system for point-of-care biosensing are also demonstrated. This study highlights the advantages of HOFs as scaffold for multiple enzyme assemblies, which has huge potential for mimicking complex cellular transformation networks in a controllable manner.

Sustainable One-Pot Immobilization of Enzymes in/on Metal-Organic Framework Materials

The industrial use of enzymes generally necessitates their immobilization onto solid supports. The well-known high affinity of enzymes for metal-organic framework (MOF) materials, together with the great versatility of MOFs in terms of structure, composition, functionalization and synthetic approaches, has led the scientific community to develop very different strategies for the immobilization of enzymes in/on MOFs. This review focuses on one of these strategies, namely, the one-pot enzyme immobilization within sustainable MOFs, which is particularly enticing as the resultant biocomposite Enzyme@MOFs have the potential to be: (i) prepared in situ, that is, in just one step; (ii) may be synthesized under sustainable conditions: with water as the sole solvent at room temperature with moderate pHs, etc.; (iii) are able to retain high enzyme loading; (iv) have negligible protein leaching; and (v) give enzymatic activities approaching that given by the corresponding free enzymes. Moreover, this methodology seems to be near-universal, as success has been achieved with different MOFs, with different enzymes and for different applications. So far, the metal ions forming the MOF materials have been chosen according to their low price, low toxicity and, of course, their possibility for generating MOFs at room temperature in water, in order to close the cycle of economic, environmental and energy sustainability in the synthesis, application and disposal life cycle.

Metal-Organic Framework-Based Composites for Protein Delivery and Therapeutics

Owing to high bioactivity and specificity, protein drugs have achieved great success in both diagnosis and therapy. The idea of delivering active proteins directly to targets is becoming more and more attractive. However, due to their large size and environmental sensitivity, it is not easy for proteins to maintain bioactivity in extracellular fluids and cross cell membrane without being damaged. A series of techniques and carriers have been developed to deliver proteins. Thereinto, metal-organic frameworks (MOFs), which are made of metal ions connected by organic linkers, are one of the most important vehicles for protein delivery. Their porous structures, stability, biocompatibility, reproducibility, and the possibility for further functionalities offer great potential for protein delivery. In this review, recent developments of protein encapsulated by MOF nanoparticles, including mechanism of treatment, structural design, loading capacity, delivery, and release properties of proteins, are summarized, and the relationship between structure and performance is emphasized. Meanwhile, further improvements and implementations are discussed.

Three-dimensional ordered magnetic macroporous metal-organic frameworks for enzyme immobilization

Metal-organic frameworks (MOFs) have been emerged as a promising support for immobilizing enzymes owing to the tunable porosity, high surface area, and structural diversity. However, most of these possess nanometer size and small pores, which are difficult to recover them from the reaction medium and present low immobilization efficiency and protein loading capacity, and high substrate diffusion limitations. Herein, a novel magnetic amino-functionalized zeolitic imidazolate framework-8 (ZIF-8) with 3D highly ordered macroporous structure was synthesized using the assembled polystyrene (PS) nanosphere monoliths as a template. Subsequently, catalase (CAT) molecules were immobilized on the surface of macroporous magnetic ZIF-8 and inside the macropores by precipitation, covalent binding and cross-linking. The resultant immobilized CAT showed high immobilization efficiency (58%) and protein loading capacity (29%), leading to 500% higher activity than the immobilized CAT on ZIF-8 (CAT/ZIF-8). Meanwhile, the immobilized CAT could be easily recovered with a magnet without obvious activity loss. The traditional CAT/ZIF-8 lost its activity after 6 cycles, whereas, the immobilized CAT retained 90% activity of its initial activity after reusing for 8 cycles, indicating excellent reusability. In conclusion, this study provides a facile and efficient approach to immobilize enzymes on/in MOFs with enhanced activity and excellent recyclability.

MOFs as Potential Matrices in Cyclodextrin Glycosyltransferase Immobilization

Cyclodextrins (CDs) and their derivatives have attracted significant attention in the pharmaceutical, food, and textile industries, which has led to an increased demand for their production. CD is typically produced by the action of cyclodextrin glycosyltransferase (CGTase) on starch. Owing to the relatively high cost of enzymes, the economic feasibility of the entire process strongly depends on the effective retention and recycling of CGTase in the reaction system, while maintaining its stability. CGTase enzymes immobilized on various supports such as porous glass beads or glyoxyl-agarose have been previously used to achieve this objective. Nevertheless, the attachment of biocatalysts on conventional supports is associated with numerous drawbacks, including enzyme leaching prominent in physical adsorption, reduced activity as a result of chemisorption, and increased mass transfer limitations. Recent reports on the successful utilization of metal-organic frameworks (MOFs) as supports for various enzymes suggest that CGTase could be immobilized for enhanced production of CDs. The three-dimensional microenvironment of MOFs could maintain the stability of CGTase while posing minimal diffusional limitations. Moreover, the presence of different functional groups on the surfaces of MOFs could provide multiple points for attachment of CGTase, thereby reducing enzyme loss through leaching. The present review focuses on the advantages MOFs can offer as support for CGTase immobilization as well as their potential for application in CD production.

A colorimetric sensing strategy based on enzyme@metal-organic framework and oxidase-like IrO2/MnO2 nanocomposite for α-glucosidase inhibitor screening

A highly sensitive colorimetric sensing strategy based on enzyme@metal-organic framework (GAA@Cu-MOF) and IrO₂/MnO₂ nanocomposite was exploited innovatively for screening of α-glucosidase (GAA) inhibitors. IrO₂/MnO₂ nanocomposite exhibits excellent oxidase-mimicking activity which can directly catalyze the oxidation of 3,3^,5,5^,-tetramethylbenzidine (TMB) into a blue product with an absorption maximum at 652 nm. And GAA@Cu-MOF can decompose L-ascorbic acid-2-O-α-D-glucopyranosyl (AAG) to ascorbic acid (AA). The produced AA can destroy the IrO₂/MnO₂ nanocomposite and reduce its oxidase-like activity. However, the generation of AA is restricted when GAA inhibitors are added to the system, which allows the oxidase-like activity of the IrO₂/MnO₂ nanocomposite to be maintained. In view of this, a method for screening of GAA inhibitors was developed. In addition to enhancing the stability of GAA, the method can also effectively avoid the potential interference of H₂O₂ in the screening process of GAA inhibitors, which helps to improve the sensitivity of the method. Therefore, highly sensitive determination for acarbose and ascorbic acid are achieved with detection limits of 6.27 nM and 1.23 μM, respectively. The proposed method was successfully applied to screen potential GAA inhibitors from oleanolic acid derivatives. Graphical abstract.

Redox Dyshomeostasis Strategy for Hypoxic Tumor Therapy Based on DNAzyme-Loaded Electrophilic ZIFs

Redox homeostasis is one of the main reasons for reactive oxygen species (ROS) tolerance in hypoxic tumors, limiting ROS-mediated tumor therapy. Proposed herein is a redox dyshomeostasis (RDH) strategy based on a nanoplatform, FeCysPW@ZIF-82@CAT Dz, to disrupt redox homeostasis, and its application to improve ROS-mediated hypoxic tumor therapy. Once endocytosed by tumor cells, the catalase DNAzyme (CAT Dz) loaded zeolitic imidazole framework-82 (ZIF-82@CAT Dz) shell can be degraded into Zn²⁺ as cofactors for CAT Dz mediated CAT silencing and electrophilic ligands for glutathione (GSH) depletion under hypoxia, both of which lead to intracellular RDH and H₂ O₂ accumulation. These "disordered" cells show reduced resistance to ROS and are effectively killed by ferrous cysteine-phosphotungstate (FeCysPW) induced chemodynamic therapy (CDT). In vitro and in vivo data demonstrate that the pH/hypoxia/H₂ O₂ triple stimuli responsive nanocomposite can efficiently kill hypoxic tumors. Overall, the RDH strategy provides a new way of thinking about ROS-mediated treatment of hypoxic tumors.

Metal-Organic Frameworks for Enzyme Immobilization: Beyond Host Matrix Materials

Enzyme immobilization in metal-organic frameworks (MOFs) as a promising strategy is attracting the interest of scientists from different disciplines with the expansion of MOFs' development. Different from other traditional host materials, their unique strengths of high surface areas, large yet adjustable pore sizes, functionalizable pore walls, and diverse architectures make MOFs an ideal platform to investigate hosted enzymes, which is critical to the industrial and commercial process. In addition to the protective function of MOFs, the extensive roles of MOFs in the enzyme immobilization are being well-explored by making full use of their remarkable properties like well-defined structure, high porosity, and tunable functionality. Such development shifts the focus from the exploration of immobilization strategies toward functionalization. Meanwhile, this would undoubtedly contribute to a better understanding of enzymes in regards to the structural transformation after being hosted in a confinement environment, particularly to the orientation and conformation change as well as the interplay between enzyme and matrix MOFs. In this Outlook, we target a comprehensive review of the role diversities of the host matrix MOF based on the current enzyme immobilization research, along with proposing an outlook toward the future development of this field, including the representatives of potential techniques and methodologies being capable of studying the hosted enzymes.

Embedding Functional Biomacromolecules within Peptide-Directed Metal-Organic Framework (MOF) Nanoarchitectures Enables Activity Enhancement

Rationally tailoring a robust artificial coating can enhance the life-time of fragile biomacromolecules. However, the coating also can restrain the activity of the guest because of the decreased substrate accessibility. Herein, we report a peptide-directed strategy that enables in situ tailoring of the MOF-shrouded biohybrids into controllable nanoarchitectures. The MOF biohybrid can be shaped from different 3D microporous architectures into a 2D mesoporous layer by a peptide modulator. Using this mild strategy, we show that the nanoarchitectures of the MOF coatings significantly affect the biological functions of the contained biomacromolecules. The biomacromolecules entrapped within the novel 2D mesoporous spindle-shaped MOFs (2D MSMOFs) have significantly increased bioactivity compared to when encased within the hitherto explored 3D microporous MOFs. The improvement results from the shortened diffusion path and enlarged pore channel in 2D MSMOFs. Meanwhile, the thin 2D MSMOF layer also can provide excellent protection of the hosted biomacromolecules or protein-scaffolded biominerals through structural confinement.

Metal-Organic Framework in Situ Post-Encapsulating DNA-Enzyme Composites on a Magnetic Carrier with High Stability and Reusability

In recent years, metal-organic frameworks (MOFs) have been extensively studied as candidate enzyme immobilization platforms. However, conventional MOF-enzyme composites usually exhibit low controllability and reusability. In this study, a novel and stable strategy for enzyme immobilization was designed by use of ZIF-8 to encapsulate in situ DNA-enzyme composites on the surface of magnetic particles (MPs). The mechanism of in situ encapsulation was discussed in detail. It was found that immobilized enzymes were involved in the growth of ZIF-8, and the DNA cross-linking agents promoted the growth of ZIF-8 on the surface of MP. The thermal, chemical, and physical stabilities of horseradish peroxidase (HRP) were all significantly enhanced after in situ encapsulation. Most importantly, this strategy was proven to be a general platform that can be used to stabilize various proteins. The in situ encapsulation strategy was expanded to immobilize a cascade of enzymes, and ZIF-8@MP_GOx-HRP possessed high selectivity and a wide linear range (25-500 μM) for glucose detection.