"Armor-Plating" Enzymes with Metal-Organic Frameworks (MOFs)

Metal-organic frameworks as advanced materials for sample preparation of bioactive peptides

Development of novel affinity materials and separation techniques is crucial for the progress of modern proteomics and peptidomics. Detection of peptides and proteins from complex matrices still remains a challenging task due to the highly complicated biological composition, low abundance of target molecules, and large dynamic range of proteins. As an emerging area of analytical science, metal-organic framework (MOF)-based separation of proteins and peptides is attracting growing interest. This minireview summarizes the recent advances in MOF-based affinity materials for the sample preparation of proteins and peptides. Some newly emerging MOF nanoreactors for the degradation of peptides and proteins are introduced. An update of MOF-based affinity materials for the isolation of glycopeptides, phosphopeptides and low-abundance endogenous peptides in the last two years is focused on. The separation mechanism is discussed along with the chemical structures of MOFs. Finally, the remaining challenges and future development of MOFs in analyzing peptides and proteins in complicated biological samples 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.

Proline-Modified UIO-66 as Nanocarriers to Enhance Candida rugosa Lipase Catalytic Activity and Stability for Electrochemical Detection of Nitrofen

Immobilization can be used to improve the stability of lipases and enhances lipase recovery and reusability, which increases its commercial value and industrial applications. Nevertheless, immobilization frequently causes conformational changes of the lipases, which decrease lipase catalytic activity. in the present work, we synthesized UIO-66 and grafted UIO-66 crystals with proline for immobilization of Candida rugosa lipase (CRL). As indicated by steady-state fluorescence microscopy, grafting of proline onto UIO-66 crystals induced beneficial conformational change in CRL. CRL immobilized on UIO-66/Pro (CRL@UIO-66/Pro) demonstrated higher enzyme activity and better recyclability than that immobilized on UIO-66 (CRL@UIO-66) in both hydrolysis (CRL@UIO-66/Pro: 0.34 U; CRL@UIO-66: 0.15 U) and transesterification (CRL@UIO-66/Pro: 0.93 U; CRL@UIO-66: 0.25 U) reactions. The higher values of k_cat and k_cat/K_m of CRL@UIO-66/Pro also showed that it had better catalytic efficiency as compared to CRL@UIO-66. It is also worth noting that CRL@UIO-66/Pro (0.93 U) demonstrated a much higher transesterification activity as compared to free CRL (0.11 U), indicating that UIO-66/Pro has increased the solvent stability of CRL. Both CRL@UIO-66 and CRL@UIO-66/Pro were also used for the fabrication of biosensors for nitrofen with a wide linear range (0-100 μM), lower limit of detection, and good recovery rate.

Engineering ribose-5-phosphate isomerase B from a central carbon metabolic enzyme to a promising sugar biocatalyst

Ribose-5-phosphate isomerase B (RpiB) was first identified in the pentose phosphate pathway responsible for the inter-conversion of ribose-5-phosphate and ribulose-5-phosphate. Though there are seldom key enzymes in central carbon metabolic system developed as useful biocatalysts, RpiB with the advantages of wide substrate scope and high stereoselectivity has become a potential biotechnological tool to fulfill the demand of rare sugars currently. In this review, the pivotal roles of RpiB in carbon metabolism are summarized, and their sequence identity and structural similarity are discussed. Substrate binding and catalytic mechanisms are illustrated to provide solid foundations for enzyme engineering. Interesting differences in origin, physiological function, structure, and catalytic mechanism between RpiB and ribose-5-phosphate isomerase A are introduced. Moreover, enzyme engineering efforts for rare sugar production are stressed, and prospects of future development are concluded briefly in the viewpoint of biocatalysis. Aided by the progresses of structural and computational biology, the application of RpiB will be promoted greatly in the preparation of valuable molecules. KEY POINTS: • Detailed illustration of RpiB's vital function in central carbon metabolism. • Potential of RpiB in sequence, substrate scope, and mechanism for application. • Enzyme engineering efforts to promote RpiB in the preparation of rare sugars.

Metal oxide integrated metal organic frameworks (MO@MOF): rational design, fabrication strategy, characterization and emerging photocatalytic applications

This review focuses on the possible synthesis route, characterization techniques, and mechanistic pathways involved in the photocatalytic applications of MO@MOFs.

Iron-Mineralization-Induced Mesoporous Metal-Organic Frameworks Enable High-Efficiency Synergistic Catalysis of Natural/Nanomimic Enzymes

Metal-organic frameworks (MOFs) have become a promising accommodation for enzyme immobilization and protection. However, the integration of multienzymes into MOFs may result in compromise of individual enzymatic activity. In this work, we report an iron mineralization strategy to facilely construct a mesoporous MOF, possessing excellent peroxidase-mimic bioactivity. Furthermore, the feasibility of in situ encapsulating natural enzymes within the developed mesoporous MOF nanozymes endows these natural/nanomimic enzyme hybrids with remarkably enhanced synergistic catalysis ability. Such activity enhancement is mainly due to (1) the fast flux rate of substances through the interconnected mesoporous channels and (2) the simultaneously increased loading amount of enzymes and iron within the MOFs caused by the iron mineralization process.

Encapsulation of Porphyrin-Fe/Cu Complexes into Coordination Space for Enhanced Selective Oxidative Dehydrogenation of Aromatic Hydrazides

The encapsulation of specific nanoentities into hollow nanomaterials derived from metal organic frameworks has attracted continuous and growing research attentions owing to their unique structural properties and unusual synergistic functions. Herein, using the phase transformation of uniform rhombi dodecahedron ZIF-67, hollow nano-shell with a well-defined morphology is successfully prepared. Particularly, the iron-oxygen complex, that is formed by the interaction between TCPP-Fe/Cu (TCPP = tetrakis(4-carboxyphenyl)-porphyrin) and oxygen, can be acted as an ideal proton acceptor for practical organic reactions. Considering the unique adaptability of hollow ZIFs (named HZ) to the transformation of encapsulated TCPP-Fe/Cu bimetallic catalytic active sites, a heterogeneous catalyst (defined as HZ@TCPP-Fe/Cu) through morphology-controlled thermal transformation and rear assemble processes is designed and constructed. Under heterogeneous conditions, HZ@TCPP-Fe/Cu serves as a multifunctional molecular selector to promote the oxidative dehydrogenation of different aromatic hydrazide derivatives with high selectivity toward primary carbon among primary, secondary, and tertiary carbons that are unachievable by other traditional homogeneous catalysts. The high catalytic activity, selectivity, and recyclability of the catalyst proposed here are attractive advantages for an alternative route to the environmentally benign transformation of aromatic hydrazides to aromatic azobenzene.

Integrating Target-Triggered Aptamer-Capped HRP@Metal-Organic Frameworks with a Colorimeter Readout for On-Site Sensitive Detection of Antibiotics

Colorimetric analytical strategies exhibit great promise in developing on-site detection methods for antibiotics, while substantial recent research efforts remain problematic due to dissatisfactory sensitivity. Taking this into account, we develop a novel colorimetric sensor for in-field detection of antibiotics by using aptamer (Apt)-capped and horseradish peroxidise (HRP)-embedded zeolitic metal azolate framework-7 (MAF-7) (Apt/HRP@MAF-7) as target recognition and signal transduction, respectively. With the substrate 3,3',5,5'-tetramethylbenzidine (TMB)-impregnated chip attached on the lid, the assay can be conveniently operated in a tube and reliably quantified by a handheld colorimeter. Hydrophilic MAF-7 can not only prevent HRP aggregation but also enhance HRP activity, which would benefit its detection sensitivity. Besides, the catalytic activity of HRP@MAF-7 can be sealed through assembling with Apt and controllably released based on the bioresponsivity via forming target-Apt complexes. Consequently, a significant color signal can be observed owing to the oxidation of colorless TMB to its blue-green oxidized form oxTMB. As a proof-of-concept, portable detection of streptomycin was favorably achieved with excellent sensitivity, which is superior to most reported methods and commercial kits. The developed strategy affords a new design pattern for developing on-site antibiotics assays and immensely extends the application of enzyme embedded metal-organic framework composites.

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.

Peptide-functionalized metal-organic framework nanocomposite for ultrasensitive detection of secreted protein acidic and rich in cysteine with practical application

In this work, we have prepared peptide-functionalized metal-organic frameworks (MOFs) as signal-amplifying tags for the detection of secreted protein acidic and rich in cysteine (SPARC). Furthermore, enzyme-MOF nanocomposites are fabricated via a coprecipitation strategy between horse radish peroxidase (HRP) and ZIF-90, where ZIF-90 is used as a protective support for HRP immobilization. Meanwhile, the peptide sequence has been designed as SPARC-binding peptide, which imparts biorecognition functionality to HRP@ZIF-90 for performing a colorimetric sensor. Therefore, during the test, HRP molecules can be quickly released from nanocomposites by acidic condition to catalyze chromogenic reaction, enabling the ultrasensitive detection of SPARC with a low detection limit of 30 fg/mL. Moreover, the content of SPARC in colon cancer tissues with different degrees of differentiation can be determined with this sensor, demonstrating that the expression of SPARC is closely related to the occurrence, invasion and metastasis of human colon cancer. These results may show the potential applications of this biosensor in SPARC fundamental research as well as clinical diagnosis in the future.

Metal-Organic Frameworks: A Potential Platform for Enzyme Immobilization and Related Applications

Enzymes, as natural catalysts with remarkable catalytic activity and high region-selectivities, hold great promise in industrial catalysis. However, applications of enzymatic transformation are hampered by the fragility of enzymes in harsh conditions. Recently, metal-organic frameworks (MOFs), due to their high stability and available structural properties, have emerged as a promising platform for enzyme immobilization. Synthetic strategies of enzyme-MOF composites mainly including surface immobilization, covalent linkage, pore entrapment and in situ synthesis. Compared with free enzymes, most immobilized enzymes exhibit enhanced resistance against solvents and high temperatures. Besides, MOFs serving as matrixes for enzyme immobilization show extraordinary superiority in many aspects compared with other supporting materials. The advantages of using MOFs to support enzymes are discussed. To obtain a high enzyme loading capacity and to reduce the diffusion resistance of reactants and products during the reaction, the mesoporous MOFs have been designed and constructed. This review also covers the applications of enzyme-MOF composites in bio-sensing and detection, bio-catalysis, and cancer therapy, which is concerned with interdisciplinary nano-chemistry, material science and medical chemistry. Finally, some perspectives on reservation or enhancement of bio-catalytic activity of enzyme-MOF composites and the future of enzyme immobilization strategies are discussed.

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.

Enzyme immobilization in highly ordered macro–microporous metal–organic frameworks for rapid biodegradation of hazardous dyes

HRP immobilized in highly ordered macro–microporous metal–organic frameworks for biodegradation of hazardous dyes selectively and rapidly.