Fine-Tuning the Micro-Environment to Optimize the Catalytic Activity of Enzymes Immobilized in Multivariate Metal-Organic Frameworks

Protein and peptide confinement within metal-organic materials

Metal-organic materials (MOMs), including both discrete metal-organic cages (MOCs) and metal-organic frameworks (MOFs), are emerging as promising materials for peptide and protein immobilisation. In particular, the ease of synthesis of MOMs alongside their well-defined and modular internal void spaces makes them appealing when considering routes to immobilise and stabilise peptides and proteins outside of biological environments whilst retaining their native activity. Here we review recent advances made in understanding the conformation of peptidic materials confined within MOMs and the enzymes@MOF constructs which show the best enzymatic performance. We highlight opportunities for further advancement in each of these areas and proposed that complementary approaches taken by the MOC and MOF communities might be fruitfully combined to advance our understanding and the development of peptide/protein@MOM applications.

Metal ion of metal-organic frameworks facilitating immobilized chitinase performance

Metal-organic framework (MOF) structures containing different metal ions and 1,3,5-trimesic acid (BTC) were fabricated, as the carriers to immobilize chitinase (Chi) for improving the performance of Chi, for the first time. It was found that the immobilization of Chi on Ca-BTC, Ba-BTC, and Ni-BTC increased its activity. Notably, the specific activity of Chi@Ca-BTC was enhanced by 2.1 times that of free Chi, reaching 16.27 U/mg. The catalytic efficiency and substrate affinity of Chi@Ca-BTC were also remarkably improved, confirmed by the kcat/Km enhancement (4.2 times) and Km value reduction (40 %). Moreover, the half-life of Chi@Ca-BTC at the optimum pH 6.0 and 55 °C was prolonged from 1.8 h to 6.8 h. More than 80 % of enzyme activity was retained after 22 cycles of usage, showing superior reusability. It was proved that Ca2+ on MOF surface enhanced the formation of hydrogen bonds between the catalytic key amino acid Glu-405 and substrate, accelerating the enzymatic catalysis. Then, Chi@Ca-BTC was employed as an outstanding biocatalytic platform for the degradation of chitin, with 8.1 times increase in conversion efficiency. Such a MOF-based immobilization strategy for improving enzymatic performance is of considerable reference value in designing advanced immobilized enzyme for various polysaccharide biocatalytic reaction.

Enzyme-loaded Fe3+-doped ZIF-90 particles as catalytic bioreactor hybrids for operating catalytic cascades

Fe3+-doped ZIF-90 (Fe3+-ZIF-90), a metal-organic framework (MOF), was synthesized and characterized. The MOF particles reveal peroxidase-like activity reflected by catalyzing the H2O2 oxidation of 3,3',5,5'-tetramethylbenzidine, TMB, to TMB˙+. Integration of the two enzymes, β-galactosidase, β-Gal, and glucose oxidase, GOx, in the Fe3+-ZIF-90 provides an organized framework allowing the operation of a three-catalyst cascade, where the β-Gal-catalyzed oxidation of lactose yields glucose and galactose, and the resulting glucose is aerobically oxidized by GOx to gluconic acid and H2O2, followed by the Fe3+-ZIF-90-catalyzed H2O2 oxidation of TMB to TMB˙+. The coupled bienzyme/nanozyme cascade in the MOFs is ca. 5-fold enhanced, as compared to a homogeneous mixture of the catalytic constituents. The enhanced catalytic activity of the enzyme cascades in the MOFs is attributed to the confined reaction framework, allowing product channeling across the multienzyme constituents and overcoming diffusion barriers. Moreover, the enzymes, acetylcholine esterase, AChE, and choline oxidase, ChOx, are encapsulated in the confined porous Fe3+-ZIF-90 particles. The catalytic cascade where the neurotransmitter acetylcholine is hydrolyzed by AChE followed by the stepwise ChOx-catalyzed oxidation of choline to betaine and H2O2, and the Fe3+-ZIF-90-catalyzed oxidation of TMB to colored TMB˙+ by H2O2 is demonstrated. The three-catalyst cascade is ca. 5-fold enhanced as compared to the mixture of separated catalysts. The integrated three-catalyst AChE/ChOx/Fe3+-ZIF-90 particles are applied as colorimetric sensors detecting the neurotransmitter acetylcholine and probing AChE inhibitors. The novelty of the systems is reflected by the assembly of multienzyme catalytic Fe3+-ZIF-90 hybrids in confined environments as bioreactor frameworks driving effective biocatalytic cascades.

Synthesis and Characterisation of Multivariate Metal-Organic Frameworks for Controlled Doxorubicin Absorption and Release

The development of drug carriers with efficient absorption and controlled delivery properties is crucial for advancing medical treatments. Metal-organic frameworks (MOFs) with tunable porosity and a large surface area represent a promising class of materials for this application. Among them, NUIG4 stands out as a biocompatible MOF that exhibits exceptionally high doxorubicin (Dox) absorption (1995 mg dox/g NUIG4) and pH-controlled release properties. In this study, we report the synthesis and characterisation of multivariate MOFs (MV-NUIG4), which are analogues of NUIG4 that maintain the same topology while incorporating different functional groups within their framework. Eight new MV-NUIG4 MOFs have been synthesised through in situ reactions of the corresponding 4-aminobenzoic acid derivative with 4-formylbenzoic acid. The compounds were thoroughly characterised using a range of techniques, including powder X-ray diffraction, infrared spectroscopy, 1H-NMR, and single-crystal X-ray crystallography. The experimental ratio of the reagents and ligand precursors for the synthesis of MV-NUIG4 MOFs matched the ratio of the linkers in the final products. These structures incorporate additional functional groups, such as methyl and hydroxyl, in varying ratios. Computational modelling was used to provide further insight into the crystal structure of the MOFs, revealing a random distribution of the functional groups in the framework. The Dox absorption and release capacity of all analogues were studied, and the results revealed that all analogues displayed high drug absorption in the range of 1234-1995 mg Dox/g MOF. Furthermore, the absorption and release rates of the drug are modulated by the ratio of functional groups, providing a promising approach for controlling drug delivery properties in MOFs.

Enhancing Multi-Enzyme Cascade Activity in Metal-Organic Frameworks via Controlled Enzyme Encapsulation

To position multi-enzymes in a core-shell structure, the conventional layer-by-layer approach is often used. However, this method is time-consuming and complex, requiring multiple steps and the isolation of intermediates at each stage. To address this challenge, a sequential strategy is introduced for the controlled encapsulation of multi-enzymes within metal-organic frameworks (MOFs), achieving a core-shell structure without the need for intermediate isolation. Synchrotron Terahertz-Far-Infrared (THz-Far-IR) spectroscopy is employed to monitor this encapsulation process. The results revealed that the first enzyme is co-precipitated within the MOFs, followed by biomineralization upon the addition of a second enzyme, achieving distinct enzyme positioning. This approach is applicable to both two-enzyme and three-enzyme cascade systems. The results demonstrate that multi-enzyme cascade activity is significantly enhanced compared to conventional one-pot and layer-by-layer approaches, owing to optimal spatial arrangement, increased surface area, and improved enzyme conformation. Furthermore, the encapsulated enzymes exhibit strong resistance to high temperatures, proteolysis, and organic solvents, along with excellent reusability, making this method highly promising for industrial biocatalytic applications.

Advanced enzyme-assembled hydrogels for the remediation of contaminated water

Enzyme-catalyzed biodegradation is an emerging green strategy for environmental remediation, although challenged by high cost and poor robustness. Herein, natural biopolymer (cellulose)-derived hydrogels concurrently doped with β-cyclodextrin and montmorillonite nanosheets that are synthesized in one-step demonstrate exceptional pollutant affinity and mechanical strength. Laccase is then stably and effectively assembled onto the hydrogels by a facile strategy based on charge-assisted H-bonding, which can be extended to other enzymes. The advanced laccase-assembled hydrogels display excellent stability and increased degradation activity achieved by strong substrate capture and rapid electron transfer. The laccase-assembled hydrogels exhibit significantly improved removal (62-fold) and degradation (52-fold) performance compared to free laccase for diverse organic pollutants (e.g., polycyclic aromatic hydrocarbons) in real wastewater. This enhanced performance is maintained despite the presence of heavy metals, other organic chemicals or dissolved organic matter. This work provides a practical strategy for designing an advanced and sustainable biodegradation tool for environmental remediation.

Catalysis Enhanced by Catalyst Wettability

Heterogeneous catalysis is a surface phenomenon where the adsorption, desorption, and transfer of reactants and products are critical for catalytic performance. Recent results show that catalyst wettability is strongly related to the adsorption, desorption, and transfer of reactants and products. In this review, we briefly summarize strategies for regulating wettability to enrich reactants, accelerate the desorption of products, and promote mass transfer in heterogeneous catalysis. In addition, we explore insights into catalyst wettability for the enhancement of catalytic performance. Finally, the concerns and challenges in this subject are outlined, and practical strategies are proposed for the regulation of catalyst wettability. We hope that this review will be helpful for designing highly efficient heterogeneous catalysts in the future.

Combining Hard Shell with Soft Core to Enhance Enzyme Activity and Resist External Disturbances

Immobilizing enzymes onto solid supports having enhanced catalytic activity and resistance to harsh external conditions is considered as a promising and critical method of broadening enzymatic applications in biosensing, biocatalysis, and biomedical devices; however, it is considerably hampered by limited strategies. Here, a core-shell strategy involving a soft-core hexahistidine metal assembly (HmA) is innovatively developed and characterized with encapsulated enzymes (catalase (CAT), horseradish peroxidase, glucose oxidase (GOx), and cascade enzymes (CAT+GOx)) and hard porous shells (zeolitic imidazolate framework (ZIF), ZIF-8, ZIF-67, ZIF-90, calcium carbonate, and hydroxyapatite). The enzyme-friendly environment provided by the embedded HmA proves beneficial for enhanced catalytic activity, which is particularly effective in preserving fragile enzymes that will have been deactivated without the HmA core during the mineralization of porous shells. The enzyme encapsulated within a core-shell particle exhibits noteworthy resilience against harsh external conditions, including heat, organic solvents, and proteinase K. Additionally, no significant alteration in the catalytic behavior of the enzyme is observed after multiple cycles of usage. This study offers a novel approach for immobilizing enzymes and rendering them resistant to harsh external conditions, with potential applications in diverse fields, including biocatalysis, bioremediation, and biomedical engineering.

High-Entropy Zeolitic Imidazolate Frameworks for Dynamic Hydrogen Isotope Separation

Entropy-driven strategy enables the systematic design of complex systems by using entropy as a quantifiable design parameter for the degree of mixing. In this study, we present mixed-linker zeolitic imidazolate frameworks (ZIFs), sod-ZIF-1 series, that features two types of six-membered rings (6MRs) with aperture sizes of 3.4 Å and 1.7 Å. By adjusting the configurational entropy, the ratio of these 6MRs can be systematically controlled, which significantly influences adsorptive properties, particularly improving the H2 affinity about 3 times throughout the series. This results in the significant enhancement of retention times in dynamic separation of hydrogen isotopes (D2/H2), even over LNG liquefaction temperature. This approach to entropy-driven pore engineering provides new opportunities for enhancing gas adsorption and separation processes.

Integrating Enzymes with Reticular Frameworks To Govern Biocatalysis

Integrating enzymes with reticular frameworks offers promising avenues for access to functionally tailorable biocatalysis. This Minireview explores recent advances in enzyme-reticular framework hybrid biocomposites, focusing on the utilization of porous reticular frameworks, including metal-organic frameworks, covalent-organic frameworks, and hydrogen-bonded organic frameworks, to regulate the reactivity of an enzyme encapsulated inside mainly by pore infiltration and in situ encapsulation strategies. We highlight how pore engineering and host-guest interfacial interactions within reticular frameworks create tailored microenvironments that substantially impact the mass transfer and enzyme conformation, leading to biocatalytic rate enhancement, or imparting enzymes with non-native biocatalytic functions, including substrate selectivity and new activity. Additionally, the feasibility of leveraging the photothermal effect of a framework to optimize the local reaction temperature and photoelectric effect to elicit diverse photoenzyme-coupled reactions is also summarized in detail, which can expand the functional repertoire of biocatalytic transformations under light irradiation. This Minireview underscores the potential of reticular frameworks as tunable and reliable platforms to govern biocatalysis, offering pathways for engineering sustainable, efficient, and selective biocatalytic reactors in pharmaceutical, environmental, and energy-related applications.

Immobilization of snailase and β-glucosidase on L-aspartic acid-modified magnetic amorphous ZIF for efficiently and sustainably producing ginsenoside compound K

Improving the catalytic efficiency and recyclability of immobilized enzyme remained a serious challenge in industrial applications. Enzyme immobilization in the amorphous zeolite imidazolate framework (aZIF) preserved high enzyme activity, but still faced separation difficulties and a low catalytic efficiency in practice. In this study, a one-pot co-precipitation method was used to form the enzyme-aZIF/magnetic nanoparticle (MNP) biocomposite by rapidly precipitating snailase (Sna) and β-glucosidase (β-G) with metal/ligand on MNP and modifying with L-aspartic acid (Asp). Thanks to Asp modification protecting the natural conformation of internal protein molecules and MNP stabilizing the conformation of active enzymes after immobilizing, Sna&β-G in the carrier had more stable conformations and higher catalytic efficiency than those in conventional ZIF-8, increasing the catalytic efficiency for converting ginsenoside Rb1 to rare ginsenoside compound K (CK) to 79.16 %. Moreover, while improving the stability of Sna&β-G, owing to the magnetism imparted by MNP, the immobilized enzyme maintained high enzyme activity and recovery after 7 cycles by rapid magnetic separation. The results provided guidance for developing immobilized Sna&β-G biocomposites with ideal catalytic efficiency and easy recovery to catalyze ginsenoside Rb1 to rare ginsenoside CK.

Trends in conductive MOFs for sensing: A review

Metal-organic frameworks (MOFs) are porous, ordered arrays formed by coordination bonds between organic ligands and metal ions or clusters. The highly tunable properties of the MOF structure and performance make it possible to meet the needs of many applications. Conductive MOFs are essential in the domain of sensing due to their electrical conductivity, porosity, and catalytic properties, offering an effective platform for detection. Numerous sensing devices that utilize conductive MOFs have been created. This text presents a thorough overview of the diverse applications of conductive MOFs within the sensing field. The results of this work provide insights into the properties and synthesis methods of conductive MOFs and the working mechanisms of sensors based on conductive MOFs, which will help to deepen the study of such materials, provide a new vision for the design and production of novel functional materials, and promote the development and application of sensors based on conductive MOFs.

Host molecules inside metal-organic frameworks: host@MOF and guest@host@MOF (Matrjoschka) materials

The controllable encapsulation of host molecules (such as porphyrin, phthalocyanine, crown ether, calixarene or cucurbituril organic macrocycles, cages, metal-organic polyhedrons and enzymes) into the pores of metal-organic frameworks (MOFs) to form host-in-host (host@MOF) materials has attracted increasing research interest in various fields. These host@MOF materials combine the merits of MOFs as a host matrix and functional host molecules to exhibit synergistic functionalities for the formation of guest@host@MOF materials in sorption and separation, ion capture, catalysis, proton/ion conduction and biosensors. (This guest@host@MOF construction is reminiscent of Russian (Matrjoschka) dolls which are nested dolls of decreasing size placed one inside another.) In this tutorial review, the advantages of MOFs as a host matrix are presented; the encapsulation approaches and general important considerations for the preparation of host@MOF materials are introduced. The state-of-the-art examples of these materials based on different host molecules are shown, and representative applications and general characterization of these materials are discussed. This review will guide researchers attempting to design functional host@MOF and guest@host@MOF materials for various applications.

In Situ Encapsulation of Atomically Precise Nanoclusters in Reticular Frameworks via Mechanochemical Synthesis

The combination of atomically precise nanoclusters (APNCs) and reticular frameworks is promising for generating component-specific nanocomposites with emergent properties. However, traditional liquid-phase synthesis often hampers this potential by damaging APNCs and limiting combination diversity. Here, mechanochemical synthesis to explore the encapsulation of diverse oil- and water-soluble APNCs within various reticular frameworks is employed, establishing a database of 21 unique APNC-framework combinations, including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), and multivariate MOFs. These framework coatings not only spatially immobilize APNCs but also secure their structures, preventing aggregation and degradation while enhancing stability and activity. Encapsulating Au25 in HOFs resulted in a remarkable 315-fold increase in catalytic activity compared to Au25 homogeneous catalyst, highlighting the framework's crucial role in catalytic enhancement. The mechanochemical synthesis strategy facilitates tailored support screening, catering to specific needs, and shows promise for developing multifunctional systems, including enzyme-APNC@frameworks material for cascade reactions.

Unlocking of Hidden Mesopores for Enzyme Encapsulation by Dynamic Linkers in Stable Metal-Organic Frameworks

Mesoporous metal-organic frameworks (MOFs) are promising supports for the immobilization of enzymes, yet their applications are often limited by small pore apertures that constrain the size of encapsulated enzymes to below 5 nm. In this study, we introduced labile linkers (4,4',4''-(2,4,6-boroxintriyl)-tribenzoate, TBTB) with dynamic boroxine bonds into mesoporous PCN-333, resulting in PCN-333-TBTB with enhanced enzyme loading and protection capabilities. The selective breaking of B-O bonds creates defects in PCN-333, which effectively expands both window and cavity sizes, thereby unlocking hidden mesopores for enzyme encapsulation. Consequently, this strategy not only increases the adsorption kinetics of small enzymes (<5 nm) such as cytochrome c (Cyt C) and horseradish peroxidase (HRP), but also enables the immobilization of various large-sized enzymes (>5 nm), such as glycoenzymes. The glycoenzymes@PCN-333-TBTB platform was successfully applied to synthesize thirteen complex oligosaccharides and polysaccharides, demonstrating high activity and enhanced enzyme stability. The dynamic linker-mediated enzyme encapsulation strategy enables the immobilization of enzymes exceeding the inherent pore size of MOFs, thus broadening the scope of enzymatic catalytic reactions achievable with MOF materials.

Engineering enzyme conformation within liquid-solid hybrid microreactors for enhanced continuous-flow biocatalysis

The artificial engineering of an enzyme's structural conformation and dynamic properties to promote its catalytic activity and stability outside cellular environments is highly pursued in industrial biotechnology. Here, we describe an elegant strategy of combining the rationally designed liquid-solid hybrid microreactor with a tailor-made polyethylene glycol functional ionic liquid (PEG-IL) microenvironment to exercise a high level of control over the configuration of enzymes for practical continuous-flow biocatalysis. As exemplified by a lipase driven kinetic resolution reaction, the obtained system exhibits a 2.70 to 30.35-fold activity enhancement compared to their batch or traditional IL-based counterparts. Also, our results demonstrate that the thermal stability of encapsulated lipase can be significantly strengthened in the presence of PEG groups, showcasing a long-term continuous-flow stability even up to 1000 h at evaluated temperature of 60 oC. Through systematic experiment and molecular dynamics simulation studies, the conformational changes of the active site cavity in the modified lipases are correlated with enzymatic properties alteration, and the pronounced effects of PEG-groups in stabilizing enzyme's secondary structures by delaying unfolding at elevated temperatures are identified. We believe that this study will guide the design of high-performance enzymatic systems, promoting their utilization in real-world biocatalysis applications.

Reticular Chemistry for Enhancing Bioentity Stability and Functional Performance

Addressing the fragility of bioentities that results in instability and compromised performance during storage and applications, reticular chemistry, specifically through metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), offers versatile platforms for stabilization and enhancement of bioentities. These highly porous frameworks facilitate efficient loading and mass transfer, offer confined environments and selective permeability for stabilization and protection, and enable finely tunable biointerfacial interactions and microenvironments for function optimization, significantly broadening the applications of various bioentities, including enzymes, nucleic acids, cells, etc. This Perspective outlines strategies for integrating bioentities with reticular frameworks, highlighting new design ideas for existing issues within these strategies. It emphasizes the crucial roles of these frameworks for bioentities in enhancing stability, boosting activity, imparting non-native functions, and synergizing bioentity systems. Concluding with a discussion of the challenges and prospects in the design, characterization, and practical applications of these biocomposites, this Perspective aims to inspire further development of high-performance biocomposites in this promising field.

Insight into the efficient loading and enhanced activity of enzymes immobilized on functionalized UiO-66

Enzyme immobilization is an effective strategy for achieving efficient and sustainable enzyme catalysis. As a kind of promising enzyme-loading materials, the systematic research on zirconium based metal organic frameworks (Zr-MOFs) about immobilization performance at molecular level is still in its initial stage. In this work, UiO-66 was functionalized with various groups (-H, -NH2, -COOH, -OH, -2OH) for the immobilization of cytochrome c (Cyt c) and antioxidant enzyme catalase (CAT). Then the effects of surface-functionalized UiO-66 derivatives on the loading efficiency, enzyme stability and catalysis kinetics were systematically investigated. In addition, the affinity constants of Cyt c and CAT towards UiO-66-series MOFs carriers were also compared. The results have shown that hydroxyl group functionalized UiO-66 represents the highest enzyme loading capacity, enhanced activity and improved stability for Cyt c and CAT possibly due to high surface area and suitable microenvironments as well as enhanced affinity towards the enzymes provided by the introduction of a single hydroxyl group. Our research would foresee immense potential of MOFs in engineering biocatalysts.

Base on photothermal interfacial molecular transfer for efficient biodiesel catalysis via enzyme@cyclodextrin metal-organic frameworks loaded MXene

Efficient, green and stable catalysis has always been the core concept of enzyme catalysis in industrial processes for manufacturing. Therefore, we construct a new strategy with photothermal interfacial molecular transfer for green and efficient biodiesel catalysis. We encapsulate Candida albicans lipase B (CalB) in a γ-cyclodextrin metal-organic framework (γ-CD-MOF) loading with Ti3C2TX by in situ growth and electrostatic assembly. The γ-CD-MOF not only protects the fragile enzyme, but also enhances the catalytic performance through the synergistic effects of porous adsorption (MOF pore structure) and interfacial enrichment (cyclodextrins host-guest assembly structure) for accelerating substrate transfer (642.6 %). The CalB@γ-CD-MOF/MXene-i activity can be regulated up to 274.6 % by exposure to near-infrared (NIR). Importantly, CalB@γ-CD-MOF/MXene-i achieves 93.3 % biodiesel conversion under NIR and maintained 86.9 % activity after 6 cycles. Meanwhile, the MXene after the CalB@γ-CD-MOF/MXene catalytic cycle can be almost completely recovered. We verify the mechanism of high catalytic activity of γ-CD-MOF and rationalize the mechanism of CD molecular channel by DFT. Therefore, this highly selective enzyme catalytic platform offers new possibilities for green and efficient preparation of bioenergy.

In Situ Biomineralization and Citric Acid Etching Strategy for Enhancing Activity of Immobilized Acetylcholinesterase

Enhancing the structural stability of an enzyme and maintaining its catalytic activity are effective ways to improve enzyme utilization and reduce the cost of drug screening. However, immobilized enzyme activity tends to decrease in existing immobilization techniques due to conformational changes and microenvironmental restrictions. In this paper, we present a facile approach to prepare immobilized acetylcholinesterase (AChE) with high activity by a ZIF-8 in situ immobilization and citric acid (CA) etching strategy. CA breaks the coordination bond of ZIF-8 and produces defects, expanding the pore space, improving substrate accessibility, and fully exposing the active site of the enzyme. The enhancement of the catalytic activity of AChE@ZIF-8-CA was about 6.10-fold compared with the free enzyme. In addition, AChE@ZIF-8-CA exhibited an excellent encapsulation efficiency and good tolerance to temperature, pH, and organic solvents. The relative activity remains at the initial 83.77% even in five repeated experiments. The strategy provides a novel and efficient way to quickly construct highly active immobilized enzymes under mild conditions.

Establishing cell suitability for high-level production of licorice triterpenoids in yeast

Yeast has been an indispensable host for synthesizing complex plant-derived natural compounds, yet the yields remained largely constrained. This limitation mainly arises from overlooking the importance of cell and pathway suitability during the optimization of enzymes and pathways. Herein, beyond conventional enzyme engineering, we dissected metabolic suitability with a framework for simultaneously augmenting cofactors and carbon flux to enhance the biosynthesis of heterogenous triterpenoids. We further developed phospholipid microenvironment engineering strategies, dramatically improving yeast's suitability for the high performance of endoplasmic reticulum (ER)-localized, rate-limiting plant P450s. Combining metabolic and microenvironment suitability by manipulating only three genes, NHMGR (NADH-dependent HMG-CoA reductase), SIP4 (a DNA-binding transcription factor)and GPP1 (Glycerol-1-phosphate phosphohydrolase 1), we enabled the high-level production of 4.92 g/L rare licorice triterpenoids derived from consecutive oxidation of β-amyrin by two P450 enzymes after fermentation optimization. This production holds substantial commercial value, highlighting the critical role of establishing cell suitability in enhancing triterpenoid biosynthesis and offering a versatile framework applicable to various plant natural product biosynthetic pathways.

Machine Learning Optimizing Enzyme/ZIF Biocomposites for Enhanced Encapsulation Efficiency and Bioactivity

In this study, we present the first example of using a machine learning (ML)-assisted design strategy to optimize the synthesis formulation of enzyme/ZIFs (zeolitic imidazolate framework) for enhanced performance. Glucose oxidase (GOx) and horseradish peroxidase (HRP) were chosen as model enzymes, while Zn(eIM)2 (eIM = 2-ethylimidazolate) was selected as the model ZIF to test our ML-assisted workflow paradigm. Through an iterative ML-driven training-design-synthesis-measurement workflow, we efficiently discovered GOx/ZIF (G151) and HRP/ZIF (H150) with their overall performance index (OPI) values (OPI represents the product of encapsulation efficiency (E in %), retained enzymatic activity (A in %), and thermal stability (T in %)) at least 1.3 times higher than those in systematic seed data studies. Furthermore, advanced statistical methods derived from the trained random forest model qualitatively and quantitatively reveal the relationship among synthesis, structure, and performance in the enzyme/ZIF system, offering valuable guidance for future studies on enzyme/ZIFs. Overall, our proposed ML-assisted design strategy holds promise for accelerating the development of enzyme/ZIFs and other enzyme immobilization systems for biocatalysis applications and beyond, including drug delivery and sensing, among others.

Molecular simulations guide immobilization of lipase on nest-like ZIFs with regulatable hydrophilic/hydrophobic surface

The catalytic performance of immobilized lipase is greatly influenced by functional support, which attracts growing interest for designing supports to achieve their promotive catalytic activity. Many lipases bind strongly to hydrophobic surfaces where they undergo interfacial activation. Herein, the behavioral differences of lipases with distinct lid structures on interfaces of varying hydrophobicity levels were firstly investigated by molecular simulations. It was found that a reasonable hydrophilic/hydrophobic surface could facilitate the lipase to undergo interfacial activation. Building on these findings, a novel "nest"-like superhydrophobic ZIFs (ZIFN) composed of hydrophobic ligands was prepared for the first time and used to immobilize lipase from Aspergillus oryzae (AOL@ZIFN). The AOL@ZIFN exhibited 2.0-folds higher activity than free lipase in the hydrolysis of p-Nitrophenyl palmitate (p-NPP). Especially, the modification of superhydrophobic ZIFN with an appropriate amount of hydrophilic tannic acid can significantly improve the activity of the immobilized lipase (AOL@ZIFN-TA). The AOL@ZIFN-TA exhibited 30-folds higher activity than free lipase, and still maintained 82% of its initial activity after 5 consecutive cycles, indicating good reusability. These results demonstrated that nanomaterials with rational arrangement of the hydrophilic/hydrophobic surface could facilitate the lipase to undergo interfacial activation and improve its activity, displaying the potential of the extensive application.

Insights on the role of blocking agent on the properties of the lipase from Thermomyces lanuginosus immobilized on heterofunctional support for hydroesterification reactions

Here, we report a study of the effect of the blocking agent on the properties of the lipase from Thermomyces lanuginosus (TLL) immobilized on a heterofunctional support (Purolite C18-ethylnediamina (EDA)- vinyl sulfone (VS)-TLL-blocking agent) in different reactions. The performance of the biocatalysts was compared to those immobilized on standard hydrophobic support (Purolite C18-TLL) and the commercial one (TLL-IM). The nature of the blocking agent (Cys, Gly and Asp) altered the enzyme features. TLL-IM always gave a comparatively worse performance, with its specificity for the oil being very different to the Purolite biocatalysts. Under optimized conditions, Purolite C18-TLL yielded 97 % of hydrolysis conversion after 4 h using a water/waste cooking soybean oil (WCSO) mass ratio of 4.3, biocatalyst load of 6.5 wt% and a temperature of 44.2 °C (without buffer or emulsification agent). In esterification reactions of the purified free fatty acids (FFAs) obtained from WCSO, the best TLL biocatalysts depended on the utilized alcohol: linear amyl alcohol was preferred by Purolite C18-TLL and Purolite C18-EDA-VS-TLL-Gly, while higher activity was achieved utilizing isoamyl alcohol as nucleophile by Purolite C18-EDA-VS-TLL-Cys, Purolite C18-EDA-VS-TLL-Asp and IM-TLL as catalysts. All the results indicate the influence of the blocking step on the final biocatalyst features.

Multivariate Silicification-Assisted Single Enzyme Structure Augmentation for Improved Enzymatic Activity-Stability Trade-Off

The ability to finely tune/balance the structure and rigidity of enzymes to realize both high enzymatic activity and long-term stability is highly desired but highly challenging. Herein, we propose the concept of the "silicazyme", where solid inorganic silica undergoes controlled hybridization with the fragile enzyme under moderate conditions at the single-enzyme level, thus enabling simultaneous structure augmentation, long-term stability, and high enzymatic activity preservation. A multivariate silicification approach was utilized and occurred around individual enzymes to allow conformal coating. To realize a high activity-stability trade-off the structure flexibility/rigidity of the silicazyme was optimized by a component adjustment ternary (CAT) plot method. Moreover, the multivariate organosilica frameworks bring great advantages, including surface microenvironment adjustability, reversible modification capability, and functional extensibility through the rich chemistry of silica. Overall silicazymes represent a new class of enzymes with promise for catalysis, separations, and nanomedicine.

A Hydrophilic Metal Azolate Coordination Polymer for In Situ Encapsulation of Haloalkane Dehalogenase with Enhanced Enzymatic Performance

Encapsulating enzymes within metal-organic frameworks such as zeolitic imidazolate framework-8 (ZIF-8) has been demonstrated to enhance enzymatic performance under harsh conditions. However, by computer-aided analysis, we revealed that highly hydrophobic organic ligands and unfavorable metal ions could greatly impair the activity of haloalkane dehalogenase DhaA by directly interacting with the catalytic sites, causing an extremely low activity of DhaA after encapsulating within ZIF-8. We also found that the presence of a protecting polymer could protect DhaA from the damage of organic ligands and metal ions and that a positively charged amino acid could increase the DhaA activity. Based on the simulations and experimental observations, we have designed to coencapsulate DhaA with poly(vinylpyrrolidone) (PVP) and lysine (Lys) within the amorphous Co-based metal azolate coordination polymer (CoCP). The as-prepared immobilized enzyme (DhaA/PVP/Lys@CoCP) exhibited significantly increased activity (91.5 times higher than that of DhaA@ZIF-8), dramatically enhanced thermostability at 50-70 °C, greatly improved catalytic performance in several organic solvent solutions, and good recyclability (over 75% of the initial activity after 10 cycles). The superiority of the immobilized enzyme was also demonstrated with a substrate frequently detected in the real world. In addition to the protective effect of PVP and positive effect of Lys, experimental and computational investigations unveiled other two favorable aspects that contributed to the enhanced enzymatic performance: (1) high hydrophilicity of the immobilization material and (2) the use of Co2+ with a minimal negative effect on DhaA. The research has thus provided a promising immobilized DhaA with favorable catalytic performance and great potential in industrial applications.

A Synergetic Pore Compartmentalization and Hydrophobization Strategy for Synchronously Boosting the Stability and Activity of Enzyme

Spatial immobilization of fragile enzymes using a nanocarrier is an efficient means to design heterogeneous biocatalysts, presenting superior stability and recyclability to pristine enzymes. An immobilized enzyme, however, usually compromises its catalytic activity because of inevasible mass transfer issues and the unfavorable conformation changes in a confined environment. Here, we describe a synergetic metal-organic framework pore-engineering strategy to trap lipase (an important hydrolase), which confers lipase-boosted stability and activity simultaneously. The hierarchically porous NU-1003, featuring interconnected mesopore and micropore channels, is precisely modified by chain-adjustable fatty acids on its mesopore channel, into which lipase is trapped. The interconnected pore structure ensures efficient communication between trapped lipase and exterior media, while the fatty acid-mediated hydrophobic pore can activate the opening conformation of lipase by interfacial interaction. Such dual pore compartmentalization and hydrophobization activation effects render the catalytic center of trapped lipase highly accessible, resulting in 1.57-fold and 2.46-fold activities as native lipase on ester hydrolysis and enantioselective catalysis. In addition, the feasibility of these heterogeneous biocatalysts for kinetic resolution of enantiomer is also validated, showing much higher efficiency than native lipase.

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.

A Multienzyme Cascade Pathway Immobilized in a Hydrogen-Bonded Organic Framework for the Conversion of CO2

The reduction of carbon dioxide to valuable chemicals through enzymatic processes is regarded as a promising approach for the reduction of carbon dioxide emissions. In this study, an in vitro multi-enzyme cascade pathway is constructed for the conversion of CO2 into dihydroxyacetone (DHA). This pathway, known as FFFP, comprises formate dehydrogenase (FDH), formaldehyde dehydrogenase (FaldDH), formolase (FLS), and phosphite dehydrogenase (PTDH), with PTDH serving as the critical catalyst for regenerating the coenzyme NADH. Subsequently, the immobilization of the FFFP pathway within the hydrogen-bonded organic framework (HOF-101) is accomplished in situ. A 1.8-fold increase in DHA yield is observed in FFFP@HOF-101 compared to the free FFFP pathway. This enhancement can be explained by the fact that within FFFP@HOF-101, enzymes are positioned sufficiently close to one another, leading to the elevation of the local concentration of intermediates and an improvement in mass transfer efficiency. Moreover, FFFP@HOF-101 displays a high degree of stability. In addition to the establishment of an effective DHA production method, innovative concepts for the tailored synthesis of fine compounds from CO2 through the utilization of various multi-enzyme cascade developments are generated by this work.

Boosting Enzyme Activity in Enzyme Metal-Organic Framework Composites

Enzymes, as highly efficient biocatalysts, excel in catalyzing diverse reactions with exceptional activity and selective properties under mild conditions. Nonetheless, their broad applications are hindered by their inherent fragility, including low thermal stability, limited pH tolerance, and sensitivity to organic solvents and denaturants. Encapsulating enzymes within metal-organic frameworks (MOFs) can protect them from denaturation in these harsh environments. However, this often leads to a compromised enzyme activity. In recent years, extensive research efforts have been dedicated to enhancing enzymatic activity within MOFs, leading to the development of new enzyme-MOF composites that not only preserve their catalytic potential but also outperform their free counterparts. This Review provides a comprehensive review on recent developments in enzyme-MOF composites with a specific emphasis on their enhanced enzymatic activity compared to free enzymes.

DNA-Directed Assembly of Hierarchical MOF-Cellulose Nanofiber Microbioreactors with "Branch-Fruit" Structures

Assembling metal-organic frameworks (MOFs) into ordered multidimensional porous superstructures promises the encapsulation of enzymes for heterogeneous biocatalysts. However, the full potential of this approach has been limited by the poor stability of enzymes and the uncontrolled assembly of MOF nanoparticles onto suitable supports. In this study, a novel and exceptionally robust Ni-imidazole-based MOF was synthesized in water at room temperature, enabling in situ enzyme encapsulation. Based on this MOF platform, we developed a DNA-directed assembly strategy to achieve the uniform placement of MOF nanoparticles onto bacterial cellulose nanofibers, resulting in a distinctive "branch-fruit" structure. The resulting hybrid materials demonstrated remarkable versatility across various catalytic systems, accommodating natural enzymes, nanoenzymes, and multienzyme cascades, thus showcasing enormous potential as universal microbioreactors. Furthermore, the hierarchical composites facilitated rapid diffusion of the bulky substrate while maintaining the enzyme stability, with ∼3.5-fold higher relative activity compared to the traditional enzyme@MOF immobilized in bacterial cellulose nanofibers.

Highly Enantioselective Catalysis by Enzyme Encapsulated in Metal Azolate Frameworks with Micelle-Controlled Pore Sizes

Encapsulating enzymes within metal-organic frameworks has enhanced their structural stability and interface tunability for catalysis. However, the small apertures of the frameworks restrict their effectiveness to small organic molecules. Herein, we present a green strategy directed by visible linker micelles for the aqueous synthesis of MAF-6 that enables enzymes for the catalytic asymmetric synthesis of chiral molecules. Due to the large pore aperture (7.6 Å), double the aperture size of benchmark ZIF-8 (3.4 Å), MAF-6 allows encapsulated enzyme BCL to access larger substrates and do so faster. Through the optimization of surfactants' effect during synthesis, BCL@MAF-6-SDS (SDS = sodium dodecyl sulfate) displayed a catalytic efficiency (Kcat/Km) that was 420 times greater than that of BCL@ZIF-8. This biocomposite efficiently catalyzed the synthesis of drug precursor molecules with 94-99% enantioselectivity and nearly quantitative yields. These findings represent a deeper understanding of de novo synthetic encapsulation of enzyme in MOFs, thereby unfolding the great potential of enzyme@MAF catalysts for asymmetric synthesis of organics and pharmaceuticals.

Biomimetic Mineralization of Large Enzymes Utilizing a Stable Zirconium-Based Metal-Organic Frameworks

Enzymes are natural catalysts for a wide range of metabolic chemical transformations, including selective hydrolysis, oxidation, and phosphorylation. Herein, we demonstrate a strategy for the encapsulation of enzymes within a highly stable zirconium-based metal-organic framework. UiO-66-F4 was synthesized under mild conditions using an enzyme-compatible amino acid modulator, serine, at a modest temperature in an aqueous solution. Enzyme@UiO-66-F4 biocomposites were then formed by an in situ encapsulation route in which UiO-66-F4 grows around the enzymes and, consequently, provides protection for the enzymes. A range of enzymes, namely, lysozyme, horseradish peroxidase, and amano lipase, were successfully encapsulated within UiO-66-F4. We further demonstrate that the resulting biocomposites are stable under conditions that could denature many enzymes. Horseradish peroxidase encapsulated within UiO-66-F4 maintained its biological activity even after being treated with the proteolytic enzyme pepsin and heated at 60 °C. This strategy expands the toolbox of potential metal-organic frameworks with different topologies or functionalities that can be used as enzyme encapsulation hosts. We also demonstrate that this versatile process of in situ encapsulation of enzymes under mild conditions (i.e., submerged in water and at a modest temperature) can be generalized to encapsulate enzymes of various sizes within UiO-66-F4 while protecting them from harsh conditions (i.e., high temperatures, contact with denaturants or organic solvents).

Metal-pyrimidine nanocubes immobilized enzymes with pH-switchable multienzyme-like activity for broad-pH-responsive sensing assay for organophosphorus pesticides

Peroxidase (POD)-like can only function in acidic environments and the pH mismatch restricts the application of enzyme-nanozyme cascade catalytic sensing platforms in the broad-pH-responsive assay for organophosphorus pesticides (OPs). Herein, the metal-pyrimidine nanocubes (MPNCs) with intrinsic pH-switchable POD-like and catalase (CAT)-like properties were synthesized via the coordination of pyrimidin-2-ol with Cu2+. Meanwhile, acetylcholinesterase (AChE) and choline oxidase (CHO) were simultaneously encapsulated in MPNCs to construct an enzyme-nanozyme cascade catalytic platform (AChE/CHO@MPNCs). AChE/CHO@MPNCs could catalyze the hydrolysis of acetylcholine to choline, which was subsequently converted to H2O2. The POD-like activity of MPNCs was dominant under acidic conditions, while the CAT-like activity prevailed under neutral and alkaline conditions, which could catalyze H2O2 to •OH and O2, respectively, then oxidizing dopamine (DA) to polydopamine quantum dots (PDA QDs) with different fluorescence characteristics. Consequently, OPs could be detected in a linear range from 0.05 to 1000 nM with a LOD of 0.015 nM in acidic environments and a linear range from 0.05 to 500 nM with a LOD of 0.023 nM in alkaline environments. Overall, our work expands the horizon of constructing enzyme@MOFs composites with high catalytic activity. Meanwhile, the intrinsic pH-switchable multienzyme-like property opens avenues to construct sensing platforms with broad-pH-responsive for OPs and other analytes detection.

Integration of Enzyme and Covalent Organic Frameworks: From Rational Design to Applications

Manufacturing is undergoing profound transformations, among which green biomanufacturing with low energy consumption, high efficiency, and sustainability is becoming one of the major trends. However, enzymes, as the "core chip" of biomanufacturing, are often handicapped in their application by their high cost, low operational stability, and nonreusability. Immobilization of enzymes is a technology that binds or restricts enzymes in a certain area with solid materials, allows them to still carry out their unique catalytic reaction, and allows them to be recycled and reused. Compared with free enzymes, immobilized enzymes boast numerous advantages such as enhanced storage stability, ease of separation, reusability, and controlled operation. Currently, commonly used supports for enzyme immobilization (e.g., mesoporous silica, sol-gel hydrogels, and porous polymer) can effectively improve enzyme stability and reduce product inhibition. However, they still face drawbacks such as potential leaching or conformational change during immobilization and poor machining performance. Especially, most enzyme carrier solid materials possess disordered structures, inevitably introducing deficiencies such as low loading capacity, hindered mass transfer, and unclear structure-property relationships. Additionally, it remains a notable challenge to meticulously design immobilization systems tailored to the specific characteristics of enzyme/reaction. Therefore, there is a significant demand for reliable solid materials to overcome the above challenges. Crystalline porous materials, particularly covalent organic frameworks (COFs), have garnered significant interest as a promising platform for immobilizing enzymes due to their unique properties, such as their crystalline nature, high porosity, accessible active sites, versatile synthetic conditions, and tunable structure. COFs create a stabilizing microenvironment that protects enzymes from denaturation and significantly enhances reusability. Nevertheless, some challenges still remain, including difficulties in loading large enzymes, reduced enzyme activities, and the limited functionality of carriers. Therefore, it is essential to develop innovative carriers and novel strategies to broaden the methods of immobilizing enzymes, enabling their application across a more diverse array of fields.The integration of enzymes with advanced porous materials for intensified performance and diverse applications is still in its infancy, and our group has done a series of pioneering works. This Account presents a comprehensive overview of recent research progress made by our group, including (i) the development of innovative enzyme immobilization strategies utilizing COFs to make the assembly and integration of enzymes and carriers more effective; (ii) rational design and construction of functional carriers for enzyme immobilization using COFs; and (iii) extensions of immobilized enzyme applications based on COFs from industrial catalysis to biomedicine and chiral separation. The integration of enzymes with functional crystalline materials offers mutual benefits and results in a performance that surpasses what either component can achieve individually. Additionally, immobilized enzymes exhibit enhanced functionality and intriguing characteristics that differ from those of free enzymes. Consistent with our research philosophy centered on integration, platform development, and engineering application, this Account addresses the critical challenges associated with enzyme immobilization using COFs while extending the applications of COFs and proposing future design principles for biomanufacturing and enzyme industry.

Structural and Functional Insights into the Biomineralized Zeolite Imidazole Frameworks

Biomineralization is a natural process of mineral formation mediated by biomacromolecules, allowing access to hierarchical structures integrating biological, chemical, and material properties. In this contribution, we comprehensively investigate the biomineralization of zeolite imidazole frameworks (ZIFs) for one-step synthesis of an enzyme-MOF biocomposite, in terms of differential crystallization behaviors, fine microstructure of resultant ZIF biominerals, the enzyme's conformation evolution, and protective effect of ZIF mineral. We discover that the biomineralization ability is ZIF organic linker dependent and the biocatalytic function is highly related to the ZIF mineral species and their distinguishable topologies and defect structures. Importantly, a side-by-side analysis suggests that the protective effect of ZIF mineral toward the hosted enzyme is highly associated with the synergistic effect of size dimension and chemical microenvironment of the ZIF pores. This work provides important insight into the ZIF-dependent biomineralization behaviors and highlights the important role of the ZIF microstructure in its biocatalytic activity and durability, which has been underestimated previously.

Biomimetic Metal-Pyrimidine Nanoflowers: Enzyme Immobilization Platforms with Boosted Activity

For the enzyme immobilization platform, enhancing enzyme activity retention while improving enzyme stability remains a challenge for sensitive sensing analysis. Herein, an in situ biomimetic immobilized enzyme carrier (metal-pyrimidine nanoflowers, MPNFs) synthesized by the coordination of DNA base derivative (2-aminopyrimidine) with Zn2+ in the aqueous phase at room temperature is developed. The biocompatibility of 2-aminopyrimidine and the hydrophilicity and green synthetic conditions of MPNFs allows the immobilized enzymes to retain above 91.2% catalytic activity. On this basis, a cascade catalytic platform is constructed by simultaneously immobilizing acetylcholinesterase (AChE), choline oxidase (CHO), and horseradish peroxidase (HRP) in MPNFs (AChE/CHO/HRP@MPNFs) for organophosphorus pesticides (OPs) colorimetric biosensing detection. The assay could specifically detect parathion-methyl within 13 min with a wider linear range (0.1-1000.0 nm) and a lower limit of detection (LOD) (0.032 nm). The remarkable stability of the immobilized enzymes is also achieved under harsh environments, room temperature storage, and recycling. Furthermore, a portable and cost-effective biosensing platform is developed by integrating AChE/CHO/HRP@MPNFs with a smartphone-assisted paper device for the on-site detection of OPs. Overall, the high catalytic activity retention and the enhanced detection performance demonstrate that MPNF is a robust carrier in enzyme immobilization and holds great promise in biosensing and other field applications.

Cellulase immobilization to enhance enzymatic hydrolysis of lignocellulosic biomass: An all-inclusive review

Cellulase-mediated lignocellulosic biorefinery plays a crucial role in the production of high-value biofuels and chemicals, with enzymatic hydrolysis being an essential component. The advent of cellulase immobilization has revolutionized this process, significantly enhancing the efficiency, stability, and reusability of cellulase enzymes. This review offers a thorough analysis of the fundamental principles underlying immobilization, encompassing various immobilization approaches such as physical adsorption, covalent binding, entrapment, and cross-linking. Furthermore, it explores a diverse range of carrier materials, including inorganic, organic, and hybrid/composite materials. The review also focuses on emerging approaches like multi-enzyme co-immobilization, oriented immobilization, immobilized enzyme microreactors, and enzyme engineering for immobilization. Additionally, it delves into novel carrier technologies like 3D printing carriers, stimuli-responsive carriers, artificial cellulosomes, and biomimetic carriers. Moreover, the review addresses recent obstacles in cellulase immobilization, including molecular-level immobilization mechanism, diffusion limitations, loss of cellulase activity, cellulase leaching, and considerations of cost-effectiveness and scalability. The knowledge derived from this review is anticipated to catalyze the evolution of more efficient and sustainable biocatalytic systems for lignocellulosic biomass conversion, representing the current state-of-the-art in cellulase immobilization techniques.

Bioinspired Cu(II) Defect Sites in ZIF-8 for Selective Methane Oxidation

Activating the C-H bonds of alkanes without further oxidation to more thermodynamically stable products, CO and CO2, is a long-sought goal of catalytic chemistry. Inspired by the monocopper active site of methane monooxygenase, we synthesized a Cu-doped ZIF-8 metal-organic framework with 25% Cu and 75% Zn in the nodes and activated it by heating to 200 °C and dosing in a stepwise fashion with O2, methane, and steam. We found that it does oxidize methane to methanol and formaldehyde. The catalysis persists through at least five cycles, and beyond the third cycle, the selectivity improves to the extent that no CO2 can be detected. Experimental characterization and analysis were carried out by PXRD, DRUV-vis, SEM, and XAS (XANES and EXAFS). The reaction is postulated to proceed at open-coordination copper sites generated by defects, and the mechanism of methanol production was explicated by density functional calculations with the revMO6-L exchange-correlation functional. The calculations reveal a catalytic cycle of oxygen-activated CuI involving the conversion of two molecules of CH4 to two molecules of CH3OH by a sequence of hydrogen atom transfer reactions and rebound steps. For most steps in the cycle, the reaction is more favored by singlet species than by triplets.

Enzyme-immobilized hierarchically porous covalent organic framework biocomposite for catalytic degradation of broad-range emerging pollutants in water

Efficient enzyme immobilization is crucial for the successful commercialization of large-scale enzymatic water treatment. However, issues such as lack of high enzyme loading coupled with enzyme leaching present challenges for the widespread adoption of immobilized enzyme systems. The present study describes the development and bioremediation application of an enzyme biocomposite employing a cationic macrocycle-based covalent organic framework (COF) with hierarchical porosity for the immobilization of horseradish peroxidase (HRP). The intrinsic hierarchical porous features of the azacalix[4]arene-based COF (ACA-COF) allowed for a maximum HRP loading capacity of 0.76 mg/mg COF with low enzyme leaching (<5.0 %). The biocomposite, HRP@ACA-COF, exhibited exceptional thermal stability (∼200 % higher relative activity than the free enzyme), and maintained ∼60 % enzyme activity after five cycles. LCMSMS analyses confirmed that the HRP@ACA-COF system was able to achieve > 99 % degradation of seven diverse types of emerging pollutants (2-mercaptobenzothiazole, paracetamol, caffeic acid, methylparaben, furosemide, sulfamethoxazole, and salicylic acid)in under an hour. The described enzyme-COF system offers promise for efficient wastewater bioremediation applications.

Enhanced Bioactivity of Enzyme/MOF Biocomposite via Host Framework Engineering

This study reports the successful development of a sustainable synthesis protocol for a phase-pure metal azolate framework (MAF-6) and its application in enzyme immobilization. An esterase@MAF-6 biocomposite was synthesized, and its catalytic performance was compared with that of esterase@ZIF-8 and esterase@ZIF-90 in transesterification reactions. Esterase@MAF-6, with its large pore aperture, showed superior enzymatic performance compared to esterase@ZIF-8 and esterase@ZIF-90 in catalyzing transesterification reactions using both n-propanol and benzyl alcohol as reactants. The hydrophobic nature of the MAF-6 platform was shown to activate the immobilized esterase into its open-lid conformation, which exhibited a 1.5- and 4-times enzymatic activity as compared to free esterase in catalyzing transesterification reaction using n-propanol and benzyl alcohol, respectively. The present work offers insights into the potential of MAF-6 as a promising matrix for enzyme immobilization and highlights the need to explore MOF matrices with expanded pore apertures to broaden their practical applications in biocatalysis.

Improving the Bioactivity and Stability of Embedded Enzymes by Covalent Organic Frameworks

De novo embedding enzymes within reticular chemistry materials have shown the enhancement of physical and chemical stability for versatile catalytic reactions. Compared to metal-organic frameworks (MOFs), covalent organic frameworks (COFs) are usually considered to be the more superior host of enzymes because of their large channels with low diffusion barriers, outstanding chemical/thermal stability, and metal-free nature. However, detailed investigations on the comparison of COFs and MOFs in enhancing biocatalytic performance have not been explored. Here, we de novo encapsulated enzymes within two COFs via a mechanochemical strategy, which avoided the extreme synthetic conditions of COFs and highly maintained the biological activities of the embedded enzymes. The enzymes@COFs biocomposites exhibited a much higher activity (3.4-14.7 times higher) and enhanced stability than those in MOFs (ZIF-8, ZIF-67, HKUST-1, MIL-53, and CaBDC), and the rate parameter (kcat/Km) of enzyme@COFs was 41.3 times higher than that of enzyme@ZIF-8. Further explorations showed that the conformation of enzymes inside MOFs was disrupted, owing to the harmful interfacial interactions between enzymes and metal ions as confirmed by ATR-FTIR, fluorescence spectroscopy, and XPS data. In contrast, enzymes that were embedded in metal-free COFs highly preserved the natural conformation of free enzymes. This study provides a better understanding of the interfacial interactions between reticular supports and enzymes, which paves a new road for optimizing the bioactivities of immobilized enzymes.

Covalent Organic Framework Based Crosslinked Porous Microcapsules for Enzymatic Catalysis

The design of porous microcapsules with selective mass transfer and mechanical robustness for enzyme encapsulation is highly desired for biocatalysis, yet the construction remains challenging. Herein, we report the facile fabrication of porous microcapsules by assembling covalent organic framework (COF) spheres at the interfaces of emulsion droplets followed by interparticle crosslinking. The COF microcapsules could offer an enclosed aqueous environment for enzymes, with size-selective porous shells that allow for the fast diffusion of substrates and products while excluding larger molecules such as protease. Crosslinking of COF spheres not only enhances the structural stability of capsules but also imparts enrichment effects. The enzymes encased in the COF microcapsules show enhanced activity and durability in organic media as verified in both batch reaction and continuous-flow reaction. The COF microcapsules offer a promising platform for the encapsulation of biomacromolecules.

Yolk-Shell and Hollow Zr/Ce-UiO-66 for Manipulating Selectivity in Tandem Reactions and Photoreactions

One of the hallmarks of multicomponent metal-organic frameworks (MOFs) is to finely tune their active centers to achieve product selectivity. In particular, obtaining bimetallic MOF hollow structures with precisely tailored redox centers under the same topology is still challenging despite a recent surge of such efforts. Herein, we present an engineering strategy named "cluster labilization" to generate hierarchically porous MOF composites with hollow structures and tunable active centers. By partially replacing zirconium with cerium in the hexanuclear clusters of UiO-66, unevenly distributed yolk-shell structures (YSS) were formed. Through acid treatment or annealing of the YSS precursor, single-shell hollow structures (SSHS) or double-shell hollow structures (DSHS) can be obtained, respectively. The active centers in SSHS and DSHS differ in their species, valence, and spatial locations. More importantly, YSS, SSHS, and DSHS with distinct active centers and microenvironments exhibit tunable catalytic activity, reversed selectivity, and high stability in the tandem reaction and the photoreaction.

Impact of Crystallinity on Enzyme Orientation and Dynamics upon Biomineralization in Metal-Organic Frameworks

Aqueous-phase co-crystallization (also known as biomimetic mineralization or biomineralization) is a unique way to encapsulate large enzymes, enzyme clusters, and enzymes with large substrates in metal-organic frameworks (MOFs), broadening the application of MOFs as enzyme carriers. The crystallinity of resultant enzyme@MOF biocomposites, however, can be low, raising a concern about how MOF crystal packing quality affects enzyme performance upon encapsulation. The challenges to overcome this concern are (1) the limited database of enzyme performance upon biomineralization in different aqueous MOFs and (2) the difficulty in probing enzyme restriction and motion in the resultant MOF scaffolds, which are related to the local crystal packing quality/density, under the interference of the MOF backgrounds. We have discovered several new aqueous MOFs for enzyme biomineralization with varied crystallinity [Jordahl, D.; Armstrong, Z.; Li, Q.; Gao, R.; Liu, W.; Johnson, K.; Brown, W.; Scheiwiller, A.; Feng, L.; Ugrinov, A.; Mao, H.; Chen, B.; Quadir, M.; Pan, Y.; Li, H.; Yang, Z. Expanding the Library of Metal-Organic Frameworks (MOFs) for Enzyme Biomineralization. ACS Appl. Mater. Interfaces 2022, 14 (46), 51619-51629, DOI: 10.1021/acsami.2c12998]. Here, we address the second challenge by probing enzyme dynamics/restriction in these MOFs at the residue level via site-directed spin labeling (SDSL)-electron paramagnetic resonance (EPR) spectroscopy, a unique approach to determine protein backbone motions regardless of the background complexity. We encapsulated a model large-substrate enzyme, lysozyme, in eight newly discovered MOFs, which possess various degrees of crystallization, via aqueous-phase co-crystallization. Through the EPR study and simulations, we found rough connections between (a) enzyme mobility/dynamics and MOF crystal properties (packing quality and density) and (b) enzyme areas exposed above each MOF and their catalytic performance. This work suggests that protein SDSL and EPR can serve as an indicator of MOF crystal packing quality/density when biomineralized in MOFs. The method can be generalized to probing the dynamics of other enzymes on other solid surfaces/interfaces and guide the rational design of solid platforms (ca. MOFs) to customize enzyme immobilization.

Hierarchical covalent organic framework-foam for multi-enzyme tandem catalysis

Covalent organic frameworks (COFs) are ideal host matrices for biomolecule immobilization and biocatalysis due to their high porosity, various functionalities, and structural robustness. However, the porosity of COFs is limited to the micropore dimension, which restricts the immobilization of enzymes with large volumes and obstructs substrate flow during enzyme catalysis. A hierarchical 3D nanostructure possessing micro-, meso-, and macroporosity could be a beneficial host matrix for such enzyme catalysis. In this study, we employed an in situ CO2 gas effervescence technique to induce disordered macropores in the ordered 2D COF nanostructure, synthesizing hierarchical TpAzo COF-foam. The resulting TpAzo foam matrix facilitates the immobilization of multiple enzymes with higher immobilization efficiency (approximately 1.5 to 4-fold) than the COF. The immobilized cellulolytic enzymes, namely β-glucosidase (BGL), cellobiohydrolase (CBH), and endoglucanase (EG), remain active inside the TpAzo foam. The immobilized BGL exhibited activity in organic solvents and stability at room temperature (25 °C). The enzyme-immobilized TpAzo foam exhibited significant activity towards the hydrolysis of p-nitrophenyl-β-d-glucopyranoside (BGL@TpAzo-foam: Km and Vmax = 23.5 ± 3.5 mM and 497.7 ± 28.0 μM min-1) and carboxymethylcellulose (CBH@TpAzo-foam: Km and Vmax = 18.3 ± 4.0 mg mL-1 and 85.2 ± 9.6 μM min-1 and EG@TpAzo-foam: Km and Vmax = 13.2 ± 2.0 mg mL-1 and 102.2 ± 7.1 μM min-1). Subsequently, the multi-enzyme immobilized TpAzo foams were utilized to perform a one-pot tandem conversion from carboxymethylcellulose (CMC) to glucose with high recyclability (10 cycles). This work opens up the possibility of synthesizing enzymes immobilized in TpAzo foam for tandem catalysis.

Harnessing Self-Repairing and Crystallization Processes for Effective Enzyme Encapsulation in Covalent Organic Frameworks

Immobilization of fragile enzymes in crystalline porous materials offers new opportunities to expand the applications of biocatalysts. However, limited by the pore size and/or harsh synthesis conditions of the porous hosts, enzymes often suffer from dimension limitation or denaturation during the immobilization process. Taking advantage of the dynamic covalent chemistry feature of covalent organic frameworks (COFs), herein, we report a preprotection strategy to encapsulate enzymes in COFs during the self-repairing and crystallization process. Enzymes were first loaded in the low-crystalline polymer networks with mesopores formed at the initial growth stage, which could offer effective protection for enzymes from the harsh reaction conditions, and subsequently the encapsulation proceeded during the self-repairing and crystallization of the disordered polymer into the crystalline framework. Impressively, the biological activity of the enzymes can be well-maintained after encapsulation, and the obtained enzyme@COFs also show superior stability. Furthermore, the preprotection strategy circumvents the size limitation for enzymes, and its versatility was verified by enzymes with different sizes and surface charges, as well as a two-enzyme cascade system. This study offers a universal design idea to encapsulate enzymes in robust porous supports and holds promise for developing high-performance immobilized biocatalysts.

Multi-compartmental MOF microreactors derived from Pickering double emulsions for chemo-enzymatic cascade catalysis

Bioinspired multi-compartment architectures are desired in synthetic biology and metabolic engineering, as credited by their cell-like structures and intrinsic ability of assembling catalytic species for spatiotemporal control over cascade reactions like in living systems. Herein, we describe a general Pickering double emulsion-directed interfacial synthesis method for the fabrication of multicompartmental MOF microreactors. This approach employs multiple liquid-liquid interfaces as a controllable platform for the self-completing growth of dense MOF layers, enabling the microreactor with tailor-made inner architectures and selective permeability. Importantly, simultaneous encapsulation of incompatible functionalities, including hydrophilic enzyme and hydrophobic molecular catalyst, can be realized in a single MOF microreactor for operating chemo-enzymatic cascade reactions. As exemplified by the Grubb' catalyst/CALB lipase driven olefin metathesis/ transesterification cascade reaction and glucose oxidase (GOx)/Fe-porphyrin catalyzed oxidation reaction, the multicompartmental microreactor exhibits 2.24-5.81 folds enhancement in cascade reaction efficiency in comparison to the homogeneous counterparts or physical mixture of individual analogues, due to the restrained mutual inactivation and substrate channelling effects. Our study prompts further design of multicompartment systems and the development of artificial cells capable of complex cellular transformations.

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.

Bioinspired Framework Catalysts: From Enzyme Immobilization to Biomimetic Catalysis

Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.