Chaperonin-Nanocaged Hemin as an Artificial Metalloenzyme for Oxidation Catalysis

Carbonic anhydrase encapsulation using bamboo cellulose scaffolds for efficient CO2 capture and conversion

Utilizing carbonic anhydrase (CA) to catalyze CO2 hydration offers a sustainable and potent approach for carbon capture and utilization. To enhance CA's reusability and stability for successful industrial applications, enzyme immobilization is essential. In this study, delignified bamboo cellulose served as a renewable porous scaffold for immobilizing CA through oxidation-induced cellulose aldehydation followed by Schiff base linkage. The catalytic performance of the resulting immobilized CA was evaluated using both p-NPA hydrolysis and CO2 hydration models. Compared to free CA, immobilization onto the bamboo scaffold increased CA's optimal temperature and pH to approximately 45 °C and 9.0, respectively. Post-immobilization, CA activity demonstrated effective retention (>60 %), with larger scaffold sizes (i.e., 8 mm diameter and 5 mm height) positively impacting this aspect, even surpassing the activity of free CA. Furthermore, immobilized CA exhibited sustained reusability and high stability under thermal treatment and pH fluctuation, retaining >80 % activity even after 5 catalytic cycles. When introduced to microalgae culture, the immobilized CA improved biomass production by ∼16 %, accompanied by enhanced synthesis of essential biomolecules in microalgae. Collectively, the facile and green construction of immobilized CA onto bamboo cellulose block demonstrates great potential for the development of various CA-catalyzed CO2 conversion and utilization technologies.

Enzymatic Electrochemical/Fluorescent Nanobiosensor for Detection of Small Chemicals

The detection of small molecules has attracted enormous interest in various fields, including the chemical, biological, and healthcare fields. In order to achieve such detection with high accuracy, up to now, various types of biosensors have been developed. Among those biosensors, enzymatic biosensors have shown excellent sensing performances via their highly specific enzymatic reactions with small chemical molecules. As techniques used to implement the sensing function of such enzymatic biosensors, electrochemical and fluorescence techniques have been mostly used for the detection of small molecules because of their advantages. In addition, through the incorporation of nanotechnologies, the detection property of each technique-based enzymatic nanobiosensors can be improved to measure harmful or important small molecules accurately. This review provides interdisciplinary information related to developing enzymatic nanobiosensors for small molecule detection, such as widely used enzymes, target small molecules, and electrochemical/fluorescence techniques. We expect that this review will provide a broad perspective and well-organized roadmap to develop novel electrochemical and fluorescent enzymatic nanobiosensors.

Ferric heme b in aqueous micellar and vesicular systems: state-of-the-art and challenges

Ferric heme b (= ferric protoporphyrin IX = hemin) is an important prosthetic group of different types of enzymes, including the intensively investigated and widely applied horseradish peroxidase (HRP). In HRP, hemin is present in monomeric form in a hydrophobic pocket containing among other amino acid side chains the two imidazoyl groups of His170 and His42. Both amino acids are important for the peroxidase activity of HRP as an axial ligand of hemin (proximal His170) and as an acid/base catalyst (distal His42). A key feature of the peroxidase mechanism of HRP is the initial formation of compound I under heterolytic cleavage of added hydrogen peroxide as a terminal oxidant. Investigations of free hemin dispersed in aqueous solution showed that different types of hemin dimers can form, depending on the experimental conditions, possibly resulting in hemin crystallization. Although it has been recognized already in the 1970s that hemin aggregation can be prevented in aqueous solution by using micelle-forming amphiphiles, it remains a challenge to prepare hemin-containing micellar and vesicular systems with peroxidase-like activities. Such systems are of interest as cheap HRP-mimicking catalysts for analytical and synthetic applications. Some of the key concepts on which research in this fascinating and interdisciplinary field is based are summarized, along with major accomplishments and possible directions for further improvement. A systematic analysis of the physico-chemical properties of hemin in aqueous micellar solutions and vesicular dispersions must be combined with a reliable evaluation of its catalytic activity. Future studies should show how well the molecular complexity around hemin in HRP can be mimicked by using micelles or vesicles. Because of the importance of heme b in virtually all biological systems and the fact that porphyrins and hemes can be obtained under potentially prebiotic conditions, ideas exist about the possible role of heme-containing micellar and vesicular systems in prebiotic times.

Insertion of Hemin into Metal-Organic Frameworks: Mimicking Natural Peroxidase Microenvironment for the Rapid Ultrasensitive Detection of Uranium

Constructing enzyme-like active sites in mimic enzyme systems is critical for achieving catalytic performances comparable to natural enzymes and can shed light on the natural development of enzymes. In this study, we described a specific hemin-based mimetic enzyme, which was facilely synthesized by the assembly of zeolitic imidazolate framework-l (ZIF-l) and hemin. The obtained hemin-based mimetic enzyme (denoted as ZIF-l-hemin) displayed enhanced peroxidase activity compared to free hemin in solution. Such excellent activity originated from the ZIF-l framework mimicking the active site cavity microenvironment of horseradish peroxidase in terms of axially coordinated histidine and distal histidine. Additionally, the constructed peroxidase mimetic was extremely resistant to a variety of severe circumstances that would normally denature natural enzymes. These characteristics made ZIF-l-hemin a potential platform for the colorimetric sensor of uranium (UO₂²⁺) with wide linear ranges (0.25-40 μM) and low limits of detection (0.079 μM). Moreover, the detection mechanism demonstrated that the coordination of uranyl ion with imidazole of ZIF-l-hemin reduced the catalytic efficiency of ZIF-l-hemin. The current work not only proposed a novel approach for fabricating artificial peroxidase but also offered facile colorimetric methods for selective radionuclide detection.

Target-Responsive Smart Nanomaterials via a Au-S Binding Encapsulation Strategy for Electrochemical/Colorimetric Dual-Mode Paper-Based Analytical Devices

Target-responsive nanomaterials attract growing interest in the application of drug delivery, bioimaging, and sensing due to the responsive releasing of guest molecules by the smart molecule gate. However, it remains a challenge to develop smart nanomaterials with simple assembly and low nonspecific leakage starting from encapsulation strategies, especially in the sensing field. Herein, Au nanoclusters (Au NCs) were first grown on porous carbon derived from ZIF-8 (PCZIF) to be employed as nanocarriers. By employing the Au NCs as linkers and aptamer (Apta) double-strand hybrids (target Apta and SH-complementary DNA) as capping units, we reported the novel target-responsive nanomaterials of Apta/Au NCs-PCZIF/hemin through Au-S binding encapsulation for sensing assays. The Au-S binding encapsulation strategy simplified the packaging procedure and reduced non-target responsive leakage. As a proof, ochratoxin A (OTA) as a model target participates in the double-strand hybrid competitive displacement reaction and triggered Apta conformation switches from a coil to a G-quadruplex structure accompanied by the dissociation of the gatekeeper. Simultaneously, the released hemin can initiate a self-assembly to form G-quadruplex/hemin DNAzyme. Interestingly, owing to DNAzyme providing electron transfer mediators and peroxidase-like activity, we proposed an electrochemical/colorimetric dual-mode paper-based analytical device (PAD) that provided self-verification to enhance reliability and accuracy, benefiting from independent signal conversion and transmission mechanism. As a consequence, the proposed dual-mode PAD could achieve sensitive electrochemical detection and visual prediction of OTA in the range of 1 pg/mL to 500 ng/mL and 50 pg/mL to 500 ng/mL, respectively. The electrochemical detection limit for OTA was as low as 0.347 pg/mL (S/N = 3). We believe that this work provides point-of-care testing (POCT) tools for a broad spectrum of applications.

Self-assembled recombinant camel serum albumin nanoparticles-encapsulated hemin with peroxidase-like activity for colorimetric detection of hydrogen peroxide and glucose

The construction of enzyme mimics using protein protection layers possesses advantages of high biocompatibility and superior catalytic activity, which is desirable for biomedical applications including diseases diagnosis. Here, from E. coli expression system, recombinant protein of camel serum albumin (rCSA) from Camelus bactrianus was successfully obtained to encapsulate hemin via the self-assemble method without additional toxic organic reagents. As compared with that of horseradish peroxidase, the produced rCSA-hemin nanoparticles exhibited enhanced enzyme-mimicking activity and stability under harsh experimental conditions. Additionally, the steady-state kinetic analysis of rCSA-hemin in the solution revealed its higher affinity to the substrates. Therefore, a colorimetric detection method of H₂O₂ and glucose was constructed with a linear range of 2.5-500 μM with an LOD of 2.39 and 2.42 μM, respectively, which was also applied for the determination of glucose in the serum samples with satisfying recovery ratio ranging from 101.1% to 112.1%. The constructed camel protein-derived nanozyme system of remarkable stability holds promising potentials for the versatile biomedical uses.

Hemin-Caged Ferritin Acting as a Peroxidase-like Nanozyme for the Selective Detection of Tumor Cells

Nanozyme is a class of artificial materials that possess enzyme-like activities and can overcome limitations of natural enzymes. However, controllability of the active sites, uniformity of the particles, and dispersion in the physiological media are still challenging for nanomaterial-based nanozymes. In this work, a protein-based nanozyme has been constructed by the encapsulation of hemin into the nanocavity of a recombinant human heavy chain ferritin (Ftn), generating a monodispersed peroxidase-mimetic nanozyme (hemin@Ftn). Hemin@Ftn possesses high peroxidase catalytic activity and high tolerance to the harsh environmental conditions, such as high temperature and chemical denaturant. Remarkably, hemin@Ftn can act as a colorimetric probe for the detection of tumor cells because it can selectively catalyze reactions in tumor cells. This protein-based nanozyme bridges the gap between natural enzymes and nanomaterial-based nanozymes by the incorporation of a catalytically active prosthetic group into a highly stable Ftn.

Enhanced recombinant Sulfurihydrogenibium yellowstonense carbonic anhydrase activity and thermostability by chaperone GroELS for carbon dioxide biomineralization

Biological carbon fixation is a feasible strategy to reduce atmospheric carbon dioxide levels (CO₂). In this platform, carbonic anhydrase (CA) enzyme is employed to accelerate the sequestration of CO₂. The present work explored the effect of chaperone GroELS and TrxA-tag on improving soluble expression of the recombinant Sulfurihydrogenibium yellowstonense CA which activity and biomineralization capability were taken into consideration. At first, the expression of GroELS using the inducible T7 promoter and constitutive J23100 promoter were investigated. The results indicated that 1.4 folds increment of soluble protein and 100% of CA activity enhancement were achieved with GroELS co-expression driven by J23100 promoter. Furthermore, the involvement of TrxA fusion tag displayed a significant enhancement of soluble protein production which was about 2.67 times higher than that of original SyCA. Besides, co-expression with GroELS intensified the thermostability of SyCA at 60 °C owing to changes in the structural conformation of the protein, which was proved by an in vitro assay. The SyCA was further entrapped and immobilized into polyacrylamide gel (i.e., PAGE-SyCA). The biomineralization capability of the PAGE-SyCA and whole-cell (WC) was compared in a two-column system. After 5 cycles of reuse, PAGE-SyCA maintained 29.8% activity and formed 774 mg of CaCO₃ solids in the B::JG strain. This study presents the recombinant engineering strategies to improve SyCA productivity, activity, thermostability, and effective carbon dioxide conversion.

Protein Nanobarrel for Integrating Chlorophyll a Molecules and Its Photochemical Performance

Taking inspiration from biology's effectiveness in nanoscale organization of chlorophylls for photosynthesis, we describe here a design for chlorophyll-protein conjugates that exploits the central hydrophobic cavity of GroEL protein nanobarrel as a binding pocket for chlorophyll. We found water-soluble conjugates of chlorophyll with GroEL could be easily generated via detergent dialysis. The number of chlorophyll units bound to GroEL is tunable by varying the equilibrium concentration of chlorophyll during dialysis. Meanwhile, it is shown that an increase in the entrapped chlorophyll amount leads to an improvement of chlorophyll-GroEL photostability. Using methyl viologen as an electron acceptor, we demonstrate that chlorophyll-GroEL has photoreduction activity, which is also switchable in on/off illumination mode. Finally, it is shown that chlorophyll-GroEL-sensitized solar cells have good photoelectric properties, yielding a high photoelectric conversion efficiency of ∼0.9%. The current strategy may be adopted for integrating other photosensitizing dyes or for other photocatalytic reactions.

Protein-caged zinc porphyrin as a carbonic anhydrase mimic for carbon dioxide capture

Zinc tetraphenylporphyrin (Zn-TPP) solubilized by GroEL protein cage was prepared as a supramolecular mimic of carbonic anhydrase (CA) for CO2 capture. It is shown that the soluble Zn-TPP-GroEL complex can be formed easily by detergent dialysis. The Zn-TPP/GroEL binding ratio was found to increase with their dialysis ratio until reaching the maximum of about 30 porphyrins per protein cage. Moreover, the complex showed hydrase activity that catalyzes the CO2 hydration in HCO3- and H+. It is further seen that the catalytic activity of Zn-TPP-GroEL was about one-half of that of a bovine CA at 25 °C. On the other hand, as the temperature was increased to 60 °C close to an industrial CO2 absorption temperature, the natural enzyme lost function while Zn-TPP-GroEL exhibited better catalytic performance indicative of a higher thermal stability. Finally, we demonstrate that the GroEL-solubilized Zn-TPP is able to accelerate the precipitation of CO2 in the form of CaCO3 and has better long-term performance than the bovine CA. Thus a new type of nano-caged system mimicking natural CAs for potential applications in carbon capture has been established.

Bactrian camel serum albumins-based nanocomposite as versatile biocargo for drug delivery, biocatalysis and detection of hydrogen peroxide

In response to extreme environmental conditions, Bactrian camels with largest population in China have evolved with the unique and extraordinary stress-tolerant mechanism in the bodies, in which the most abundantly secreted serum albumins contribute to an important role in diverse physiological activities such as maintaining osmotic pressure and transporting endogenous/exogenous molecules. In this study, we have for the first time purified Chinese Bactrian camel serum albumins (CSA) aimed at exploring their biomedical application. The mass spectrometric as well as structural analysis of CSA have revealed the sequence consensus and alpha-helix abundant structures among its heterologous proteins. Using desolvation methods, CSA-based nanoparticles have been prepared to encapsulate two substrate molecules including Doxorubicin (Dox) and hemin, which confers the versatility of nanocomposite. As drug delivery matrix, the Dox-loaded CSA nanoparticles displayed sustained release behaviors of DOX with the decreased cytotoxicity detected by both CCK-8 assay and real-time cell analysis. The CSA-hemin nanoparticles exhibited superior catalytic activities in the oxidation of Orange II comparable with horse radish peroxidase following a ping-pong mechanism. Furthermore, the constructed CSA-hemin nanoparticles were applied for the spectroscopic detection of H₂O₂ resulting in a wide linear calibration curve ranging from 5 to 400 μM with a detection limit of 3.32 μM.

Supramolecular Construction of Biohybrid Nanozymes Based on the Molecular Chaperone GroEL as a Promiscuous Scaffold

The study of enzymatic reactions in a confined space can provide valuable insight into the natural selection of nanocompartments for biocatalytic processes. Design of nanozyme capsules with the barrel-shaped protein cage of GroEL has been proposed as a promising means to constrain chemical reactions in a spatiotemporally controllable manner. Herein, we further demonstrate with hemin that the open GroEL cavity can provide a favorable microenvironment for shielding hydrophobic catalytically active species. Meanwhile, it is shown that the GroEL-caged hemin nanozyme not only has a significantly higher catalytic activity than merely dispersed hemin but also exhibits substrate specificity in the model oxidation reactions, which is a merit lacking in natural hemoproteins. To understand the underlying mechanism behind this supramolecular assembly, molecular docking and molecular dynamics simulations were performed to study the detailed interactions of hemin with the protein cage. This revealed the most likely binding mode and preferred binding residues in the paired hydrophobic α-helices lining the GroEL cavity which are genetically encoded for substrate capture. Finally, we demonstrate that the hemin-GroEL nanozyme has great potential in label-free fluorometric molecular detection when combined with suitable substrates such as homovanillic acid. We believe that our strategy is an advantageous tool for studying confined biocatalytic kinetics as simple mimics of protein-based organelles found in nature and for designing diverse nanozymes or bio-nanoreactors with the promiscuous GroEL binding cavity.

Oil-in-water Pickering emulsions using a protein nano-ring as high-grade emulsifiers

Pickering emulsion-based delivery of liposoluble bioactive ingredients employing protein nanoparticles as biocompatible emulsifiers is a promising choice for food, cosmetic, and medical industries. This paper reports a novel design of a protein nano-ring (termed SR') derived from chaperonin GroEL as an emulsifying agent, which has a naturally evolved hydrophobic binding rim in addition to its well-defined shape. It is shown that SR' adsorbed at rosemary oil/water interface and formed stable oil-in-water Pickering emulsions, with dispersed droplet size being dependent on the SR' concentration and oil/water ratio as well. The optimal formulation yielding stable nano-emulsions was determined to be at a SR' concentration between 0.30 wt.% and 0.45 wt.%, and an oil/water ratio of 0.05-0.20 (v/v). Meanwhile, we demonstrate that nano-sized Pickering droplets could be easily prepared irrespective of the examined external factors including pH, temperature and ionic strength, with the lowest droplet sizes being produced at pH = 7.0, temperature ≤ 40 °C, and ionic strength (NaCl concentration) ≤ 50 mM. Besides, rheological analysis revealed the gelation propensity of SR'-stabilized emulsions with high oil/water ratios, an advantageous property that would further enhance the emulsion stability. Finally, it is shown that the SR' emulsified system is able to protect β-carotene, which was used as a model of bioactive but labile compound. This work, in the context of the current drive for biocompatibility and sustainability, is believed to provide opportunities for emulsion-based applications to switch towards greener solutions.

Highly Surface-Active Chaperonin Nanobarrels for Oil-in-Water Pickering Emulsions and Delivery of Lipophilic Compounds

Stabilization of Pickering emulsions via particles of biological origin exhibits a great potential to be widely applied in food, cosmetic, or biomedicine formulation because of their excellent biocompatibility, biodegradability, and functional properties. This paper describes the successful development of a bioderived GroEL protein nanobarrel as a Pickering stabilizer and its protective properties on β-carotene in dispersed oil phase, as a model of labile bioactive compounds. It is shown that the GroEL nanobarrel is highly surface-active and allows the formation of Pickering emulsion by physical adsorption at the oil/water interface. The optimized formulation for generating a stable submicron oil droplet by ultrasonication includes a GroEL concentration of 0.05-0.45 wt % with an oil/water volume ratio of 0.05-0.35. The as-prepared Pickering emulsion shows pH-responsive emulsification/demulsification transition and excellent stability at temperatures less than 65 °C and ionic strength (with NaCl addition) up to 500 mM. Meanwhile, the emulsion tends to form a gel-like network structure with the oil/water ratio increasing. Finally, we demonstrate that possible factors of oxidant, reducing agent, UV radiation, and sucrose have sequentially decreasing to no effect on the stability of β-carotene encapsulated in GroEL-stabilized Pickering emulsion and that higher GroEL concentration can significantly reduce β-carotene degradation rate, thus ensuring more efficient long-term storage. We believe that the emulsion system supported by the GroEL nanobarrel could be developed to a viable tool for delivering lipophilic bioactive compounds.

Functionalization of GroEL nanocages with hemin for label-free colorimetric assays

The design of functionalized protein nanocages as enzyme mimics is a relatively new and promising field as these biogenic nanocapsules are inherently monodisperse with precise architectures, unattainable by purely synthetic processes. This work explored the catalytic properties of the bacterial GroEL-nanocaged hemin and its performance in label-free colorimetric assays. It is demonstrated that the hemin-GroEL biohybrid has peroxidase-like activity and follows the typical Michaelis-Menten kinetics and ping-pong mechanism in the model sensing processes. The open nature and nanoreactor effect of the GroEL cage and the addition of ATP are shown to significantly influence the catalytic activity. For glucose detection with the hemin-GroEL complex, the linearity between the analyte concentration and UV-vis absorption was determined to range from 0 to 200 μM with a limit of detection (LOD) of ~ 12 μM under the defined conditions. In addition, the colorimetric detection of catechol led to a linear dynamic range of 0-120 μM with a LOD of ~ 17 μM. This indicates that the same assay could be used as a sensing platform for the detection or even oxidative removal of phenolic contaminants in the presence of H₂O₂. Thus, GroEL cage is a valuable tool for the development of nanozymes and practical applications such as clinical analysis and environmental monitoring.

Metallopolymers for advanced sustainable applications

Since the development of metallopolymers, there has been tremendous interest in the applications of this type of materials. The interest in these materials stems from their potential use in industry as catalysts, biomedical agents in healthcare, energy storage and production as well as climate change mitigation. The past two decades have clearly shown exponential growth in the development of many new classes of metallopolymers that address these issues. Today, metallopolymers are considered to be at the forefront for discovering new and sustainable heterogeneous catalysts, therapeutics for drug-resistant diseases, energy storage and photovoltaics, molecular barometers and thermometers, as well as carbon dioxide sequesters. The focus of this review is to highlight the advances in design of metallopolymers with specific sustainable applications.

Protein cage assembly across multiple length scales

Within the materials science community, proteins with cage-like architectures are being developed as versatile nanoscale platforms for use in protein nanotechnology. Much effort has been focused on the functionalization of protein cages with biological and non-biological moieties to bring about new properties of not only individual protein cages, but collective bulk-scale assemblies of protein cages. In this review, we report on the current understanding of protein cage assembly, both of the cages themselves from individual subunits, and the assembly of the individual protein cages into higher order structures. We start by discussing the key properties of natural protein cages (for example: size, shape and structure) followed by a review of some of the mechanisms of protein cage assembly and the factors that influence it. We then explore the current approaches for functionalizing protein cages, on the interior or exterior surfaces of the capsids. Lastly, we explore the emerging area of higher order assemblies created from individual protein cages and their potential for new and exciting collective properties.

Probing the interaction mechanisms between transmembrane peptides and the chaperonin GroEL with fluorescence anisotropy

Proper translocation, membrane insertion and folding are crucial biophysical steps in the biogenesis of functional transmembrane peptides/proteins (TMPs). ATP-dependent chaperonins are able to regulate each of these processes, but the underlying mechanisms remain unclear. In this work, interaction between the bacterial chaperonin GroEL and a synthetic fluorescent transmembrane peptide was investigated by fluorescence anisotropy. Binding of the peptide with GroEL resulted in increased fluorescence anisotropy and intensity. The dissociation constant and binding stoichiometry, as assessed by titration of the peptide with GroEL, were estimated to be 0.6±0.2μM and 2.96±0.35, respectively. Complementary study with the single-ring version of GroEL confirmed the high-affinity peptide binding, and indicates that the two GroEL rings may function alternatively in binding the peptides. The co-chaperonin GroES was found to be effective at releasing the peptides initially bound to GroEL with the help of ATP. Moreover, our observation with the single-ring GroEL mutant demonstrated that during the encapsulation of GroEL by GroES, the bound peptides may either be confined in the cage thus formed, or escape outside. Competitive binding experiments indicated that the peptides studied interact with GroEL through the paired helices H and I on its apical domain. Our spectroscopic studies revealed some basic mechanisms of interaction between transmembrane peptides and GroEL, which would be instrumental for deciphering the chaperonin-mediated TMP biogenesis.

Synthesis and coordination studies of 5-(4′-carboxyphenyl)-10,15,20-tris(pentafluorophenyl)porphyrin and its pyrrolidine-fused chlorin derivative

An efficient strategy was developed to obtain carboxyphenyl porphyrin, chlorins and metal complexes, with potential applications in photonics and biology.