Enhanced activity of acetyl CoA synthetase adsorbed on smart microgel: an implication for precursor biosynthesis

Designing Configurable Soft Microgelsomes as a Smart Biomimetic Protocell

Self-assembly is an intriguing aspect of primitive cells. The construction of a semipermeable compartment with a robust framework of soft material capable of housing an array of functional components for chemical changes is essential for the fabrication of synthetic protocells. Microgels, loosely cross-linked polymer networks, are suitable building blocks for protocell capsule generation due to their porous structure, tunable properties, and assembly at the emulsion interface. Here, we present an interfacial assembly of microgel-based microcompartments (microgelsomes, MGC) that are defined by a semipermeable, temperature-responsive elastic membrane formed by densely packed microgels in a monolayer. The water-dispersible microgelsomes can thermally shuttle between 10 and 95 °C while retaining their structural integrity. Importantly, the microgelsomes exhibited distinct properties of protocells, such as cargo encapsulation, semipermeable membrane, DNA amplification, and membrane-gated compartmentalized enzymatic cascade reaction. This versatile approach for the construction of biomimetic microcompartments augments the protocell library and paves the way for programmable synthetic cells.

Bactericidal and biocatalytic temperature responsive microgel based self-cleaning membranes for water purification

The present study focuses on creating an antimicrobial and biocatalytic smart gating membrane by synthesizing unique core-shell microgels. The core-shell microgels are synthesized by grafting short chains of poly(ethylenimine) (PEI) onto a poly((N-isopropyl acrylamide)-co-glycidyl methacrylate)) (P(NIPAm-co-GMA)) core. Subsequently, the produced microgels are utilized as a substrate for synthesizing and stabilizing silver nanoparticles (Ag NPs) through an in-situ approach. These Ag NPs immobilized microgels are then suction filtered over a polyethylene terephthalate (PET) track-etched support to create cross-linked composite microgel membranes (CMMs). After structural and permeation characterization of the prepared CMMs, the laccase enzyme is then covalently grafted to the surface of the membrane and tested for its effectiveness in degrading Reactive red-120 dye. The laccase immobilized biocatalytic CMMs show effective degradation of the Reactive red-120 by 71%, 48%, and 34% at pH 3, 4, and 5, respectively. Furthermore, the immobilized laccase enzyme showed better activity and stability in terms of thermal, pH, and storage compared to the free laccase, leading to increased reusability. The unique combination of Ag NPs and laccase on a thermoresponsive microgel support resulted in a responsive self-cleaning membrane with excellent antimicrobial and dye degradation capabilities for environmentally friendly separation technology.

Hydrogel-Based Multi-enzymatic System for Biosynthesis

Biosynthesis involving multi-enzymatic reactions is usually an efficient and economic method to produce plentiful important molecules. To increase the product yield in biosynthesis, the involved enzymes can be immobilized to carriers for enhancing enzyme stability, increasing synthesis efficiency and improving enzyme recyclability. Hydrogels with three-dimensional porous structures and versatile functional groups are promising carriers for enzyme immobilization. Herein, we review the recent advances of the hydrogel-based multi-enzymatic system for biosynthesis. First, we introduce the strategies of enzyme immobilization in hydrogel, including the pros and cons of the strategies. Then we overview the recent applications of the multi-enzymatic system for biosynthesis, including cell-free protein synthesis (CFPS) and non-protein synthesis, especially high value-added molecules. In the last section, we discuss the future perspective of the hydrogel-based multi-enzymatic system for biosynthesis.

Pd Nanoparticle-Loaded Smart Microgel-Based Membranes as Reusable Catalysts

In this work, palladium-loaded smart membranes made by UV cross-linking of thermoresponsive microgels are prepared to obtain a reusable, catalytically active material which can, for example, be implemented in chemical reactors. The membranes are examined with respect to their coverage of a supporting mesh via atomic force microscopy measurements. Force indentation mapping was performed in the dried, collapsed state and in the swollen state in water to determine the Young modulus. Furthermore, we compare the catalytic activity of the membrane with the corresponding suspended colloidal nanoparticle microgel hybrids. For this purpose, the reduction of 4-nitrophenol is an established model reaction to quantify the catalytic activity by UV-vis spectroscopy. The membrane is embedded inside a continuous stirred tank reactor equipped for continuous monitoring of the reaction progress. Although catalysis with membranes shows lower catalytic activity than freely dispersed particles, membranes allow straightforward separation and recycling of the catalyst. The fabricated membranes in this work show no decrease in catalytic activity between several cycles, unlike free particles. The feasible and durable deposition of catalytically active inter-cross-linked microgel particles on commercial nylon meshes as supporting scaffolds, as demonstrated in this work, is promising for up-scaling of continuous industrial processes.

Synthesis of Anionic Nanogels for Selective and Efficient Enzyme Encapsulation

Polyelectrolyte nanogels containing cross-linked ionic polymer networks feature both soft environment and intrinsic charges which are of great potential for enzyme encapsulation. In this work, well-defined poly(acrylic acid) (PAA) nanogels have been synthesized based on a facile strategy, namely, electrostatic assembly directed polymerization (EADP). Specifically, AA monomers are polymerized together with a cross-linker in the presence of a cationic-neutral diblock copolymer as the template. Effects of control factors including pH, salt concentration, and cross-linking degree have been investigated systematically, based on which the optimal preparation of PAA nanogels has been established. The obtained nanogel features not only compatible pocket for safely loading enzymes without disturbing their structures, but also abundant negative charges which enable selective and efficient encapsulation of cationic enzymes. The loading capacities of PAA nanogels for cytochrome (cyt c) and lysozyme are 100 and 125 μg/mg (enzyme/nanogel), respectively. More notably, the PAA network seems to modulate a favorable microenvironment for cyt c and induces 2-fold enhanced activity for the encapsulated enzymes, as indicated by the steady-state kinetic assay. Our study reveals the control factors of EADP for optimal synthesis of anionic nanogels and validates their distinctive advances with respect to efficient loading and activation of cationic enzymes.

Regenerable temperature-responsive biocatalytic nanofiltration membrane for organic micropollutants removal

Biocatalytic nanofiltration membranes (BNMs) exhibit great potentials in organic micropollutants removal attributed to its synergistic effect between enzyme catalysis and membrane separation. However, the difficulties in regeneration of the BNMs halted their economic practicality. Inspired by cell membranes with stimuli-responsive channels, we have developed the temperature-responsive BNMs with nanogating function by poly(N-isopropyl acrylamide) (PNIPAM) modification. PNIPAM modification increases the geometric confinement of the support layer to enzymes, thus improving enzyme loading, inhibiting enzyme leakage, and preventing membrane permeability decline caused by enzyme excess migration and aggregation. By optimizing the concentration of reaction monomers, modification time, and strategies, the PNIPAM-based BNMs show high bisphenol A (BPA) removal efficiency and long-term stability. Furthermore, the PNIPAM-polyethyleneimine-based BNMs can be easily regenerated at 38°C, and the laccase activity and BPA removal efficiency are fully recovered. This work would promote the real application of BNMs in bioconversion, drug delivery, and biosensors.

Biocatalytic self-assembled synthetic vesicles and coacervates: From single compartment to artificial cells

Compartmentalization is an intrinsic feature of living cells that allows spatiotemporal control over the biochemical pathways expressed in them. Over the years, a library of compartmentalized systems has been generated, which includes nano to micrometer sized biomimetic vesicles derived from lipids, amphiphilic block copolymers, peptides, and nanoparticles. Biocatalytic vesicles have been developed using a simple bag containing enzyme design of liposomes to multienzymes immobilized multi-vesicular compartments for artificial cell generation. Additionally, enzymes were also entrapped in membrane-less coacervate droplets to mimic the cytoplasmic macromolecular crowding mechanisms. Here, we have discussed different types of single and multicompartment systems, emphasizing their recent developments as biocatalytic self-assembled structures using recent examples. Importantly, we have summarized the strategies in the development of the self-assembled structure to improvise their adaptivity and flexibility for enzyme immobilization. Finally, we have presented the use of biocatalytic assemblies in mimicking different aspects of living cells, which further carves the path for the engineering of a minimal cell.

Regulated Polyelectrolyte Nanogels for Enzyme Encapsulation and Activation

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

One-pot surfactant-free modulation of size and functional group distribution in thermoresponsive microgels

Control over the size and functional group distribution of soft responsive hydrogel particles is essential for applications such as drug delivery, catalysis and chemical sensing. Traditionally, targeted functional group distributions are achieved with semi-batch techniques which require specialized equipment, while the preparation of size-tailored particles typically involves the use of surfactants. Herein, we present a simple and robust surfactant-free method for the modulation of size and carboxylic acid functional group distribution in poly(N-isopropylacrylamide) thermoresponsive microgels, employing reaction pH as the single experimental parameter. The varying distributions of carboxylic acid residues arise due to differences in kinetic reactivity, which are a function of the degree of dissociation of methacrylic acid, and thus of reaction pH. Incorporated charged residues induce a surfactant-like action during the particle nucleation stage, and impact the final particle size. Characterization with dynamic light scattering, and electron microscopy consistently supports the pH-tailored morphology of the microgels. A mathematical model which accounts for particle deformation on the imaging substrate also shows excellent agreement with the experimental results.

Hairy Particles with Immobilized Enzymes: Impact of Particle Topology on the Catalytic Activity

Enzymes are described as ideal green biocatalysts because they are highly specific and selective. However, their practical application is hampered because of the low stability and missing reusability of free enzymes. One method to overcome these problems is the immobilization of enzymes onto carriers. Although numerous publications discuss different immobilization strategies, optimization of these carriers for the highest enzyme activity and loading capacity, enzyme selectivity, reusability, and reactor system configuration still remains a challenging task. In this contribution, we aim to address the role of the core-shell particle design with respect to their geometry as well as the polymer shell thickness on the immobilization of biomolecules. We discovered that spherical particles with a core diameter of 200 nm and intermediate shell thickness as well as platelet-like particles exhibited excellent results with a maximum immobilization yield of laccase from Trametes versicolor of up to 92% and an activity on the carrier material of 5.722 U/(g particle). Especially, the platelet-like particles offered a scalable and convenient alternative for the immobilization of laccase. Circular dichroism measurements proved that the secondary structure of the enzyme is not impaired by immobilization onto all kinds of carrier particles. Moreover, the immobilized laccase was successfully used for the decolorization of Cibacron blue P-3R in up to 18 cycles. Finally, particle separation was achieved via citrate-induced flocculation within 10 min. This detailed study contributes to the understanding of rational design of catalytically active hybrid materials and their effective performance at interfaces for applications in textile industry and environmental technologies.

Acetyl-CoA production by encapsulated pyruvate ferredoxin oxidoreductase in alginate hydrogels

Pyruvate ferredoxin oxidoreductase from Citrobacter sp. S-77 (PFOR_S77) was purified in order to develop a method for acetyl-CoA production. Although the purified PFOR_S77 showed high O₂-sensitivity, the activity could be remarkably stabilized in anaerobic conditions. PFOR_S77 was effectively immobilized on ceramic hydroxyapatite (PFOR_S77-HA) with an efficiency of more than 96%, however, after encapsulation of PFOR_S77-HA in alginate, the rate of catalytic acetyl-CoA production was highly reduced to 36% when compared to that of the free enzyme. However, the operational stability of the PFOR_S77-HA in alginate hydrogels was remarkable, retaining over 68% initial activity even after ten repeated cycles. The results suggested that the PFOR_S77-HA hydrogels have a high potential for biotechnological application.

Functional Microgels and Microgel Systems

Microgels are macromolecular networks swollen by the solvent in which they are dissolved. They are unique systems that are distinctly different from common colloids, such as, e.g., rigid nanoparticles, flexible macromolecules, micelles, or vesicles. The size of the microgel networks is in the range of several micrometers down to nanometers (then sometimes called "nanogels"). In a collapsed state, they might resemble hard colloids but they can still contain significant amounts of solvent. When swollen, they are soft and have a fuzzy surface with dangling chains. The presence of cross-links provides structural integrity, in contrast to linear and (hyper)branched polymers. Obviously, the cross-linker content will allow control of whether microgels behave more "colloidal" or "macromolecular". The combination of being soft and porous while still having a stable structure through the cross-linked network allows for designing microgels that have the same total chemical composition, but different properties due to a different architecture. Microgels based, e.g., on two monomers but have either statistical spatial distribution, or a core-shell or hollow-two-shell morphology will display very different properties. Microgels provide the possibility to introduce chemical functionality at different positions. Combining architectural diversity and compartmentalization of reactive groups enables thus short-range coexistence of otherwise instable combinations of chemical reactivity. The open microgel structure is beneficial for uptake-release purposes of active substances. In addition, the openness allows site-selective integration of active functionalities like reactive groups, charges, or markers by postmodification processes. The unique ability of microgels to retain their colloidal stability and swelling degree both in water and in many organic solvents allows use of different chemistries for the modification of microgel structure. The capability of microgels to adjust both their shape and volume in response to external stimuli (e.g., temperature, ionic strength and composition, pH, electrochemical stimulus, pressure, light) provides the opportunity to reversibly tune their physicochemical properties. From a physics point of view, microgels are particularly intriguing and challenging, since their intraparticle properties are intimately linked to their interparticle behavior. Microgels, which reveal interface activity without necessarily being amphiphilic, develop even more complex behavior when located at fluid or solid interfaces: the sensitivity of microgels to various stimuli allows, e.g., the modulation of emulsion stability, adhesion, sensing, and filtration. Hence, we envision an ever-increasing relevance of microgels in these fields including biomedicine and process technology. In sum, microgels unite properties of very different classes of materials. Microgels can be based on very different (bio)macromolecules such as, e.g., polysaccharides, peptides, or DNA, as well as on synthetic polymers. This Account focuses on synthetic microgels (mainly based on acrylamides); however, the general, fundamental features of microgels are independent of the chemical nature of the building moieties. Microgels allow combining features of chemical functionality, structural integrity, macromolecular architecture, adaptivity, permeability, and deformability in a unique way to include the "best" of the colloidal, polymeric, and surfactant worlds. This will open the door for novel applications in very different fields such as, e.g., in sensors, catalysis, and separation technology.

Polyethylenimine: a very useful ionic polymer in the design of immobilized enzyme biocatalysts

This review discusses the possible roles of polyethylenimine (PEI) in the design of improved immobilized biocatalysts from diverse perspectives.

Bienzymatic Sequential Reaction on Microgel Particles and Their Cofactor Dependent Applications

We report, the preparation and characterization of bioconjugates, wherein enzymes pyruvate kinase (Pk) and l-lactic dehydrogenase (Ldh) were covalently bound to poly(N-isopropylacrylamide)-poly(ethylenimine) (PNIPAm-PEI) microgel support using glutaraldehyde (GA) as the cross-linker. The effects of different arrangements of enzymes on the microgels were investigated for the enzymatic behavior and to obtain maximum Pk-Ldh sequential reaction. The dual enzyme bioconjugates prepared by simultaneous addition of both the enzymes immobilized on the same microgel particles (PL), and PiLi, that is, dual enzyme bioconjugate obtained by combining single-enzyme bioconjugates (immobilized pyruvate kinase (Pi) and immobilized lactate dehydrogenase (Li)), were used to study the effect of the assembly of dual enzymes systems on the microgels. The kinetic parameters (Km, kcat), reaction parameters (temperature, pH), stability (thermal and storage), and cofactor dependent applications were studied for the dual enzymes conjugates. The kinetic results indicated an improved turn over number (kcat) for PL, while the kcat and catalytic efficiency was significantly decreased in case of PiLi. For cofactor dependent application, in which the ability of ADP monitoring and ATP synthesis by the conjugates were studied, the activity of PL was found to be nearly 2-fold better than that of PiLi. These results indicated that the influence of spacing between the enzymes is an important factor in optimization of multienzyme immobilization on the support.