Polyelectrolyte Multilayers Enhance the Dry Storage and pH Stability of Physically Entrapped Enzymes

Single-Particle and Single-Molecule Characterization of Immobilized Enzymes: A Multiscale Path toward Optimizing Heterogeneous Biocatalysts

Heterogeneous biocatalysis is highly relevant in biotechnology as it offers several benefits and practical uses. To leverage the full potential of heterogeneous biocatalysts, the establishment of well-crafted protocols, and a deeper comprehension of enzyme immobilization on solid substrates are essential. These endeavors seek to optimize immobilized biocatalysts, ensuring maximal enzyme performance within confined spaces. For this aim, multidimensional characterization of heterogeneous biocatalysts is required. In this context, spectroscopic and microscopic methodologies conducted at different space and temporal scales can inform about the intraparticle enzyme kinetics, the enzyme spatial distribution, and the mass transport issues. In this Minireview, we identify enzyme immobilization, enzyme catalysis, and enzyme inactivation as the three main processes for which advanced characterization tools unveil fundamental information. Recent advances in operando characterization of immobilized enzymes at the single-particle (SP) and single-molecule (SM) levels inform about their functional properties, unlocking the full potential of heterogeneous biocatalysis toward biotechnological applications.

Layer-by-layer assembly of poly-l-lysine/hyaluronic acid protein reservoirs on poly(glycerol sebacate) surfaces

The modification of biomaterial surfaces has become increasingly relevant in the context of ongoing advancements in tissue engineering applications and the development of tissue-mimicking polymer materials. In this study, we investigated the layer-by-layer (LbL) deposition of polyelectrolyte multilayer protein reservoirs consisting of poly-l-lysine (PLL) and hyaluronic acid (HA) on the hydrophobic surface of poly(glycerol sebacate) (PGS) elastomer. Using the methods of isothermal titration calorimetry and surface plasmon resonance, we systematically investigated the interactions between the polyelectrolytes and evaluated the deposition process in real time, providing insight into the phenomena associated with film assembly. PLL/HA LbL films deposited on PGS showed an exceptional ability to incorporate bone morphogenetic protein-2 (BMP-2) compared to other growth factors tested, thus highlighting the potential of PLL/HA LbL films for osteoregenerative applications. The concentration of HA solution used for film assembly did not affect the thickness and topography of the (PLL/HA)10 films, but had a notable impact on the hydrophilicity of the PGS surface and the BMP-2 release kinetics. The release kinetics were successfully described using the Weibull model and hyperbolic tangent function, underscoring the potential of these less frequently used models to compare the protein release from LbL protein reservoirs.

Uricase Crowding via Polyelectrolyte Layers Coacervation for Carbon Fiber-Based Electrochemical Detection of Uric Acid

Urate oxidase (UOx) surrounded by synthetic macromolecules, such as polyethyleneimine (PEI), poly(allylamine hydrochloride) (PAH), and poly(sodium 4-styrenesulfonate) (PSS) is a convenient model of redox-active biomacromolecules in a crowded environment and could display high enzymatic activity towards uric acid, an important marker of COVID-19 patients. In this work, the carbon fiber electrode was modified with Prussian blue (PB) redox mediator, UOx layer, and a layer-by-layer assembled polyelectrolyte film, which forms a complex coacervate consisting of a weakly charged polyelectrolyte (PEI or PAH) and a highly charged one (PSS). The film deposition process was controlled by cyclic voltammetry and scanning electron microscopy coupled with energy-dispersive X-ray analysis (at the stage of PB deposition) and through quartz crystal microbalance technique (at latter stages) revealed uniform distribution of the polyelectrolyte layers. Variation of the polyelectrolyte film composition derived the following statements. (1) There is a linear correlation between electrochemical signal and concentration of uric acid in the range of 10^-4-10^-6 M. (2) An increase in the number of polyelectrolyte layers provides more reproducible values for uric acid concentration in real urine samples of SARS-CoV-2 patients measured by electrochemical enzyme assay, which are comparable to those of spectrophotometric assay. (3) The PAH/UOx/PSS/(PAH/PSS)₂-coated carbon fiber electrode displays the highest sensitivity towards uric acid. (4) There is a high enzyme activity of UOx immobilized into the hydrogel nanolayer (values of the Michaelis-Menten constant are up to 2 μM) and, consequently, high affinity to uric acid.

Advancing bone tissue engineering one layer at a time: a layer-by-layer assembly approach to 3D bone scaffold materials

The layer-by-layer (LbL) assembly technique has shown excellent potential in tissue engineering applications. The technique is mainly based on electrostatic attraction and involves the sequential adsorption of oppositely charged electrolyte complexes onto a substrate, resulting in uniform single layers that can be rapidly deposited to form nanolayer films. LbL has attracted significant attention as a coating technique due to it being a convenient and affordable fabrication method capable of achieving a wide range of biomaterial coatings while keeping the main biofunctionality of the substrate materials. One promising application is the use of nanolayer films fabricated by LbL assembly in the development of 3-dimensional (3D) bone scaffolds for bone repair and regeneration. Due to their versatility, nanoscale films offer an exciting opportunity for tailoring surface and bulk property modification of implants for osseous defect therapies. This review article discusses the state of the art of the LbL assembly technique, and the properties and functions of LbL-assembled films for engineered bone scaffold application, combination of multilayers for multifunctional coatings and recent advancements in the application of LbL assembly in bone tissue engineering. The recent decade has seen tremendous advances in the promising developments of LbL film systems and their impact on cell interaction and tissue repair. A deep understanding of the cell behaviour and biomaterial interaction for the further development of new generations of LbL films for tissue engineering are the most important targets for biomaterial research in the field. While there is still much to learn about the biological and physicochemical interactions at the interface of nano-surface coated scaffolds and biological systems, we provide a conceptual review to further progress in the LbL approach to 3D bone scaffold materials and inform the future of LbL development in bone tissue engineering.

Intraparticle Kinetics Unveil Crowding and Enzyme Distribution Effects on the Performance of Cofactor-Dependent Heterogeneous Biocatalysts

Multidimensional kinetic analysis of immobilized enzymes is essential to understand the enzyme functionality at the interface with solid materials. However, spatiotemporal kinetic characterization of heterogeneous biocatalysts on a microscopic level and under operando conditions has been rarely approached. As a case study, we selected self-sufficient heterogeneous biocatalysts where His-tagged cofactor-dependent enzymes (dehydrogenases, transaminases, and oxidases) are co-immobilized with their corresponding phosphorylated cofactors [nicotinamide adenine dinucleotide phosphate (NAD(P)H), pyridoxal phosphate (PLP), and flavin adenine dinucleotide (FAD)] on porous agarose microbeads coated with cationic polymers. These self-sufficient systems do not require the addition of exogenous cofactors to function, thus avoiding the extensive use of expensive cofactors. To comprehend the microscopic kinetics and thermodynamics of self-sufficient systems, we performed fluorescence recovery after photobleaching measurements, time-lapse fluorescence microscopy, and image analytics at both single-particle and intraparticle levels. These studies reveal a thermodynamic equilibrium that rules out the reversible interactions between the adsorbed phosphorylated cofactors and the polycations within the pores of the carriers, enabling the confined cofactors to access the active sites of the immobilized enzymes. Furthermore, this work unveils the relationship between the apparent Michaelis-Menten kinetic parameters and the enzyme density in the confined space, eliciting a negative effect of molecular crowding on the performance of some enzymes. Finally, we demonstrate that the intraparticle apparent enzyme kinetics are significantly affected by the enzyme spatial organization. Hence, multiscale characterization of immobilized enzymes serves as an instrumental tool to better understand the in operando functionality of enzymes within confined spaces.

Mimicking natural strategies to create multi-environment enzymatic reactors: From natural cell compartments to artificial polyelectrolyte reactors

Engineering microenvironments for sequential enzymatic reactions has attracted specific interest within different fields of research as an effective strategy to improve the catalytic performance of enzymes. While in industry most enzymatic reactions occur in a single compartment carrier, living cells are however able to conduct multiple reactions simultaneously within confined sub-compartments, or organelles. Engineering multi-compartments with regulated environments and transformation properties enhances enzyme activity and stability and thus increases the overall yield of final products. In this review, we discuss current and potential methods to fabricate artificial cells for sequential enzymatic reactions, which are inspired by mechanisms and metabolic pathways developed by living cells. We aim to advance the understanding of living cell complexity and its compartmentalization and present solutions to mimic these processes in vitro. Particular attention has been given to layer-by-layer assembly of polyelectrolytes for developing multi-compartments. We hope this review paves the way for the next steps toward engineering of smart artificial multi-compartments with adoptive stimuli-responsive properties, mimicking living cells to improve catalytic properties and efficiency of the enzymes and enhance their stability.

Faster Surface Ligation Reactions Improve Immobilized Enzyme Structure and Activity

During integration into materials, the inactivation of enzymes as a result of their interaction with nanometer size denaturing "hotspots" on surfaces represents a critical challenge. This challenge, which has received far less attention than improving the long-term stability of enzymes, may be overcome by limiting the exploration of surfaces by enzymes. One way this may be accomplished is through increasing the rate constant of the surface ligation reaction and thus the probability of immobilization with reactive surface sites (i.e., ligation efficiency). Here, the connection between ligation reaction efficiency and the retention of enzyme structure and activity was investigated by leveraging the extremely fast reaction of strained trans-cyclooctene (sTCOs) and tetrazines (Tet). Remarkably, upon immobilization via Tet-sTCO chemistry, carbonic anhydrase (CA) retained 77% of its solution-phase activity, while immobilization via less efficient reaction chemistries, such as thiol-maleimide and azide-dibenzocyclooctyne, led to activity retention of only 46% and 27%, respectively. Dynamic single-molecule fluorescence tracking methods further revealed that longer surface search distances prior to immobilization (>0.5 μm) dramatically increased the probability of CA unfolding. Notably, the CA distance to immobilization was significantly reduced through the use of Tet-sTCO chemistry, which correlated with the increased retention of structure and activity of immobilized CA compared to the use of slower ligation chemistries. These findings provide an unprecedented insight into the role of ligation reaction efficiency in mediating the exploration of denaturing hotspots on surfaces by enzymes, which, in turn, may have major ramifications in the creation of functional biohybrid materials.

Single molecule characterization of anomalous transport in a thin, anisotropic film

The diffusion of small, charged molecules incorporated in an anisotropic polyelectrolyte multilayer (PEM) was tracked in three dimensions by combining single-molecule fluorescence localization (to characterize lateral diffusion) with Förster resonance energy transfer (FRET) between diffusing molecules and the supporting surface (to measure diffusion in the surface-normal direction). Analysis of the surface-normal diffusion required model-based statistical analysis to account for the inherently noisy FRET signal. Combining these distinct single-molecule methods, which are inherently sensitive to different length-scales, permitted simultaneous characterization of severely anisotropic diffusion, which was more than three orders of magnitude slower in the surface-normal direction. We hypothesize that the anomalously slow surface-normal diffusion was related to the periodic distribution of charge in the PEM, which created electrostatic barriers. The motion was strongly subdiffusive, with anomalous temporal scaling exponents in lateral and normal directions, suggesting a connection to the transient, random fractal conformation of polymer chains in the film's matrix.

Chitosan-based CLEAs from Aspergillus niger type A feruloyl esterase: high-productivity biocatalyst for alkyl ferulate synthesis

The enzymatic synthesis of alkyl ferulates is an important reaction in cosmetic and pharmaceutical chemistries, since it may allow to expand the biorefinery concept valorizing biomass wastes enriched in ferulic acid. However, robust biocatalysts for that purpose are scarce. Herein, we have immobilized the type A feruloyl esterase from Aspergillus niger (AnFaeA) as cross-linked enzyme aggregates, employing chitosan as co-feeder (ChCLEAs). High immobilization yields and relative activity recovery were attained in all assessed conditions (> 93%). Furthermore, we enhanced the thermal stability of the soluble enzyme 32-fold. AnFaeA-ChCLEAs were capable to quantitatively perform the solvent-free direct esterification of short- to medium-chain alkyl ferulates (C4-C12) in less than 24 h. By raising the operational temperature to 50 °C, AnFaeA-ChCLEAs transformed 350 mM ferulic acid into isopentyl ferulate with a space-time yield of 46.1 g of product × L^-1 × day^-1, 73-fold higher than previously reported. The overall sustainability of this alkyl ferulate production bioprocess is supported by the high total turnover number (TTN 7 × 10⁵) and the calculated green metrics (E factor = 30). Therefore, we herein present a robust, efficient, and versatile heterogeneous biocatalyst useful for the synthesis of a wide diversity of alkyl ferulates. KEY POINTS: • CLEAs of feruloyl esterase A from A. niger using chitosan as co-feeder were obtained. • Microenvironment of the biocatalysts allowed to obtain C1 to C18 alkyl ferulates. • Biocatalyst at boundary conditions showed a high productivity of 46 g/L day. Graphical Abstract.