Structure and activity of magnetic cross-linked enzyme aggregates of bovine carbonic anhydrase as promoters of enzymatic CO 2 capture

Direct Biocatalytic Processes for CO2 Capture as a Green Tool to Produce Value-Added Chemicals

Direct biocatalytic processes for CO₂ capture and transformation in value-added chemicals may be considered a useful tool for reducing the concentration of this greenhouse gas in the atmosphere. Among the other enzymes, carbonic anhydrase (CA) and formate dehydrogenase (FDH) are two key biocatalysts suitable for this challenge, facilitating the uptake of carbon dioxide from the atmosphere in complementary ways. Carbonic anhydrases accelerate CO₂ uptake by promoting its solubility in water in the form of hydrogen carbonate as the first step in converting the gas into a species widely used in carbon capture storage and its utilization processes (CCSU), particularly in carbonation and mineralization methods. On the other hand, formate dehydrogenases represent the biocatalytic machinery evolved by certain organisms to convert CO₂ into enriched, reduced, and easily transportable hydrogen species, such as formic acid, via enzymatic cascade systems that obtain energy from chemical species, electrochemical sources, or light. Formic acid is the basis for fixing C₁-carbon species to other, more reduced molecules. In this review, the state-of-the-art of both methods of CO₂ uptake is assessed, highlighting the biotechnological approaches that have been developed using both enzymes.

Carbonic anhydrase as a tool to mitigate global warming

The global average temperature breaks the record every year, and this unprecedented speed at which it is unfolding is causing serious climate change which in turn impacts the lives of humans and other living organisms. Thus, it is imperative to take immediate action to limit global warming. Increased CO2 emission from the industrial sector that relies on fossil fuels is the major culprit. Mitigating global warming is an uphill battle that involves an integration of technologies such as switching to renewable energy, increasing the carbon sink capacity, and implementing carbon capture and sequestration (CCS) on major sources of CO2 emissions. Among all these methods, CCS is globally accepted as a potential technology to address this climate change. CCS using carbonic anhydrase (CA) is gaining momentum due to its advantages over other conventional CCS technologies. CA is a metalloenzyme that catalyses a fundamental reaction for life, i.e. the interconversion of bicarbonate and protons from carbon dioxide and water. The practical application of CA requires stable CAs operating under harsh operational conditions. CAs from extremophilic microbes are the potential candidates for the sequestration of CO2 and conversion into useful by-products. The soluble free form of CA is expensive, unstable, and non-reusable in an industrial setup. Immobilization of CA on various support materials can provide a better alternative for application in the sequestration of CO2. The present review provides insight into several types of CAs, their distinctive characteristics, sources, and recent developments in CA immobilization strategies for application in CO2 sequestration.

(Magnetic) Cross-Linked Enzyme Aggregates of Cellulase from T. reesei: A Stable and Efficient Biocatalyst

Cross-linked enzyme aggregates (CLEAs) represent an effective tool for carrier-free immobilization of enzymes. The present study promotes a successful application of functionalized magnetic nanoparticles (MNPs) for stabilization of cellulase CLEAs. Catalytically active CLEAs and magnetic cross-linked enzyme aggregates (mCLEAs) of cellulase from Trichoderma reesei were prepared using glutaraldehyde (GA) as a cross-linking agent and the catalytic activity and stability of the CLEAs/mCLEAs were investigated. The influence of precipitation agents, cross-linker concentration, concentration of enzyme, addition of bovine serum albumin (BSA), and addition of sodium cyanoborohydride (NaBH₃CN) on expressed activity and immobilization yield of CLEAs/mCLEAs was studied. Particularly, reducing the unsaturated Schiff's base to form irreversible linkages is important and improved the activity of CLEAs (86%) and mCLEAs (91%). For increased applicability of CLEAs/mCLEAs, we enhanced the activity and stability at mild biochemical process conditions. The reusability after 10 cycles of both CLEAs and mCLEAs was investigated, which retained 72% and 65% of the initial activity, respectively. The thermal stability of CLEAs and mCLEAs in comparison with the non-immobilized enzyme was obtained at 30 °C (145.65% and 188.7%, respectively) and 50 °C (185.1% and 141.4%, respectively). Kinetic parameters were determined for CLEAs and mCLEAs, and the K_M constant was found at 0.055 ± 0.0102 mM and 0.037 ± 0.0012 mM, respectively. The maximum velocity rate (V_max) was calculated as 1.12 ± 0.0012 µmol/min for CLEA and 1.17 ± 0.0023 µmol/min for mCLEA. Structural characterization was studied using XRD, SEM, and FT-IR. Catalytical properties of immobilized enzyme were improved with the addition of reducent NaBH₃CN by enhancing the activity of CLEAs and with addition of functionalized aminosilane MNPs by enhancing the activity of mCLEAs.

Carbonic anhydrase to boost CO2 sequestration: Improving carbon capture utilization and storage (CCUS)

CO₂ Capture Utilization and Storage (CCUS) is a fundamental strategy to mitigate climate change, and carbon sequestration, through absorption, can be one of the solutions to achieving this goal. In nature, carbonic anhydrase (CA) catalyzes the CO₂ hydration to bicarbonates. Targeting the development of novel biotechnological routes which can compete with traditional CO₂ absorption methods, CA utilization has presented a potential to expand as a promising catalyst for CCUS applications. Driven by this feature, the search for novel CAs as biocatalysts and the utilization of enzyme improvement techniques, such as protein engineering and immobilization methods, has resulted in suitable variants able to catalyze CO₂ absorption at relevant industrial conditions. Limitations related to enzyme recovery and recyclability are still a concern in the field, affecting cost efficiency. Under different absorption approaches, CA enhances both kinetics and CO₂ absorption yields, besides reduced energy consumption. However, efforts directed to process optimization and demonstrative plants are still limited. A recent topic with great potential for development is the CA utilization in accelerated weathering, where industrial residues could be re-purposed towards becoming carbon sequestrating agents. Furthermore, research of new solvents has identified potential candidates for integration with CA in CO₂ capture, and through techno-economic assessments, CA can be a path to increase the competitiveness of alternative CO₂ absorption systems, offering lower environmental costs. This review provides a favorable scenario combining the enzyme and CO₂ capture, with possibilities in reaching an industrial-like stage in the future.

Immobilization of carbonic anhydrase for CO2 capture and utilization

Carbonic anhydrase (CA) is an excellent candidate for novel biocatalytic processes based on the capture and utilization of CO₂. The setup of efficient methods for enzyme immobilization makes CA utilization in continuous bioreactors increasingly attractive and opens up new opportunities for the industrial use of CA. The development of efficient processes for CO₂ capture and utilization (CCU) is one of the most challenging targets of modern chemical reaction engineering. In the general frame of CCU processes, the interest in the utilization of immobilized CA as a biocatalyst for augmentation of CO₂ reactive absorption has grown consistently over the last decade. The present mini-review surveys and discusses key methodologies for CA immobilization aimed at the development of heterogeneous biocatalysts for CCU. Advantages and drawbacks of covalent attachment on fine granular solids, immobilization as cross-linked enzyme aggregates, and "in vivo" immobilization methods are presented. In particular, criteria for optimal selection of CA-biocatalyst and design of CO₂ absorption units are presented and discussed to highlight the most effective solutions. Perspectives on biocatalytic CCU processes that can include the use of CA in an enzymatic reactive CO₂ absorption step are eventually presented with a special focus on two examples of CO₂ fixation pathways: hybrid enzyme-microalgae process and enzyme cascade for the production of carboxylic acids. KEY POINTS: • Covalent immobilization techniques applied to CA are effective for CO₂ ERA. • Biocatalyst type and morphology must be selected considering CO₂ ERA conditions. • Immobilized CA can offer novel routes to CO₂ capture and direct utilization.

In vivo immobilized carbonic anhydrase and its effect on the enhancement of CO2 absorption rate

Reactive absorption into aqueous solutions promoted by carbonic anhydrase (CA, E.C. 4.2.1.1.) has been often proposed as a post-combustion CO₂ capture process. The state of the art reveals the need for efficient biocatalyst based on carbonic anhydrase that can be used to further develop CO₂ capture and utilization technologies. The present study is focused on the use of a thermostable CA-based biocatalyst. The carbonic anhydrase SspCA, from the thermophilic bacterium Sulfurihydrogenibium yellowstonense, was in vivo immobilized as membrane-anchored protein (INPN-SspCA) on the outer membrane of Escherichia coli cells. The dispersed biocatalyst, made by cell membrane debris, was characterized in terms of its contribution to the enhancement of CO₂ absorption in carbonate/bicarbonate alkaline buffer at operating conditions relevant for industrial CO₂ capture processes. The amount of immobilized enzyme, estimated by SDS-PAGE, resulted in about 1 mg enzyme/g membrane debris. The apparent kinetics of the biocatalyst was characterized through CO₂ absorption tests in a stirred cell lab-scale reactor assuming a pseudo-homogeneous behaviour of the biocatalyst. At 298 K, the assessed values of the second-order kinetic constant ranged between 0.176 and 0.555 L∙mg^-1∙s^-1. Reusability of the biocatalyst after 24 h showed the absence of free enzyme release in the alkaline solvent. Moreover, the equilibration of dispersed cell membrane debris against the alkaline buffer positively affected the performances of the heterogeneous biocatalyst. These results encourage further studies on the in vivo immobilized SspCA aimed at optimizing the enzyme loading on the cell membrane and the handling of the biocatalyst in the CO₂ absorption reactors.

Cross-linking of carbonic anhydrase and formate dehydrogenase based on amino acid specific recognition: Conversion of carbon dioxide to formic acid

Inspired by the cascades performed in vivo, the assembly of multiple enzymes in vitro has strongly moved into the focus of researchers in the field of biocatalysis. In this study, a new, mild and accurate enzyme cross-linking method is revealed. Microbial transglutaminase (MTG) acts as a "cross-linking medium" by identifying the amide group of the glutamine and the primary amine group of lysine in the artificial peptide tags specifically to form an iso-peptide bond. Here, carbonic anhydrase (CA) and formate dehydrogenase (FDH) with different peptide tags that can be recognized by MTG were linked together to obtain different proportions of cross-linked enzymes for efficient conversion of greenhouse gas carbon dioxide to formic acid. After cross-linking, we obtained "one-to-one" and "one-to-more" cross-linked enzyme aggregates. There is a minor residual loss of the two enzymes, the remaining enzyme activity of CA is more than 93%, and the remaining enzyme activity of FDH is more than 84%. In particular, the overall catalytic efficiency of the cross-linked enzyme is increased by 5.8 times compared with free enzymes and the thermal stability of FDH at different temperatures is improved. The applied strategy demonstrates the potential application of MTG in multi-enzyme assembly and synthetic biology.

Magnetic Cross-Linked Enzyme Aggregates of a Transpeptidase-Specialized Variant (N450D) of Bacillus licheniformis γ-Glutamyl Transpeptidase: An Efficient and Stable Biocatalyst for l-Theanine Synthesis

γ-Glutamyl transpeptidase (GGT) catalyzes the transfer of glutathione’s γ-glutamyl group and related γ-glutamyl amides to water, amino acids or peptides, and utilizes a conserved Thr residue to process its own polypeptide chain into a large and a small subunit that then assemble to produce a catalytically competent enzyme. In this study, the magnetic cross-linked enzyme aggregates (mCLEAs) of a transpeptidase-specialized variant (N450D) of Bacillus licheniformis GGT were successfully prepared with optimized process parameters viz.1.25:1 (v/v) of isopropanol to N450D (0.3 mg/mL) ratio/0.02:1 (w/w) of enzyme to 3-aminopropyl triethoxysilane (APTES)-coated magnetic nanoparticle ratio/20 mM of glutaraldehyde. The prepared magnetic nanoparticles and immobilized enzyme (N450D-mCLEAs) were characterized by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, field-emission scanning electron microscope integrated with energy dispersive X-ray spectroscopy (FESEM/EDS), and superparamagnetic analysis. As compared with free enzyme, N450D-mCLEAs displayed significantly higher heat resistance at temperatures of 55 and 60 °C, and had a greater stability over a storage period of one month. The immobilized enzyme could also be reused for 10 consecutive biocatalytic cycles with no significant reduction in the percent yield of l-theanine. Conclusively, this immobilization strategy surely provides a meaningful glance of developing N450D-mediated biocatalysis for the production of physiologically important γ-glutamyl compounds.

High throughput acoustic microfluidic mixer controls self-assembly of protein nanoparticles with tuneable sizes

HYPOTHESIS

Protein nanoparticles have attracted increased interest due to their broad applications ranging from drug delivery and vaccines to biocatalysts and biosensors. The morphology and the size of the nanoparticles play a crucial role in determining their suitability for different applications. Yet, effectively controlling the size of the nanoparticles is still a significant challenge in their manufacture. The hypothesis of this paper is that the assembly conditions and size of protein particles can be tuned via a mechanical route by simply modifying the mixing time and strength, while keeping the chemical parameters constant.

EXPERIMENTAL

We use an acoustically actuated, high throughput, ultrafast, microfluidic mixer for the assembly of protein particles with tuneable sizes. The performance of the acoustic micro-mixer is characterized via Laser Doppler Vibrometry and image processing. The assembly of protein nanoparticles is monitored by dynamic light scattering (DLS) and transmission electron microscopy (TEM).

FINDINGS

By changing actuation parameters, the turbulence and mixing in the microchannel can be precisely varied to control the initiation of protein particle assembly while the solution conditions of assembly (pH and ionic strength) are kept constant. Importantly, mixing times as low as 6 ms can be achieved for triggering protein assembly in the microfluidic channel. In comparison to the conventional batch process of assembly, the acoustic microfluidic mixer approach produces smaller particles with a more uniform size distribution, promising a new way to manufacture protein particles with controllable quality.

Stabilized and Immobilized Carbonic Anhydrase on Electrospun Nanofibers for Enzymatic CO2 Conversion and Utilization in Expedited Microalgal Growth

Carbonic anhydrases convert CO₂ to bicarbonate at a high turnover rate up to 10⁶ s^-1, but their actual applications in CO₂ conversion processes are hampered by their poor stability. This study reports highly loaded and stabilized bovine carbonic anhydrase (bCA) upon being immobilized onto electrospun polymer nanofibers in the form of enzyme precipitate coating (EPC). The EPC protocol, consisting of enzyme covalent attachment, precipitation, and cross-linking, maintained 65.3% of initial activity even after being incubated in aqueous solution at room temperature under shaking at 200 rpm for 868 days. EPC also showed strong resistance to the treatment of the metal chelation agent, ethylenediaminetetraacetic acid, and molecular dynamic simulation was carried out to elucidate the prevention of metal leaching from the active site of bCA upon being cross-linked in the form of EPC. Highly stable EPC with high bCA loading was employed for the conversion of bubbling CO₂ to bicarbonate, and the bicarbonate solution was utilized as a carbon source for expedited microalgae growth in a separate bioreactor. The addition of EPC in the bubbling CO₂ reactor resulted in 134 and 231% accelerated microalgae growths compared to the controls with and without 25 mM sodium bicarbonate, respectively. EPC with high enzyme loading and unprecedentedly successful stabilization of enzyme stability has a great potential to be used for the development of various enzyme-mediated CO₂ conversion and utilization technologies.

User-Tailored Metal-Organic Frameworks as Supports for Carbonic Anhydrase

Carbonic anhydrase (CA) was previously proposed as a green alternative for biomineralization of carbon dioxide (CO₂). However, enzyme's fragile nature when in synthetic environment significantly limits such industrial application. Herein, we hypothesized that CA immobilization onto flexible and hydrated "bridges" that ensure proton-transfer at their interfaces leads to improved activity and kinetic behavior and potentially increases enzyme's feasibility for industrial implementation. Our hypothesis was formulated considering that water plays a key role in the CO₂ hydration process and acts as both the reactant as well as the rate-limiting step of the CO₂ capture and transformation process. To demonstrate our hypothesis, two types of user-synthesized organic metallic frameworks [metal-organic frameworks (MOFs), one hydrophilic and one hydrophobic] were considered as model supports and their surface characteristics (i.e., charge, shape, curvature, size, etc.) and influence on the immobilized enzyme's behavior were evaluated. Morphology, crystallinity and particle size, and surface area of the model supports were determined by scanning electron microscopy, dynamic light scattering, and nitrogen adsorption/desorption measurements, respectively. Enzyme activity, kinetics, and stability at the supports interfaces were determined using spectroscopical analyses. Analysis showed that enzyme functionality is dependent on the support used in the immobilization process, with the enzyme immobilized onto the hydrophilic support retaining 72% activity of the free CA, when compared with that immobilized onto the hydrophobic one that only retained about 28% activity. Both CA-MOF conjugates showed good storage stability relative to the free enzyme in solution, with CA immobilized at the hydrophilic support also revealing increased thermal stability and retention of almost all original enzyme activity even after heating treatment at 70 °C. In contrast, free CA lost almost half of its original activity when subject to the same conditions. This present work suggests that MOFs tunable hydration conditions allow high enzyme activity and stability retention. Such results are expected to impact CO₂ storage and transformation strategies based on CA and potentially increase user-integration of enzyme-based green technologies in mitigating global warming.

Evaluation of Strategies to Produce Highly Porous Cross-Linked Aggregates of Porcine Pancreas Lipase with Magnetic Properties

The preparation of highly porous magnetic crosslinked aggregates (pm-CLEA) of porcine pancreas lipase (PPL) is reported. Some strategies to improve the volumetric activity of the immobilized biocatalyst were evaluated, such as treatment of PPL with enzyme surface-modifying agents (polyethyleneimine or dodecyl aldehyde), co-aggregation with protein co-feeders (bovine serum albumin and/or soy protein), use of silica magnetic nanoparticles functionalized with amino groups (SMNPs) as separation aid, and starch as pore-making agent. The combination of enzyme surface modification with dodecyl aldehyde, co-aggregation with SMNPs and soy protein, in the presence of 0.8% starch (followed by hydrolysis of the starch with α-amylase), yielded CLEAs expressing high activity (immobilization yield around 100% and recovered activity around 80%), high effectiveness factor (approximately 65% of the equivalent free enzyme activity) and high stability at 40 °C and pH 8.0, i.e., PPL CLEAs co-aggregated with SMNPs/bovine serum albumin or SMNPs/soy protein retained 80% and 50% activity after 10 h incubation, respectively, while free PPL was fully inactivated after 2 h. Besides, highly porous magnetic CLEAs co-aggregated with soy protein and magnetic nanoparticles (pm-SP-CLEAs) showed good performance and reusability in the hydrolysis of tributyrin for five 4h-batches.