Enzymatic conversion of CO(2) to bicarbonate in functionalized mesoporous silica

Carbonic Anhydrase Robustness for Use in Nanoscale CO2 Capture Technologies

Continued dependence on crude oil and natural gas resources for fossil fuels has caused global atmospheric carbon dioxide (CO2) emissions to increase to record-setting proportions. There is an urgent need for efficient and inexpensive carbon sequestration systems to mitigate large-scale emissions of CO2 from industrial flue gas. Carbonic anhydrase (CA) has shown high potential for enhanced CO2 capture applications compared to conventional absorption-based methods currently utilized in various industrial settings. This study aims to understand structural aspects that contribute to the stability of CA enzymes critical for their applications in industrial processes, which require the ability to withstand conditions different from those in their native environments. Here, we evaluated the thermostability and enzyme activity of mesophilic and thermophilic CA variants at different temperature conditions and in the presence of atmospheric gas pollutants like nitrogen oxides and sulfur oxides. Based on our enzyme activity assays and molecular dynamics simulations, we see increased conformational stability and CA activity levels in thermostable CA variants incubated week-long at different temperature conditions. The thermostable CA variants also retained high levels of CA activity despite changes in solution pH due to increasing NO and SO2 concentrations. A loss of CA activity was observed only at high concentrations of NO/SO2 that possibly can be minimized with the appropriate buffered solutions.

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.

Engineering Olefin-Linked Covalent Organic Frameworks for Photoenzymatic Reduction of CO2

It is of profound significance concerning the global energy and environmental crisis to develop new techniques that can reduce and convert CO₂ . To address this challenge, we built a new type of artificial photoenzymatic system for CO₂ reduction, using a rationally designed mesoporous olefin-linked covalent organic framework (COF) as the porous solid carrier for co-immobilizing formate dehydrogenase (FDH) and Rh-based electron mediator. By adjusting the incorporating content of the Rh electronic mediator, which facilitates the regeneration of nicotinamide cofactor (NADH) from NAD⁺ , the apparent quantum yield can reach as high as 9.17±0.44 %, surpassing all reported NADH-regenerated photocatalysts constructed by crystalline framework materials. Finally, the assembled photocatalyst-enzyme coupled system can selectively convert CO₂ to formic acid with high efficiency and good reusability. This work demonstrates the first example using COFs to immobilize enzymes for artificial photosynthesis systems that utilize solar energy to produce value-added chemicals.

Towards Sustainable Oxalic Acid from CO2 and Biomass

To quickly and drastically reduce CO2 emissions and meet our ambitions of a circular future, we need to develop carbon capture and storage (CCS) and carbon capture and utilization (CCU) to deal with the CO2 that we produce. While we have many alternatives to replace fossil feedstocks for energy generation, for materials such as plastics we need carbon. The ultimate circular carbon feedstock would be CO2 . A promising route is the electrochemical reduction of CO2 to formic acid derivatives that can subsequently be converted into oxalic acid. Oxalic acid is a potential new platform chemical for material production as useful monomers such as glycolic acid can be derived from it. This work is part of the European Horizon 2020 project "Ocean" in which all these steps are developed. This Review aims to highlight new developments in oxalic acid production processes with a focus on CO2 -based routes. All available processes are critically assessed and compared on criteria including overall process efficiency and triple bottom line sustainability.

Enzymatic characteristics of immobilized carbonic anhydrase and its applications in CO2 conversion

Native carbonic anhydrase (CA) has been widely used in several different applications due to its catalytic function in the interconversion of carbon dioxide (CO₂) and carbonic acid. However, subject to its stability and recyclability, native CA often deactivates when in harsh environments, which restricts its applications in the commercial market. Maintaining the stability and high catalytic activity of CA is challenging. Immobilization provides an effective route that can improve enzymatic stability. Through the interaction of covalent bonds and van der Waals forces, water-soluble CA can be combined with various insoluble supports to form water-insoluble immobilized CA so that CA stability and utilization can be greatly improved. However, if the immobilization method or immobilization condition is not suitable, it often leads to a decrease in CA activity, reducing the application effects on CO₂ conversion. In this review, we discuss existing immobilization methods and applications of immobilized CA in the environmental field, such as the mineralization of carbon dioxide and multienzyme cascade catalysis based on CA. Additionally, prospects in current development are outlined. Because of the many outstanding and superior properties after immobilization, CA is likely to be used in a wide variety of scientific and technical areas in the future.

Immobilized carbonic anhydrase: preparation, characteristics and biotechnological applications

Carbonic anhydrase (CA) is an essential metalloenzyme in living systems for accelerating the hydration and dehydration of carbon dioxide. CA-catalyzed reactions can be applied in vitro for capturing industrially emitted gaseous carbon dioxide in aqueous solutions. To facilitate this type of practical application, the immobilization of CA on or inside solid or soft support materials is of great importance because the immobilization of enzymes in general offers the opportunity for enzyme recycling or long-term use in bioreactors. Moreover, the thermal/storage stability and reactivity of immobilized CA can be modulated through the physicochemical nature and structural characteristics of the support material used. This review focuses on (i) immobilization methods which have been applied so far, (ii) some of the characteristic features of immobilized forms of CA, and (iii) biotechnological applications of immobilized CA. The applications described not only include the CA-assisted capturing and sequestration of carbon dioxide, but also the CA-supported bioelectrochemical conversion of CO₂ into organic molecules, and the detection of clinically important CA inhibitors. Furthermore, immobilized CA can be used in biomimetic materials synthesis involving cascade reactions, e.g. for bone regeneration based on calcium carbonate formation from urea with two consecutive reactions catalyzed by urease and CA.

Immobilization of Carbonic Anhydrase in Glass Micropipettes and Glass Fiber Filters for Flow-Through Reactor Applications

There are various ways of immobilizing carbonic anhydrase (CA) on solid materials. One of the final aims is to apply immobilized CA for the catalytic hydration of carbon dioxide (CO₂) as a first step in the conversion of gaseous CO₂ into solid products. The immobilization method investigated allows a straightforward, stable, and quantifiable immobilization of bovine erythrocyte carbonic anhydrase (BCA) on silicate surfaces. The method is based on the use of a water-soluble, polycationic second-generation dendronized polymer with on average 1000 repeating units, abbreviated as de-PG2₁₀₀₀. Several copies of BCA were first covalently linked to de-PG2₁₀₀₀ through stable bisaryl hydrazone (BAH) bonds. Then, the de-PG2₁₀₀₀-BAH-BCA conjugates obtained were adsorbed noncovalently either on microscopy glass coverslips, inside glass micropipettes, or in porous glass fiber filters. The apparent density of the immobilized BCA on the glass surfaces was about 8-10 pmol/cm². In all three cases, the immobilized enzyme was highly active and stable when tested with p-nitrophenyl acetate as a model enzyme substrate at room temperature. The micropipettes and the glass fiber filters were applied as flow-through systems for continuous operation at room temperature. In the case of the glass fiber filters, the filters were placed inside a homemade flow-through filter holder which allows flow-through runs with more than one filter connected in series. This offers the opportunity of increasing the substrate conversion by increasing the number of BCA-containing filters.

Immobilization of carbonic anhydrase on polyvinylidene fluoride membranes

In recent years, the application of carbonic anhydrase (CA) in CO₂ removal has attracted great interest. However, obtaining high enzyme recovery activity is difficult in existing immobilization techniques. In this work, water plasma-treated poly(vinylidene fluoride) (PVDF) membranes were modified via 3-aminopropyl triethoxy silane (KH550) or γ-(2, 3-epoxypropoxy) propyl trimethoxy silane (KH560), and then CA was attached. The immobilization process was optimized, and the catalytic properties of PVDF-attached CA were characterized. The maximum activity recovery of PVDF-KH550-CA was 60%, whereas that of PVDF-KH560-CA was 33%. The K_m values of PVDF-KH550-CA, PVDF-KH560-CA, and free enzyme were 9.97 ± 0.37, 12.5 ± 0.2, and 6.18 ± 0.23 mM, respectively, and their K_cat /K_m values were 206 ± 2, 117 ± 5, and 488 ± 4 M^-1 ·Sec^-1 . PVDF-attached CA shows excellent storage stability and reusability, and their half-life values were 82 and 78 days at 4 °C. At 25 °C, they were 50 and 37 days, respectively. PVDF-KH550-CA and PVDF-KH560-CA retained approximately 85% and 72% of the initial activity after undergoing 10 cycles. In the presence of them, the generation rates of CaCO₃ were 76% and 65% of the free CA system, which were 1.6 and 1.3 times that of the blank system, respectively. Its role in accelerating CO₂ sequestration holds great promise for its practical application.

Structural and biophysical characterization of the α-carbonic anhydrase from the gammaproteobacterium Thiomicrospira crunogena XCL-2: insights into engineering thermostable enzymes for CO2 sequestration

Biocatalytic CO2 sequestration to reduce greenhouse-gas emissions from industrial processes is an active area of research. Carbonic anhydrases (CAs) are attractive enzymes for this process. However, the most active CAs display limited thermal and pH stability, making them less than ideal. As a result, there is an ongoing effort to engineer and/or find a thermostable CA to fulfill these needs. Here, the kinetic and thermal characterization is presented of an α-CA recently discovered in the mesophilic hydrothermal vent-isolate extremophile Thiomicrospira crunogena XCL-2 (TcruCA), which has a significantly higher thermostability compared with human CA II (melting temperature of 71.9°C versus 59.5°C, respectively) but with a tenfold decrease in the catalytic efficiency. The X-ray crystallographic structure of the dimeric TcruCA shows that it has a highly conserved yet compact structure compared with other α-CAs. In addition, TcruCA contains an intramolecular disulfide bond that stabilizes the enzyme. These features are thought to contribute significantly to the thermostability and pH stability of the enzyme and may be exploited to engineer α-CAs for applications in industrial CO2 sequestration.

Towards efficient chemical synthesis via engineering enzyme catalysis in biomimetic nanoreactors

Biocatalysis with immobilized enzymes as catalysts holds enormous promise in developing more efficient and sustainable processes for the synthesis of fine chemicals, chiral pharmaceuticals and biomass feedstocks. Despite the appealing potentials, nowadays the industrial-scale application of biocatalysts is still quite modest in comparison with that of traditional chemical catalysts. A critical issue is that the catalytic performance of enzymes, the sophisticated and vulnerable catalytic machineries, strongly depends on their intracellular working environment; however the working circumstances provided by the support matrix are radically different from those in cells. This often leads to various adverse consequences on enzyme conformation and dynamic properties, consequently decreasing the overall performance of immobilized enzymes with regard to their activity, selectivity and stability. Engineering enzyme catalysis in support nanopores by mimicking the physiological milieu of enzymes in vivo and investigating how the interior microenvironment of nanopores imposes an influence on enzyme behaviors in vitro are of paramount significance to modify and improve the catalytic functions of immobilized enzymes. In this feature article, we have summarized the recent advances in mimicking the working environment and working patterns of intracellular enzymes in nanopores of mesoporous silica-based supports. Especially, we have demonstrated that incorporation of polymers into silica nanopores could be a valuable approach to create the biomimetic microenvironment for enzymes in the immobilized state.

Enzymes immobilized in mesoporous silica: a physical-chemical perspective

Mesoporous materials as support for immobilized enzymes have been explored extensively during the last two decades, primarily not only for biocatalysis applications, but also for biosensing, biofuels and enzyme-controlled drug delivery. The activity of the immobilized enzymes inside the pores is often different compared to that of the free enzymes, and an important challenge is to understand how the immobilization affects the enzymes in order to design immobilization conditions that lead to optimal enzyme activity. This review summarizes methods that can be used to understand how material properties can be linked to changes in enzyme activity. Real-time monitoring of the immobilization process and techniques that demonstrate that the enzymes are located inside the pores is discussed by contrasting them to the common practice of indirectly measuring the depletion of the protein concentration or enzyme activity in the surrounding bulk phase. We propose that pore filling (pore volume fraction occupied by proteins) is the best standard for comparing the amount of immobilized enzymes at the molecular level, and present equations to calculate pore filling from the more commonly reported immobilized mass. Methods to detect changes in enzyme structure upon immobilization and to study the microenvironment inside the pores are discussed in detail. Combining the knowledge generated from these methodologies should aid in rationally designing biocatalyst based on enzymes immobilized in mesoporous materials.

Heated proteins are still active in a functionalized nanoporous support

Even under heated conditions, the nearly native conformation and activity of a protein can be hoarded in a functionalized nanoporous support via non-covalent interaction. Surprisingly, the protein released from the heated protein-nanoporous composite can maintain its nearly native conformation and activity, while free proteins are permanently denatured under the same treatment.