Enzyme inspired complexes for industrial CO2 capture: Opportunities and challenges

Addressing the Effectiveness and Molecular Mechanism of the Catalytic CO2 Hydration in Aqueous Solutions by Nickel Nanoparticles

Hydration of carbon dioxide in water solution is the rate limiting step for the CO2 mineralization process, a process which is at the base of many carbon capture and utilization (CCU) technologies aiming to convert carbon dioxide to added-value products and mitigate climate change. Here, we present a combined experimental and computational study to clarify the effectiveness and molecular mechanism by which nickel nanoparticles, NiNPs, may enhance CO2 hydration in aqueous solutions. Contrary to previous literature, our kinetic experiments recording changes of pHs, conductivity, and dissolved carbon dioxide in solution reveal a minimal effect of the NiNPs in catalyzing CO2 hydration. Our atomistic simulations indicate that the Ni metal surface can coordinate only a limited number of water molecules, leaving uncoordinated metal sites for the binding of carbon dioxide or other cations in solution. This deactivates the catalyst and limits the continuous re-formation of a hydroxyl-decorated surface, which was a key chemical step in the previously suggested Ni-catalyzed hydration mechanism of carbon dioxide in aqueous solutions. At our experimental conditions, which expand the investigation of NiNP applicability toward a wider range of scenarios for CCU, NiNPs show a limited catalytic effect on the rate of CO2 hydration. Our study also highlights the importance of the solvation regime: while Ni surfaces may accelerate carbon dioxide hydration in water restricted environments, it may not be the case in fully hydrated conditions.

Computational analysis of the effect of Gly100Ala mutation on the thermostability of SazCA

Maintaining the protein stability upon mutation is a challenging task in protein engineering. In the present computational study, we induced a single point Gly100Ala mutation in SazCA and examined the factors governing the stability and flexibility of the mutated form, and compared it to that of the wildtype using molecular dynamics simulations. We observed higher structural stability and lesser residual mobility in the mutated SazCA. Improved H-bonding due to Gly100Ala was observed. Ala100 was responsible for the increased helical contents in the mutated SazCA while Gly100 compromised the secondary structure contents in the wildtype. A strong network of salt bridges and high local ordering of the solvent molecules at the protein surface contributed to the enhanced stability of the mutated protein. Our simulations conclusively highlight Gly100Ala mutation as a step towards designing a more robust and thermostable SazCA.Communicated by Ramaswamy H. Sarma.

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.

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.

Simultaneous removal of CO2, NOx and SOx using single stage absorption column

Capturing flue gases often require multiple stages of scrubbing, increasing the capital and operating costs. So far, no attempt has been made to study the absorption characteristics of all the three gases (NO, SO₂ and CO₂) in a single stage absorption unit at alkaline pH conditions. We have attempted to capture all the three gases with a single wet scrubbing column. The absorption of all three gases with sodium carbonate solution promoted with oxidizers was investigated in a tall absorption column. The absorbance was found to be 100% for CO₂, 30% for NO and 95% for SO₂ respectively. The capture efficiency of sodium carbonate solution was increased by 40% for CO₂ loading, with the addition of oxidizer. Absorption kinetics and reaction pathways of all the three gases were discussed individually in detail.

Biocatalytic CO2 Absorption and Structural Studies of Carbonic Anhydrase under Industrially-Relevant Conditions

The unprecedently high CO₂ levels in the atmosphere evoke the urgent need for development of technologies for mitigation of its emissions. Among the alternatives, the biocatalytic route has been claimed as one of the most promising. In the present work, the carbonic anhydrase from bovine erythrocytes (BCA) was employed as a model enzyme for structural studies in an aqueous phase at alkaline pH, which is typical of large-scale absorption processes under operation. Circular dichroism (CD) analysis revealed a high enzymatic stability at pH 10 with a prominent decrease of the melting temperature above this value. The CO₂ absorption capacity of the aqueous solutions were assessed by online monitoring of pressure decay in a stainless-steel cell, which indicated a better performance at pH 10 with a kinetic rate increase of up to 43%, as compared to non-biocatalytic conditions. Even low enzyme concentrations (0.2 mg g^-1) proved to be sufficient to improve the overall CO₂ capture process performance. The enzyme-enhanced approach of CO₂ capture presents a high potential and should be further studied.

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.

Kinetic study of catalytic CO2 hydration by metal-substituted biomimetic carbonic anhydrase model complexes

The rapid rise of the CO₂ level in the atmosphere has spurred the development of CO₂ capture methods such as the use of biomimetic complexes that mimic carbonic anhydrase. In this study, model complexes with tris(2-pyridylmethyl)amine (TPA) were synthesized using various transition metals (Zn²⁺, Cu²⁺ and Ni²⁺) to control the intrinsic proton-donating ability. The pK_a of the water coordinated to the metal, which indicates its proton-donating ability, was determined by potentiometric pH titration and found to increase in the order [(TPA)Cu(OH₂)]²⁺ < [(TPA)Ni(OH₂)]²⁺ < [(TPA)Zn(OH₂)]²⁺. The effect of pK_a on the CO₂ hydration rate was investigated by stopped-flow spectrophotometry. Because the water ligand in [(TPA)Zn(OH₂)]²⁺ had the highest pK_a, it would be more difficult to deprotonate it than those coordinated to Cu²⁺ and Ni²⁺. It was, therefore, expected that the complex would have the slowest rate for the reaction of the deprotonated water with CO₂ to form bicarbonate. However, it was confirmed that [(TPA)Zn(OH₂)]²⁺ had the fastest CO₂ hydration rate because the substitution of bicarbonate with water (bicarbonate release) occurred easily.

Biomimetic Approach to CO2 Reduction

The development of artificial photosynthetic technologies able to produce solar-fuels from CO₂ reduction is a fundamental task that requires the employment of specific catalysts being accomplished. Besides, effective catalysts are also demanded to capture atmospheric CO₂, mitigating the effects of its constantly increasing emission. Biomimetic transition metal complexes are considered ideal platforms to develop efficient and selective catalysts to be implemented in electrocatalytic and photocatalytic devices. These catalysts, designed according to the inspiration provided by nature, are simple synthetic molecular systems capable of mimic features of the enzymatic activity. The present review aims to focus the attention on the mechanistic and structural aspects highlighted to be necessary to promote a proper catalytic activity. The determination of these characteristics is of interest both for clarifying aspects of the catalytic cycle of natural enzymes that are still unknown and for developing synthetic molecular catalysts that can readily be applied to artificial photosynthetic devices.