Team:AHUT China/Background

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     Project Background

Background:

     Nowadays, greenhouse effect is the most important problem that people are facing, which attracts great attention from governments. CO2 accounts for about two-thirds of the total greenhouse gas, and it is the most important gas which causes the greenhouse effect. As a result, various measures to control carbon dioxide emissions, such as CO2 capture, are becoming more and more important. The main methods of CO2 capture include solvent absorption, physical adsorption and membrane separation.
    Solvent absorption is mainly based on chemical absorption. Some chemical absorbents can react with CO2 to form compounds and separate CO2 from the flue gas containing the absorbent. However, this method also has the following disadvantages: the solution is easy to oxidize and decompose; the solution is highly corrosive and easy to corrode the instrument; large energy consumption and high operation cost.
Physical adsorption method collects CO2 according to the adsorption characteristics of different components of gas to solid adsorbent. However, this method requires a large number of adsorbent to maintain the operation of this process, and the adsorbent has poor selectivity, low adsorption capacity and low efficiency, resulting in high operating costs and few practical applications.
    Membrane separation method is based on the polymer film to different gas components of different permeability to achieve the purpose of separation of different gas components. The membranes used can be divided into organic membranes and inorganic membranes. Among them, the organic membrane has strong selectivity for the gas components, simple assembly, but poor heat resistance and corrosion resistance. The inorganic membrane, on the contrary, has good heat resistance and corrosion resistance, but its assembly is complex. In general, the CO2 captured by this method is low in purity , requiring multiple purification and less application in industry.
    The above CO2 capture technologies all have the disadvantages of high cost, low efficiency and poor circulability. These unavoidable disadvantages hinder their application in production and life. Therefore, new technologies are urgently needed, and the technology of carbonic anhydrase (CA) capture makes up for the shortage of other methods. Carbonic anhydrase, a metal enzyme containing Zn2+, can catalyze CO2 and H2O to produce HCO3- (as shown in figure 1). Carbonic anhydrase catalyzes faster than other types of enzymes. The range of carbonic anhydrase catalytic rates is 104 to 106 reactions per second. Among various sources of carbonic anhydrase, human carbonic anhydrase has the highest catalytic efficiency.




    

Fig1_Catalytic mechanism of carbonic anhydrase (CA2)




    The molecular weight of carbonic anhydrase is about 30KDa, which is composed of a single peptide chain and contains about 260 amino acids. Each enzyme molecule contains one Zn2+. The structure is ellipsoid, with a pouch cavity in the middle about 1.5nm deep and a cavity opening about 2.0nm wide. Zn2+ binds at the bottom of the cavity. At present, the most studied type of carbonic anhydrase is cosine-family CA, also known as CA2. Its main secondary structure is in its enzyme molecule 10 pali-fold. It is because of their existence that the enzyme structure is divided into two parts. Many key amino acid residues in enzyme molecules are related to their activity. In addition to extension-folding, the surface of the enzyme molecules is also distributed in the form of an icy-helical structure, which is usually a short structure (Fig 2).




    

Fig2_The structure of carbonic anhydrase (CA2)




    Compared with other methods, carbonic anhydrase capture technology is specific and can capture CO2 from other gases. In addition, this method is environmentally friendly. Carbonic anhydrase converts CO2 into bicarbonate, which can meet the growth demand of plants and microorganisms. When CO2 is converted to bicarbonate, bicarbonate can combine with calcium ions to form calcium carbonate, which is stably stored underground.

    Compared with other methods, carbonic anhydrase capture technology is more efficient. The main rate-limiting step of general CO2 capture technology is the hydration reaction of CO2, while carbonic anhydrase can significantly increase the hydration reaction rate of CO2, thus improving the CO2 capture efficiency.

     Although carbonic anhydrase capture technology has high efficiency, it still has some limitations. Because most natural carbonic anhydrases are too sensitive to the reaction environment and are not thermally stable. However, when carbonic anhydrase is involved in CO2 capture, the ambient temperature is generally 65 ° C, while natural carbonic anhydrase cannot remain stable at this temperature, and the enzyme activity is lost after multiple cycles.The price of carbonic anhydrase is relatively expensive, and frequent replacement will greatly increase the cost of capture, which limits the wide spread of carbonic anhydrase capture technology. So the key now is to look for the thermal stability of carbonic anhydrase.
In order to find the high efficiency catalysis and high stability of carbonic anhydrase, in this project, we use the high efficiency catalysis characteristic of human carbonic anhydrase 2 (hereinafter referred to as CA2), use molecular simulation method to optimize its amino acid sequence, and design the CA2 mutant with high activity and high stability. The project includes the following aspects:
1) molecular simulation;
2) construction of escherichia coli strains expressing wild-type and mutant CA2;
3) expression and purification of CA2;
4) practical application of CA2: CO2 capture.



References:

[1]Rahman F A, Aziz M M A, Saidur R, et al. Pollution to solution: Capture and sequestration of carbon dioxide (CO2) and its utilization as a renewable energy source for a sustainable future[J]. Renewable & Sustainable Energy Reviews, 2017, 71:112-126.
[2]Claudiu T. Supuran. Structure and function of carbonic anhydrases[J]. Biomolecular Biochemical journal, 2016, 473(14):2023-2032.
[3]Lionetto M G, Caricato R, Erroi E, et al. Potential application of carbonic anhydrase activity in bioassay and biomarker studies [J]. Chemistry & Ecology, 2006, 22(sup1): S119-S25.
[4]Migliardini F, De L V, Carginale V, et al. Biomimetic CO2 capture using a highly thermostable bacterial α-carbonic anhydrase immobilized on a polyurethane foam [J]. Journal of Enzyme Inhibition & Medicinal Chemistry, 2014, 29(1):146.