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Construction of mutant human carbonic anhydrase 2 (CA2 (L203K)) expression plasmid

Because wild-type CA2 has the fastest reaction rate at 37 °C and loses its activity at 50 °C, so it may be not suitable for using wide type CA2 to capture CO2 under industrial operating conditions. Therefore, we use molecular simulation to design new high-efficiency and stable carbonic anhydrases by improving their catalytic properties and stability. Basing on the simulation results above, we finally determined that the suitable mutation site of CA2 with high and stable activity was L203K (the 203th leucine mutated into lysine).

Therefore, we constructed an expression vector containing CA2 (L203K) coding sequence for following activity assay (Fig. 1). The obtained recombinant vector was verified by restriction enzyme digestion (Fig. 2) and sequencing.

    

Fig. 1 Map of CA2 (L203K) recombinant vector

Fig. 2 Agarose Gel Electrophoresis of CA2(L203K) recombinant plasmid and its identification by enzyme digestion (NdeⅠand Hind Ⅲ). Lane M: DNA marker; Lane 1: CA2 (L203K) recombinant plasmid; Lane 2: enzyme digestion band of CA2 (L203K), the length was 825 bp (the arrow indicated).

Induced expression of CA2 (L203K) protein

The CA2 (L203K) expression plasmid was transformed into E. coli BL21 (DE3), and its expression was induced with IPTG, and identified by SDS-PAGE analysis. The results showed that CA2 (L203K) could be expressed in BL21 (DE3) strain and existed in soluble form in the cell lysate supernatant (Fig. 3).

    

Fig. 3 SDS-PAGE analysis for CA2 (L203K) cloned in pET-30a(+) vector and expressed in BL21(DE3) strain.

Purification of CA2 (L203K) protein

In order to detect the enzyme activity of CA2 (L203K) protein, we further purify the crude protein extract by nickel column to obtain purified CA2 (L203K) protein. CA2 (L203K) was purified with high purity as indicated by a significant single protein band after SDS-PAGE and Western blot (Fig. 4).

    

Fig. 4 SDS-PAGE and Western blot analysis of CA2 (L203K) protein. Lane 1: Negative control; Lane 2: purified CA2 (L203K) protein.

Enzyme activity assay of CA2-WT and CA2 (L203K) protein

Next, we determined the enzymatic activities of wild-type and mutant CA2 by colorimetric and esterase methods. As indicated in Fig. 5, specific activity of mutant CA2 was about 2 times greater than that of wild-type enzyme. The kinetic constants (Km and Vmax) were calculated for esterase activity assay, and the result showed that CA2 (L203K) protein has a higher activity than CA2-WT (Fig. 6).

    

Fig. 5 Colorimetric assay of CA2 activity

    

Fig. 6 Esterase activity analysis of CA2 protein

Thermal stability studies of CA2-WT and CA2 (L203K) protein

We then investigated the effect of temperature on CA2 activity by esterase activity assay. As shown in Fig. 7, as the temperature increases, especially at 55 °C and 65 °C, the enzymatic activity of CA2-WT was significantly decreased, while the mutant CA2 still retain relatively high activity, indicating that CA2 (L203K) was more stable at high temperature and retained its activity.

    

Fig. 7 Activity of purified CA2-WT and CA2 (L203K) protein under indicated temperatures and time points.

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