Nano SiO was a kind of versatile material for enzyme

Nano-SiO2 was a kind of versatile material for enzyme immobilization because of its excellent properties, including low cost, lack of toxicity, high stability, large specific surface area and high biocompatibility [10]. After the immobilization on nano-SiO2, the selectivity of enzymatic reactions would be improved greatly and the immobilized enzyme also enjoyed the ability to resist the attack of microbial [11]. Enzyme could be immobilized on nano-SiO2 through different methods, and the most common way was to form covalent bonds by linker molecules, such as glutaraldehyde between the enzyme molecule and the carrier surface [12]. After immobilization, enzymatic properties are very different from those of free CU CPT 4a synthesis [13], such as optimal temperature, pH and stability. Besides, because of a limited exposure to environmental factors and to the constrains of polypeptide conformational freedom as a consequence of the interactions with the pore walls, the silica nanoparticles can facilitate enhanced enzyme stability [4]. Therefore, nano-SiO2 was a promising material for the application of enzyme in industry. In the past few years, Metal-Organic Frameworks (MOFs) have been developed a new type of nanomaterials to encapsulate free enzyme due to novel structures and potentially useful properties [14,15]. The high surface area, pore volume and tunable structures suggest huge potential of MOFs as the supporting materials for enzyme immobilization in industry [[15], [16], [17], [18], [19]]. Once the enzyme was immobilized in MOFs, the biochemical properties would be improved greatly, such as the thermostability, reusability and resistance ability against organic solvents or proteases [20]. In particular, ZIF-8 is one of the most promising nanomaterials in MOFs due to its merits to protect enzyme from denaturation while maintaining the bioactivity [14]. Lyu immobilized cyto-chrome c (Cyt c) by ZIF-8 to increase the peroxidase activity [21]; Wang immobilized glucose oxidase (GOx) and NiPd hollow nanoparticles simultaneously by ZIF-8 as the carrier to establish a mimic multi-enzyme system [22]. The β-glucosidase (EC 3.2.1.21) is a widespread enzyme that catalyze the hydrolysis of glycosides to produce glucose and aglycone [23,24]. It has been widely found in both prokaryotes and eukaryotes [[25], [26], [27]]. Nowadays, β-glucosidase has been applied in several industrial applications, some of which are biodegradation of cellulose [28] and ethanol production [29]. In this study, the β-glucosidase was chosen as the model enzyme to be immobilized by nano-SiO2 and ZIF-8, then the properties of the two kinds of immobilized enzymes were compared to evaluate the catalytic activity, stability and reusability.
Experimental
Results
Discussion A novel β-glucosidase from Agrocybe aegirit was purified by ultrafiltration and anion exchange. The β-glucosidase was immobilized by SiO2 nanoparticles and ZIF-8 via crosslinking and encapsulation method, respectively. In this study, the enzymatic properties of the nano-SiO2@β-glucosidase and the β-glucosidase@ZIF-8 were compared with free β-glucosidase. The optimal pH and temperature of the nano-SiO2@β-glucosidase and the β-glucosidase@ZIF-8 had no obvious difference from that of free β-glucosidase. After immobilization, the stability of β-glucosidase was improved greatly. In particular, the remarkable increase in the enzymatic activity of β-glucosidase@ZIF-8 at pH 8.0 was achieved. The possible situation is that the internal environment of ZIF-8 led to the growth of pH. The residual activity of the β-glucosidase@ZIF-8 in pH and temperature tolerance investigations showed significant improvement over free β-glucosidase. This enhancement could be attributed to the encapsulation of β-glucosidase by ZIF-8, which prevented the enzyme from leaking into solution. According to Liang [35], the rigid structure of ZIF-8 restricted the structural rearrangement as the temperature increased that led to denaturation of enzyme. Moreover, there might be other factors that affected the stability of immobilized β-glucosidase such as the morphology and size of immobilized enzyme.