Full Text - Optimal immobilization of β-glucosidase into chitosan beads using response surface methodology

Optimal immobilization of β-glucosidase into chitosan beads using response surface methodology

¹Huazhong Agricultural University, College of Food Science and Technology, Hongshan District, Wuhan, China
²Wuhan Grain Administration, Wuhan, Hubei, China

Financial support: This work was financially supported by the Innovation Foundation of Huazhong Agricultural University, China (Grant No. 2013QC032).

Keywords: β-glucosidase; characters; chitosan beads; immobilization; response surface methodology.

Background: β-Glucosidase is known as an effective catalyst for the hydrolysis of various glycosides and immobilization is one of the most efficient strategies to improve its activity recovery and properties.

Results: Crosslinking-adsorption-crosslinking method was employed to immobilize β-glucosidase into chitosan beads and response surface methodology (RSM) was used to optimize the immobilized conditions of the maximum activity recovery. Enzyme concentration and adsorption time were found to be significant influence factors, and the maximum activity recovery (50.75%) obtained from response surface methodology was in excellent agreement with experimental value (50.81%). Furthermore, various characteristics of immobilized β-glucosidase were evaluated. Compared to the free β-glucosidase, the immobilized enzyme exhibited broader pH and temperature ranges, enhanced thermal stability, better storage stability and reusability and higher accessibility of the substrate to the immobilized β-glucosidase.

Conclusion: Response surface methodology (RSM) was proved to be much economical for optimum immobilization of β-glucosidase into chitosan beads.

β-glucosidase (EC 3.2.1.21), widely existed in various types of living organisms, is known as an effective catalyst for the hydrolysis of various glycosides (Njokweni et al. 2012). Recently, it was reported to be used for enhancement of aroma in wine (Palmeri and Spagna, 2007), tea (Su et al. 2010), fruit (Fan et al. 2011), hydrolysis of cellulose (Shewale, 1982), and bioconversion of isoflavone glycosides to aglycones (Rekha and Vijayalakshmi, 2010). It is currently receiving much attention for their potential industrial applications. However, there are still some limitations for the widely application of β-glucosidase, because of its structure instability and the high cost of recovery of enzymes for repeated applications (Nisha et al. 2012; Dicosimo et al. 2013; Sheldon and van Pelt, 2013). Immobilization of β-glucosidase is considered to be the promising method to overcome these drawbacks.

Immobilized enzymes are versatile catalysts in the laboratory and in an industrial scale. Immobilization facilitates the efficient recovery and reuse of costly enzymes (Sheldon, 2007). Additional advantages include enhanced stability, great operational control, flexibility of reactor design, and easy separation from catalyst and unreacted substrate contamination. Chitosan is an inexpensive, inert, hydrophilic, biocompatible support, and it is considered to be an excellent carrier for enzyme immobilization (Çetinus and Öztop, 2003; Krajewska, 2004). The presence of amino groups of chitosan facilitates covalent binding of enzymes. Immobilization can be carried out either by entrapment into chitosan beads (Freeman and Dror, 1994) or by covalent binding to transparent chitosan films (Çetinus and Öztop, 2000) or by using glutaraldehyde from the formation of the Schiff’s base (Ravi Kumar, 1999). β-glucosidase has been reported to be immobilized in chitosan gels, but the maximum activity recovery of immobilization was only 26.9% (Spagna et al. 2002). So it is of great importance to further improve the activity recovery of immobilized β-glucosidase into chitosan beads for industrial application.

The designing of an efficient immobilized enzyme is a multivariate process involving many factors that could affect immobilization efficiency. Response surface methodology (RSM) is a statistically designed experimental protocol for developing, improving, and optimizing processes (Bouaid et al. 2009). It helps to identify the effect of the interactions of different design variables on the response when they are varied simultaneously. Furthermore, central composite design (CCD), the most successful factorial design, was also used for the optimization of parameters with a limited number of experiments and minimum costs (Singh et al. 2008). Recently, RSM has been successfully used for the optimization of several immobilization processes, including immobilization of Gluconobacter oxydans in Ca-alginate gel (Wu et al. 2010), Thermomyces lanuginosus lipase on styrene-divinylbenzene copolymer (Aybastier and Demir, 2010), acetylcholine esterase on ceramic packing (Ebrahimi et al. 2010) and chick pea β-galactosidase to alkylamine glass (Kishore and Kayastha, 2012).

In this research, β-glucosidase was immobilized into chitosan beads and response surface methodology (RSM) was used to optimize the immobilized conditions of β-glucosidase for the maximum activity recovery. Furthermore, various properties for free and immobilized β-glucosidase such as the Ph-activity curve, the temperature-activity curve, thermal stability, storage stability and reusability were evaluated.

β-Glucosidase (2.18 U/mg) was purchased from Sigma-Aldrich Co. LLC. p-NPG was provided by Shanghai Source Leaves Biological Technology Co., LTD. Chitosan (Degree of deacetylation 90%) was obtained from Wuhan Huashun Biological Technology Co., LTD. Glutaraldehyde (GA) was bought from Sinopharm Chemical Reagent Co., Ltd. All other chemicals were of analytical grade.

The pretreated chitosan beads were prepared based on a reported method with minor modification (Çetinus and Öztop, 2003; Hung et al. 2003). Chitosan flakes (2.0 g) were dissolved in 100 mL of acetic acid solution (5.0%, v/v) by needles (0.7 x 30 TWLB). The resulting solution was added into 200 mL of NaOH solution (2.0 M) containing 40 mL of ethanol through a needle with a diameter of 0.7 mm at room temperature, and the chitosan gelled spheres formed instantaneously. After hardening, the pretreated chitosan beads were separated and washed with deionized water until the filtrate became neutral (pH 7.0). The diameter of the pretreated beads was approximately 1.5-1.8 mm.

Then the pretreated chitosan beads (0.1 g) were added into 10 mL of glutaraldehyde solution (3.0%) and the mixtures were stirred at 25ºC and 150 rpm for 2 hrs. The crosslinking chitosan beads were separated on a filter and washed with 100 mM phosphate-citric acid buffer (pH 5.0).

Direct adsorption. 0.1 g pretreated chitosan beads were added into 1 mL β-glucosidase solution (0.1 mg/mL) and the solution was stirred at 25ºC, 150 rpm for 5.0 hrs. Then the chitosan beads with adsorbed enzyme were separated and washed thrice with 100 mM phosphate-citric acid buffer (pH 5.0) to remove unbound enzymes.

Adsorption-crosslinking. The procedure of adsorption was the same as the above description. After direct adsorption, the washed chitosan beads with adsorbed enzyme were added into 10 mL of 3.0% (v/v) glutaraldehyde solution. The mixture was stirred at 25ºC and 150 rpm for 2.0 hrs. The immobilized enzyme was separated and washed thrice with 100 mM phosphate-citric acid buffer (pH 5.0).

Crosslinking-adsorption. 0.1 g crosslinking chitosan beads and 1 mL β-glucosidase solution (0.1 mg/ml) were mixed and stirred at 25ºC, 150 rpm for 5.0 hrs. The crosslinking chitosan beads with adsorbed enzyme were washed thrice with 100 mM phosphate-citric acid buffer (pH 5.0) to remove unbound enzymes.

Crosslinking-adsorption-crosslinking. The main procedure was the combination of crosslinking-adsorption and adsorption-crosslinking.

The activity of β-glucosidase was determined by monitoring the release of p-NPG. A total of 100 μL of enzyme solution was incubated with 900 μL of 100 mM phosphate-citric acid buffer (pH 5.0) containing 10 mM p-NPG for 15 min at 50ºC. The amount of p-nitrophenol released was measured at λ = 400 nm after addition of 2 M Na₂CO₃ to the reaction mixtures. One unit of p-NPG-hydrolyzing activity was defined as the amount of enzyme equivalent to release 1 μmol of p-nitrophenol per minute.

The activity of immobilized β-glucosidase was measured by the same method as above, except that 100 μL of the enzyme solution was replaced by 100 μL phosphate-citric acid buffer and a given amount of immobilized enzyme.

The immobilized β-glucosidase activity recovery was calculated as: activity recovery (%) = (total activity of immobilized enzyme/total activity of enzyme solution) x 100%.

Factorial design was performed using Minitab Statistical Software (Minitab 15) to screen the significant influence factors. The influence factors were chosen after a series of preliminary experiments (data not shown). Effects of various factors on immobilization of β-glucosidase into chitosan beads were evaluated using 2-level-10-factor experimental design with 16 experiments.

Response surface methodology was used to determine optimum conditions of immobilization to acquire the maximum activity recovery, and a 3-level-2-factor central composite design with 14 experiments was performed. All experiments were repeated at least three times. Minitab Statistical Software was used both in experimental design and the statistical analysis of the results.

The optimum pH of free and immobilized β-glucosidase was determined in 100 mM Na₂HPO₄-C₆H₈O₇ buffer (pH 3.0-7.0). The optimum temperature of free and immobilized β-glucosidase was determined by assaying the enzyme activity at various temperatures (30-80ºC) in 100 mM phosphate-citric acid buffer (pH 5.0). The thermal stability of free and immobilized β-glucosidase was evaluated as follow: The β-glucosidase was kept in a water bath for 1 hr at different temperatures (30-80ºC), and followed by residual activity assay under standard conditions.

The Michaelis constant (Km) and maximum velocity (Vmax) of free and immobilized β-glucosidase were measured by assaying the enzyme activity at different p-NPG concentrations. The Km and Vmax were calculated using Lineweaver-Burk plot (Lineweaver and Burk, 1934).

The activity of free and immobilized β-glucosidase were evaluated after a long period (50 days) storage in phosphate-citric acid buffer (pH 5.0) at 4ºC under different time intervals.

For reusability assessment, stored immobilized β-glucosidase was reused 10 times and the residual activity was measured. After each reaction, the immobilized sample was washed with 100 mM Na₂HPO₄-C₆H₈O₇ buffer (pH 5.0) to remove any attached substrate. All conditions were kept constant for every batch.

In present work, four different covalent binding methods were used to immobilize β-glucosidase, and the results of the activity recovery are listed in Table 1. According to these results, the crosslinking-adsorption-crosslinking method was the best one (Entry 4), and the activity recovery reached 32.55%. The loss of activation may be caused by the restriction of the support which resulted in limited mobility and accessibility of the active sites (Fan et al. 2002). Consequently, the crosslinking-adsorption-crosslinking method was chosen to immobilize β-glucosidase in the following experiments.

Screening of the significant factors by factorial design. To further improve the activity recovery of immobilized β-glucosidase, a 2-level-10-factor factorial design was firstly employed to screen the significant factors for immobilization of β-glucosidase, and the response was set as the activity recovery of immobilized β-glucosidase (Table 2). Table 3 shows their analysis of variance and model-fitting results. Among those factors, enzyme concentration and adsorption time were significant factors (P value < 0.05, entries 7 and 8).

Optimization of immobilized conditions by response surface methodology. Based on the analysis of the factorial design, enzyme concentration and adsorption time significantly affected the immobilization of β-glucosidase into chitosan beads. Thus, these two factors were further optimized using response surface methodology to obtain the maximum activity recovery of immobilized β-glucosidase. To assess the influence of each factor and their interactions, a central composite design of 6 centre points with 14 experiments was performed. The levels of each factor are given in Table 4 and the experimental data on the basis of activity recovery are recorded in Table 5.

By regression analysis (Table 6), a quadratic polynomial equation was established to explain the relationship between the activity recovery and independent variables as follows:

Y = 0.430546-0.10608 x A + 0.049386 x B - 0.04129 x A²- 0.01102 x A x B - 0.11533 x B²

Where Y, A and B represent the activity recovery, enzyme concentration and adsorption time, respectively. Regression analysis (Table 6) showed a coefficient of determination (R²) value of 0.9996, indicating that this model could explain 99.96% of the variability in activity recovery. The adjusted R² value (0.9992) was also high, meaning that this model has high significance. The predicted R² value (0.9970) was considered reasonable to predict responses by using the model. The P-value serves as a tool for checking the significance of each coefficient. A smaller P-value indicates a higher level of significance for the corresponding coefficient (Li et al. 2007). From regression analysis (Table 6), it can be also observed that the regression coefficients of the linear terms (A, B), the quadratic terms (A², B²) and the interaction term (A x B) were all significant at 1% level, implying that above coefficients have noticeable effects on the immobilization of β-glucosidase.

The summary of analysis of variance (ANOVA) for the model of activity recovery is given in Table 7. The calculated F ratio was 3233.61 and P value was 0.000, implying that the model is highly significant and satisfying. The large value of F indicates that the most of the variation in the response can be explained by the regression equation. The associated P value was used to estimate whether F was large enough to indicate statistical significance. If P value was lower than 0.05, it indicates that the model was statistically significant (Ranjan et al. 2009). Moreover, the lack of fit F ratio was 1.98 and P value was 0.259, indicating that the lack of fit was also statistically significant.

In order to understand the interaction between enzyme concentration and adsorption time as well as determine the maximum activity recovery corresponding to the optimum conditions of both variables, their response surface and contour plots are represented in Figure 1 and Figure 2, which indicates the response surface and contour plots have the same results. Optimal conditions for enzyme concentration and adsorption time were 67.15 μg/mL and 5.54 hrs, respectively, leading to the maximum activity recovery of 50.75%. A verification experiment was performed under the optimal conditions to validate the reliability and accuracy of the model. The experimental activity recovery was found to be 50.81%, which is in excellent agreement with the value predicted by the model based on central composite design (50.75%). Consequently, this model could be considered reliable and accurate for predicting the activity recovery of immobilization of β-glucosidase. The activity recovery of immobilized β-glucosidase on chitosan in previous report was only 26.90% (Spagna et al. 2002). Evidently, the activity recovery of immobilized β-glucosidase on chitosan beads has been obviously improved in our study. Furthermore, the activity recovery in our study is much higher than that of immobilized β-glucosidase on other various carriers, such as sodium alginate (28.20%) (Fan et al. 2011), S-layer (37.00%) (Balogh et al. 2005), Eupergit C (30.00%) (Tu et al. 2006), and sol-gel beads (28.00%) (O'Neill et al. 2002).

Optimum pH. The optimal pH of free and immobilized β-glucosidase was investigated at diverse pH (3.0-7.0) at the same temperature. As shown in Figure 3, the optimal pH of free and immobilized β-glucosidase were both 5.0, but the immobilized β-glucosidase has a broader pH range of high activity.

Optimum temperature and thermal stability. The optimal temperature of free and immobilized β-glucosidase was investigated in the temperatures range of 30-80ºC. Figure 4 shows that the optimal temperature is 60ºC for the free β-glucosidase and 70ºC for the immobilized β-glucosidase. This shift toward high temperature for the immobilized β-glucosidase was similar with the published literatures (Yodoya et al. 2003; Jung et al. 2012). Moreover, Immobilization β-glucosidase evidently widened the working temperature range and the relative activity of the immobilized β-glucosidase still remained over 84% at 40-70ºC. The reason for increased temperature stability maybe because that immobilization of β-glucosidase increased the rigidity of the enzyme and the support had a protection effect at the high temperatures at which enzyme deactivation took place (Yodoya et al. 2003; Singh et al. 2011).

The thermal stability of free and immobilized β-glucosidase was investigated after incubating for 1 hr at diverse temperatures (30-80ºC). As shown in Figure 5, the activity of the immobilized β-glucosidase decreased much more slowly than that of the free β-glucosidase at the temperature range from 50ºC to 60ºC. The free enzyme only retained 35% of its initial activity, whereas the immobilized enzyme was found to keep almost 64% of its initial activity. Since the immobilization increased the rigidity of the enzyme, the heat-resistance of immobilized β-glucosidase increased considerably (Chiou and Wu, 2004; Dwevedi and Kayastha, 2009). Thus, compared to the original free enzyme, the immobilized enzyme could work in harsh environmental conditions with less activity loss (Spagna et al. 2002; Reddy and Kayastha, 2006; Dwevedi and Kayastha, 2009).

As expected, the K_m and V_max values of β-glucosidase were significantly affected after immobilization into chitosan beads. As shown in Figure 6, Kinetic parameters, K_m values of the free and immobilized β-glucosidase were found to be 3.12 and 2.51 mM, respectively. The decrease in K_m of immobilized β-glucosidase may be due to the affinity change of the enzyme to its substrate or conformational change in the immobilization of β-glucosidase into chitosan beads, which caused higher accessibility of the substrate to the immobilized β-glucosidase. Similar results were also observed in β-glucosidase immobilization on alginate (Su et al. 2010; Fan et al. 2011).

The storage stability of an enzyme is a prior condition to evaluate its industrial applicability. The free β-glucosidase lost more than 80% of its initial activity after storage for 50 days, whereas the immobilized β-glucosidase was found to lose less than 20% of its initial activity (Figure 7a). The results showed that the immobilized β-glucosidase exhibited higher stability than the free one, and similar phenomenon has also been documented recently (Su et al. 2010; Fan et al. 2011; González-Pombo et al. 2011).

The reusability of immobilized β-glucosidase is of key importance for their industrial application (Gupta et al. 2009; Zhang et al. 2010). As indicated in Figure 7b, the activity of immobilized enzymes was retained more than 89% after 10 reuses. Evidently, the immobilized β-glucosidase has a more stable performance in reuse. The activity loss of immobilized enzyme upon reuse could be caused by the weakening of binding strength between the enzyme and supporter. Moreover, the immobilized enzyme might be distorted by the frequent encountering between the active site and substrate, and thereby its catalytic efficiency might be lowered (Dwevedi and Kayastha, 2009). Similar results about reusability for the immobilized β-glucosidase were reported in other immobilization studies (O'Neill et al. 2002; Singh et al. 2011).

This is the first report about optimization of β-glucosidase immobilization into chitosan beads by response surface methodology. After immobilized by crosslinking-adsorption-crosslinking method, a factorial design was applied to successfully select two significant factors including enzyme concentration and adsorption time, and then RSM was employed to obtain the optimal levels of enzyme concentration (67.15 μg/ml) and adsorption time (5.54 hrs). The predicted maximum activity recovery (50.75%) is in excellent agreement with the experimental value (50.81%), which is much higher than that of other immobilized methods reported before. The immobilized β-glucosidase showed improved enzymatic properties than the free form, such as broader pH and temperature ranges, enhanced thermal stability, higher accessibility of the substrate, better storage stability and superior reusability. Improved activity recovery and enzymatic properties of immobilized β-glucosidase in present study will promote its industrial application.

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