Optimized Conditions for Preparing a Heterogeneous Biocatalyst via Cross-Linked Enzyme Aggregates (CLEAs) of β-Glucosidase from Aspergillus niger

This study mainly aims to find the optimal conditions for immobilizing a non-commercial β-glucosidase from Aspergillus niger via cross-linked enzyme aggregates (CLEAs) by investigating the effect of cross-linking agent (glutaraldehyde) concentration and soy protein isolate/enzyme ratio (or spacer/enzyme ratio) on the catalytic performance of β-glucosidase through the central composite rotatable design (CCRD). The influence of certain parameters such as pH and temperature on the hydrolytic activity of the resulting heterogeneous biocatalyst was assessed and compared with those of a soluble enzyme. The catalytic performance of both the soluble and immobilized enzyme was assessed by hydrolyzing ρ-nitrophenyl-β-D-glucopyranoside (ρ-NPG) at pH 4.5 and 50 °C. It was found that there was a maximum recovered activity of around 33% (corresponding to hydrolytic activity of 0.48 U/mL) in a spacer/enzyme ratio of 4.69 (mg/mg) using 25.5 mM glutaraldehyde. The optimal temperature and pH conditions for the soluble enzyme were 60 °C and 4.5, respectively, while those for CLEAs of β-glucosidase were between 50 and 65 °C and pH 3.5 and 4.0. These results reveal that the immobilized enzyme is more stable in a wider pH and temperature range than its soluble form. Furthermore, an improvement was observed in thermal stability after immobilization. After 150 days at 4 °C, the heterogeneous biocatalyst retained 80% of its original activity, while the soluble enzyme retained only 10%. The heterogeneous biocatalyst preparation was also characterized by TG/DTG and FT-IR analyses that confirmed the introduction of carbon chains via cross-linking. Therefore, the immobilized biocatalyst prepared in this study has improved enzyme stabilization, and it is an interesting approach to preparing heterogeneous biocatalysts for industrial applications.