Optimization of extracellular ethanol-tolerant β-glucosidase production from a newly isolated Aspergillus sp. DHE7 via solid state fermentation using jojoba meal as substrate: purification and biochemical characterization for biofuel preparation

Background: The increasing demand and the continuous depletion in fossil fuels have persuaded researchers to investigate new sources of renewable energy. Bioethanol produced from cellulose could be a cost-effective and a viable alternative to petroleum. It is worth note that beta-glucosidase plays a key role in the hydrolysis of cellulose and therefore in the production of bioethanol. This study aims to investigate a simple and standardized method for maximization of extracellular beta-glucosidase production from a novel fungal isolate under solid-state fermentation using agro-industrial residues as the sole source of carbon and nitrogen. Furthermore, purification and characterization of beta-glucosidase were performed to determine the conditions under which the enzyme displayed the highest performance. Results: A fungus identified genetically as a new Aspergillus sp. DHE7 was found to exhibit the highest extracellular beta-glucosidase production among the sixty fungal isolates tested. Optimization of culture conditions improved the enzyme biosynthesis by 2.1-fold (174.6 +/- 5.8 U/g of dry substrate) when the fungus grown for 72 h at 35 degrees C on jojoba meal with 60% of initial substrate moisture, pH 6.0, and an inoculum size of 2.54 x 10(7) spores/mL The enzyme was purified to homogeneity through a multi-step purification process. The purified beta-glucosidase is monomeric with a molecular mass of 135 kDa as revealed by the SDS-PAGE analysis. Optimum activity was observed at 60 degrees C and pH of 6.0, with a remarkable pH and thermal stability. The enzyme retained about 79% and 53% of its activity, after 1 h at 70 degrees C and 80 degrees C, respectively. The purified beta-glucosidase hydrolysed a wide range of substrates but displaying its greater activity on p-nitrophenyl-beta-D-glucopyranoside and cellobiose. The values of K-m and V-max on p-nitrophenyl beta-D-glucopyranoside were 0.4 mM and 232.6 U/mL, respectively. Purified beta-glucosidase displayed high catalytic activity (improved by 25%) in solutions contained ethanol up to 15%. Conclusion: beta-glucosidase characteristics associated with its ability to hydrolyse cellobiose, underscore its utilization in improving the quality of food and beverages. In addition, taking into consideration that the final concentration of ethanol produced by the conventional methods is about 10%, suggests its use in ethanol-containing industrial processes and in the saccharification processes for bioethanol production.

Automated summary

A novel fungal isolate was identified as the most efficient in producing extracellular beta-glucosidase among sixty isolates tested in this study. Optimal conditions for enzyme biosynthesis were determined, leading to a 2.1-fold increase in enzyme production. The purified beta-glucosidase displayed high catalytic activity and stability, making it a promising candidate for various industrial processes, particularly in ethanol production and food quality improvement.