Optimizing bioethanol production from hassawi rice straw with Aspergillus sp. NAS51 cellulosic enzyme and in silico homology modeling

Abstract

Researchers' attention has been greatly focused on the use of lignocellulosic wastes to produce bioethanol and biogas due to the depletion of non-renewable energy resources. In our effort to find a potent cellulase-producing fungal strain, the fungus NAS51 was isolated among eight isolates from a sponge collected from the Red Sea, Jeddah, and selected as it displayed the highest cellulase activity including (FP-ase, CMC-ase, and β-glucosidase enzymes) at levels of 3.13 U/ml, 2.52 U/ml, and 0.69 U/ml, respectively. The fungus was identified morphologically and genetically by sequencing its 18SrRNA gene as Aspergillus sp. NAS51. The cellulase activity of Aspergillus sp. NAS51 was optimized and maximum enzyme production was obtained at initial pH7, temp 30^oC, incubation period 11 days, moisture content 70%, urea as a nitrogen source, and K2HPO4 (2 g/L). The crude cellulases from strain NAS51 were characterized, and results showed that the enzyme is stable over a wide pH range (6–10), with peak activity at pH 7.0 and 40 °C. The cellulase gene has been sequenced and the protein 3D structure was generated via in silico homology modeling. Determination of binding sites and biological annotations of the constructed protein was carried out via COACH and COFACTOR based on the I-TASSER structure prediction. To reach the maximum enzyme hydrolysis, the rice straw collected from Al-Ahsa, Kingdom of Saudi Arabia was pretreated with NaOH 1.5% to remove lignin and to enhance the saccharification process by Cellulase. The saccharified product was measured using HPLC, fermented by S. cerevisiae and the bioethanol yield produced from the fermentation was 0.454 mL ethanol/g fermentable sugars. This study indicates the potential application of fungal enzymes such as cellulase enzymes in biofuel generation and waste management.