Production of cellulases and xylanases by Humicola grisea var. thermoidea and application in sugarcane bagasse arabinoxylan hydrolysis

Abstract

Humicola grisea var. thermoidea is a deuteromycete that secretes a large spectrum of hydrolytic enzymes when grown on lignocellulosic residues. Agricultural residues are rich in lignocellulosic material and can be used as an alternate source for the production of different value-added products. The fungus was grown on sugarcane bagasse (SCB), wheat brain (WB), rice straw (RS) and corn cob (CC) as carbon source. The major endoxylanase, cellobiohydrolase, arabinofuranosidase and avicelase activities were observed when the fungus had grown on WB, while the major endoglucanase, FPAse and β-xylosidase activities on SCB. Secreted proteins by H. grisea upon growth on WB (HgWB) and SCB (HgSCB) by 144 h were analyzed by SDS/PAGE and Zymogram. The results suggested that HgWB and HgSCB presented a different pool of cellulases and xylanases. Cellulases and xylanases are used in enzyme cocktail to cellulose hydrolysis in cellulosic ethanol plants from SCB, hemicelluloses currently represents the largest polysaccharide fraction wasted in these plants. H. grisea enzymes (HgWB and HgSCB) and recombinant hemicellulases (endoxylanase GH11 – HXYN2, β-xylosidase – HXYLA and arabinofuranosidase ABF3) were employed to design hemicellulases cocktail which were employed to recovery arabinoxylan (AX) from steam explosion sugarcane bagasse (seSCB). The hydrolysis of seSCB with hemicellulases cocktail proved to be efficient and the release of reducing sugars (RS) from AX arriving at 91% conversion and the highest hydrolysis yields were obtained when using 4.200 U g−1 (Units per gram substrate) of HXYN2, 10 U g−1 of HXYLA and 4 U g−1 of ABF3. After AX hydrolysis, the solid fraction containing cellulose was hydrolyzed with commercial cellulases and presented 28% increase in hydrolysis yield (glucose production) comparing to seSCB hydrolysis without enzymatic pre-treatment. A scanning electron microscope (SEM) was an important tool to observe the modifications in seSCB after steam explosion treatment and hydrolysis. SEM images revealed that steam explosion caused considerable damage to the fibers, such as the relaxation, fragmentation and peeling of the superficial layers of the cellulose and after enzymatic hydrolysis can be observed a greater number of cracks and more brittle in the structure. The results demonstrated that enzymatic pre-treatment with hemicellulases cocktail allowed recovery of pentoses from AX which can be converted in diverse products and improved glucose production from cellulose fraction.