Mode of action and properties of the [beta]-xylosidases from Talaromyces emersonii and Trichoderma reesei

Enzymatic hydrolysis of arabinoxylan is an important prerequisite for the utilization of hemicellulose for ethanol fermentation or for making the low calorie sweetener xylitol by catalytic hydrogenation of the generated xylose. This study focus on cloning and characterization of two industrial relevant [beta]-xylosidases (1,4-[beta]-D-xylan xylohydrolase, EC 3.2.1.37) from Talaromyces emersonii ([beta]XTE) and Trichoderma reesei ([beta]XTR) and a comparison of these in relation to hemicellulose hydrolysis using an industrial relevant substrate. Both [beta]-xylosidases were expressed in A. oryzae and subsequently purified. During the enzymatic hydrolysis of xylobiose, the reaction product of both enzymes was found to be [beta]-D-xylose proving that the hydrolysis is proceeding via a retaining reaction mechanism. Based on sequence similarities and glycosyl hydrolases family membership, the active site residues of [beta]XTE and [beta]XTR are predicted to be Asp 242 and Glu 441, and Asp 264 and Glu 464, respectively. The involvement in catalysis of these carboxyls was examined by modification using the carbodiimide-nucleophile procedure resulting in a complete inactivation of both enzymes. The degree of xylose release from vinasse, an ethanol fermentation by-product, by [beta]XTE and [beta]XTR was 12.1% and 7.7%, respectively. Using the [beta]-xylosidases in combination with the multicomponent enzyme product Ultraflo L, resulted in 41.9% and 40.8% release of xylose, respectively indicating a strong synergistic effect between the exo-acting [beta]-xylosidases and the endo-1,4-[beta]-xylanases and [alpha]-L-arabinofuranosidase in Ultraflo L. There seems to be no measurable differences between the two [beta]-xylosidases when used in this specific application despite the differences in specific activity and kinetic properties.