17-15: Role of Acremonium cellulolyticus xylanase (Xyl10A) in enzymatic hydrolysis of rice straw

　　Acremonium cellulolyticus, isolated by AIST in 1982, is one of the high cellulolytic enzyme-producing fungi. Five types of cellulolytic enzymes (two cellobiohydrolases: Cel6A and Cel7A; two endoglucanases: Cel5A and Cel7B; and xylanase: Xyl10A) and b-glucosidase (Bgl3A) were identified as core enzymes of A. cellulolyticus for lignocellulosic biomass degradation. To investigate the role of Xyl10A in enzymatic hydrolysis of lignocellulosic biomass, we purified and characterized the recombinant xylanase (Xyl10A).

　　Xyl10A was expressed in A. cellulolyticus by chromosomal integration under the control of glucoamylase promoter. At the purification step, three different peaks for xylanase were observed on hydrophobic column chromatography and individually purified. Molecular weights of these Xyl10As were estimated as 53 kDa (Xyl10A-I), 51 kDa (Xyl10A-II), and 50 kDa (Xyl10A-III) by SDS-PAGE. Specific activities of the purified Xyl10A-I, -II, and –III were 251, 250, and 395 U/mg toward birch wood xylan, respectively, and their optimum pH and temperature were almost the same (pH 5.0 and 75 °C).

　　The synergistic effect between Xyl10A and Cel7A was evaluated in the hydrolysis of wet disk-milled rice straw. The enzyme mixture was supplemented with purified Bgl3A. After 12 hr reaction, the amount of glucose production was increased more than 5-fold in the mixture of 80% Cel7A and 20% Xyl10A compared with Cel7A alone. This result suggests that enzymatic xylan removal by Xyl10A plays critical role in enhancement of cellulose hydrolysis in xylan-rich lignocellulosic biomass such as rice straw.