Xylo-oligosaccharides are competitive inhibitors of cellobiohydrolase I from Thermoascus aurantiacus

► Strong inhibition of cellulases by XOS was attributed to the inhibition of the activity of cellobiohydrolases by XOS. ► Stronger inhibitory effects by XOS were observed on CBHII originating from Trichoderma reesei than on CBHI from Thermoascus aurantiacus. ► Xylobiose and xylotriose were competitive inhibitors of cellobiohydrolase I from T. aurantiacus. ► Xylobiose exhibited stronger inhibitory effect than xylotriose on cellobiohydrolase I from T. aurantiacus. The effects of xylo-oligosaccharides (XOS) and xylose on the hydrolytic activities of cellulases, endoglucanase II (EGII, originating from Thermoascus aurantiacus), cellobiohydrolase I (CBHI, from T. aurantiacus), and cellobiohydrolase II (CBHII, from Trichoderma reesei) on Avicel and nanocellulose were investigated. After the addition of XOS, the amounts of cellobiose, the main product released from Avicel and nanocellulose by CBHI, decreased from 0.78 and 1.37mg/ml to 0.59 and 1.23mg/ml, respectively. During hydrolysis by CBHII, the amounts of cellobiose released from the substrates were almost cut in half after the addition of XOS. Kinetic experiments showed that xylobiose and xylotriose were competitive inhibitors of CBHI. The results revealed that the strong inhibition of cellulase by XOS can be attributed to the inhibitory effect of XOS especially on cellobiohydrolase I. The results indicate the necessity to totally hydrolyze xylo-oligosaccharides into the less inhibitory product, xylose, to increasing hydrolytic efficiency.