A Highly Glucose Tolerant ß-Glucosidase from Malbranchea pulchella (MpBg3) Enables Cellulose Saccharification

beta -glucosidases catalyze the hydrolysis beta -1,4, beta -1,3 and beta -1,6 glucosidic linkages from non-reducing end of short chain oligosaccharides, alkyl and aryl beta -D-glucosides and disaccharides. They catalyze the rate-limiting reaction in the conversion of cellobiose to glucose in the saccharification of cellulose for second-generation ethanol production, and due to this important role the search for glucose tolerant enzymes is of biochemical and biotechnological importance. In this study we characterize a family 3 glycosyl hydrolase (GH3) beta -glucosidase (Bgl) produced by Malbranchea pulchella (MpBgl3) grown on cellobiose as the sole carbon source. Kinetic characterization revealed that the MpBgl3 was highly tolerant to glucose, which is in contrast to many Bgls that are completely inhibited by glucose. A 3D model of MpBgl3 was generated by molecular modeling and used for the evaluation of structural differences with a Bgl3 that is inhibited by glucose. Taken together, our results provide new clues to understand the glucose tolerance in GH3 beta -glucosidases.