A P-glucosidase gene (bg13) from Streptomyces sp. QM-B814 (American Type Culture Collection 11238) has been cloned by functional complementation of a P-glucosidase-negative mutant of Streptomyces lividans. An open-reading frame of 1440 nucleotides encoding a polypeptide of 479 amino acids was found by sequencing. The encoded protein (Bg13) shows extensive similarity (over 45% identity) with P-glycosidases from family-1 glycosyl hydrolases. The cloned enzyme, purified following ammonium sulphate precipitation and two chromatographic steps, is monomeric with molecular mass 52.6 kDa, as determined by mass spectrometry, and an isoelectric point of PI 4.4. The enzyme appears to be a P-glucosidase with broad substrate specificity, is active on cellooligomers, and performs transglycosylation reactions. The estimated apparent K,,, values for p-nitrophenyl-P-D-glucopyranoside and cellobiose are 0.27 mM and 7.9 mM, respectively. The K, values for glucose and 6-gluconolactone, using p-nitrophenyl-P-D-glucopyranoside as a substrate, are 65 mM and 0.08 mM, respectively. The purified enzyme has a pH optimum of pH 6.5 and the temperature optimum for activity is 50°C. P-Glucosidases (P-glucoside glucohydrolase) are enzymes that formally catalyze the transfer of glucosyl groups between oxygen nucleophiles. Under physiological conditions, such a transfer reaction generally results in the hydrolysis of a P-D-glucosidic bond. P-Glucosidases, more specifically cellobiases, have been traditionally referred to as one of the three enzyme classes constituting the cellulase system [ 1, 21. Thus, in cellulolytic processes, P-glucosidases catalyze the hydrolysis of cellobiose and cellooligosaccharides to glucose. Cellulolysis is a process effectively performed in nature through the synergism of microbial communities, but its industrial exploitation is not efficiently feasible to date because of diverse reasons including, among others, enzyme availability, enzymic stability in reactor conditions, the complexity of natural substrates, and the heterogeneity of cellulase systems. The interest in studying P-glucosidases in the context of cellulolysis lies in the following factors: (a) the step they catalyze is rate limiting since cellobiose is an inhibCorrespondence to
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