Substrate specificity and transglucosylation catalyzed by cycad beta-glucosidase

Initial rates of transglucosylation with diglucosides and diglucose-azoxyglycosides as acceptor by cycad beta-glucosidase were tentatively obtained. The formation of beta-1,3 glucosidic linkage was predominant, except for neocycasin A (beta-laminaribioside of methylazoxymethanol, MAM) as an acceptor. With neocycasin A as an acceptor, beta-1,4 and beta-1,6 glucosidic linkages were formed but beta-1,3 linkage was not. Whereas with laminaribiose as acceptor, laminaritriose and triose with beta-1,6 linkage were formed, but triose with beta-1,4 linkage was not. On the other hand, with other diglucoses and neocycasin B (beta-gentiobioside of MAM) as acceptor, all the linkages formed were beta-1,3 glucosidic. The aglycone of azoxyglycosides, MAM, affected the kind of linkages formed in the trisaccharides. When initial rates of the linkage formation of the transglucosylation at 100 mM acceptor were compared with the hydrolysis rates obtained by Lineweaver-Burk plot, the order of formation rates of the di- and tri-glucosides by transglucosylation was the same as obtained for the hydrolysis parameter, kcat/Km. Km values for various substrates could be grouped according to the kind of the linkages (beta-1,3, beta-1,4, and beta-1,6) first split by the enzyme.