Structural elements responsible for the glucosidic linkage-selectivity of a glycoside hydrolase family 13 exo-glucosidase

Glycoside hydrolase family 13 contains exo-glucosidases specific for α-(1 → 4)- and α-(1 → 6)-linkages including α-glucosidase, oligo-1,6-glucosidase, and dextran glucosidase. The α-(1 → 6)-linkage selectivity of Streptococcus mutans dextran glucosidase was altered to α-(1 → 4)-linkage selectivity through site-directed mutations at Val195, Lys275, and Glu371. V195A showed 1300-fold higher kcat/Km for maltose than wild-type, but its kcat/Km for isomaltose remained 2-fold higher than for maltose. K275A and E371A combined with V195A mutation only decreased isomaltase activity. V195A/K275A, V195A/E371A, and V195A/K275A/E371A showed 27-, 26-, and 73-fold higher kcat/Km for maltose than for isomaltose, respectively. Consequently, the three residues are structural elements for recognition of the α-(1 → 6)-glucosidic linkage.