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 k cat / K m for maltose than wild‐type, but its k cat / K m 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 k cat / K m for maltose than for isomaltose, respectively. Consequently, the three residues are structural elements for recognition of the α‐(1 → 6)‐glucosidic linkage.