Characterization of Three Fungal Isomaltases Belonging to Glycoside Hydrolase Family 13 That Do not Show Transglycosylation Activity

α-1,6-Glucosidase (isomaltase) belongs to glycoside hydrolase (GH) families 13 and 31. Genes encoding 3 isomaltases belonging to GH family 13 were cloned from filamentous fungi, Aspergillus oryzae (agl1), A. niger (agdC),and Fusarium oxysporum (foagl1), and expressed in Escherichia coli. The enzymes hydrolyzed isomaltose and α-glucosides preferentially at a neutral pH, but did not recognize maltose, trehalose, and dextran. The activity of AgdC and Agl1 was inhibited in the presence of 1 % glucose, while Foagl1 was more tolerant to glucose than the other two enzymes were. The three fungal isomaltases did not show transglycosylation when isomaltose was used as the substrate and a similar result was observed for AgdC and Agl1 when p-nitrophenyl-α-glucoside was used as the substrate.