α-Glucosidase from Aspergillus niger (AgdA; typical α-1,4-glucosidase) is known to industrially produce α-(1→6)-glucooligosaccharides. This fungus also has another α-glucosidase-like protein, AgdB. To learn its function, wild-type AgdB was expressed in Pichia pastoris. However, the enzyme displayed two electrophoretic forms due to heterogeneity of N-glycosylation at Asn354. The deglycosylation mutant N354D shared the same properties with wild-type AgdB. N354D demonstrated hydrolytic specificity toward α-(1→3)- and α-(1→4)-glucosidic linkages, indicating that AgdB is an α-1,3-/α-1,4-glucosidase. N354D-catalyzed transglucosylation from maltose was analyzed in short- and long-term reactions, enabling us to learn the transglucosylation specificity and product accumulation, respectively. A short-term reaction (<15 min) synthesized 3II- O-α-glucosyl-maltose and maltotriose, indicating α-1,3-/α-1,4-transferring specificity. A long-term reaction (<24 h) accumulated kojibiose and nigerose using formed glucose as an acceptor substrate. AgdA and AgdB are distinct α-glucosidases. At a high concentration of glucose added exogenously, AgdB largely generated the rare sugars kojibiose and nigerose (exhibiting beneficial physiological functions) with 19% and 24% yields from maltose, respectively.
Keywords: Aspergillus niger; kojibiose; nigerose; transglucosylation; α-glucosidase.