Heterologous Production and Characterization of Arthrobacter globiformis T6 Isomalto-dextranase

Efficient production of an isomalto-dextranase from Arthrobacter globiformis T6 has been achieved in Escherichia coli. The combination of an expression vector lacking the region for the signal peptide, cultivation at 25°C, and optimization of E. coli cells allows us to produce the isomalto-dextranase at more than 1000 times the amount under the original conditions, which then enables us to characterize the enzyme in detail. The primary structure of the isomalto-dextranase from A. globiformis T6 has a distant similarity with enzymes belonging to glycosyl hydrolase family 27, which comprises mainly α-galactosidases and α-N-acetylgalactosaminidases. Therefore, the reaction of the isomalto-dextranase for melibiose, a substrate for α-galactosidases, has also been investigated. The isomalto-dextranase did not hydrolyze melibiose or p-nitrophenyl α-D-galactoside. The Ki values for isomaltose and maltose were 2.3 and 7.8 mM, respectively, while melibiose scarcely inhibited the activity of the isomalto-dextranase. Moreover, melibiose was a poor acceptor for the transglycosylation with dextran, and the maximum accumulation of the transglycosylation product was 12-fold less than that for isomaltose. The findings indicated here that the isomalto-dextranase is highly specific for the glucosyl moiety in both hydrolysis and transglycosylation.