Transisomaltosylation activity of a bacterial isomalto-dextranase.

A bacterial isomalto-dextranase, described previously as a new type of dextranase different from the known 1, 6-α-D-glucan 6-glucanohydrolase [EC 3. 2. 1. 11], was found to be a configuration-retaining exo-glucanase so far as judged from the downward mutarotation shown by products in a dextran digest, and from the lower activity of the enzyme on lesspolymerized isomaltodextrins according to one of the criteria proposed by Reese et al. The dextranase was observed to cause not only transisomaltosylation (isomaltotetraose formation in dextran digests, isomaltotriose formation in dextran digests containing glucose and transisomaltosylation among isomaltodextrins) but also isomaltose condensation to isomaltotetraose in concentrated solutions. These activities shown by the isomalto-dextranase are in keeping with the novel concept that carbohydrases are catalysts of glycosylation (glycosylhydrogen interchange), proposed by Hehre and his coworkers. The relative ease of isomaltose condensation catalyzed by the enzyme appears due to the exergonic nature of the reaction. A free energy change value of ca. -1200 cal/mole was obtained for the condensation.