Functional Characteristics of a Bacterial Dextrin Dextranase from Acetobacter capsulatum ATCC 11894.

Abstract

Dextrin dextranase(DDase, EC 2.4.1.2) was extracellularly secreted in a culture medium of Acetobacter capsulatum ATCC 11894 containing both glucose and a small amount of dextrin as the essential carbon sources. The enzyme was simply purified by a high-speed refrigerated centrifugation, and immediately dialyzed against 50mM acetate buffer(pH4.5). The purified DDase gave a single protein band on both Native- and SDS-PAGE. The molecular mass of the purified enzyme was estimated to be about 152kDa(SDS-PAGE). The optimum pH and temperature of the enzyme were 5.2 and 38.DEG.C., respectively. The Km(mM) and Vmax (mg dextran/mg protein/min) values for maltooligosaccharides(DP=3-7) and short-chain amylose(DP=17.3) were estimated to be about 10.2, 1.74; 6.41, 2.56; 3.34, 2.64; 2.59, 2.39; 1.66, 2.17; 0.12, 2.23, respectively. The conversion rate of maltodextrins into dextran increased with the increases in the DP number of donor substrates. The maximum yield of product dextran reached 73.9% by using short-chain amylose as a substrate. DDase showed a strong affinity on the sugars having non-reducing terminal linked with either .ALPHA.-1,4- or .ALPHA.-1,6-glucosidic bond. The affinity of enzyme on acceptor substrates increased with the increases in the DP number of sugars tested. Various oligosaccharides were formed, when DDase reacted on a maltose-dextran mixture. The ratio of .ALPHA.-1,4- to .ALPHA.-1,6-glucosidic linkages in a product dextran molecule was calculated to be about 1:20. The average molecular mass of product dextran was estimated to be about 1270kDa. The chemical structures of synthesized glucooligosaccharides(DP=3-7) from a maltose-dextran mixture were 4-O-.ALPHA.-isomaltosyl-D-glucose, 4-O-.ALPHA.-isomaltotriosyl-D-glucose, 4-O-.ALPHA.-isomaltotetraosyl-D-glucose, 4-O-.ALPHA.-isomaltopentaosyl-D-glucose and 4-O-.ALPHA.-isomaltohexaosyl-D-glucose. On the basis of many experimental data, we propose the possible transglucosylation actions of DDase which consist of five reaction routes.