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 50 mM acetate buffer (pH 4.5). 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 152 kDa (SDS-PAGE). The optimum pH and temperature of the enzyme were 5.2 and 38°C, respectively. The K_m (mM) and V_max (mg dextran/mg protein/min) values for mal-tooligosaccharides (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 sub-strates. 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 α-1, 4- to α-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 α-1, 4- to α-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 1270 kDa. The chemical structures of synthesized glucooligo-saccharides (DP =3-7) from a maltose-dextran mixture were 4-ο-α-isomaltosyl-D-glucose, 4-ο-α-isomaltotriosyl-D-glucose, 4-ο-α-isomaltotetraosyl-D-glucose, 4-ο-α-isomaltopentaosyl-D-glucose and 4-ο-α-isomaltohexaosyl-D-glucose. On the basis of many experimental data, we propose the possible transglucosylation actions of DDase which consist of five reaction routes.