Acceptor reactions of alternansucrase from Leuconostoc mesenteroides NRRL B-1355

Abstract

Extracellular glucansucrases from various bacterial sources, including Leuconostoc mesenteroides, have been shown to catalyze the transfer of glucosyl groups from sucrose to low-molecular-weight acceptor sugars, forming a series of oligosaccharides. An extracellular glucansucrase recently isolated from Leuconostoc mesenteroides NRRL B-1355 synthesizes a polysaccharide consisting of alternating α-(1→6)- and α-(1→3)-linked d-glucose residues. We have found that this enzyme preparation is capable of forming both α-(1→6)- and α-(1→3)-linked acceptor-products in the presence of a number of low-molecular-weight acceptor sugars. d-Glucose gave isomaltose only, no nigerose being formed. Similarly, methyl α- d-glucoside, methyl β- d-glucoside, maltose, and nigerose gave methyl α-isomaltoside, methyl β-isomaltoside, panose, and 6 2- O-α- d-glucosylnigerose, respectively. However, isomaltose gave both isomaltotriose and 3 2- O-α- d-glucosylisomaltose. These initial acceptor-products may also act as acceptors, and the structures of the products of higher d.p. show that a (1→3)-α- d-glucosidic bond is formed only when the nonreducing glucose acceptor group is linked through an α-(1→6) bond to another glucose residue. Experiments using [ 14C]sucrose and unlabeled acceptors showed that acceptor reactions occur by transfer of a glucosyl group from sucrose to O-3 or O-6 of the glucosyl group at the nonreducing end of the acceptor saccharide. If a “good” acceptor is defined as one that gives rise to a greater amount of oligosaccharide and less polysaccharide, then the acceptors tested may be listed as follows: maltose ≥ nigerose > methyl α- d-glucoside > isomaltose > d-glucose > methyl β- d-glucoside.