The extension of α-d-1,3-branch linkages by 1,3-α-d-glucan synthase from Streptococcus sobrinus

In the presence of an acceptor, 1,3-α-d-glucan synthase of Streptococcus sobrinus synthesizes water-insoluble glucans from sucrose. Under such conditions, 1,3-α-d-glucoside linkages were extended without any change in the glucose-residue number between the 1,3,6-branch points on the acceptor. From these results, the mechanisms of water-insoluble-glucan formation were proposed as follows: (i) the attachment of an acceptor to the glucan binding sites of 1,3-α-d-glucan synthase occurs during the initiation of the reaction, and concurrently determines the positions of the branched portions of 1,3,6 on the acceptor, and (ii) the 1,3-α-d-glucoside linkage extends from these positions.