Catherine Taylor

Productivity of four α-d-glucosyltransferases released by Streptococcus sobrinus under defined conditions in continuous culture

Abstract The distribution and activity of four different α- d -glucosyltransferases (GTF) were determined in culture filtrates of Streptococcus sobrinus grown in the chemostat. The rate of production of each enzyme depended on the growth rate, the pH of the medium, the limiting nutrient and on the presence of Tween 80. Changes in the relative proportion of GTF were reflected in the altered structures and adhesive qualities of the water-insoluble α- d -glucans synthesized from sucrose by the culture filtrates.

Structures of water-soluble α-d-glucans synthesized from sucrose by glucosyltransferases isolated from Streptococcus sobrinus culture filtrates

Abstract From three strains of Streptococcus sobrinus -K1-R, -6715-13-201 and -6715-13-27, grown in continuous culture under a variety of defined conditions, three different glucosyltransferases (GTF): GTF-S1, GTF-S3 and GTF-S4, were isolated. The enzymes were used to synthesize, from sucrose, soluble α- d -glucans S1, S3 and S4 respectively, of quite different structure. Methylation analysis, NMR spectroscopy and enzymic hydrolysis were used to determine the structural features of the S1, S3 and S4 polysaccharides. The GTF-S1-type enzymes synthesized highly branched (up to ≈ 34%) glucans having single -(1 → 3)-α-linked d -glucosyl residues attached to approximately one in two of the -(1 → 3)-α-linked d -glucosyl units of the main chain. The GTF-S3 enzymes produced low molecular weight linear -(1 → 3)-α-linked glucans. The GTF-S4 enzymes, released from strains K1-R and 6715-13-201 only in the presence of the surfactant Tween 80, form S4 glucans with branching ranging from ≈ 8 to 11%. Enzymic hydrolysis with an endo-(1 → 3)-α- d -glucanase was used to show that side chains consisting of a sequence of -(1 → 3)-α-linked- d -glucosyl residues are attached to -(1 → 6)-α-linked- d -glucosyl residues of the backbone, in S4 glucans from strains K1-R and 6715-13-201. Side chains of single -(1 → 3)-α-linked glucosyl residues are also present in these S4 glucans, as shown by NMR studies. Strain 6715-13-27 released GTF-S4 enzymes in the absence of Tween 80. The S4 glucans formed by these enzymes have the single side chains almost exclusively, and undergo very limited hydrolysis with the -(1 → 3)-α-glucanase.

Action of Endo-(1 → 6)-α-d-glucanases on the soluble dextrans produced by three extracellular α-d-glucosyltransferases of Streptococcus sobrinus

Abstract Four different α- d -glucosyltransferases (GTF) have been obtained from culture filtrates of Streptococcus sobrinus strains grown in the chemostat at pH 6·5 in complex medium supplemented with Tween 80. Three of the enzymes, GTF-S1, GTF-S3 and GTF-S4, converted sucrose into soluble glucans. Their limit of hydrolysis with endodextranase, the proportion of linear to branched oligosaccharides among the end products of enzymic degradation, and methylation analysis, all supported the view that the glucans were dextrans. The S1-dextrans were highly branched (32% of α-(1 → 3)-branch points), S3-dextrans were linear, and the branching of S4-dextrans was intermediate in value (9%). The enzymes that catalyze the synthesis of three such diverse dextrans were thus proved to be three different GTF, each with a characteristic specificity. Conditions of growth in the chemostat could be varied to provide maximum yields of either GTF-S1, -S3 or -S4.