A D-glucosylated form of dextransucrase: preparation and characteristics

Characterization of the Different Dextransucrase Activities Excreted in Glucose, Fructose, or Sucrose Medium by Leuconostoc mesenteroides NRRL B-1299

When grown in glucose or fructose medium in the absence of sucrose, Leuconostoc mesenteroides NRRL B-1299 produces two distinct extracellular dextransucrases named glucose glucosyltransferase (GGT) and fructose glucosyltransferase (FGT). The production level of GGT and FGT is 10 to 20 times lower than that of the extracellular dextransucrase sucrose glucosyltransferase (SGT) produced on sucrose medium (traditional culture conditions). GGT and FGT were concentrated by ultrafiltration before sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. Their molecular masses were 183 and 186 kDa, respectively, differing from the 195 kDa of SGT. The structural analysis of the dextran produced from sucrose and of the oligosaccharides synthesized by acceptor reaction in the presence of maltose showed that GGT and FGT are two different enzymes not previously described for this strain. The polymer synthesized by GGT contains 30% alpha(1-->2) linkages, while FGT catalyzes the synthesis of a linear dextran only composed of alpha(1-->6) linkages.

Dextran molecular size and degree of branching as a function of sucrose concentration, pH, and temperature of reaction of Leuconostoc mesenteroides B-512FMCM dextransucrase

Reactions of Leuconostoc mesenteroides B-512FMCM dextransucrase with increasing concentrations of sucrose, from 0.1 to 4.0 M, gave a decreasing amount of high-molecular weight dextran (HMWD) (>10(6) Da) with a concomitant increase in low-molecular weight dextran (LMWD) (<10(5) Da). At 0.1 M sucrose, pH 5.5, and 28 degrees C, 99.8% of the dextran had a MW>10(6) Da and at 4.0 M sucrose, 69.9% had a MW<10(5) Da and 30.1% had a MW>10(6) Da, giving a bimodal distribution. The degree of branching increased from 5% for 0.1 M sucrose to 16.6% for 4.0 M sucrose. The temperature had very little effect on the size of the dextran, which was >10(6) Da, but it had a significant effect on the degree of branching, which was 4.8% at 4 degrees C and increased to 14.7% at 45 degrees C. Both the molecular weight (MW) and the degree of branching were not significantly affected by different pH values between 4.5 and 6.0.

A circularly permuted alpha-amylase-type alpha/beta-barrel structure in glucan-synthesizing glucosyltransferases

A motif of amino acid residues, located at the active site and specific beta-strands in alpha-amylases, is recognized in alpha-1,3- and alpha-1,6-glucan-synthesizing glucosyltransferases, leading to the conclusion that these enzymes contain an alpha/beta-barrel closely related to the (beta/alpha)8-fold of the alpha-amylase superfamily. The secondary structure elements of the transferase barrel, however, are circularly permuted to start with an alpha-helix equivalent to helix 3 in the alpha-amylases. Thus, the transferase counterpart of the long third beta-->alpha connection--constituting a domain in the alpha-amylases--is divided to precede and succeed the barrel. This architectural arrangement may be coupled to sucrose scission and glucosyl transfer. The involvement in the mechanism in glucosyltransferases of active site residues recurring in amylolytic enzymes is discussed.

Photolabeling of dextransucrase from Streptococcus sanguis with p-azidophenyl alpha-D-glucopyranoside

Dextransucrase from Streptococcus sanguis ATCC 10558 was photolabeled using p-azidophenyl alpha-D-glucopyranoside with an apparent rate constant of inactivation of 1.40 min-1. The dissociation constant for this compound, which acts as an acceptor molecule in the enzymatic reaction, is 90 microM. Apparently two acceptor binding sites exist on dextransucrase as shown by (i.) photolabeling the enzyme with p-azidophenyl-alpha-D-[5,6-3H]glucopyranoside and (ii.) fluorescence titration experiments.

The activity of glucosyltransferase adsorbed onto saliva-coated hydroxyapatite

This study aimed to determine physical and kinetic properties of glucosyltransferase (GTF) adsorbed onto hydroxyapatite (HA) surfaces. For development of a solid-phase enzyme assay, 4.0-mg samples of washed HA powder were exposed to centrifuged whole saliva (WSHA) or buffer, and subsequently exposed to a GTF solution. The activities of GTF adsorbed to HA and that remaining in solution were measured. WSHA was more effective in adsorbing GTF than was naked HA. Enzyme activity on the surface of WSHA was enhanced; more activity was detected on WSHA than was apparently removed from solution. A similar effect was observed when GTF was adsorbed to naked HA from a mixture with lysozyme or saliva; however, no enhancement was seen when GTF was adsorbed from a mixture with albumin. Compared with GTF in solution, adsorbed GTF displayed activity over a much wider range of pH values. Temperature-activity profiles indicated that GTF adsorbed to surfaces had a lower temperature optimum (40 degrees C) than did soluble enzyme (45 degrees C), and that the bound enzyme was more resistant to adverse effects of heat at elevated temperatures. The majority of glucan made by GTF adsorbed to parotid saliva-coated HA remained attached to the surface. The activity of lysozyme adsorbed to HA was reduced by adsorption of GTF to the same surface and was almost completely abolished by formation of glucans by the adsorbed GTF. These results suggest that soluble bacterial enzymes found in saliva can be incorporated into pellicle, interact with host-derived molecules on the surfaces of teeth, express enzymatic activity, and potentially influence the biological properties of pellicle.

Dynamic reactivities of dextransucrase

Dextransucrase, from Streptococcus sanguis ATCC 10558, was immobilized on hydroxylapatite and was "charged" in short pulses with labeled sucrose, as previously described [V. K. Parnaik, G. A. Luzio, D. A. Grahame, S. L. Ditson, and R. M. Mayer (1983) Carbohydr. Res. 121, 257-268]. The "charged" enzyme has been shown to contain both bound glucose and gluco-oligosaccharides. The reactivity of this form of the enzyme has been studied, and shown to have unexpected behavior. Earlier pulse-chase experiments [J. F. Robyt, B. K. Kimble, and T. F. Walseth (1979) Arch. Biochem. Biophys. 165, 634-640; S. L. Ditson and R. M. Mayer (1984) Carbohydr. Res. 126, 170-175], carried out with high concentrations of unlabeled sucrose in the chase, resulted in a rapid decrease in isotope at the reducing termini of enzyme-bound oligosaccharides. However, in the present work, in which the pulsed enzyme was chased with low concentrations of unlabeled sucrose, we observed an increase in the radioactive reducing termini. The possibility that this was due to the enzymatic hydrolysis of dextran has been ruled out. Data presented demonstrate that the enzyme catalyzes the depolymerization of the bound oligosaccharides. Individual glucosyl residues of the oligosaccharides are transferred to acceptors, such as added maltose to form a trisaccharide, or water to form glucose. Similarly, the glucosyl residues can be transferred to added fructose to form sucrose. The studies also provide evidence that the oligosaccharides are slowly released from the enzyme. The ability of the enzyme to catalyze the reverse of the glucosyl transfer reaction involving acceptors was also examined. It was observed that glucose residues transferred by dextransucrase to an acceptor can also be removed to produce sucrose when fructose is added.

A D-glucosylated form of dextransucrase: demonstration of partial reactions

A D-glucosylated form of dextransucrase, whose preparation and characteristics have just been reported in Carbohydr. Res., was employed in a series of studies designed to explore the question of whether the bound sugars participate in the reactions catalyzed by the enzyme. When exposed to maltose, a good acceptor-substrate, monomeric D-glucosyl groups were rapidly transferred to the disaccharide, affording a trisaccharide. In the absence of an acceptor, monomeric D-glucose was released from the enzyme by hydrolysis. In a reaction with D-fructose, the charged enzyme catalyzed the formation of sucrose. Finally, in the presence of unlabeled sucrose, monomeric D-glucosyl groups were chased into enzyme-associated oligomers. Evidence is also presented which indicates that the various pathways for the bound D-glucosyl groups are competitive. The significance of these observations is discussed.