The mechanism of dextransucrase action. Direction of dextran biosynthesis

Kinetic modeling of oligosaccharide synthesis catalyzed by leuconostoc mesenteroides NRRL B-1299 dextransucrase

The kinetic behavior of soluble and insoluble forms of dextransucrase from Leuconostoc mesenteroides NRRL B-1299 was investigated with sucrose as substrate and maltose as acceptor. To study the parameters involved, a kinetic model was applied that was previously developed for L. mesenteroides NRRL B-512F dextransucrase. There are significant correlations between the parameters of the soluble form of B-1299 dextransucrase and those calculated for the B-512F enzyme; that is, their properties are comparable and differ from those of the insoluble form of B-1299 dextransucrase. Whereas the calculated parameters for high maltose concentrations describe the kinetic behavior very well, the time curves for low maltose concentrations were not described correctly. Therefore, the parameters were calculated separately for the two ranges. Copyright 1999 John Wiley & Sons, Inc.

Modelling of oligosaccharide synthesis by dextransucrase

Dextransucrase catalyses the formation of dextran, but also of numerous oligosaccharides from sucrose and different acceptors, if appropriate conditions are chosen. Much experimental work has been carried out and a scheme of reactions and a mathematical model have been developed to describe the complex kinetic behaviour of the enzyme. A computer program was used to calculate the parameters of the model from a broad range of experimental data, investigating a large number of kinetic tests with the acceptors maltose and fructose. The results lead to design considerations for a continuous reactor system with immobilized dextransucrase to produce leucrose, a disaccharide of industrial interest.

Characterization of hyaluronan synthase from a human glioma cell line

In the present study we describe a method to prepare membranes with high hyaluronan synthase activity from human glioma cells by pretreatment of the cells with both testicular hyaluronidase and 4-phorbol 12-myristate 13-acetate (PMA). A 23-fold increase in hyaluronan synthase activity was detected in comparison to untreated cells. Using isolated membranes as a source of hyaluronan synthase activity we demonstrate that chain elongation occurs at the reducing end of the hyaluronan molecule. We also present a method to solubilize hyaluronan synthase in active form with 1% digitonin. The solubilized synthase synthesized shorter hyaluronan chains than the membrane bound enzyme. Partial purification of the solubilized enzyme on a Superdex-200 column revealed a 12-fold increase in specific activity. Affinity purified polyclonal antibodies, raised against a synthetic peptide corresponding to the carboxy-terminus of the deduced protein sequence of human hyaluronan synthase recognized a 66 kDa component in the purified preparations. The elution position of the solubilized hyaluronan synthesizing activity immediately after V0 corresponding to a molecular mass of about 600 kDa, suggested that the 66 kDa enzyme forms a complex with other components which may have accessory or regulatory roles during hyaluronan synthesis.

Effect of Leuconostoc mesenteroides NRRL B-512F dextransucrase carboxy-terminal deletions on dextran and oligosaccharide synthesis

Dextransucrase (DSR-S) from Leuconostoc mesenteroides NRRL B-512F is a glucosyltransferase that catalyzes synthesis of soluble dextran from sucrose. In the presence of efficient acceptor molecules, such as maltose, the reaction pathway is shifted toward glucooligosaccharide synthesis. Like glucosyltransferases from oral streptococci, DSR-S possesses a C-terminal glucan-binding domain composed of a series of tandem repeats. In order to determine the role of the C-terminal region of DSR-S in dextran or oligosaccharide synthesis, four DSR-S genes with deletions at the 3' end were constructed. The results showed that the C-terminal region modulated the initial velocity of dextran synthesis but that the K(m) for sucrose, the optimum pH, and the activation energy were all unaffected by the deletions. The C-terminal domain modulated the rate of oligosaccharide synthesis whatever acceptor molecule was used (a good acceptor molecule such as maltose or a poor acceptor molecule such as fructose). The C-terminal domain seemed to play no role in the catalytic process in dextran and oligosaccharide synthesis. In fact, it seems that the role of the C-terminal domain of DSR-S may be to facilitate the translation of dextran and oligosaccharides from the catalytic site.

Studies on the inactivation of Leuconostoc mesenteroides NRRL B-512F dextransucrase by o-phthalaldehyde: evidence for the presence of an essential lysine residue at the active site

The kinetics of inactivation of Leuconostoc mesenteroides NRRL B-512F dextransucrase by o-phthalaldehyde showed that the reaction followed pseudo-first order reaction. The loss of enzyme activity was concomitant with an increase in fluorescence at 417 nm indicating that the inhibition involved the reaction of an epsilon-amino and a thiol group of the enzyme leading to the formation of an isoindole derivative. The stoichiometry of inactivation showed that one isoindole derivative was formed per enzyme molecule. The substrates sucrose and glucose provided protection against o-phthalaldehyde inactivation which was also corroborated by fluorescence studies. Dextransucrase was not inactivated by 5,5'-dithiobis(2-nitrobenzoic acid), showing that the cysteine present in close proximity to the lysine is not essential for enzyme activity. Denaturation of dextransucrase by urea or heat treatment prior to o-phthalaldehyde addition resulted in a decrease of fluorescence intensity indicating that the native conformation of the enzyme is essential for isoindole derivative formation. These results established that a lysine residue is present at the active site and is essential for the activity of dextransucrase.

Structural characterization of the maltose acceptor-products synthesized by Leuconostoc mesenteroides NRRL B-1299 dextransucrase

The glucooligosaccharides (GOS), produced by Leuconostoc mesenteroides NRRL B-1299 dextransucrase through an acceptor reaction with maltose and sucrose, were purified by reverse phase chromatography. Logarithmic plots of retention time vs. dp of the GOS gave three parallel lines suggesting the existence of at least three families of homologous molecules. The structure (13C and 1H NMR spectroscopy) and reactivity of the purified molecules of the three families were investigated. All the products bear a maltose residue at the reducing end. The GOS in the first family (named OD) contained additional glucosyl residues all alpha-(1-->6) linked. The smallest molecule in this first series was panose or alpha-D-glucopyranosyl-(1-->6)-D-maltose (dp 3). All the OD molecules were shown to be good acceptors for dextransucrase in the presence of sucrose. The second family, named R, was composed of linear GOS containing alpha-(1-->6)-linked glucosyl residues and a terminal alpha-(1-->2)-linked residue at the non-reducing end of the molecule; the smallest molecule in this family was alpha-D-glucopyranosyl-(1-->2)-D-panose (dp 4). The third family, R', was formed of GOS containing additional residues linked through alpha-(1-->6) linkages that constitute the linear chain, and an alpha-(1-->2)-branched residue located on the penultimate element of the chain, near the non-reducing end. The smallest molecule in this series is alpha-D-glucopyranosyl-(1-->6)-[alpha-D-glucopyranosyl-(1-->2)]-alpha-D- glucopyranosyl-(1-->6)-D-panose, dp 6. R and R' GOS are very poor acceptors for L. mesenteroides NRRL B-1299 dextransucrase. This study makes it possible to suggest a rather simple reaction scheme, where molecules Ri, R'i and ODi of the same dp all result from the glucosylation of the same GOS: ODi-l.

Inactivation of Leuconostoc mesenteroids NRRL B-512F dextransucrase by specific modification of lysine residues with pyridoxal-5'-phosphate

Dextransucrase from Leuconostoc mesenteroides NRRL B-512F was inactivated by pyridoxal-5'-phosphate (PLP). The inactivation was reversible in as much as the loss of enzyme activity was completely reversed by prolonged dialysis. PLP-modified dextransucrase after reduction with sodium borohydride showed a characteristic fluorescence emission maximum at 397 nm when excited at 325 nm. The stoichiometric results indicated that four lysine residues are modified by PLP under the experimental conditions. These results established for the first time that lysine residues are essential for the activity of dextransucrase.

Properties of Leuconostoc mesenteroides B-512FMC constitutive dextransucrase

Leuconostoc mesenteroides B-512FMC, a constitutive mutant for dextransucrase, was grown on glucose, fructose, or sucrose. The amount of cell-associated dextransucrase was about the same for the three sugars at different concentrations (0.6% and 3%). Enzyme produced in glucose medium was adsorbed on Sephadex G-100 and G-200, but much less enzyme was adsorbed when it was produced in sucrose medium. Sephadex adsorption decreased when the glucose-produced enzyme was preincubated with dextrans of molecular size greater than 10 kDa. The release of dextransucrase activity from Sephadex by buffer (20 mM acetate, pH 5.2) was the highest at 28 degrees-30 degrees C. The addition of dextran to the enzyme stimulated dextran synthesis but had very little effect on the temperature or pH stability. Dextransucrase purified by ammonium sulfate precipitation, hydroxyapatite chromatography, and Sephadex G-200 adsorption did not contain any carbohydrate, and it synthesized dextran, showing that primers are not necessary to initiate dextran synthesis. The purified enzyme had a molecular size of 184 kDa on SDS-PAGE. On standing at 4 degrees C for 30 days, the native enzyme was dissociated into three inactive proteins of 65, 62, and 57 kDa. However, two protein bands of 63 and 59 kDa were obtained on SDS-PAGE after heat denaturation of the 184-kDa active enzyme at 100 degrees C. The amount of 63-kDa protein was about twice that of 59-kDa protein. The native enzyme is believed to be a trimer of two 63-kDa and one 59-kDa monomers.

Production and selection of mutants of Leuconostoc mesenteroides constitutive for glucansucrases

After chemical mutagenesis using ethyl methane sulfonate, we isolated mutants constitutive for glucansucrases from Leuconostoc mesenteroides NRRL B-512FM, B-1142, and B-1355. Those mutants produced glucansucrases when grown on D-glucose as well as on sucrose. They produced higher glucansucrase activities (3 to 22 times) when grown on D-glucose than the parent strains grown on sucrose. Glucansucrases from mutants B-1355C and B-1142C grown on glucose formed glucans that were highly resistant to Penicillium dextranase hydrolysis. Mutant B-512FMC dextransucrase formed the same kind of dextran as the parent strain; however, it showed higher thermal stability, even when dextran was absent.

Control of the synthesis of dextran and acceptor-products by Leuconostoc mesenteroides B-512FM dextransucrase

In the maltose-acceptor reaction of Leuconostoc mesenteroides B-512FM dextransucrase, some of the D-glucose moieties of sucrose are diverted from the synthesis of dextran and are transferred to the nonreducing end of maltose to form panose. Glucose is also transferred to panose and to subsequent acceptor products to give a homologous series of isomaltosyl dextrins attached alpha-(1-->6) to maltose. Three experimental parameters were studied to obtain quantitative information about the yield and distribution of acceptor products and the yield of dextran: (a) the ratio of maltose to sucrose, (b) the concentration of maltose and sucrose, and (c) the amount of enzyme. The reactions were run with [14C]sucrose and the amount of each acceptor product and the amount of dextran synthesized were determined for (a), (b), and (c) by TLC separation and measurement of the radioactivity with a PhosphorImager. It was found that an increase in the ratio of maltose to sucrose increased the amount of acceptor products with a concomitant decrease in the synthesis of dextran. Further, as the ratio was increased, the number of acceptor-products decreased. When the concentrations of maltose and sucrose were increased and the ratio was maintained at 1:1, there also was a decrease in the amount of dextran and an increase in the amount of acceptor-products. In addition, there was a decrease in the amount of dextran and an increase in the amount and number of acceptor-products when the amount of enzyme was increased. The first acceptor-product can be exclusively obtained without the formation of any dextran, by using a specific ratio and concentration of maltose and sucrose and a specified amount of enzyme.

Interpretation of dextransucrase inhibition at high sucrose concentrations

When acceptor reactions were carried out at high sucrose concentrations (> or = 200 mM), dextran synthesis was inhibited and the acceptor reactions were increased. A model, based on the known mechanisms of dextran synthesis and acceptor reactions, is proposed to explain the inhibition of dextran synthesis and the increase in the acceptor products at high sucrose concentrations. According to the model, sucrose binds to a third, low-affinity binding site, allosterically changing the conformation of the active site so that dextran cannot be formed but acceptor products can be formed.

Formation of alpha-(1-->6), alpha-(1-->3), and alpha-(1-->2) glycosidic linkages by dextransucrase from Streptococcus sanguis in acceptor-dependent reactions

Dextransucrase from Streptococcus sanguis 10558 was found to synthesize alpha-(1-->6), alpha-(1-->3), and alpha-(1-->2) linkages during an acceptor-dependent glucosyl transfer reaction. Normally, new glucosyl residues are added at C-6 of monosaccharide acceptors. However, sugars blocked at C-6 also can serve as good acceptors. The disaccharide and trisaccharide products formed when methyl 6-bromo-6-deoxy-alpha-D-glucopyranoside was used as acceptor were isolated and characterized. Both were found to contain only alpha-(1-->3) glycosidic bonds. This supports the hypothesis that when C-6 is blocked the acceptor binds to the enzyme in a flipped orientation, resulting in an approximate exchange in space of the C-3 and C-6, thereby putting C-3 adjacent to the active site. The second alpha-(1-->3) links in the trisaccharide are formed by a single-chain mechanism without release of the intermediate disaccharide. With maltose as acceptor, new glucosyl residues are added at C-6'. However, if that position is blocked with a bromine atom, the resulting compound, 6'-bromo-6'-deoxy-maltose, can still serve as an acceptor. The product in this case was isolated and characterized. The new glycosidic link was found to be alpha-(1-->2).

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.

A facile purification of Leuconostoc mesenteroides B-512FM dextransucrase

Leuconostoc mesenteroides NRRL B-512F has been mutated by treatment with N-nitrosoguanidine. The resulting mutant (designated as B-512FM) produces 300 times as much enzyme as the parent strain. B-512FM dextransucrase was treated extensively with Sigma crude dextranase, followed by column chromatography on Bio-Gel A-5m. The purified dextransucrase had a specific activity of 84 IU/mg, a 100-fold purification with 42% yield, and was shown by SDS-PAGE to have a single protein of molecular weight of 158,000 with dextransucrase activity. The procedure has been used to produce purified enzyme for sequencing. The molecular weight of 158,000 agrees with that calculated from its amino acid sequence.

Evidence for the presence of two distinct sites of sucrose hydrolysis and glucosyl transfer activities on 1,3-alpha-D-glucan synthase of Streptococcus mutans

1,3-alpha-D-Glucan synthase of Streptococcus mutans catalyzes both the hydrolysis of sucrose to glucose and fructose, and the glucosyl transfer to glucosyl polymers to yield water-insoluble glucan. The enzyme catalyzes only sucrose hydrolysis, however, in the absence of 1,6-alpha-D-glucan as an acceptor. In the present study, we found that glucosyl transfer activity was completely inhibited by the antiserum against isolated 1,3-alpha-D-glucan synthase but that the sucrose hydrolysis activity was not. The antiserum did not impair the binding of the enzyme to the acceptor. These findings indicate that sucrose hydrolysis and glucosyl transfer occur at two distinct sites on the enzyme.

Essential histidine residues in dextransucrase: chemical modification by diethyl pyrocarbonate and dye photo-oxidation

Treatment of Leuconostoc mesenteroides B-512F dextransucrase with diethyl pyrocarbonate (DEP) at pH 6.0 and 25 degrees or photo-oxidation in the presence of Rose Bengal or Methylene Blue at pH 6.0 and 25 degrees, caused a rapid decrease of enzyme activity. Both types of inactivation followed pseudo-first-order kinetics. Enzyme partially inactivated by DEP could be completely reactivated by treatment with 100 mM hydroxylamine at pH 7 and 4 degrees. The presence of dextran partially protected the enzyme from inactivation. At pH 7 or below, DEP is relatively specific for the modification of histidine. DEP-modified enzyme showed an increased absorbance at 240 nm, indicating the presence of (ethoxyformyl)ated histidine residues. DEP modification of the sulfhydryl group of cysteine and of the phenolic group of tyrosine was ruled out by showing that native and DEP-modified enzyme had the same number of sulfhydryl and phenolic groups. DEP modification of the epsilon-amino group of lysine was ruled out by reaction at pH 6 and reactivation with hydroxylamine, which has no effect on DEP-modified epsilon-amino groups. The photo-oxidized enzyme showed a characteristic increase in absorbance at 250 nm, also indicating that histidine had been oxidized, and no decrease in the absorbance at 280 nm, indicating that tyrosine and tryptophan were not oxidized. A statistical, kinetic analysis of the data on inactivation by DEP showed that two histidine residues are essential for the enzyme activity. Previously, it was proposed that two nucleophiles at the active site attack bound sucrose, to give two covalent D-glucosyl-enzyme intermediates. We now propose that in addition, two imidazolium groups of histidine at the active site donate protons to the leaving, D-fructosyl moieties. The resulting imidazole groups then facilitate the formation of the alpha-(1----6)-glycosidic linkage by abstracting protons from the C-6-OH groups, and become reprotonated for the next series of reactions.

[Stepwise study procedure of sugar substitutes--preliminary study with enzymes. 4. Glucosyltransferases of Streptococcus mutans AHT]

A continuous procedure for the simultaneous enzymatic measurement of the release of free fructose and glucose was adapted to the kinetic conditions of the synthesis of polysaccharides from sucrose by glucosyltransferases from Streptococcus mutans AHT. Initial velocities, Km of sucrose, and the efficiency of the formation of glucans from sucrose can be determined. Longtime incubations with the isolation of soluble and insoluble glucans as an established method were compared with the new procedure. As examples of the effect of sugar substitutes on glucosyltransferases, data on leucrose, nystose, Palatinit, xylitol, leucritol and polyglucose PL-3 are presented. The results provide a preliminary assessment of sugar substitutes such as non-cariogenic sweeteners.

A novel glucosyltransferase from Streptococcus mutans produces oligo-isomaltosaccharides

Streptococcus mutans secretes a sucrose-independent branalphang enzyme that utilizes isomaltosaccharides as donors for branalphang formation on dextran. Although the branching enzyme is necessary for the formation of extracellular polysaccharide complexes, the source of the donor for the enzyme is unknown. In this study, we purified a novel glucosyltransferase from S. mutans and characterized its properties. The glucosyltransferase was primer independent 1,6-alpha-D-glucan synthase, which produced oligo-isomaltosaccharides. The enzyme was thought to be a source of donor for the branching enzyme in S. mutans.

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.

Functional molecular size and structure of dextransucrase by radiation inactivation and gel electrophoresis

Robyt et al. have proposed a mechanism for dextransucrase in which dextran is synthesized by the cooperative action of two equivalent nucleophiles (Robyt, J.F., Kimble, B.K. and Walseth, T.F. (1974) Arch. Biochem. Biophys. 165, 634-640). To distinguish between the possibilities that the enzyme is a monomer bearing both nucleophiles, or a dimer with each subunit bearing one nucleophile, the molecular weight of the enzyme was determined by SDS-polyacrylamide gel electrophoresis and by radiation inactivation. Two major forms of dextransucrase from Leuconostoc mesenteroides NRRL B-512F were found on SDS-polyacrylamide gel electrophoresis, with Mr 177 000 and 158 000, and sometimes a minor form with Mr 168 000. No form of dextransucrase smaller than Mr 158 000 was found, either in the presence or absence of dextran T10, although levansucrase was detected at Mr 92 000 and 116 000. On irradiation with 60Co, dextransucrase behaved as a single species with a maximum size of Mr 201 000. Because Mr 201 000 is much smaller than the minimum dimer size of Mr 316 000 (= 2 X 158 000), it is concluded that both nucleophiles are probably located on the same peptide, rather than one on each subunit of a dimer, and that peptide association is probably not required for dextran synthesis.

Difference in mode of inhibition between alpha-D-xylosyl beta-D-fructoside and alpha-isomaltosyl beta-D-fructoside in synthesis of glucan by Streptococcus mutans D-glucosyltransferase

Both alpha-isomaltosyl beta-D-fructoside and alpha-D-xylosyl beta-D-fructoside show strong inhibition of the synthesis of water-insoluble and water-soluble D-glucans from sucrose by a partially purified preparation of a D-glucosyltransferase (GTase) from Streptococcus mutans 6715; however, the inhibitory modes differ substantially. In the presence of alpha-isomaltosyl beta-D-fructoside, the production of reducing sugars and the consumption of sucrose are remarkably enhanced, compared with a control of sucrose alone. Under these conditions, a large proportion of low-molecular-weight glycan (lmwg) and a series of nonreducing oligosaccharides (both containing D-fructosyl groups or residues) are produced. In contrast, in the presence of alpha-D-xylosyl beta-D-fructoside, the production of reducing sugars and the sucrose consumption are strikingly suppressed, and no lmwg or oligosaccharides are produced. Thus, it may be concluded that alpha-isomaltosyl beta-D-fructoside acts as an alternative acceptor for the D-glucosyl and/or D-glucanosyl transfer reactions of the enzyme, and serves to lessen the formation of insoluble and soluble D-glucan, although it stimulates the transferring activity of the enzyme. On the other hand, alpha-D-xylosyl beta-D-fructoside competitively inhibits the sucrose-splitting activity of the enzyme as an analog to sucrose, and thereby diminishes the synthesis of D-glucan.

Inhibition of Streptococcus mutans 6715 glucosyltransferases by sucrose analogs modified at positions 6 and 6'

Sucrose derivatives modified at position 6 (6-deoxysucrose, 6-thiosucrose, 6,6'-dithiodisucrose, and 6,6'-dideoxy-6,6'-difluorosucrose) were tested as inhibitors of the two Streptococcus mutans 6715 glucosyltransferases. 6-Deoxysucrose was the best inhibitor studied, competitively inhibiting the soluble-D-glucan forming enzyme (GTF-S) and the insoluble-D-glucan forming enzyme (GTF-I) with Ki values one order of magnitude lower than the sucrose Km values. 6-Thiosucrose was also a competitive inhibitor for both enzymes. 6,6'-Dithiodisucrose and 6,6'-dideoxy-6,6'-difluorosucrose only inhibited GTF-I; 6,6'-dithiodisucrose gave mixed inhibition and 6,6'-dideoxy-6,6'-difluorosucrose gave uncompetitive inhibition. 6-Thiosucrose was a substrate for both enzymes to produce acceptor products when acceptors were present. GTF-I synthesized de novo a water-insoluble, (1----3)-6-thio-alpha-D-glucan from 6-thiosucrose.

The formation of alpha-D-(1----3) branch linkages by a D-glucansucrase from Streptococcus mutans 6715 producing a soluble D-glucan

An exocellular D- glucansucrase that synthesizes a water-soluble, alpha-D-(1----6)-linked D-glucan having a high proportion of alpha-D-(1----3) branches was purified from the culture broth of Streptococcus mutans 6715. The rate of incorporation of D-[14C]glucose from [14C]sucrose into D-glucan of high molecular weight by this enzyme was increased (stimulated) by the presence of exogenous Leuconostoc mesenteroides B- 512F dextran, and it was found that this dextran could act as an acceptor. A highly branched dextran, containing 45-50% of alpha-D-(1----3) branch linkages, did not stimulate the enzyme nearly so much as B- 512F dextran, which has a low degree (5%) of alpha-D-(1----3) branches. We interpret this as evidence that the stimulating effects of dextran are not due to priming. If they were, the more highly branched dextran should have produced the greatest stimulation per unit weight, because a much greater number of nonreducing-end, priming sites would be available. We show that the D- glucansucrase was capable of transferring D-glucosyl groups from sucrose to B- 512F dextran to form alpha-D-(1----3) branches, thereby rendering the dextran more resistant to hydrolysis by endodextranase . The presence of 1.6M ammonium sulfate caused the enzyme to synthesize a D-glucan having a much higher percentage of alpha-D-(1----3) linkages.

Stabilization of dextransucrase from Leuconostoc mesenteroides NRRL B-512F by nonionic detergents, poly(ethylene glycol) and high-molecular-weight dextran

Dextransucrase (sucrose: 1,6-alpha-D-glucan 6-alpha-D-glucosyltransferase, EC 2.4.1.5) (3 IU/ml culture supernatant) was obtained by a modification of the method of Robyt and Walseth (Robyt, J.F. and Walseth, T.F. (1979) Carbohydr. Res. 68, 95-111) from a nitrosoguanidine mutant of Leuconostoc mesenteroides NRRL B-512F selected for high dextransucrase production. Dialyzed, concentrated culture supernatant (crude enzyme) was treated with immobilized dextranase (EC 3.2.1.11) and chromatographed on a column of Bio-Gel A-5m. The resulting, purified enzyme lost activity rapidly at 25 degrees C or on manipulation, as did the crude enzyme when diluted below 1 U/ml. Both enzyme preparations could be stabilized by low levels of high-molecular-weight dextran (2 micrograms/ml), poly(ethylene glycol) (e.g., 10 micrograms/ml PEG 20 000), or nonionic detergents (e.g., 10 micrograms/ml Tween 80). The stabilizing capacity of poly(ethylene glycol) and of dextran increased with molecular weight. Calcium had no stabilizing action in the absence of other additions, but reduced the inactivation that occurred in the presence of 0.5% bovine serum albumin or high concentrations (greater than 0.1%) of Triton X-100. In summary, dextransucrase could be stabilized against activity losses caused by heating or by dilution through the addition of low concentrations of nonionic polymers (dextran, PEG 20000, methyl cellulose) or of nonionic detergents at or slightly below their critical micelle concentrations.

The synthesis of some fluoro derivatives of sucrose

2,3,1',3'4',6'-Hexa-O-benzylsucrose was obtained by mild acid-catalysed hydrolysis of the 4,6-O-isopropylidene derivative and then converted into its 4,6-di-O-mesyl derivative. Selective displacement of this disulphonate with fluoride anion (from tetrabutylammonium fluoride) then afforded the 6-fluoro-4-mesylate. Removal of the protecting groups yielded 6-deoxy-6-fluorosucrose, which was characterised as its crystalline hepta-acetate. A derivative of 6-deoxy-6-fluoro-galacto-sucrose was formed when the above 6-fluoro-4-mesylate was subjected to nucleophilic displacement with benzoate anion.

A D-glucosylated form of dextransucrase: preparation and characteristics

Dextransucrase was treated with [14C]sucrose, and the product applied to gel-permeation columns. In the absence of the detergents SDS and Triton X-100, poor recovery of enzyme was observed; however, that enzyme which was recovered was labeled. In the presence of detergents, recovery was increased, but the material appeared to be a large aggregate (mol. wt. greater than 5 X 10(6) ). In addition, the ratio of D-glucose to enzyme suggested that a polymer had been formed. Disc-gel electrophoresis in the presence of a mixture of SDS and Triton X-100 showed similar results, and indicated that the aggregate was disrupted upon treatment with dextranase. Native enzyme that had been immobilized on hydroxylapatite could also be labeled with [14C]sucrose, and the labeling followed saturation kinetics. The labeled protein could be released from the gel with 8M urea, but was aggregated. Radioactive sugars, free from protein, could be released by heating the labeled enzyme. The sugars released consisted of a mixture of D-glucose with oligosaccharides having an average chain-length of 17 D-glucosyl residues. The significance of these observations is discussed.

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.

Mechanism of synthesis of D-glucans by D-glucosyltransferases from Streptococcus mutans 6715

Two glucosyltransferases from Streptococcus mutans 6715 were purified and separated. One of the glucosyltransferases synthesized an insoluble glucan, and the other, a soluble glucan. The enzymes were immobilized on Bio-Gel P-2 beads, and the mechanism of glucan synthesis was studied by pulse and chase techniques with 14C-sucrose. Label was associated with the immobilized enzymes. The label could be quantitatively released by heating at pH 2. Analysis of the labeled products from the pulse experiment showed labeled glucose and labeled glucan; the chase experiment showed labeled glucan and a significant decrease in labeled glucose. The glucans from the pulse and the chase experiments were separated from glucose by chromatography on Bio-Gel P-6. They were reduced with sodium borohydride, and the products hydrolyzed with acid. Analysis of the labeled products from the reduced and hydrolyzed, pulsed glucans showed labeled glucose and labeled glucitol; label in the glucitol was greatly decreased in the chase experiment. These experiments showed that glucose and glucan were covalently attached to the active site of the enzymes during synthesis, and that the glucose was being transferred to the reducing end of the glucan chain. A mechanism for the synthesis of the glucans is proposed in which there are two catalytic groups on each enzyme that holds glucosyl and glucanosyl units. During synthesis, the glucosyl and glucanosyl units alternate between the two sites, giving elongation of the glucans from the reducing end. The addition of increasing amounts of B-512F dextran to the insoluble-glucan-forming glucosyltransferase produced a decrease in the proportion of insoluble glucan formed and a concomitant increase in a soluble glucan. The total amount of glucan synthesized (soluble plus insoluble) was increased 1.6 times over the amount of insoluble glucan formed when no exogenous dextran was added. It is shown that the addition of B-512F dextran affects the solubility of the synthesized alpha-(1 to 3)-glucan by accepting alpha-(1-3)-glucan chains at various positions along the dextran chain, to give a soluble, graft polymer.