Dextransucrase: donor substrate reactions

Catechol glucosides act as donor/acceptor substrates of glucansucrase enzymes of Lactobacillus reuteri

Previously, we have shown that the glucansucrase GtfA-ΔN enzyme of Lactobacillus reuteri 121, incubated with sucrose, efficiently glucosylated catechol and we structurally characterized catechol glucosides with up to five glucosyl units attached (te Poele et al. in Bioconjug Chem 27:937-946, 2016). In the present study, we observed that upon prolonged incubation of GtfA-ΔN with 50 mM catechol and 1000 mM sucrose, all catechol had become completely glucosylated and then started to reappear. Following depletion of sucrose, this glucansucrase GtfA-ΔN used both α-D-Glcp-catechol and α-D-Glcp-(1→4)-α-D-Glcp-catechol as donor substrates and transferred a glucose unit to other catechol glycoside molecules or to sugar oligomers. In the absence of sucrose, GtfA-ΔN used α-D-Glcp-catechol both as donor and acceptor substrate to synthesize catechol glucosides with 2 to 10 glucose units attached and formed gluco-oligosaccharides up to a degree of polymerization of 4. Also two other glucansucrases tested, Gtf180-ΔN from L. reuteri 180 and GtfML1-ΔN from L. reuteri ML1, used α-D-Glcp-catechol and di-glucosyl-catechol as donor/acceptor substrate to synthesize both catechol glucosides and gluco-oligosaccharides. With sucrose as donor substrate, the three glucansucrase enzymes also efficiently glucosylated the phenolic compounds pyrogallol, resorcinol, and ethyl gallate; also these mono-glucosides were used as donor/acceptor substrates.

Oligosaccharide synthesis by dextransucrase: new unconventional acceptors

The acceptor reactions of dextransucrase offer the potential for a targeted synthesis of a wide range of di-, tri- and higher oligosaccharides by the transfer of a glucosyl group from sucrose to the acceptor. We here report on results which show that the synthetic potential of this enzyme is not restricted to 'normal' saccharides. Additionally functionalized saccharides, such as alditols, aldosuloses, sugar acids, alkyl saccharides, and glycals, and rather unconventional saccharides, such as fructose dianhydride, may also act as acceptors. Some of these acceptors even turned out to be relatively efficient: alpha-D-glucopyranosyl-(1-->5)-D-arabinonic acid, alpha-D-glucopyranosyl-(1-->4)-D-glucitol, alpha-D-glucopyranosyl-(1-->6)-D-glucitol, alpha-D-glucopyranosyl-(1-->6)-D-mannitol, alpha-D-fructofuranosyl-beta-D-fructofuranosyl-(1,2':2,3')-dianhydride, 1,5-anhydro-2-deoxy-D-arabino-hex-1-enitol ('D-glucal'), and may therefore be of interest for future applications of the dextransucrase acceptor reaction.

Inhibition of dextransucrase by alpha-D-glucose derivatives

alpha-D-Glucopyranosyl fluoride was modified at positions 2, 3, or 5 and these analogs were tested as substrates and inhibitors of dextransucrase from Leuconostoc mesenteroides B-512F. The analogs studied were 2-deoxy-2-fluoro-alpha-D-glucopyranosyl fluoride, 3-deoxy-3-fluoro-alpha-D-glucopyranosyl fluoride, 3-deoxy-3-thio-alpha-D-glucopyranosyl fluoride, and 5-thio-alpha-D-glucopyranosyl fluoride. Kinetic constants for alpha-D-glucopyranosyl fluoride were also determined. None of the alpha-D-glucopyranosyl fluorides were accepted as substrates for dextransucrase. 2-Deoxy-2-fluoro-alpha-D-glucopyranosyl fluoride, 3-deoxy-3-fgluoro-alpha-D-glucopyranosyl fluoride, and 3-deoxy-3-thio-alpha-D-glucopyranosyl fluoride were competitive inhibitors with Ki values of 63, 93, and 53 mM, respectively. The Km for alpha-D-glucopyranosyl fluoride was found to be 26 mM. The data indicate that the hydroxyl groups at C2 and C3 are important for proper binding of alpha-D-glucopyranosyl fluoride for the active site of dextransucrase and that the C3-hydroxyl probably acts as a hydrogen-bond donor.

Specificity of acceptor binding to Leuconostoc mesenteroides B-512F dextransucrase: binding and acceptor-product structure of alpha-methyl-D-glucopyranoside analogs modified at C-2, C-3, and C-4 by inversion of the hydroxyl and by replacement of the hydroxyl with hydrogen

The specificity of acceptor binding to the active site of dextransucrase was studied by using alpha-methyl-D-glucopyranoside analogs modified at C-2, C-3, and C-4 positions by (a) inversion of the hydroxyl group and (b) replacement of the hydroxyl group with hydrogen. 2-Deoxy-alpha-methyl-D-glucopyranoside was synthesized from 2-deoxyglucose; 3- and 4-deoxy-alpha-methyl-D-glucopyranosides were synthesized from alpha-methyl-D-glucopyranoside; and alpha-methyl-D-allopyranoside was synthesized from D-glucose. The analogs were incubated with [14C]sucrose and dextransucrase, and the products were separated by thin-layer chromatography and quantitated by liquid scintillation spectrometry. Structures of the acceptor products were determined by methylation analyses and optical rotation. The relative effectiveness of the acceptor analogs in decreasing order were 2-deoxy, 2-inverted, 3-deoxy, 3-inverted, 4-inverted, and 4-deoxy. The enzyme transfers D-glucopyranose to the C-6 hydroxyl of analogs modified at C-2 and C-3, to the C-4 hydroxyl of 4-inverted, and to the C-3 hydroxyl of 4-deoxy analogs of alpha-methyl-D-glucopyranoside. The data indicate that the hydroxyl group at C-2 is not as important for acceptor binding as the hydroxyl groups at C-3 and C-4. The hydroxyl group at C-4 is particularly important as it determines the binding orientation of the alpha-methyl-D-glucopyranoside ring.

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.

Interaction of deoxyhalosucrose derivatives with dextransucrase

Members of a series of deoxyhalosucrose analogs substituted at one, two, or three primary carbon atoms with bromine or chlorine were prepared. Dextransucrase isolated from Streptococcus sanguis was separately treated with 6-bromo-6-deoxysucrose, 6,6'-dibromo-6,6'-dideoxysucrose, 6,1',6'-tribromotrideoxysucrose, and 6,6'-dichlorodideoxysucrose, in order to determine if they were inactivators. Variation in time of exposure, and in the concentration of the sucrose analogs, did not yield significant irreversible inactivation. In supplementary studies, it was found that the compounds serve as weak, reversible inhibitors.

Effect of maltose on glucan synthesis by glucosyltransferases of Streptococcus mutans

The effects of added maltose on the activities of a preparation of crude glucosyltransferases (GTases) and purified dextransucrase (DS) were investigated to elucidate the inhibition mechanism of maltose on the synthesis of water-insoluble glucan (ISG) in Streptococcus mutans HS-6. Tri- and tetra-saccharides produced by crude GTases from sucrose in the presence of maltose were identified as panose (4-alpha-isomaltosylglucose) and 4-alpha-isomaltotriosylglucose which were responsible for the activity of DS involved in crude GTases. Kinetic studies on crude GTases in the presence of maltose showed similar results to those of DS except that the synthesis of ISG in the crude GTases was inhibited. Comparative studies of soluble products of crude GTases and DS in the presence of maltose were performed employing gel filtration on Sephadex G-15. The existence of oligosaccharides above hexasaccharide was revealed as the products of DS but not of crude GTases. These findings were interpreted in terms of the previously proposed mechanism of ISG synthesis by S. mutans, i.e., ISG should be synthesized from the preformed soluble glucan. It was indicated that oligosaccharides above hexasaccharide are utilized for ISG synthesis in the crude GTases system. From these results, the inhibitory mechanism of added maltose on ISG synthesis by crude GTases is considered as follows: DS synthesizes a series of 4-alpha-isomaltodextrinylglucose from sucrose and maltose, and the increase of added maltose results in the decrease of oligosaccharides responsible for synthesis of ISG.