Mutagenesis of asp-569 of glucosyltransferase I glucansucrase modulates glucan and oligosaccharide synthesis

Mutations in Amino Acid Residues of Limosilactobacillus reuteri 121 GtfB 4,6-α-Glucanotransferase that Affect Reaction and Product Specificity

Limosilactobacillus reuteri 121 4,6-α-glucanotransferase (Lr121 4,6-α-GTase), belonging to the glycosyl hydrolase (GH) 70 GtfB subfamily, converts starch and maltodextrins into linear isomalto/malto polysaccharides (IMMPs) with consecutive (α1 → 6) linkages. The recent elucidation of its crystal structure allowed identification and analysis of further structural features that determine its reaction and product specificity. Herein, sequence alignments between GtfB enzymes with different product linkage specificities (4,6-α-GTase and 4,3-α-GTase) identified amino acid residues in GH70 homology motifs, which may be critical for reaction and product specificity. Based on these alignments, four Lr121 GtfB-ΔN mutants (I1020M, S1057P, H1056G, and Q1126I) were constructed. Compared to wild-type Lr121 GtfB-ΔN, mutants S1057P and Q1126I had considerably improved catalytic efficiencies. Mutants H1056G and Q1126I showed a 9% decrease and an 11% increase, respectively, in the ratio of (α1 → 6) over (α1 → 4) linkages in maltodextrin-derived products. A change in linkage type (e.g., (α1 → 6) linkages to (α1 → 3) linkages) was not observed. The possible functional roles of these Lr121 GtfB-ΔN residues located around the acceptor substrate-binding subsites are discussed. The results provide new insights into structural determinants of the reaction and product specificity of Lr121 GtfB 4,6-α-GTase.

Structure-function relationships of family GH70 glucansucrase and 4,6-α-glucanotransferase enzymes, and their evolutionary relationships with family GH13 enzymes

Lactic acid bacteria (LAB) are known to produce large amounts of α-glucan exopolysaccharides. Family GH70 glucansucrase (GS) enzymes catalyze the synthesis of these α-glucans from sucrose. The elucidation of the crystal structures of representative GS enzymes has advanced our understanding of their reaction mechanism, especially structural features determining their linkage specificity. In addition, with the increase of genome sequencing, more and more GS enzymes are identified and characterized. Together, such knowledge may promote the synthesis of α-glucans with desired structures and properties from sucrose. In the meantime, two new GH70 subfamilies (GTFB- and GTFC-like) have been identified as 4,6-α-glucanotransferases (4,6-α-GTs) that represent novel evolutionary intermediates between the family GH13 and "classical GH70 enzymes". These enzymes are not active on sucrose; instead, they use (α1 → 4) glucans (i.e. malto-oligosaccharides and starch) as substrates to synthesize novel α-glucans by introducing linear chains of (α1 → 6) linkages. All these GH70 enzymes are very interesting biocatalysts and hold strong potential for applications in the food, medicine and cosmetic industries. In this review, we summarize the microbiological distribution and the structure-function relationships of family GH70 enzymes, introduce the two newly identified GH70 subfamilies, and discuss evolutionary relationships between family GH70 and GH13 enzymes.

Characterization of the Functional Roles of Amino Acid Residues in Acceptor-binding Subsite +1 in the Active Site of the Glucansucrase GTF180 from Lactobacillus reuteri 180

α-Glucans produced by glucansucrase enzymes hold strong potential for industrial applications. The exact determinants of the linkage specificity of glucansucrase enzymes have remained largely unknown, even with the recent elucidation of glucansucrase crystal structures. Guided by the crystal structure of glucansucrase GTF180-ΔN from Lactobacillus reuteri 180 in complex with the acceptor substrate maltose, we identified several residues (Asp-1028 and Asn-1029 from domain A, as well as Leu-938, Ala-978, and Leu-981 from domain B) near subsite +1 that may be critical for linkage specificity determination, and we investigated these by random site-directed mutagenesis. First, mutants of Ala-978 (to Leu, Pro, Phe, or Tyr) and Asp-1028 (to Tyr or Trp) with larger side chains showed reduced degrees of branching, likely due to the steric hindrance by these bulky residues. Second, Leu-938 mutants (except L938F) and Asp-1028 mutants showed altered linkage specificity, mostly with increased (α1 → 6) linkage synthesis. Third, mutation of Leu-981 and Asn-1029 significantly affected the transglycosylation reaction, indicating their essential roles in acceptor substrate binding. In conclusion, glucansucrase product specificity is determined by an interplay of domain A and B residues surrounding the acceptor substrate binding groove. Residues surrounding the +1 subsite thus are critical for activity and specificity of the GTF180 enzyme and play different roles in the enzyme functions. This study provides novel insights into the structure-function relationships of glucansucrase enzymes and clearly shows the potential of enzyme engineering to produce tailor-made α-glucans.

Residue Leu940 has a crucial role in the linkage and reaction specificity of the glucansucrase GTF180 of the probiotic bacterium Lactobacillus reuteri 180

Highly conserved glycoside hydrolase family 70 glucansucrases are able to catalyze the synthesis of α-glucans with different structure from sucrose. The structural determinants of glucansucrase specificity have remained unclear. Residue Leu(940) in domain B of GTF180, the glucansucrase of the probiotic bacterium Lactobacillus reuteri 180, was shown to vary in different glucansucrases and is close to the +1 glucosyl unit in the crystal structure of GTF180-ΔN in complex with maltose. Herein, we show that mutations in Leu(940) of wild-type GTF180-ΔN all caused an increased percentage of (α1→6) linkages and a decreased percentage of (α1→3) linkages in the products. α-Glucans with potential different physicochemical properties (containing 67-100% of (α1→6) linkages) were produced by GTF180 and its Leu(940) mutants. Mutant L940W was unable to form (α1→3) linkages and synthesized a smaller and linear glucan polysaccharide with only (α1→6) linkages. Docking studies revealed that the introduction of the large aromatic amino acid residue tryptophan at position 940 partially blocked the binding groove, preventing the isomalto-oligosaccharide acceptor to bind in an favorable orientation for the formation of (α1→3) linkages. Our data showed that the reaction specificity of GTF180 mutant was shifted either to increased polysaccharide synthesis (L940A, L940S, L940E, and L940F) or increased oligosaccharide synthesis (L940W). The L940W mutant is capable of producing a large amount of isomalto-oligosaccharides using released glucose from sucrose as acceptors. Thus, residue Leu(940) in domain B is crucial for linkage and reaction specificity of GTF180. This study provides clear and novel insights into the structure-function relationships of glucansucrase enzymes.

Construction of Chimeric Glucansucrases for Analyzing Substrate-binding Regions That Affect the Structure of Glucan Products

A gene that encodes dextransucrase S (dsrS) from Leuconostoc mesenteroides NRRL B-512F encodes a glucansucrase dextransucrase S (DSRS) which mainly produces water-soluble glucan (dextran), while the dsrT5 gene derived from dsrT of the B-512F strain encodes an enzyme dextransucrase T5 (DSRT5), which mainly produces water-insoluble glucan. Tyr340-Asn510 of DSRS and Tyr307-Asn477 of DSRT5 (Site 1), Lys696-Gly768 of DSRS and Lys668-Gly740 of DSRT5 (Site 2), and Asn917-Lys1131 of DSRS and Asn904-Lys1118 of DSRT5 (Site 3) were exchanged and six different chimeric enzymes were constructed. Water-soluble glucan produced by recombinant DSRS was composed of 64% 6-linked glucopyranoside (Glcp), 9% 3,6-linked Glcp, and 13% 4-linked Glcp. Water-insoluble glucan produced by recombinant DSRT5 was composed of 47% 6-linked Glcp and 43% 3-linked Glcp. All of the chimeric enzymes produced glucans different from the ones produced by their parental enzymes. Some of the glucans produced by chimeric enzymes were extremely changed. The Site 1 chimeric enzyme of DSRS (STS1) produced water-soluble glucan composed mostly of 6-linked Glcp. That of DSRT5 (TST1) produced water-insoluble glucan composed mostly of 4-linked Glcp. The Site 3 chimeric enzyme of DSRS (STS3) produced mainly water-insoluble glucan, DSRT5 (TST3) produced mainly water-soluble glucans, and all of the glucan fractions consisted of 3-Glcp, 4-Glcp, and 6-Glcp. The amounts of the three linkages in the water-soluble glucan produced by TST3 were about 1:1:1. Site 1 was assumed to be important for making or avoiding making alpha-1,4 linkages, while Site 3 was assumed to be important for determining the kinds of glucosyl linkages made.

Extending synthetic routes for oligosaccharides by enzyme, substrate and reaction engineering

The integration of all relevant tools for bioreaction engineering has been a recent challenge. This approach should notably favor the production of oligo- and polysaccharides, which is highly complex due to the requirements of regio- and stereoselectivity. Oligosaccharides (OS) and polysaccharides (PS) have found many interests in the fields of food, pharmaceuticals, and cosmetics due to different specific properties. Food, sweeteners, and food ingredients represent important sectors where OS are used in major amounts. Increasing attention has been devoted to the sophisticated roles of OS and glycosylated compounds, at cell or membrane surfaces, and their function, e.g., in infection and cancer proliferation. The challenge for synthesis is obvious, and convenient approaches using cheap and readily available substrates and enzymes will be discussed. We report on new routes for the synthesis of oligosaccharides (OS), with emphasis on enzymatic reactions, since they offer unique properties, proceeding highly regio- and stereoselective in water solution, and providing for high yields in general.

Characterization and biocompatibility of glucan: a safe food additive from probiotic Lactobacillus plantarum DM5

BACKGROUND

Exopolysaccharide produced by lactic acid bacteria are the subject of an increasing number of studies for their potential applications in the food industry as stabilizing, bio-thickening and immunostimulating agents. In this regard, the authors isolated an exopolysaccharide producing probiotic lactic acid bacterium from fermented beverage Marcha of north eastern Himalayas.

RESULTS

The isolate Lactobacillus plantarum DM5 showed extracellular glucansucrase activity of 0.48 U mg⁻¹ by synthesizing natural exopolysaccharide glucan (1.87 mg mL⁻¹) from sucrose. Zymogram analysis of purified enzyme confirms the presence of glucosyltransferase of approximately 148 kDa with optimal activity of 18.7 U mg⁻¹ at 30 °C and pH 5.4. The exopolysaccharide was purified by gel permeation chromatography and had an average molecular weight of 1.11 × 10⁶ Da. Acid hydrolysis and structural characterization of exopolysaccharide revealed that it was composed of d-glucose residues, containing 86.5% of α-(1→6) and 13.5% of α-(1→3) linkages. Rheological study exhibited a shear thinning effect of glucan appropriate for food additives. A cytotoxicity test of glucan on human embryonic kidney 293 (HEK 293) and human cervical cancer (HeLa) cell lines revealed its nontoxic biocompatible nature.

CONCLUSION

This is the first report on the structure and biocompatibility of homopolysaccharide α-D-glucan (dextran) from probiotic Lactobacillus plantarum strain and its unique physical and rheological properties that facilitate its application in the food industry as viscosifying and gelling agent.

Combinatorial engineering of dextransucrase specificity

We used combinatorial engineering to investigate the relationships between structure and linkage specificity of the dextransucrase DSR-S from Leuconostoc mesenteroides NRRL B-512F, and to generate variants with altered specificity. Sequence and structural analysis of glycoside-hydrolase family 70 enzymes led to eight amino acids (D306, F353, N404, W440, D460, H463, T464 and S512) being targeted, randomized by saturation mutagenesis and simultaneously recombined. Screening of two libraries totaling 3.6.10⁴ clones allowed the isolation of a toolbox comprising 81 variants which synthesize high molecular weight α-glucans with different proportions of α(1→3) linkages ranging from 3 to 20 %. Mutant sequence analysis, biochemical characterization and molecular modelling studies revealed the previously unknown role of peptide ⁴⁶⁰DYVHT⁴⁶⁴ in DSR-S linkage specificity. This peptide sequence together with residue S512 contribute to defining +2 subsite topology, which may be critical for the enzyme regiospecificity.

A novel phosphotransferase system of Streptococcus mutans is responsible for transport of carbohydrates with α-1,3 linkage

The most common type of carbohydrate-transport system in Streptococcus mutans is the phosphoenolpyruvate-sugar phosphotransferase system (PTS). Fourteen PTS exist in S. mutans UA159. Several studies have shown that microorganisms growing in biofilms express different genes compared with their planktonic counterparts. In this study, we showed that one PTS of S. mutans was expressed in sucrose-grown biofilms. Furthermore, the same PTS was also responsible for the transport and metabolism of disaccharide nigerose (3-O-α-d-glucopyranosyl-d-glucose). Additionally, the results indicate that the studied PTS might be involved in the transport and metabolism of carbohydrates synthesized by glucosyltransferase B and glucosyltransferase C of S. mutans. To our knowledge, this is the first report that shows PTS transport of a disaccharide (and possibly extracellular oligosaccharides) with α-1,3 linkage.

Bioengineering of Leuconostoc mesenteroides glucansucrases that gives selected bond formation for glucan synthesis and/or acceptor-product synthesis

The variations in glucosidic linkage specificity observed in products of different glucansucrases appear to be based on relatively small differences in amino acid sequences in their sugar-binding acceptor subsites. Various amino acid mutations near active sites of DSRBCB4 dextransucrase from Leuconostoc mesenteroides B-1299CB4 were constructed. A triple amino acid mutation (S642N/E643N/V644S) immediately next to the catalytic D641 (putative transition state stabilizing residue) converted DSRBCB4 enzyme from the synthesis of mainly α-(1→6) dextran to the synthesis of α-(1→6) glucan containing branches of α-(1→3) and α-(1→4) glucosidic linkages. The subsequent introduction of mutation V532P/V535I, located next to the catalytic D530 (nucleophile), resulted in the synthesis of an α-glucan containing increased branched α-(1→4) glucosidic linkages (approximately 11%). The results indicate that mutagenesis can guide glucansucrase toward the synthesis of various oligosaccharides or novel polysaccharides with completely altered linkages without compromising high transglycosylation activity and efficiency.

Crystal structure of glucansucrase from the dental caries pathogen Streptococcus mutans

Glucansucrase (GSase) from Streptococcus mutans is an essential agent in dental caries pathogenesis. Here, we report the crystal structure of S. mutans glycosyltransferase (GTF-SI), which synthesizes soluble and insoluble glucans and is a glycoside hydrolase (GH) family 70 GSase in the free enzyme form and in complex with acarbose and maltose. Resolution of the GTF-SI structure confirmed that the domain order of GTF-SI is circularly permuted as compared to that of GH family 13 α-amylases. As a result, domains A, B and IV of GTF-SI are each composed of two separate polypeptide chains. Structural comparison of GTF-SI and amylosucrase, which is closely related to GH family 13 amylases, indicated that the two enzymes share a similar transglycosylation mechanism via a glycosyl-enzyme intermediate in subsite -1. On the other hand, novel structural features were revealed in subsites +1 and +2 of GTF-SI. Trp517 provided the platform for glycosyl acceptor binding, while Tyr430, Asn481 and Ser589, which are conserved in family 70 enzymes but not in family 13 enzymes, comprised subsite +1. Based on the structure of GTF-SI and amino acid comparison of GTF-SI, GTF-I and GTF-S, Asp593 in GTF-SI appeared to be the most critical point for acceptor sugar orientation, influencing the transglycosylation specificity of GSases, that is, whether they produced insoluble glucan with α(1-3) glycosidic linkages or soluble glucan with α(1-6) linkages. The structural information derived from the current study should be extremely useful in the design of novel inhibitors that prevent the biofilm formation by GTF-SI.

Mutans streptococcal infection induces salivary antibody to virulence proteins and associated functional domains

The interplay between mucosal immune responses to natural exposure to mutans streptococci and the incorporation and accumulation of these cariogenic microorganisms in oral biofilms is unclear. An initial approach to explore this question would be to assess the native secretory immunity emerging as a consequence of Streptococcus mutans infection. To this end, we analyzed salivary immunoglobulin A (IgA) antibody to mutans streptococcal glucosyltransferase (Gtf) and glucan binding protein B (GbpB) and to domains associated with enzyme function and major histocompatibility complex (MHC) class II binding in two experiments. Salivas were collected from approximately 45-day-old Sprague-Dawley rats, which were then infected with S. mutans SJ32. Infection was verified and allowed to continue for 2 to 2.5 months. Salivas were again collected following the infection period. Pre- and postinfection salivas were then analyzed for IgA antibody activity using peptide- or protein-coated microsphere Luminex technology. S. mutans infection induced significant levels of salivary IgA antibody to Gtf (P < 0.002) and GbpB (P < 0.001) in both experiments, although the levels were usually far lower than the levels achieved when mucosal immunization is used. Significantly (P < 0.035 to P < 0.001) elevated levels of postinfection salivary IgA antibody to 6/10 Gtf peptides associated with either enzyme function or MHC binding were detected. The postinfection levels of antibody to two GbpB peptides in the N-terminal region of the six GbpB peptides assayed were also elevated (P < 0.031 and P < 0.001). Interestingly, the patterns of the rodent response to GbpB peptides were similar to the patterns seen in salivas from young children during their initial exposure to S. mutans. Thus, the presence of a detectable postinfection salivary IgA response to mutans streptococcal virulence-associated components, coupled with the correspondence between rat and human mucosal immune responsiveness to naturally presented Gtf and GbpB epitopes, suggests that the rat may be a useful model for defining mucosal responses that could be expected in humans. Under controlled infection conditions, such a model could prove to be helpful for unraveling relationships between the host response and oral biofilm development.

Fusion proteins comprising the catalytic domain of mutansucrase and a starch-binding domain can alter the morphology of amylose-free potato starch granules during biosynthesis

It has been shown previously that mutan can be co-synthesized with starch when a truncated mutansucrase (GtfICAT) is directed to potato tuber amyloplasts. The mutan seemed to adhere to the isolated starch granules, but it was not incorporated in the starch granules. In this study, GtfICAT was fused to the N- or C-terminus of a starch-binding domain (SBD). These constructs were introduced into two genetically different potato backgrounds (cv. Kardal and amf), in order to bring GtfICAT in more intimate contact with growing starch granules, and to facilitate the incorporation of mutan polymers in starch. Fusion proteins of the appropriate size were evidenced in starch granules, particularly in the amf background. The starches from the various GtfICAT/SBD transformants seemed to contain less mutan than those from transformants with GtfICAT alone, suggesting that the appended SBD might inhibit the activity of GtfICAT in the engineered fusion proteins. Scanning electron microscopy showed that expression of SBD-GtfICAT resulted in alterations of granule morphology in both genetic backgrounds. Surprisingly, the amf starches containing SBD-GtfICAT had a spongeous appearance, i.e., the granule surface contained many small holes and grooves, suggesting that this fusion protein can interfere with the lateral interactions of amylopectin sidechains. No differences in physico-chemical properties of the transgenic starches were observed. Our results show that expression of granule-bound and "soluble" GtfICAT can affect starch biosynthesis differently.

Role of asparagine 1134 in glucosidic bond and transglycosylation specificity of reuteransucrase from Lactobacillus reuteri 121

Glucansucrases from lactic acid bacteria convert sucrose into various alpha-glucans that differ greatly with respect to the glucosidic bonds present (e.g. dextran, mutan, alternan and reuteran). This study aimed to identify the structural features of the reuteransucrase from Lactobacillus reuteri 121 (GTFA) that determine its reaction specificity. We here report a detailed mutational analysis of a conserved region immediately next to the catalytic Asp1133 (putative transition-state stabilizing) residue in GTFA. The data show that Asn1134 is the main determinant of glucosidic bond product specificity in this reuteransucrase. Furthermore, mutations at this position greatly influenced the hydrolysis/transglycosylation ratio. Changes in this amino acid expands the range of glucan and gluco-oligosaccharide products synthesized from sucrose by mutant GTFA enzymes.

Structure/function relationship of homopolysaccharide producing glycansucrases and therapeutic potential of their synthesised glycans

The capability of lactic acid bacteria (LAB) to produce exopoly- and oligosaccharides was and is the subject of expanding research efforts. Due to their physicochemical properties and health-promoting potential, exopoly- and oligosaccharides from food-grade LAB can be used in the food and other industries and may have additional medical applications. In the last years, many LAB have been screened for their ability to produce exopoly- and oligosaccharides, and several glycosyltransferases involved in their biosynthesis have been characterised at biochemical and genetic levels. These research efforts aim to exploit the full potential of these organisms and to understand the structure/function relationship of glycosyltransferases. The latter knowledge is a prerequisite for the production of tailored exopoly- and oligosaccharides for the diverse applications. This review will survey the results of recent works on the structure/function relationship of homopolysaccharide producing glycosyltransferases and the therapeutic potential of their synthesised exopoly- and oligosaccharides.

Structure-function relationships of glucansucrase and fructansucrase enzymes from lactic acid bacteria

Lactic acid bacteria (LAB) employ sucrase-type enzymes to convert sucrose into homopolysaccharides consisting of either glucosyl units (glucans) or fructosyl units (fructans). The enzymes involved are labeled glucansucrases (GS) and fructansucrases (FS), respectively. The available molecular, biochemical, and structural information on sucrase genes and enzymes from various LAB and their fructan and alpha-glucan products is reviewed. The GS and FS enzymes are both glycoside hydrolase enzymes that act on the same substrate (sucrose) and catalyze (retaining) transglycosylation reactions that result in polysaccharide formation, but they possess completely different protein structures. GS enzymes (family GH70) are large multidomain proteins that occur exclusively in LAB. Their catalytic domain displays clear secondary-structure similarity with alpha-amylase enzymes (family GH13), with a predicted permuted (beta/alpha)(8) barrel structure for which detailed structural and mechanistic information is available. Emphasis now is on identification of residues and regions important for GS enzyme activity and product specificity (synthesis of alpha-glucans differing in glycosidic linkage type, degree and type of branching, glucan molecular mass, and solubility). FS enzymes (family GH68) occur in both gram-negative and gram-positive bacteria and synthesize beta-fructan polymers with either beta-(2-->6) (inulin) or beta-(2-->1) (levan) glycosidic bonds. Recently, the first high-resolution three-dimensional structures have become available for FS (levansucrase) proteins, revealing a rare five-bladed beta-propeller structure with a deep, negatively charged central pocket. Although these structures have provided detailed mechanistic insights, the structural features in FS enzymes dictating the synthesis of either beta-(2-->6) or beta-(2-->1) linkages, degree and type of branching, and fructan molecular mass remain to be identified.

Biochemical and molecular characterization of Lactobacillus reuteri 121 reuteransucrase

Lactobacillus reuteri strain 121 uses sucrose for synthesis of a unique, soluble glucan ('reuteran') with mainly alpha-(1-->4) glucosidic linkages. The gene (gtfA) encoding this glucansucrase enzyme had previously been characterized. Here, a detailed biochemical and molecular analysis of the GTFA enzyme is presented. This is believed to be the first report describing reuteransucrase enzyme kinetics and the oligosaccharides synthesized with various acceptors. Alignments of the GTFA sequence with glucansucrases from Streptococcus and Leuconostoc identified conserved amino-acid residues in the catalytic core critical for enzyme activity. Mutants Asp1024Asn, Glu1061Gln and Asp1133Asn displayed 300- to 1000-fold-reduced specific activities. To investigate the role of the relatively large N-terminal variable domain (702 amino acids) and the relatively short C-terminal putative glucan-binding domain (267 amino acids, with 11 YG repeats), various truncated derivatives of GTFA (1781 amino acids) were constructed and characterized. Deletion of the complete N-terminal variable domain of GTFA (GTFA-Delta N) had little effect on reuteran characteristics (size, distribution of glycosidic linkages), but the initial transferase activity of the mutant enzyme increased drastically. Sequential C-terminal deletions (up to six YG repeats) in GTFA-Delta N also had little effect on reuteran characteristics. However, enzyme kinetics drastically changed. Deletion of 7, 8 or 11 YG repeats resulted in dramatic loss of total enzyme activity (43-, 63- and 1000-fold-reduced specific activities, respectively). Characterization of sequential C-terminal deletion mutants of GTFA-Delta N revealed that the C-terminal domain of reuteransucrase has an important role in glucan binding.

Towards a more versatile alpha-glucan biosynthesis in plants

Starch is an important storage polysaccharide in many plants. It is composed of densely packed alpha-glucans, consisting of 1,4- and 1,4,6-linked glucose residues. The starch polymers are used in many industrial applications. The biosynthetic machinery for assembling the granule has been manipulated in many different ways to gain insight into the process of starch biosynthesis and to engineer starches with improved functionalities. With respect to the latter, two generic technologies with great potential have been developed: (i) introduction of new linkage types in starch polymers (1,3- and 1,6-linkages), and (ii) engineering granule-boundness. The toolbox to engineer this new generation of starch polymers is discussed.

Molecular characterization of DSR-E, an alpha-1,2 linkage-synthesizing dextransucrase with two catalytic domains

A novel Leuconostoc mesenteroides NRRL B-1299 dextransucrase gene, dsrE, was isolated, sequenced, and cloned in Escherichia coli, and the recombinant enzyme was shown to be an original glucansucrase which catalyses the synthesis of alpha-1,6 and alpha-1,2 linkages. The nucleotide sequence of the dsrE gene consists of an open reading frame of 8,508 bp coding for a 2,835-amino-acid protein with a molecular mass of 313,267 Da. This is twice the average mass of the glucosyltransferases (GTFs) known so far, which is consistent with the presence of an additional catalytic domain located at the carboxy terminus of the protein and of a central glucan-binding domain, which is also significantly longer than in other glucansucrases. From sequence comparison with family 70 and alpha-amylase enzymes, crucial amino acids involved in the catalytic mechanism were identified, and several original sequences located at some highly conserved regions in GTFs were observed in the second catalytic domain.

Facilitated intranasal induction of mucosal and systemic immunity to mutans streptococcal glucosyltransferase peptide vaccines

Synthetic peptide vaccines which are derived from functional domains of Streptococcus mutans glucosyltransferases (GTF) have been shown to induce protective immunity in Sprague-Dawley rats after subcutaneous injection in the salivary gland region. Since mucosal induction of salivary immunity would be preferable in humans, we explored methods to induce mucosal antibody in the rat to the GTF peptide vaccines HDS and HDS-GLU after intranasal administration. Several methods of facilitation of the immune response were studied: the incorporation of peptides in bioadhesive poly(D,L-lactide-coglycolide) (PLGA) microparticles, the use of monoepitopic (HDS) or diepitopic (HDS-GLU) peptide constructs, or the use of mucosal adjuvants. Salivary immunoglobulin A (IgA) responses were not detected after intranasal administration of diepitopic HDS-GLU peptide constructs in alum or after incorporation into PLGA microparticles. However, significant primary and secondary salivary IgA and serum IgG antibody responses to HDS were induced in all rats when cholera holotoxin (CT) or a detoxified mutant Escherichia coli heat-labile enterotoxin (R192G LT) were intranasally administered with HDS peptide constructs in PLGA. Coadministration of LT with HDS resulted in predominantly IgG2a responses in the serum, while coadministration with CT resulted in significant IgG1 and IgG2a responses to HDS. Serum IgG antibody, which was induced to the HDS peptide construct by coadministration with these adjuvants, also bound intact mutans streptococcal GTF in an enzyme-linked immunosorbent assay and inhibited its enzymatic activity. Thus, immune responses which are potentially protective for dental caries can be induced to peptide-based GTF vaccines after mucosal administration if combined with the CT or LT R192G mucosal adjuvant.

Involvement of Gln937 of Streptococcus downei GTF-I glucansucrase in transition-state stabilization

Multiple alignment of deduced amino-acid sequences of glucansucrases (glucosyltransferases and dextransucrases) from oral streptococci and Leuconostoc mesenteroides has shown them to share a well-conserved catalytic domain. A portion of this domain displays homology to members of the alpha-amylase family (glycoside hydrolase family 13), which all have a (beta/alpha)8 barrel structure. In the glucansucrases, however, the alpha-helix and beta-strand elements are circularly permuted with respect to the order in family 13. Previous work has shown that amino-acid residues contributing to the active site of glucansucrases are situated in structural elements that align with those of family 13. In alpha-amylase and cyclodextrin glucanotransferase, a histidine residue has been identified that acts to stabilize the transition state, and a histidine is conserved at the corresponding position in all other members of family 13. In all the glucansucrases, however, the aligned position is occupied by glutamine. Mutants of glucosyltransferase I were constructed in which this glutamine, Gln937, was changed to histidine, glutamic acid, aspartic acid, asparagine or alanine. The effects on specific activity, ability to form glucan and ability to transfer glucose to a maltose acceptor were examined. Only histidine could substitute for glutamine and maintain Michaelis-Menten kinetics, albeit at a greatly reduced kcat, showing that Gln937 plays a functionally equivalent role to the histidine in family 13. This provides additional evidence in support of the proposed alignment of the (beta/alpha)8 barrel structures. Mutation at position 937 altered the acceptor reaction with maltose, and resulted in the synthesis of novel gluco-oligosaccharides in which alpha1,3-linked glucosyl units are joined sequentially to maltose.