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

Bacterial α-Glucan and Branching Sucrases from GH70 Family: Discovery, Structure-Function Relationship Studies and Engineering

Glucansucrases and branching sucrases are classified in the family 70 of glycoside hydrolases. They are produced by lactic acid bacteria occupying very diverse ecological niches (soil, buccal cavity, sourdough, intestine, dairy products, etc.). Usually secreted by their producer organisms, they are involved in the synthesis of α-glucans from sucrose substrate. They contribute to cell protection while promoting adhesion and colonization of different biotopes. Dextran, an α-1,6 linked linear α-glucan, was the first microbial polysaccharide commercialized for medical applications. Advances in the discovery and characterization of these enzymes have remarkably enriched the available diversity with new catalysts. Research into their molecular mechanisms has highlighted important features governing their peculiarities thus opening up many opportunities for engineering these catalysts to provide new routes for the transformation of sucrose into value-added molecules. This article reviews these different aspects with the ambition to show how they constitute the basis for promising future developments.

Different strategies to co-immobilize dextransucrase and dextranase onto agarose based supports: Operational stability study

Co-immobilization is a groundbreaking technique for enzymatic catalysis, sometimes strategic, as for dextransucrase and dextranase. In this approach, dextranase hydrolytic action removes the dextran layer that covers dextransucrase reactive groups, improving the immobilization. Another advantage is the synergic effect of the two enzymes towards prebiotic oligosaccharides production. Thus, both enzymes were co-immobilized onto the heterobifunctional support Amino-Epoxy-Glyoxyl-Agarose (AMEG) and the ion exchanger support monoaminoethyl-N-ethyl-agarose (Manae) at pH 5.2 and 10, followed or not by glutaraldehyde treatment. This work is the first attempt to immobilize dextransucrase under alkaline conditions. The immobilized dextransucrase on AMEG support at pH 10 (12.78 ± 0.70 U/g) presents a similar activity of the biocatalyst produced at pH 5.2 (14.95 ± 0.82 U/g). The activity of dextranase immobilized onto Manae was 5-fold higher than the obtained onto AMEG support. However, the operational stability test showed that the biocatalyst produced on AMEG at pH 5.2 kept >60% of both enzyme activities for five batches. The glutaraldehyde treatment was not worthwhile to improve the operational stability of this biocatalyst.

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.

Maximization of dextransucrase activity expressed in E. coli by mutation and its functional characterization

A novel dextransucrase gene, DSRN, was obtained by ultrasoft X-ray treatment of the DSRB742 gene. The DSRN gene was further mutated via site-directed mutagenesis producing four mutants: DSRN1 (F196S), DSRN2 (Y346N), DSRN3 (K395T) and DSRN4 (P980T). Dextransucrases derived from DSRB742 and its mutants were expressed in E. coli and affinity-purified using dextran to give 80% purity. They had specific activities of 0.6-17 U/mg with Km values of 18-88 mM. DSRB742 had the lowest (0.02 s(-1) x mM(-1)) and DSRN1 had the highest (0.13 s(-1) x mM(-1)) Kcat/Km values. DSRN3 had the highest enzymatic transglycosylation efficiency with maltose (63% of theoretical), gentiobiose (39%), or salicine (40%).

Construction and characterization of chimeric enzymes of kojibiose phosphorylase and trehalose phosphorylase from Thermoanaerobacter brockii

Chimeric phosphorylases were constructed of the kojibiose phosphorylase (KP) gene and the trehalose phosphorylase (TP) gene from Thermoanaerobacter brockii. Four chimeric enzymes had KP activity, and another had TP activity. Chimera V-III showed not TP, but KP activity, although only 125 amino acid residues in 785 residues of chimera V-III were from that of KP. Chimera V-III had 1% of the specific activity of the wild-type KP. Furthermore, the temperature profile and kinetic parameters of chimera V-III were remarkably changed as compared to those of the wild-type KP. The results of the molecular mass of chimera V-III using GPC (76,000 Da) strongly suggested that the chimera V-III protein exists as a monomer in solution, whereas wild-type KP and TP are hexamer and dimer structures, respectively. The result of the substrate specificity for phosphorolysis was that the chimera acted on nigerose, sophorose and laminaribiose, in addition to kojibiose. Furthermore, chimera V-III was also able to act on sophorose and laminaribiose in the absence of inorganic phosphate, and produced two trisaccharides, beta-D-glucosyl-(1-->6)-laminaribiose and laminaritriose, from laminaribiose.

Changes in linkage pattern of glucan products induced by substitution of Lys residues in the dextransucrase

Dextransucrase S (DSRS) is the only active glucansucrase that has been found in Leuconostoc mesenteroides NRRL B-512F strain. Native DSRS produces mainly 6-linked glucopyranosyl residue (Glcp), while Escherichia coli recombinant DSRS was observed to produce a glucan consisting of 70% 6-linked Glcp and 15% 3,6-Glcp. Lys residues were introduced at the N-terminal end of the core domain by site-directed mutagenesis. In glucans produced by the one-point mutants T350K and S455K, the amount of 6-linked Glcp was increased to about 85% of the total glucan produced, more similar in structure to native B-512F dextran. The double mutant T350K/S455K produced adhesive, water-insoluble glucan with 77% 6-linked Glcp, 8% 3,6-linked Glcp and 4% 2,6-linked Glcp. The T350K/S455K mutant exhibited a 10-fold increase in glucosyltransferase activity over those of the parental DSRS-His(6) and its T350K and S455K mutants. This is the first report demonstrating a change in the properties of a dextransucrase or a related glucosyltransferase through simple site-directed mutagenesis to create 2,6-linked Glcp.