Modification of alternan by dextranase

Lactic acid bacteria-derived α-glucans: From enzymatic synthesis to miscellaneous applications

Lactic acid bacteria (LAB) are capable of producing a variety of exopolysaccharide α-glucans, such as dextran, mutan, reuteran, and alternan. Their structural diversity allows LAB-derived α-glucans to hold vast commercial value and application potential in the food, cosmetic, medical, and biotechnology fields, garnering much attention in recent years. Glycoside Hydrolase 70 family (GH70) enzymes are efficient tools for the biosynthesis of α-glucans with various sizes, linkage compositions, and degrees of branching, using renewable and low-cost sucrose and starch as substrates. To date, plenty of various LAB-derived GH70 glucansucrases (especially dextransucrase) have been biochemically characterized to synthesize α-glucans from sucrose with a variety of structural organizations. This review mainly aimed at the biotechnological synthesis of α-glucans using GH70 family enzymes and their diverse (potential) applications. The purification, structural analysis and physicochemical properties of α-glucan polysaccharides were reviewed in detail. Synchronously, some new insights and future perspectives of LAB-derived α-glucans enzymatic synthesis and applications were also discussed. To expand the range of applications, the physicochemical properties and bioactivities of LAB-derived α-glucans, other than dextran, should be further explored. Additionally, screening novel GH70 subfamily starch-acting enzymes is conducive to expanding the repertoire of α-glucans.

A specific oligosaccharide-binding site in the alternansucrase catalytic domain mediates alternan elongation

Microbial α-glucans produced by GH70 (glycoside hydrolase family 70) glucansucrases are gaining importance because of the mild conditions for their synthesis from sucrose, their biodegradability, and their current and anticipated applications that largely depend on their molar mass. Focusing on the alternansucrase (ASR) from Leuconostoc citreum NRRL B-1355, a well-known glucansucrase catalyzing the synthesis of both high- and low-molar-mass alternans, we searched for structural traits in ASR that could be involved in the control of alternan elongation. The resolution of five crystal structures of a truncated ASR version (ASRΔ2) in complex with different gluco-oligosaccharides pinpointed key residues in binding sites located in the A and V domains of ASR. Biochemical characterization of three single mutants and three double mutants targeting the sugar-binding pockets identified in domain V revealed an involvement of this domain in alternan binding and elongation. More strikingly, we found an oligosaccharide-binding site at the surface of domain A, distant from the catalytic site and not previously identified in other glucansucrases. We named this site surface-binding site (SBS) A1. Among the residues lining the SBS-A1 site, two (Gln700 and Tyr717) promoted alternan elongation. Their substitution to alanine decreased high-molar-mass alternan yield by a third, without significantly impacting enzyme stability or specificity. We propose that the SBS-A1 site is unique to alternansucrase and appears to be designed to bind alternating structures, acting as a mediator between the catalytic site and the sugar-binding pockets of domain V and contributing to a processive elongation of alternan chains.

Effect of a single point mutation on the interaction of glucans with a glucansucrase from Leuconostoc mesenteroides NRRL B-1118

Our previous work showed that substitution of an amino acid that is coupled with the +2 subsite adjacent to the transition stabilizer of a glucansucrase, which produces a water-insoluble glucan, resulted in significant changes in the structures and yields of the water-insoluble glucans produced. We now describe how these changes affect the ability of the glucansucrase to bind to exogenous glucans, and how these glucans can influence the yield, product structures, and kinetics of the mutant glucansucrases. The activity of the wild-type enzyme, with threonine at position 654, is not significantly activated by added dextran, and the yield of water-insoluble glucan from sucrose is only slightly increased by dextran. Mutant T654Y is not affected at all by the addition of dextran. However, several mutant enzymes exhibit markedly lower yields of glucan relative to the wild type; these lower yields can be partially or completely overcome by the addition of water-soluble dextran. Although evidence indicates that the soluble dextran is incorporated into water-insoluble glucan, the increased yields cannot be accounted for solely by incorporation of the dextran into insoluble product. Furthermore, these DsrI mutants are significantly activated by exogenous glucans. The addition of dextran does not markedly change the KM for sucrose in the mutant enzymes, but does increase the Vmax of the reaction. These effects apparently depend on the presence of unbranched sequences of α1→6-linked D-glucose units in the glucan.

Polysaccharide Modification through Green Technology: Role of Endodextranase in Improving the Physicochemical Properties of (1→3)(1→6)-α-D-Glucan

The structure and properties of bioengineered (1→3)(1→6)-α-D-glucan subjected to endodextranase treatment were investigated. Upon enzyme treatment, OD220 and Mw decreased substantially during the first 60 min and thereafter slowed as the modification progressed. Compared to the native glucan, the modified sample solution had a lighter opalescent, bluish-white color. The morphological analysis revealed that bioengineered glucan produced quite a few small particles after hydrolysis. The molecular weight distribution curve gradually shifted to the low Mw region with a significant broadening distribution, and the chain hydrolysis reaction followed a combination of zeroth- and first-order processes. The NMR results showed some specific α-1,6 linkages of glucan chains were cleaved with enzyme treatment. The viscosity of modified glucan solution was markedly reduced, and the Newtonian plateaus were also observed at high shear rates (10-100 1/s). The above results suggested that the modified (1→3)(1→6)-α-D-glucan showed a tailor-made solution character similar to that of arabic gum and would be used as a novel food gum substitute in the design of artificial carbohydrate-based foods.