Alternansucrase acceptor products

Catalytic action of alternansucrase on sucrose under in vitro simulated gastric conditions

Alternansucrase, a glucosyltransferase, is currently used to produce slowly digestible alternan oligosaccharides or maltooligosaccharides from sucrose. These oligosaccharides are popular for food fortification to lower postprandial glucose levels. This study aimed to explore the enzymatic reaction of alternansucrase in simulated in vitro gastric reaction conditions. Under the studied conditions, SucroSEB (a model enzyme for alternansucrase) hydrolyzed the sucrose and transglycosylated the glucose to produce glucans, both in the absence and presence of acceptors. The preference of the acceptor was maltose˃ raffinose˃ lactose. The rate of sucrose hydrolysis was significantly higher in the presence of maltose (p = 0.024). The glucans formed during the reaction included oligomers (DP 3-10) and polymers (DP ≥ 11), both of which increased over time. These glucans contained α-1,3 and α-1,6 glycosidic linkages, confirmed by 1H and 13C NMR. They were slowly and partially digestible in the presence of rat intestinal extract in contrast to the complete and rapid digestion of starch. The glucans formed after a longer gastric reaction time exhibited higher dietary fiber potential (19.145 ± 4.77 %; 60 min) compared to those formed during the initial phase (2.765 ± 0.19 %; 15 min). Overall, this study demonstrated the efficacy of SucroSEB in converting sucrose to slowly and partially digestible glucans under simulated in vitro gastric conditions.

Unravelling Regioselectivity of Leuconostoc citreum ABK-1 Alternansucrase by Acceptor Site Engineering

Alternansucrase (ALT, EC 2.4.1.140) is a glucansucrase that can generate α-(1,3/1,6)-linked glucan from sucrose. Previously, the crystal structure of the first alternansucrase from Leuconostoc citreum NRRL B-1355 was successfully elucidated; it showed that alternansucrase might have two acceptor subsites (W675 and W543) responsible for the formation of alternating linked glucan. This work aimed to investigate the primary acceptor subsite (W675) by saturated mutagenesis using Leuconostoc citreum ABK-1 alternansucrase (LcALT). The substitution of other residues led to loss of overall activity, and formation of an alternan polymer with a nanoglucan was maintained when W675 was replaced with other aromatic residues. Conversely, substitution by nonaromatic residues led to the synthesis of oligosaccharides. Mutations at W675 could potentially cause LcALT to lose control of the acceptor molecule binding via maltose-acceptor reaction-as demonstrated by results from molecular dynamics simulations of the W675A variant. The formation of α-(1,2), α-(1,3), α-(1,4), and α-(1,6) linkages were detected from products of the W675A mutant. In contrast, the wild-type enzyme strictly synthesized α-(1,6) linkage on the maltose acceptor. This study examined the importance of W675 for transglycosylation, processivity, and regioselectivity of glucansucrases. Engineering glucansucrase active sites is one of the essential approaches to green tools for carbohydrate modification.

Synthesis of potential prebiotic α-glucooligosaccharides using microbial glucansucrase and their in vitro fecal fermentation

Novel α-glucooligosaccharides were synthesized by the acceptor reaction of Leuconostoc citreum SK24.002 glucansucrase with maltose and sucrose. The impact of synthesis conditions, including the ratio of sucrose to maltose and the substrate concentration, on the formation of α-glucooligosaccharides was evaluated. Under the optimized experimental conditions, the yield of a mixture of α-glucooligosaccharides with DP 3-5 reached approximately 56.4% with a concentration of 170.7 mg mL-1. Each of these α-glucooligosaccharides was purified, and the structures were assigned as follows: α-D-Glcp-(1,6)-α-D-Glcp-(1,4)-D-Glcp (DP3), α-D-Glcp-(1,3)-α-D-Glcp-(1,6)-α-D-Glcp-(1,4)-D-Glcp (DP4), and α-D-Glcp-(1,6)-α-D-Glcp-(1,3)-α-D-Glcp-(1,6)-α-D-Glcp-(1,4)-D-Glcp (DP5), respectively. For these three structurally different oligosaccharides, the fermentation selectivity by fecal bacteria was determined in anaerobic batch culture. Fructooligosaccharide (FOS) was used as a positive prebiotic control. Similar to FOS, all three α-glucooligosaccharides selectively stimulated the proliferation of Bifidobacteria and Lactobacilli compared with the control. DP3 exhibited the strongest prebiotic ability to increase the Bifidobacterium and Lactobacillus population, whereas DP5 produced the most short-chain fatty acids. In addition, DP4 produced the highest butyrate concentration and resulted in the lowest acetate : propionate ratio. These results suggested that the enzymatically synthesized α-glucooligosaccharides were potential prebiotics, underlining correlations between the structural features of oligosaccharides and their impact on the metabolism of fecal microbiota.

Synthesis of glucooligosaccharides with prebiotic potential by glucansucrase URE 13-300 acceptor reactions with maltose, raffinose and lactose

In the present work, we report an efficient synthesis of glucooligosaccharides (GOSs) with prebiotic potential by novel glucansucrase URE 13-300 from Leuconostoc mesenteroides URE 13 strain. The highest total yield of GOSs with degree of polymerization (DP) from 3 to 6 was obtained with maltose as an acceptor and maltose/sucrose (M/S) ratio 1-136 g/L. An efficient modulation of GOSs composition is achieved by varying the M/S ratio. At M/S = 1, 2, 4 and 7 the content of DP3 products gradually increase from 54.50 to 91.70%. When the M/S ratio was decreased the synthesis of DP>3 GOSs is predominant and reaches 75.60% (M/S = 0.25). In addition, the maltose derived GOSs with DP>3, as well as raffinose and lactose glucosylation products have a branched structure which is prerequisite for increased prebiotic potential. The synthesized GOSs were efficiently metabolized by probiotic strains of Lb. plantarum S26, Lb. brevis S27 and Lb. sakei S16, and the calculated values of specific growth rate (μ) were nearly identical to this on glucose media, when maltose derived GOSs were used as a carbohydrate source. Strain specific features were observed in the utilization of the synthesized GOSs, as well as in the production of lactic acid and acetic acid.

Production of isomelezitose from sucrose by engineered glucansucrases

Certain lactic acid bacteria produce glycosyltransferases known as glucansucrases, which synthesize α-D-glucans via glucosyl transfer from sucrose. We recently reported on the formation of the unusual trisaccharide isomelezitose in low yields by a variety of glucansucrases. Isomelezitose is a rare non-reducing trisaccharide, with the structure α-d-glucopyranosyl- (1→6)-β-d-fructofuranosyl-(2↔1)-α-d-glucopyranoside. In this work, we describe the synthesis of isomelezitose in high yields by variants of glucansucrases engineered to contain a single point mutation at a key leucine residue involved in acceptor substrate binding. Some variants produce isomelezitose in yields up to 57%. This method is amenable to large-scale production of isomelezitose for food, industrial and biomedical applications.

Isomelezitose formation by glucansucrases

Several glucansucrases were surveyed for their ability to produce isomelezitose, a trisaccharide with the structure α-D-glucopyranosyl (1 → 6) β-D-fructofuranosyl (2 ↔ 1) α-D-glucopyranoside. Nearly all strains tested, with one exception, produced at least trace levels of isomelezitose. Yields were low but significant, ranging from less than 1% to approximately 5% based on sucrose. This trisaccharide may arise in either of two ways: glucopyranosyl transfer to the 6^Fru-OH position of sucrose, or to the anomeric OH position of isomaltulose. This study indicates that isomelezitose formation may be a general phenomenon of many glucansucrase reactions.

Synthesis and characterization of isomaltulose-derived oligosaccharides produced by transglucosylation reaction of Leuconostoc mesenteroides dextransucrase

This paper reports the efficient enzymatic synthesis of a homologous series of isomaltulose-derived oligosaccharides with degrees of polymerization ranging from 3 to 9 through the transglucosylation reaction using a dextransucrase from Leuconostoc mesenteroides B-512F. The total oligosaccharide yield obtained under optimal conditions was 41-42% (in weight with respect to the initial amount of isomaltulose) after 24-48 h of reaction. Nuclear magnetic resonance (NMR) structural characterization indicated that dextransucrase specifically transferred glucose moieties of sucrose to the C-6 of the nonreducing glucose residue of isomaltulose. Likewise, monitoring the progression of the content of each individual oligosaccharide indicated that oligosaccharide acceptor products of low molecular weight acted in turn as acceptors for further transglucosylation to yield oligosaccharides of a higher degree of polymerization. The produced isomaltulose-derived oligosaccharides can be considered as isomalto-oligosaccharides (IMOs) because they are linked by only α-(1→6) bonds. In addition, having isomaltulose as the core structure, these IMO-like structures could possess appealing bioactive properties that could find potential applications as functional food ingredients.

Glucansucrases: three-dimensional structures, reactions, mechanism, α-glucan analysis and their implications in biotechnology and food applications

Glucansucrases are extracellular enzymes that synthesize a wide variety of α-glucan polymers and oligosaccharides, such as dextran. These carbohydrates have found numerous applications in food and health industries, and can be used as pure compounds or even be produced in situ by generally regarded as safe (GRAS) lactic acid bacteria in food applications. Research in the recent years has resulted in big steps forward in the understanding and exploitation of the biocatalytic potential of glucansucrases. This paper provides an overview of glucansucrase enzymes, their recently elucidated crystal structures, their reaction and product specificity, and the structural analysis and applications of α-glucan polymers. Furthermore, we discuss key developments in the understanding of α-glucan polymer formation based on the recently elucidated three-dimensional structures of glucansucrase proteins. Finally we discuss the (potential) applications of α-glucans produced by lactic acid bacteria in food and health related industries.

Glucosylation of raffinose via alternansucrase acceptor reactions

The glucansucrase known as alternansucrase [EC 2.4.1.140] can transfer glucosyl units from sucrose to raffinose to give good yields of oligosaccharides, which may serve as prebiotics. The main products were the tetrasaccharides alpha-d-Glcp-(1-->3)-alpha-d-Galp-(1-->6)-alpha-d-Glcp-(1<-->2)-beta-d-Fruf and alpha-d-Glcp-(1-->4)-alpha-d-Galp-(1-->6)-alpha-d-Glcp-(1<-->2)-beta-d-Fruf in ratios ranging from 4:1 to 9:1, along with lesser amounts of alpha-d-Glcp-(1-->6)-alpha-d-Galp-(1-->6)-alpha-d-Glcp-(1<-->2)-beta-d-Fruf. Ten unusual pentasaccharide structures were isolated. Three of these arose from glucosylation of the major tetrasaccharide product, two each from the minor tetrasaccharides, and three were the result of glucosylations of the fructose acceptor product leucrose or isomaltulose. The major pentasaccharide product arose from glucosylation of the major tetrasaccharide at position 4 of the fructofuranosyl unit, to give a subunit structure analogous to that of maltulose. A number of hexasaccharides and higher oligosaccharides were also produced. Unlike alternansucrase, dextransucrase [EC 2.4.1.5] gave only a single tetrasaccharide product in low yield, and no significant amounts of higher oligosaccharides. The tetrasaccharide structure from dextransucrase was found to be alpha-d-Glcp-(1-->4)-alpha-d-Galp-(1-->6)-alpha-d-Glcp-(1<-->2)-beta-d-Fruf, which is at odds with the previously published structure.

Acceptor products of alternansucrase with gentiobiose. Production of novel oligosaccharides for food and feed and elimination of bitterness

In the presence of suitable acceptor molecules, dextransucrase makes a homologous series of oligosaccharides in which the isomers differ by a single glucosyl unit, whereas alternansucrase synthesizes one trisaccharide, two tetrasaccharides, etc. Previously, we showed that alternansucrase only forms certain isomers of DP>4 from maltose in measurable amounts, and that these oligosaccharides belong to the oligoalternan series rather than the oligodextran series. We now demonstrate that the acceptor products from gentiobiose, also formed in good yields (nearly 90% in unoptimized reactions), follow a pattern similar to those formed from maltose. The initial product is a single trisaccharide, alpha-d-Glcp-(1-->6)-beta-d-Glcp-(1-->6)-d-Glc. Two tetrasaccharides were formed in approximately equal quantities: alpha-d-Glcp-(1-->3)-alpha-d-Glcp-(1-->6)-beta-d-Glcp-(1-->6)-d-Glc and alpha-d-Glcp-(1-->6)-alpha-d-Glcp-(1-->6)-beta-d-Glcp-(1-->6)-d-Glc. Just one pentasaccharide was isolated from the reaction mixture, alpha-d-Glcp-(1-->6)-alpha-d-Glcp-(1-->3)-alpha-d-Glcp-(1-->6)-beta-d-Glcp-(1-->6)-d-Glc. Our hypothesis that the enzyme is incapable of forming two consecutive alpha-(1-->3) linkages, and does not form products with more than two consecutive alpha-(1-->6) linkages, apparently applies to other acceptors as well as to maltose. The glucosylation of gentiobiose reduces or eliminates its bitter taste.