Structures of heterooligosaccharides synthesized by levansucrase

Integrative growth physiology and transcriptome profiling of probiotic Limosilactobacillus reuteri KUB-AC5

Limosilactobacillus reuteri KUB-AC5 has been widely used as probiotic in chicken for Salmonella reduction. However, a preferable carbon source and growth phase is poorly characterized underlying metabolic responses on growth and inhibition effects of L. reuteri KUB-AC5. This study therefore aimed to investigate transcriptome profiling of L. reuteri KUB-AC5 revealing global metabolic responses when alteration of carbon sources and growth phases. Interestingly, L. reuteri KUB-AC5 grown under sucrose culture showed to be the best for fast growth and inhibition effects against Salmonella Enteritidis S003 growth. Towards the transcriptome profiling and reporter proteins/metabolites analysis, the results showed that amino acid transport via ABC systems as well as sucrose metabolism and transport are key metabolic responses at Logarithmic (L)-phase of L. reuteri KUB-AC5 growth. Considering the Stationary (S)-phase, we found the potential reporter proteins/metabolites involved in carbohydrate metabolism e.g., levansucrase and levan. Promisingly, levansucrase and levan were revealed to be candidates in relation to inhibition effects of L. reuteri KUB-AC5. Throughout this study, L. reuteri KUB-AC5 had a metabolic control in acclimatization to sucrose and energy pools through transcriptional co-regulation, which supported the cell growth and inhibition potentials. This study offers a perspective in optimizing fermentation condition through either genetic or physiological approaches for enhancing probiotic L. reuteri KUB-AC5 properties.

Understanding the transfer reaction network behind the non-processive synthesis of low molecular weight levan catalyzed by Bacillus subtilis levansucrase

Under specific reaction conditions, levansucrase from Bacillus subtilis (SacB) catalyzes the synthesis of a low molecular weight levan through the non-processive elongation of a great number of intermediates. To deepen understanding of the polymer elongation mechanism, we conducted a meticulous examination of the fructooligosaccharide profile evolution during the levan synthesis. As a result, the formation of primary and secondary intermediates series in different reaction stages was observed. The origin of the series was identified through comparison with product profiles obtained in acceptor reactions employing levanbiose, blastose, 1-kestose, 6-kestose, and neo-kestose, and supported with the isolation and NMR analyses of some relevant products, demonstrating that all of them are inherent products during levan formation from sucrose. These results allowed to establish the network of fructosyl transfer reactions involved in the non-processive levan synthesis. Overall, our results reveal how the relaxed acceptor specificity of SacB during the initial steps of the synthesis is responsible for the formation of several levan series, which constitute the final low molecular weight levan distribution.

Intrinsic Levanase Activity of Bacillus subtilis 168 Levansucrase (SacB)

Levansucrase catalyzes the synthesis of fructose polymers through the transfer of fructosyl units from sucrose to a growing fructan chain. Levanase activity of Bacillus subtilis levansucrase has been described since the very first publications dealing with the mechanism of levan synthesis. However, there is a lack of qualitative and quantitative evidence regarding the importance of the intrinsic levan hydrolysis of B. subtilis levansucrase and its role in the levan synthesis process. Particularly, little attention has been paid to the long-term hydrolysis products, including its participation in the final levan molecules distribution. Here, we explored the hydrolytic and transferase activity of the B. subtilis levansucrase (SacB) when levans produced by the same enzyme are used as substrate. We found that levan is hydrolyzed through a first order exo-type mechanism, which is limited to a conversion extent of around 30% when all polymer molecules reach a structure no longer suitable to SacB hydrolysis. To characterize the reaction, Isothermal Titration Calorimetry (ITC) was employed and the evolution of the hydrolysis products profile followed by HPLC, GPC and HPAEC-PAD. The ITC measurements revealed a second step, taking place at the end of the reaction, most probably resulting from disproportionation of accumulated fructo-oligosaccharides. As levanase, levansucrase may use levan as substrate and, through a fructosyl-enzyme complex, behave as a hydrolytic enzyme or as a transferase, as demonstrated when glucose and fructose are added as acceptors. These reactions result in a wide variety of oligosaccharides that are also suitable acceptors for fructo-oligosaccharide synthesis. Moreover, we demonstrate that SacB in the presence of levan and glucose, through blastose and sucrose synthesis, results in the same fructooligosaccharides profile as that observed in sucrose reactions. We conclude that SacB has an intrinsic levanase activity that contributes to the final levan profile in reactions with sucrose as substrate.

Sugarcane molasses and yeast powder used in the Fructooligosaccharides production by Aspergillus japonicus-FCL 119T and Aspergillus niger ATCC 20611

Different concentrations of sucrose (3-25% w/v) and peptone (2-5% w/v) were studied in the formulation of media during the cultivation of Aspergillus japonicus-FCL 119T and Aspergillus niger ATCC 20611. Moreover, cane molasses (3.5-17.5% w/v total sugar) and yeast powder (1.5-5% w/v) were used as alternative nutrients for both strains' cultivation. These media were formulated for analysis of cellular growth, beta-Fructosyltransferase and Fructooligosaccharides (FOS) production. Transfructosylating activity (U ( t )) and FOS production were analyzed by HPLC. The highest enzyme production by both the strains was 3% (w/v) sucrose and 3% (w/v) peptone, or 3.5% (w/v) total sugars present in cane molasses and 1.5% (w/v) yeast powder. Cane molasses and yeast powder were as good as sucrose and peptone in the enzyme and FOS (around 60% w/w) production by studied strains.

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.

The levansucrase and inulosucrase enzymes of Lactobacillus reuteri 121 catalyse processive and non-processive transglycosylation reactions

Bacterial fructosyltransferase (FTF) enzymes synthesize fructan polymers from sucrose. FTFs catalyse two different reactions, depending on the nature of the acceptor, resulting in: (i) transglycosylation, when the growing fructan chain (polymerization), or mono- and oligosaccharides (oligosaccharide synthesis), are used as the acceptor substrate; (ii) hydrolysis, when water is used as the acceptor. Lactobacillus reuteri 121 levansucrase (Lev) and inulosucrase (Inu) enzymes are closely related at the amino acid sequence level (86 % similarity). Also, the eight amino acid residues known to be involved in catalysis and/or sucrose binding are completely conserved. Nevertheless, these enzymes differ markedly in their reaction and product specificities, i.e. in β(2→6)- versus β(2→1)-glycosidic-bond specificity (resulting in levan and inulin synthesis, respectively), and in the ratio of hydrolysis versus transglycosylation activities [resulting in glucose and fructooligosaccharides (FOSs)/polymer synthesis, respectively]. The authors report a detailed characterization of the transglycosylation reaction products synthesized by the Lb. reuteri 121 Lev and Inu enzymes from sucrose and related oligosaccharide substrates. Lev mainly converted sucrose into a large levan polymer (processive reaction), whereas Inu synthesized mainly a broad range of FOSs of the inulin type (non-processive reaction). Interestingly, the two FTF enzymes were also able to utilize various inulin-type FOSs (1-kestose, 1,1-nystose and 1,1,1-kestopentaose) as substrates, catalysing a disproportionation reaction; to the best of our knowledge, this has not been reported for bacterial FTF enzymes. Based on these data, a model is proposed for the organization of the sugar-binding subsites in the two Lb. reuteri 121 FTF enzymes. This model also explains the catalytic mechanism of the enzymes, and differences in their product specificities.

Investigations of the transfructosylation reaction by fructosyltransferase from B. subtilis NCIMB 11871 for the synthesis of the sucrose analogue galactosyl-fructoside

The exo-fructosyltransferase produced from B. subtilis NCIMB 11871 strain transfers the fructose moiety from donor alpha12 linked saccharides such as sucrose, raffinose and stachyose to the acceptor d-galactose, leading to the sucrose analogue, galactosyl-fructoside. Here, we report detailed kinetic studies. The enzyme showed a remarkably high optimal temperature at 50 degrees C and was effectively immobilised on Eupergit C 250 L and Trisopor-Amino. This is also the first report about the equilibrium of the transfructosylation reaction, its activation energy determination, the structure of the product and its preparative scale isolation.

Production of long-chain levan by a sacC insertional mutant from Bacillus subtilis 327UH

A hyper extracellular protein producer, Bacillus subtilis 327UH, produced large amounts of levan in a medium containing 20% sucrose, and the yield of levan after 10 hours was more than 60%, when based on the fructose amount of sucrose. After transformation of 327UH with a levanase-deficient 168SC (sacC::Cm(r)) chromosomal DNA, a Cm(r) transformant 327UHSC (sacC::Cm(r) degSU(Hy)) produced 3 times longer levan than that of the wild type.

Purification and characterization of a Bacillus sp. SAM1606 thermostable alpha-glucosidase with transglucosylation activity

We purified a novel alpha-glucosidase to homogeneity from an Escherichia coli recombinant transformed with the alpha-glucosidase gene from thermophilic Bacillus sp. SAM1606. The enzyme existed as mono- and multimeric forms of a promoter protein with a relative molecular weight of 64,000 and isoelectric point of 4.6. We isolated a monomeric form of the enzyme and characterized it. The enzyme was unique among the known alpha-glucosidases in both broad substrate specificity and high thermostability. The enzyme hydrolysed a variety of O-alpha-D-glucopyranosides such as nigerose, maltose, isomaltose, sucrose, and trehalose efficiently. The molecular activity (k0) and the Michaelis constant (Km) values at 55 degrees C and pH 6.0 for sucrose were 54.6 s-1 and 5.3 mM, respectively. The optimum pH and temperature for hydrolysis were pH 5.5 and 75 degrees C, respectively. The enzyme exhibited a high transglucosylation activity: it reacted with 1.8 M sucrose at 60 degrees C for 70 h to yield oligosaccharides containing theanderose in a maximum yield of 35% (w/w). High thermostability of the enzyme (stable up to 65 degrees C at pH 7.2 for 10 min) permits the transglucosylation reaction at high temperatures, which would be beneficial for continuous production of oligosaccharides from sucrose.

Osmoregulation in Agrobacterium tumefaciens: accumulation of a novel disaccharide is controlled by osmotic strength and glycine betaine

We have investigated the mechanism of osmotic stress adaptation (osmoregulation) in Agrobacterium tumefaciens biotype I (salt-tolerant) and biotype II (salt-sensitive) strains. Using natural-abundance 13C nuclear magnetic resonance spectroscopy, we identified all organic solutes that accumulated to significant levels in osmotically stressed cultures. When stressed, biotype I strains (C58, NT1, and A348) accumulated glutamate and a novel disaccharide, beta-fructofuranosyl-alpha-mannopyranoside, commonly known as mannosucrose. In the salt-sensitive biotype II strain K84, glutamate was observed but mannosucrose was not. We speculate that mannosucrose confers the extra osmotic tolerance observed in the biotype I strains. In addition to identifying the osmoregulated solutes that this species synthesizes, we investigated the ability of A. tumefaciens to utilize the powerful osmotic stress protectant glycine betaine when it is supplied in the medium. Results from growth experiments, nuclear magnetic resonance spectroscopy, and a 14C labeling experiment demonstrated that in the absence of osmotic stress, glycine betaine was metabolized, while in stressed cultures, glycine betaine accumulated intracellularly and conferred enhanced osmotic stress tolerance. Furthermore, when glycine betaine was taken up in stressed cells, its accumulation caused the intracellular concentration of mannosucrose to drop significantly. The possible role of osmoregulation of A. tumefaciens in the transformation of plants is discussed.

Microbial levan

Levans are natural polymers of the sugar fructose found in many plants and microbial products. Like dextrans, they are formed as an undesirable by-product of sugar juice processing. On the other hand, levans, which can only be produced from sucrose, have potential industrial applications as thickeners and encapsulating agents and could provide additional, valuable products from sugarcane juice. A strain of B. polymyxa (NRRL B-18475) produced a high yield of polysaccharide when grown on sucrose solution. Hydrolysis and subsequent analyses showed the product to consist entirely of D-fructose. 13C-NMR and methylation analyses indicated the products to be a beta(2----6)-linked polymer of fructose, with 12% branching. The polysaccharide has a Mr of approximately 2 million and is readily soluble in water. Levan has not been utilized, but if developed, could be useful in food and other industrial applications.