Molecular and enzymatic characterization of a levan fructotransferase from Microbacterium sp. AL-210

Glycoside hydrolase family 32 enzymes from Bombella spp. catalyze the formation of high-molecular weight fructans from sucrose

AIMS

Acetic acid bacteria of the genus Bombella have not been reported to produce exopolysaccharides (EPS). In this study, the formation of fructans by B. apis TMW 2.1884 and B. mellum TMW 2.1889 was investigated.

METHODS AND RESULTS

Out of eight strains from four different Bombella species, only B. apis TMW 2.1884 and B. mellum TMW 2.1889 showed EPS formation with 50 g l-1 sucrose as substrate. Both EPS were identified as high-molecular weight (HMW) polymers (106-107 Da) by asymmetric flow field-flow fractionation coupled to multi angle laser light scattering and UV detecors (AF4-MALLS/UV) and high performance size exclusion chromatography coupled to MALLS and refractive index detectors (HPSEC-MALLS/RI) analyses. Monosaccharide analysis via trifluoroacetic acid hydrolysis showed that both EPS are fructans. Determination of glycosidic linkages by methylation analysis revealed mainly 2,6-linked fructofuranose (Fruf) units with additional 2,1-linked Fruf units (10%) and 2,1,6-Fruf branched units (7%). No glycoside hydrolase (GH) 68 family genes that are typically associated with the formation of HMW fructans in bacteria could be identified in the genomes. Through heterologous expression in Escherichia coli Top10, an enzyme of the GH32 family could be assigned to the catalysis of fructan formation. The identified fructosyltransferases could be clearly differentiated phylogenetically and structurally from other previously described bacterial fructosyltransferases.

CONCLUSIONS

The formation of HMW fructans by individual strains of the genus Bombella is catalyzed by enzymes of the GH32 family. Analysis of the fructans revealed an atypical structure consisting of 2,6-linked Fruf units as well as 2,1-linked Fruf units and 2,1,6-Fruf units.

Purification and biochemical characteristics of a novel fructosyltransferase with a high FOS transfructosylation activity from Aspergillus oryzae S719

Fructooligosaccharides (FOS) have widely used for the manufacture of low-calorie and functional foods, because they can inhibit intestinal pathogenic microorganism growth and increase the absorption of Ca²⁺ and Mg²⁺. In this study, the novel fructosyltransferase (FTase) from Aspergillus oryzae strain S719 was successfully purified and characterized. The specific activity of the final purified material was 4200 mg^-1 with purification ratio of 66 times and yield of 26%. The molecular weight of FTase of A. oryzae S719 was around 95 kDa by SDS-PAGE, which was identified as a type of FTase by Mass Spectrometry (MS). The purified FTase had optimum temperature and pH of 55 °C and 6.0, respectively. The FTase showed to be stable with more than 80% of its original activity at room temperature after 12 h and maintaining activity above 90% at pH 4.0-11.0. The Km and kcat values of the FTase were 310 mmol L^-1 and 2.0 × 10³ min^-1, respectively. The FTase was activated by 5 mmol L^-1 Mg²⁺ and 10 mmol L^-1 Na⁺ (relative activity of 116 and 114%, respectively), indicating that the enzyme was Mg²⁺ and Na⁺ dependent. About 64% of FOS was obtained by the purified FTase under 500 g L^-1 sucrose within 4 h of reaction time, which was the shortest reaction time to be reported regarding the purified enzyme production of FOS. Together, these results indicated that the FTase of A. oryzae S719 is an excellent candidate for the industrial production of FOS.

An overview of levan-degrading enzyme from microbes

Functional carbohydrates are ideal substitutes for table sugar and make up a large share of the worldwide functional food market because of their numerous physiological benefits. Growing attention has been focused on levan, a β-(2,6) fructan that possesses more favorable physicochemical properties, such as lower intrinsic viscosity and greater colloidal stability, than β-(2,1) inulin. Levan can be used not only as a functional carbohydrate but also as feedstock for the production of levan-type fructooligosaccharides (L-FOSs). Three types of levan-degrading enzymes (LDEs), including levanase (EC 3.2.1.65), β-(2,6)-fructan 6-levanbiohydrolase (LF2ase, EC 3.2.1.64), and levan fructotransferase (LFTase, EC 4.2.2.16), play significant roles in the biological production of L-FOSs. These three enzymes convert levan into different L-FOSs, levanbiose, and difructose anhydride IV (DFA IV), respectively. The prebiotic properties of both L-FOSs and DFA IV have been confirmed in recent years. Although levanase, LF2ase, and LFTase belong to the same O-glycoside hydrolase 32 family (GH32), their catalytic properties and product spectra differ significantly. In this paper, recent studies on these LDEs are reviewed, including those investigating microbial source and catalytic properties. Additionally, comparisons of LDEs, including those of their differing cleavage behavior and applications for different L-FOSs, are presented in detail.

Recent advances on the difructose anhydride IV preparation from levan conversion

Difructose anhydride IV (DFA IV) is a cyclic disaccharide consisting of two fructose residues, which is obtained from levan conversion with levan fructotransferase (LFTase) and rarely found in nature as a low-calorie sugar substitute. Some beneficial effects of DFA IV connected with its consumption have been described. The article reviews the properties and physiological functions of DFA IV, levan conversion, resources and properties of LFTase and the produced methods of DFA IV. LFTase as a relatively novel enzyme and its molecular evolution are discussed as well. The aim is to better understand a novel sugar-substituting sweetener of DFA IV as a food additive.

Production of a single cyclic type of fructooligosaccharide structure by inulin-degrading Paenibacillus sp. LX16 newly isolated from Jerusalem artichoke root

A novel inulin‐degrading bacterium was isolated from a soil sample collected on Jerusalem artichoke roots. It is a Gram‐positive, aerobic, motile and central endospore‐forming straight rod, and exhibits phenotypic properties being consistent with its classification in the genus Paenibacillus. The predominant cellular fatty acids were anteiso‐C15:0, C16:0 and anteiso‐C17:0. This strain represents a novel species of the genus Paenibacillus on the basis of phenotypic data together with phylogenetic analysis, and it is here designated as LX16 and deposited in China centre for type collection, China (= CCTCC 2015256). Strain LX16 could produce a cyclofructooligosaccharide fructanotransferase catalysing the formation of one type of fructooligosaccharide (FOS) from inulin. The FOS was identified as a cyclofructooligosaccharide with a degree of polymerization of 6. Such homology in inulin degradation products may be beneficial for the functional FOS production.

From fructans to difructose dianhydrides

Fructans are the polymers of fructose molecules, normally having a sucrose unit at what would otherwise be the reducing terminus. Inulin and levan are two basic types of simple fructan, which contain β-(2, 1) and β-(2, 6) fructosyl-fructose linkage, respectively. Fructans not only can serve as soluble dietary fibers for food industry, but also may be biologically converted into high-value products, especially high-fructose syrup and fructo-oligosaccharides. In recent years, much attention has been focused on production of difructose dianhydrides (DFAs) from fructans. DFAs are cyclic disaccharides consisting of two fructose units with formation of two reciprocal glycosidic linkages. They are expected to have promising properties and beneficial effects on human health. DFAs can be produced from fructans by fructan fructotransferases. Inulin fructotransferase (IFTase) (DFA III-forming) and IFTase (DFA I-forming) catalyze the DFA III and DFA I production from inulin, respectively, and levan fructotransferase (LFTase) (DFA IV-forming) catalyzes the production of DFA IV from levan. In this article, the DFA-producing microorganisms are summarized, relevant studies on various DFAs-producing enzymes are reviewed, and especially, the comparisons of the enzymes are presented in detail.

Structural and functional basis for substrate specificity and catalysis of levan fructotransferase

Levan is β-2,6-linked polymeric fructose and serves as reserve carbohydrate in some plants and microorganisms. Mobilization of fructose is usually mediated by enzymes such as glycoside hydrolase (GH), typically releasing a monosaccharide as a product. The enzyme levan fructotransferase (LFTase) of the GH32 family catalyzes an intramolecular fructosyl transfer reaction and results in production of cyclic difructose dianhydride, thus exhibiting a novel substrate specificity. The mechanism by which LFTase carries out these functions via the structural fold conserved in the GH32 family is unknown. Here, we report the crystal structure of LFTase from Arthrobacter ureafaciens in apo form, as well as in complexes with sucrose and levanbiose, a difructosacchride with a β-2,6-glycosidic linkage. Despite the similarity of its two-domain structure to members of the GH32 family, LFTase contains an active site that accommodates a difructosaccharide using the -1 and -2 subsites. This feature is unique among GH32 proteins and is facilitated by small side chain residues in the loop region of a catalytic β-propeller N-domain, which is conserved in the LFTase family. An additional oligosaccharide-binding site was also characterized in the β-sandwich C-domain, supporting its role in carbohydrate recognition. Together with functional analysis, our data provide a molecular basis for the catalytic mechanism of LFTase and suggest functional variations from other GH32 family proteins, notwithstanding the conserved structural elements.

Chemical and enzymatic approaches to carbohydrate-derived spiroketals: di-D-fructose dianhydrides (DFAs)

Di-D-fructose dianhydrides (DFAs) comprise a unique family of stereoisomeric spiro-tricyclic disaccharides formed upon thermal and/or acidic activation of sucrose- and/ or D-fructose-rich materials. The recent discovery of the presence of DFAs in food products and their remarkable nutritional features has attracted considerable interest from the food industry. DFAs behave as low-caloric sweeteners and have proven to exert beneficial prebiotic nutritional functions, favouring the growth of Bifidobacterium spp. In the era of functional foods, investigation of the beneficial properties of DFAs has become an important issue. However, the complexity of the DFA mixtures formed during caramelization or roasting of carbohydrates by traditional procedures (up to 14 diastereomeric spiroketal cores) makes evaluation of their individual properties a difficult challenge. Great effort has gone into the development of efficient procedures to obtain DFAs in pure form at laboratory and industrial scale. This paper is devoted to review the recent advances in the stereoselective synthesis of DFAs by means of chemical and enzymatic approaches, their scope, limitations, and complementarities.

Chemical and enzymatic approaches to carbohydrate-derived spiroketals: di-D-fructose dianhydrides (DFAs)

Di-D-fructose dianhydrides (DFAs) comprise a unique family of stereoisomeric spiro-tricyclic disaccharides formed upon thermal and/or acidic activation of sucrose- and/ or D-fructose-rich materials. The recent discovery of the presence of DFAs in food products and their remarkable nutritional features has attracted considerable interest from the food industry. DFAs behave as low-caloric sweeteners and have proven to exert beneficial prebiotic nutritional functions, favouring the growth of Bifidobacterium spp. In the era of functional foods, investigation of the beneficial properties of DFAs has become an important issue. However, the complexity of the DFA mixtures formed during caramelization or roasting of carbohydrates by traditional procedures (up to 14 diastereomeric spiroketal cores) makes evaluation of their individual properties a difficult challenge. Great effort has gone into the development of efficient procedures to obtain DFAs in pure form at laboratory and industrial scale. This paper is devoted to review the recent advances in the stereoselective synthesis of DFAs by means of chemical and enzymatic approaches, their scope, limitations, and complementarities.

Molecular cloning of levan fructotransferase gene from Arthrobacter ureafaciens K2032 and its expression in Escherichia coli for the production of difructose dianhydride IV

AIMS

To clone and overexpress a novel levan fructotransferase gene lftA from Arthrobacter ureafaciens K2032.

METHODS AND RESULTS

The lftA gene, encoding a levan fructotransferase (LFTase) of 521 amino acids (aa) residues, was cloned from the genomic DNA of A. ureafaciens K2032, and overexpressed in Escherichia coli. The recombinant LFTase overexpressed in E. coli was then used to produce a difructose dianhydride (DFA IV) from levan. DFA IV crystals with 97% purity could be obtained from the reaction mixture in 83.7% yield by using a natural crystallization method.

CONCLUSIONS

The lftA gene cloned from A. ureafaciens K2032 encode a novel levan fructotransferase which produces difructose dianhydride (DFA IV) from levan.

SIGNIFICANCE AND IMPACT OF THE STUDY

Levan fructotransferase is a useful enzyme with great promise in the production of DFA IV and various fructosides.

Molecular cloning of the gene for 2,6-beta-D-fructan 6-levanbiohydrolase from Streptomyces exfoliatus F3-2

The gene encoding a 2,6-beta-D-fructan 6-levanbiohydrolase (LF2ase) (EC 3.2.1.64) that converts levan into levanbiose was cloned from the genomic DNA of Streptomyces exfoliatus F3-2. The gene encoded a signal peptide of 37 amino acids and a mature protein of 482 amino acids with a total length of 1560 bp and was successfully expressed in Escherichia coli. The similarities of primary structure were observed with levanases from Clostridium acetobutylicum, Bacillus subtilis, B. stearothermophilus (51.0-54.3%) and with LF2ase from Microbacterium levaniformans (53.9%). The enzyme from S. exfoliatus F3-2 shared the conserved six domains and the completely conserved five amino acid residues with family 32 glycosyl hydrolases, which include levanase, inulinase, and invertase. These observations led to the conclusion that the enzyme belongs to family 32 glycosyl hydrolases.

Cloning and characterization of a levanbiohydrolase from Microbacterium laevaniformans ATCC 15953

An extracellular levanbiohydrolase gene, levM, from Microbacterium laevaniformans ATCC 15953 was cloned and its nucleotide sequence was determined. Nucleotide sequence analysis of this gene revealed a 1863 bp open reading frame coding for a protein of 621 amino acids. The deduced amino acid sequence of the levM gene exhibited 28-47% sequence identities with levanases, levanfructotransferases, and inulinases. The LevM was overexpressed by using a T7 promoter in Escherichia coli BL21 (DE3) and purified 24-fold from culture supernatant. The molecular weight of this enzyme was 68,800 Da based on the sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The optimum pH and temperature of this enzyme for levan degradation was pH 6.0 and 30 degrees C, respectively. Thin-layer and high-performance liquid chromatography analyses proved that the enzyme produced mostly levanbiose from levan in an exo-acting manner. The recombinant enzyme also hydrolyzed inulin, 1-kestose, and nystose, indicating that the enzyme cleaves not only beta-2,6-linkage of levan but also beta-2,1-linkage of fructooligosaccharides. This is the first report on a gene encoding a levanbiohydrolase that produces levanbiose as a major degradation product.