Purification and properties of a heat-stable inulin fructotransferase from Arthrobacter ureafaciens

Metabolism of Inulin via Difructose Anhydride I Pathway in Microbacterium flavum

Difructose anhydride I (DFA-I) can be produced from inulin, with DFA-I-forming inulin fructotransferase (IFTase-I). However, the metabolism of inulin through DFA-I remains unclear. To clarify this pathway, several genes of enzymes related to this pathway in the genome of Microbacterium flavum DSM 18909 were synthesized, and the corresponding enzymes were encoded, purified, and investigated in vitro. After inulin is decomposed to DFA-I by IFTase-I, DFA-I is hydrolyzed to inulobiose by DFA-I hydrolase. Inulobiose is then hydrolyzed by β-fructofuranosidase to form fructose. Finally, fructose enters glycolysis through fructokinase. A β-fructofuranosidase (MfFFase1) clears the byproducts (sucrose and fructo-oligosaccharides), which might be partially hydrolyzed by fructan β-(2,1)-fructosidase/1-exohydrolase and another fructofuranosidase (MfFFase2). Exploring the DFA-I pathway of inulin and well-studied enzymes in vitro extends our basic scientific knowledge of the energy-providing way of inulin, thereby paving the way for further investigations in vivo and offering a reference for further nutritional investigation of inulin and DFA-I in the future.

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.

Two Types of Inulin Fructotransferases

Inulin is a polysaccharide contained in chicory, dahlia, and other plants. An oligosaccharide DFA III is produced from inulin using a microbial enzyme, inulin fructotransferase (DFA III producing) [EC 2.4.1.93]. The oligosaccharide DFAIII has a unique functionality that accelerates the assimilation of minerals (Ca, Fe, and so on) from intestines. Therefore, it has a potential for the improvement of osteoporosis and iron deficiency anemia. The production of DFA III was industrialized in 2004 in Japan. Another oligosaccharide DFA I is produced from inulin by another enzyme, inulin fructotransferase (DFA I producing) [EC 2.4.1.200]. The oligosaccharide DFA I has half the sweetness of sucrose. The genes of the two enzymes were cloned and the nucleotide sequences were determined. The deduced amino acid sequences of two enzyme genes had a homology of 49.8%.

Purification and substrate specificities of a fructanase from Kluyveromyces marxianus isolated from the fermentation process of Mezcal

A fructanase, produced by a Kluyveromyces marxianus strain isolated during the fermentation step of the elaboration process of "Mezcal de Guerrero" was purified and biochemically characterized. The active protein was a glycosylated dimer with a molecular weight of approximately 250 kDa. The specific enzymatic activity of the protein was determined for different substrates: sucrose, inulin, Agave tequilana fructan, levan and Actilight® and compared with the activity of Fructozyme®. The hydrolysis profile of the different substrates analyzed by HPAEC-PAD showed that the enzyme has different affinities over the substrates tested with a sucrose/inulin enzymatic activity ratio (S/I) of 125. For the hydrolysis of Agave tequilana fructans, the enzyme also showed a higher enzymatic activity and specificity than Fructozyme®, which is important for its potential application in the tequila industry.

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.

Structural and functional insights into intramolecular fructosyl transfer by inulin fructotransferase

Inulin fructotransferase (IFTase), a member of glycoside hydrolase family 91, catalyzes depolymerization of beta-2,1-fructans inulin by successively removing the terminal difructosaccharide units as cyclic anhydrides via intramolecular fructosyl transfer. The crystal structures of IFTase and its substrate-bound complex reveal that IFTase is a trimeric enzyme, and each monomer folds into a right-handed parallel beta-helix. Despite variation in the number and conformation of its beta-strands, the IFTase beta-helix has a structure that is largely reminiscent of other beta-helix structures but is unprecedented in that trimerization is a prerequisite for catalytic activity, and the active site is located at the monomer-monomer interface. Results from crystallographic studies and site-directed mutagenesis provide a structural basis for the exolytic-type activity of IFTase and a functional resemblance to inverting-type glycosyltransferases.