Fructans levan

Fructans (levans) are natural polymers of fructose, found in many plants and microbial products. Like dextrans, they can be formed as an imdesirable microbial byproduct in the processing of sugar juice and have deleterious effects on processing. On the other hand, fructans, which... 

This enzyme [EC 3.2.1.65], also known as 2,6-/3-D-fructan fructanohydrolase, catalyzes the random hydrolysis of 2,6-j8-D-fructofuranosidic hnkages in 2,6-/3-D-fructans (levans) containing more than three fructosyl units. 

Fructans are polysaccharides composed of o-fructofuranose units. They are important in short-term energy reserves for grasses and some plants. Inulin, found in dahlias, and levans from grasses are examples of fructans. Levans are short linear polysaccharides composed of (3 2 1 linked fructose units as illustrated in structure 9.21. 

The maximum production of the D-fructan, levan, has been shown to occur during the mid-exponential to the late exponential phase of growth of Actinomyces viscosus and is followed by a rapid decline as a result of the production of levan hydrolase activity.The levan produced by Streptococcus mutans OMZ 176 appears to have a structure similar to other known D-fructans and inulin. On the basis of chemical shift displacements of the n.m.r. spectra, resulting from 0-substitution at specific carbon atoms, resonances have been assigned to the carbon atoms of the j3-D-fructofuranosyl residues in levans. A variety of different levans show almost identical n.m.r. spectra in contrast to a wide divergence in the corresponding spectra of dextrans. ... 

The plant fructans are of two main types, the linear P2,6-fructans (levans) and the linear P2,l-fructans (inulins). A third class contains both types of linkage and is highly branched. Bacterial levans, by contrast, are mostly composed of a predominance of P2,6 sequences with P2,l branches at rather less frequent intervals than in the branched fructans of plants. Whereas bacterial levans have molecular weights often in excess of one million daltons, the plant fructans seldom reach one-hundreth of this size. 

Levan-sucrase isolated from species of Aerdbacter or Badllits synthesizes a fructan (levan) from sucrose or raffinose but not from D-fructose, invert... 

The biosynthesis and degradation of fructans by microbial organisms has been reviewed in detail recently.35 Additionally, a review of the production of di-D-fructose dianhydrides from inulin and levan by enzymes has been published in Japanese.36 This account is therefore limited to a general overview. 

Two extracellular D-fructans, (2- 6)-linked S-D-fructofuranan or levan and the less common corresponding (2 l)-linked polysaccharide, of the inulin type, are elaborated by different bacteria. These polysaccharides are formed from sucrose by the action of sucrose fructosyltransferases. Terminal )S-D-fructofuranosyl groups are present in some bacterial heteropolysacchar-... 

Polymers of D-fructose are important carbohydrate reserves in a number of plants. Inulins and levans are two major types that differ in structure. D-Fructans require only relatively mild conditions for their hydrolysis, for example, levan was qualitatively hydrolyzed by hot, dilute, aqueous oxalic acid. Permethylated fructans could be hydrolyzed with 2 M CF3CO2H for 30 min at 60°. Fructan oligosaccharides were hydrolyzed in dilute sulfuric acid (pH 2) at 70 (see Ref. 53) or 95° (0.1 M). D-Fructans from timothy haplocorm (where they comprise 63% of the water-soluble carbohydrates) could be hydrolyzed with 0.01 M hydrochloric acid at 98°. 

The polysaccharide consisted entirely of fructose methylatlon analysis showed that the primary fructose linkages were P(2- 6) fructofuranosyl linkages. Carbon 13 nmr showed the product to be a levan type fructan. 

Fructan was harvested by precipitation from the culture broth by addition of ethanol or isopropanol. Acetone and methanol can also be used. The yield and consistency of the product varied depending on the amount of alcohol added. The fructan started to precipitate at the medium/alcohol v/v ratio of 1 1.2, and the yield peaked at about 1 1.5. Further increase in the ratio hardened the fructan and made the product less fluid. Slightly less isopropanol was needed than ethanol to precipitate levan (fructan). Although most of the bacterial cells, unfermented sugars, and other solubles remained in the aqueous alcohol phase, pre-removal of microbial cells by centrifuging was needed to obtain a pure form of fructan. The product was further purified by repeated precipitation and dissolution in water, followed by dialysis or ultrafiltration. The final product was an... 

Structure. The nmr spectra, shown in Figure 2, indicates that essentially all fructose molecules in the polymers are in the same conformation. In Table I, nmr peaks from fructan are compared to peaks from known inulin 0-(l->2) linked) and bacterial levan (P-(2->6) linked). Data clearly show the fructan to be of the p-(2- 6) type (27). (Sec Table II.)... 

In water, inulin undergoes reversion from a more soluble to a less soluble form, in the manner of retrograding starch (Whistler and Smart, 1953) it is slightly soluble in organic solvents. Fructans are easily hydrolyzed by acid. Levan is the branched isomer of inulin. 

Polysaccharides that exclusively contain D-fructose are known as fructans and there are two known kinds, inulin and levan. Inulin is a polysaccharide containing -D-fructofuranose linked (2 1) [118]. Inulins are found in the roots and tubers of the family of plants known as the Compositae, which includes asters, dandelions, dahlias, cosmos, burdock, goldenrod, chicory, lettuce, and Jerusalem artichokes. Other sources are from the Liliacae family, which includes lily bulbs, onion, hyacinth, and tulip bulbs. Inulins are also produced by certain species of algae [119]. Several bacterial strains of Streptococcus mutans also produce an extracellular inulin from sucrose [120]. 

Inulin, levan, graminan, phlein, andkestoses are classified as fructans [12]. Inulin exists either as a linear, branched, or cyclic fructan whereas the structure of the remaining fructans are linear and branched (O Table 2) [13]. 

Fructans are oligomers and polymers formed by polymerizing fructose from transport metabolite sucrose on one out of three possible starter trioses 1-kestose yields inulin-type (2 1)-p-D-fructans, 6-kestose yields levan-type (2 6)-p-D-fructans (Fig. 3A), and neo-kes-tose yields mixcD-type and/or branched (2 1), (2 6)-p-D-fructans (Fig. 3C). Pronounced fructan metabolism is found in composites (chicory, Jerusalem artichoke), Liliaceae (onion, chives, garlic), cereals (wheat, barley, rye, oat), Asparagaceae (asparagus), Amaryllidaceae (banana), and Agavaceae (agave). 

Phleins, Levans and Fructans. The strictly p-(2- 6)-linked polymer of fructofuranoside units attached to sucrose is termed phlein. There appear to be no experimental investigations of its structure, although some theoretical work with the primitive AMI basis set suggested a helix for the p-(2- 6)-linked fructofuranose heptamer. ... 

Bacillus subtilis levansucrase (sucrose 2,6-/8-D-fructan 6-)8-D-fructosyltrans-ferase, EC 2.4.1.10) catalyzes fructosyl transfer from sucrose to levan (154). In the absence of a fructosyl acceptor, the primary reaction is sucrose hydrolysis, although a limited amount of self-initiated levan synthesis occurs as well (155). As with sucrose phosphorylase, acceptor specificity is broad a number of saccharides and other nucleophiles are suitable fructosyl acceptors (154-158). The complete amino acid sequence of the approximately SO-kDa enzyme has been determined by both protein (159) and gene (160) sequence analyses. The three-dimensional structure at 3.8 A reveals a rod- or ellipsoid-shaped protein with a length some four times the diameter (161). 

Fructose occurs rarely in the bacterial polysaccharide. It has been found in the LPS of several Vibrio species [108] and in the K4 [109] and Kll [110] capsular antigens of Escherichia coli. In all cases, fructose appears as a terminal residue. In several plant species, fructans consisting of p-D-fructofuranosyl units are present as important storage polymers. The structure of the polysaccharide isolated from B. caryophylli is different from those of the above polysaccharides however, as levan can be isolated as bacterial exopolysaccharides, this polysaccharide may be a side-product of levan biosynthesis. 

Particular difficulties exist, however, in purifying dextrans synthesized by many streptococci, because the bacteria simultaneously elaborate exocellular fructans (levansDextrans and levans synthesized concomitantly by Leuconostoc micro-organisms have been separated by fractionally precipitating the polysaccharides from aqueous ethanol. These separative procedures are, however, only effective when considerable differences in molecular weight exist between the main fractions of the two polysaccharides, and, as a result, they are of limited value in effecting separation of levans from many streptococcal dextrans. Reports that some oral bacteria are able to metabolize levans suggest that... 

---