Levan synthesis

Utilize the suciose-containing trisaccharide raffiaose [512-69-6] as a substrate for levan synthesis, releasing the disaccharide melihiose as a side-product. 

The exact nature of levansucrase activity08 is not clear. It differs in certain respects from invertase, polymerase, fructosaccharase, and phosphorylase. Possibly the aldoside part of the substrate molecule is replaced by an enzyme-linked group, and partial decomposition of this levan precursor to aldose and ketose may furnish the energy necessary for levan synthesis. 

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). 

Non-reducing disaccharide. Synth, by the transferase action of Aerobacter levani-cum levansucrase on raffmose in the presence of xylose. Also formed by the reversal of normal levan synthesis when Bacillus siibtilis levansucrase acts on levan plus xylose. 

Levan Synthesis. Oligosaccharides of the levan type (fructosyl-2-6-fructose) are synthesized by fructosidases. The synthesis of larger polysaccharides is accomplished by special enzymes that appear to have higher aflSnity for long chains than for short ones. Levan-synthesizing enzymes have been studied in preparations from Aerahacter levanicum. ... 

A second Gram positive system that has been studied in some detail is levansucrase synthesis in B. subtilis. This extracellular enzyme catalyses transfructorylation from sucrose to various acceptors which results in levan synthesis and sucrose hydrolysis. 

Levan synthesis from sucrose was found to be strongly inhibited by D-glucose, D-galactose, D-xylose, L-arabinose, maltose, and lactose, but not... 

Inorganic phosphate is not required for levan synthesis, and it has no effect on the rate of reaction no esterification of phosphate can be detected. It is therefore considered that levan synthesis is the result of the transfer of fructose units from sucrose to a suitable acceptor through the mediation of a fructose-enzyme complex. 

Some antigens, such as type 3 pneumococcal polysaccharide, EPS and other polymeric substances such as dextrans (poly-D-glucose) and levan (poly-D-fructose) can induce antibody synthesis without the assistance of TH cells. These are known as T-independent (Ti) antigens. Only one class of immunoglobulin (IgM) is synthesized and there is a weak memory response. 

One of the recent fundamental advances in carbohydrate chemistry has been enzymic synthesis in vtiro this has now been realized with dextran8 and with levan.8 However, since phosphate sugars are not involved in the enzymic syntheses of these two bacterial polysaccharides, it is obvious that phosphorylation is just one process for the natural synthesis of polysaccharides. 

The synthesis of cellulose by A. xylinum from various polyalcohols14 is accompanied by the formation of carbon dioxide, formic acid, nonvolatile acids, ketoses and sometimes ethanol. The much greater variety of substrates suitable for cellulose synthesis, as compared with the small number for dextran or levan, may account for the widespread natural occurrence of cellulose. 

Indirect evidence of the reversal of enzymic synthesis of levan has recently been obtained.10Ba Enzyme preparations from B. subtilis were shown to contain an enzyme capable of hydrolyzing levan (in addition to the levan-synthesizing enzyme) in a system composed of D-glucose, levan, yeast invertase (to hydrolyze sucrose formed) and the B. subtilis enzyme preparation. 

The literature contains many references to the synthesis of levulosans by bacterial action. These substances are known as levans and so far they have all been found to be polysaccharides consisting of fructofura-nose residues in which the second or reducing carbon of one is joined to the sixth carbon of an adjacent residue. The levans have been discussed in a preceding chapter (page 225). 

It is of interest that similar enzymes, capable of exchanging glycosidic linkages, appear to be involved in the synthesis of some polysaccharides. Thus the polysaccharide dextran is formed from sucrose by enzyme preparations from Leuconostoc46 while the polysaccharide levan is produced from sucrose or raffinose by enzymes of other bacteria.49 The reactions may be written as follows ... 

Thus when o-glucose was added to a solution containing levan, yeast invertase and an enzyme preparation from Bacillus subtUis, it was found that the decomposition of the levan became accelerated, presumably through the synthesis and decomposition of sucrose47 as follows ... 

Myeloma proteins occur in animal tumors of various types. The synthesis of these proteins can be induced in some experimental animals by injection of mineral oil, or the implantation of plastic discs.161 Myeloma proteins were shown to possess antibody activity, with specificity for different types of substances.161 Some of these proteins were found to combine with such carbohydrates as bacterial, cell-wall polysaccharides, lipopolysaccharides, dextrans, levans, and galactans.162 Myeloma proteins that combine with galactans or dextrans possess anti-galactan activity,163,164 or anti-dextran activity,165-167 and have been studied most extensively. 

Four trimethyl-D-fructoses are known but only one of them is crystalline. Crystalline 1,3,4-trimethyl-D-fructose has been obtained from methylated levan,10 1,4,6-trimethyl-D-fructose from methylated melezi-tose and by synthesis, 3,4,6-trimethyl-D-fructose from methylated inulin and by synthesis, and 3,4,6-trimethyl-D-fructose has been synthesized. 

Fig. 8 Different products with sucrose analogues as substrates [91]. Enzymatic synthesis of levans headed with different glycopyranosides by levansucrases of different sources. 6-kestose by a Ls variant N252A, and 1-kestose, 1-nystose analogues by P-fructofuranosidase of A. niger...

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