1-Kestose synthesis

Kestose synthesis by 1-SST is limited by the availability of sucrose in the 0 to 100 mol m 3 range (Cairns and Ashton, 1991 Van den Ende and Van Laere, 1993). Therefore, high sucrose concentrations favor the rate of the first polymerization step and indirectly the synthesis of longer-chain-length polymers due to an elevation in 1-kestose concentration, which acts as a fructosyl donor. Glucose is converted to sucrose by sucrose synthase in the cytosol (Pollock, 1986 Wiemken et al., 1986) (Figure 10.18). 

Fig. 8. Different products with sucrose analogues as substrates.115 Enzymatic synthesis of 1-kestose, 1-nystose, and their analogues by /(-fructofuranosidase of A. niger. Structures of fructo-oligosaccharides (A) commercial products, (B) mannose- (C) galactose-, and (D) xylose-substituted analogues.

Dickerson, 1966 Wiemken et al., 1986). Therefore, low sucrose content is essential during fructan synthesis otherwise, 1-FFT merely transfers the fructosyl unit from 1-kestose to sucrose, forming the same products as the reactants (i.e., 1-kestose and sucrose). Both enzymes (1-SST and 1-FFT) are localized in the vacuole (Carpita et al., 1991 Darwen and John, 1989 Frehner et al., 1984). 

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...

This obvious structural difference between plant and bacterial inulin has its origin in the individual synthesis related system. Feedstock for both inulins is sucrose. However, the plant inulin production is a two-step reaction, starting with a sucrose-1-fructosyltransferase (1-SST). One sucrose molecule acts as donor and a second one as acceptor of a fructosyl unit. This leads to the formation of the trisaccharide 1-kestose. Catalyzed by a fructan-fructan-1-fructosyltransferase (1-FFT), fructosyl units are shuffled between the 1-kestose and higher polymeric p-(2 1) linked fructan molecules in the second step. Repetition of this step results in inulin with (3-(2 1) linkages only [129-132]. 

Only the fructosyltransferase (FTF, EC 2. 4. 1. 9) is required in bacteria for the synthesis of bacterial inulin. The enzyme shuffles fructosyl units from one sucrose molecule (acting as druior) to another sucrose molecule, 1-kestose, and higher polymeric p-(2 1) linked fructan molecules, respectively (acting as acceptor). This enzyme partly leads to p-(2—>6) linkages, which results in branches within the inulin molecule [130, 133]. 

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