Sucrose, inulin biosynthesis

Inulin biosynthesis in Jerusalem artichoke occurs via the combined action of two enzymes l-sucrose sucrose fructosyltransferase (1-SST) and l-fructan fructan fructosyltransferase (1-FFT). 1-SST catalyzes the synthesis of inulins of a low degree of polymerization, while 1-FFT catalyzes the synthesis of ffuctans of a degree of polymerization up to 50 (Sevenier et al., 2002a). High... 

Figure 4.73 Biosynthesis of inulin from sucrose. The wavy line illustrates the polymer main chain.

Fructosans consist predominantly, but not exclusively, of fructose. They are synthesized according to a principle similar to that which we saw, for example, in the case of raffinose. In that case a galactose unit was attached to a primer sucrose molecule. Examination of the structure of the fructosans shows that a large number of fructose units are attached to the fructose component of a sucrose molecule. We shall see presently that the sucrose molecule acts as primer for the biosynthesis. The fructose units can be linked to each other in different ways which makes it necessary to distinguish between the inulin type and the phlein type. 

The most important compound is inulin, which occurs as reserve material, particularly in Compositae. It consists of a chain of 32-34 fructose units which are linked to each other through jS-glycosidic 1-2 linkages. Fructose is present in the form of a fructofuranose. At one end of the molecule there is a sucrose unit. The biosynthesis of inulin proceeds by the attachment of fructose units, one after the other, to the sucrose molecule. The fructose donor is presumably a sugar nucleotide, probably UDP-fructose. The presence of UDP-fructose in the inulin-con-taining bulbs of dahlias is consistent with this supposition. Thus, our... 

---