Synthesis and Degradation of D-Fructans

Information on the metabolism of D-fructans is derived mainly from examination of inulin. Sucrose-sucrose 1-fructosyltransferase (SST) and transfructosylase (FFT) are responsible for the synthesis of D-fructans, whereas hydrolases A and B are involved in their breakdown. 

SST is found to catalyze the formation of a trisaccharide from sucrose, as shown by equation 3.200,201 The enzyme has a high specificity for sucrose, with little or no activity for trisaccharide as a donor, 

D-Fructans are hydrolyzed by two D-fructofuranosidases designated hydrolase A and B.203-205 Enzymic hydrolysis breaks only the ft-D-(2 — l)-linkage between a terminal D-fructofuranosyl group and its adjacent, D-fructofuranosyl chain unit as shown in equation 4. 

Sucrose is the sole source for synthesis of D-fructan in artichoke tubers.206 Thus, the level of this disaccharide can play an important role in the regulation of enzyme activities.202,203,207 Sucrose strongly inhibits the enzymic hydrolysis of inulin by D-fructan hydrolases, as 

UDP-D-fructose is found in dahlia and Jerusalem artichoke tubers. However, it is not known whether UDP-D-fructose is involved in the formation of D-fructan in a manner similar to UDP-D-glucose involvement in the biosynthesis of a-D-glucans.210-212 

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