Nature Of Fructans

Members of the family Poales which includes wheat and barley produce mixed linked fructan, that is fructan composed of both (2-1) and (2-6) linked P-D fructosyl units (Fig. 2A,B) (9, 29) A few Poales produce levan, an oligomer having (2-6)-linked fructosyl units, or the neoseries which is an oligomer having fructosyl units on either end of the glucose moiety of sucrose, (Fig. 2C) (29, 30). While differences in structure may initially seem a mute point, recent reports suggest specificity of structure by plant part, for example wheat stems having (2-6) linkages dominating over (2-1) branch points (10, 31), but (2-1) linkages dominating in leaves (31). In barley a cloned gene encoding the enzyme 6-SFT (sucrose:fructan 6-fructosyltransferase) produces bifurcose and 6-kestose, but this enzyme, cannot account for the different oligofructan linkage structures found in barley (32). With different genes encoding enzymes having linkage specificity, regulation could be complex and structure will be important. For example protein extracts from transformed protoplast of tobacco (Nicotiana plumbaginifolia) having the cloned 1-SST of onion (Allium cepa) in combination with protein extracts from protoplasts transformed using the onion fructan:fructan 6G-fructosyltransferase produced structurally different fructan than the combination of extracts from transformed protoplast having the 6-SFT of barley (33). A specific enzyme activity hydrolyzing (2-6) linkages has been purified from oat (Avena sativa cv Fulghum, 34) Lolium perenne L. (35) and an enzyme capable of hydrolyzing both (2-1) and (2-6) linkages from Lolium rigidium Gaudin (36). An enzyme specific for hydrolysis of (2-1) linkages has also been purified in wheat (37) and from barley an enzyme capable of crijOH


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