In some plants assimilates from the leaves are exported as sugar alcohols or oligosaccharides of the raffinose family
Not all plants use sucrose for the translocation of assimilates from the leaves to other parts of the plant. In some plants photo assimilates are translocated as sugar alcohols, also called polyols, including sorbitol and mannitol (Fig. 9.19). Rosaceae (including orchard trees in temperate regions) translocate assimilates in the form of sorbitol (Fig. 9.19). Other plants, such as squash, several deciduous trees (e.g., lime, hazelnut, elm), and olive trees, translocate in their sieve tubes oligosaccharides of the raffinose family. In these oligosaccharides sucrose is linked by a glycosidic bond to one or more galactose molecules (Fig. 9.20). Oligosaccharides of the raffinose family include raffinose with one, stachyose with two, and verbascose with three galactose residues. These oligosaccharides also serve as storage compounds and, for example, in pea and bean seeds make up 5% to 15% of the dry matter. Humans do not have the enzymes that cata-lyze the hydrolysis of -galactosides and are therefore unable to digest oli-gosaccharides of the raffinose family. When these sugars are ingested, they are decomposed in the last section of the intestines by anaerobic bacteria, which metabolize the sugars and release digestive gases.
The galactose required for raffinose synthesis is formed by epimerization of UDP-glucose (Fig. 9.21). UDP-glucose epimerasecatalyzes the oxidation of the OH-group in position 4 of the glucose molecule by NAD+ , which is tightly bound to the enzyme. Remaining bound to the enzyme the intermedi-ate is subsequently reduced to galactose. As the reaction is reversible, UDP-glucose epimerase catalyzes an equilibrium between glucose and galactose. The galactose residue is transferred by a transferase to the cyclic alcohol myo-inositol producing galactinol. Myo-inositol-galactosyl-transferases cat-alyze the transfer of the galactose residue from galactinol to sucrose, to syn-thesize raffinose, and correspondingly also stachyose and verbascose.
sucrose + galactinol → raffinose + myo-inositol
raffinose + galactinol → stachyose + myo-inositol
stachyose + galactinol → verbascose + myo-inositol