Legume seeds contain two major classes of soluble α-galactosides, namely the raffinose family of oligosaccharides and galactosyl cyclitols. They are biosynthetically linked by the enzyme stachyose synthase [galactinol-raffinose galactosyltransferase], which not only forms stachyose from raffinose and galactinol, but also catalyses the galactinol-dependent synthesis of galactosyl cyclitols. Stachyose synthase enzymes purified from seeds of adzuki bean (Vigna angularis) and lentil, respectively, were similar with respect to their physicochemical properties but differed substantially in their substrate specificity. The enzyme from lentil (a pinitol producing plant) was able to catalyse the formation of galactopinitol A from smallcap˜D-pinitol, while stachyose synthase from adzuki bean was able to synthesize galactosyl ononitol. The lentil enzyme was additionally able to form ciceritol (a digalactosyl pinitol) from galactopinitol A and galactinol. Moreover, both galactosyl cyclitols could substitute for galactinol as galactosyl donors in the synthesis of stachyose. Thus, stachyose synthase accounts for the synthesis of most galactosyl cyclitols and holds a central position in the interconversion of oligosaccharides in legume seeds. Analysis of mRNA and protein abundance, enzyme activity and metabolite levels during seed development suggested that the oligosaccharide composition of mature seeds is not only determined by total stachyose synthase activity, but also by the concentration of precursors (cyclitols, galactinol, and raffinose).

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