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stachyose biosynthesis

General Background Stachyose and raffinose are two members of the so-called 'raffinose series' of sucrosyl oligosaccharides (also known as raffinose family oligosaccharides, RFOs). Sucrosyl oligosaccharides represent a large portion of primary oligosaccharides in plants (defined as oligosaccharides synthesized by the action of a glycosyl transferase that mediates the transfer of a glycopyranosyl residue to either the glucopyranosyl or fructofuranosyl moiety of sucrose). They are differentiated from secondary oligosaccharides which are generated by the hydrolysis of higher oligosaccharides, polysaccharides or heterosides. For review, see |CITS:[KANDLER82]|). Members of the raffinose series occur at least in traces in each plant family; it is one of the most widespread sucrosyl oligosaccharide series in flowering plants and might even be ubiquitous (see |CITS:[KANDLER82]| for review). This series comprises |FRAME:CPD-1099|, |FRAME:CPD-170|, |FRAME:CPD-8065|,|FRAME:CPD-8066| (see also |FRAME:PWY-5342|) as well as several other compounds with a higher degree of polymerization (DP; the highest is a nonasaccharide; none of these higher-DP oligosaccharides in the series have not yet been named). The sugars of this series consist of alpha1,6-linked chains of D-galactose attached to the 6-glucosyl position of sucrose. They are synthesized in leaves, roots and tubers. Raffinose is usually found in all parts of the plants including seeds, unless another sucrosyl oligosaccharide series prevails. Stachyose is often the main oligosaccharide in storage organs although variations occur between species. Functionally, these soluble carbohydrates are used for carbon transport and storage by the plant, although they have also been reported as possibly acting as protective agents during maturation of drying seeds |CITS:[HORBOWICZ94]| and during cold stress |CITS:[12223781],[11115899]|. Sugars of this series have long been considered as undesirable non-digestible factors that promote flatulence. Recently, however, it has been suggested that they might have beneficial effects on the gut microflora |CITS:[TORTUERO97]|. Moreover, they have been suggested for non-food applications (organ preservation) |CITS:[11308150]|. Enzymes of the pathway: The first step of the biosynthesis starts with the formation of the unusual galactosyl donor |FRAME:CPD-458|. The enzyme allowing the formation of this compound is galactinol synthase, which catalyzes the transfer of a galactosyl residue from UDP-galactose onto myo-inositol [UDP-galactose:myo-inositol (1-alpha-D) galactosyltransferase]. Several enzymes with this activity have been characterized and purified. The only known function of galactinol is in the biosynthesis of RFOs; it has therefore been postulated that galactinol synthase might be the committed and regulatory step for the synthesis of these compounds. The following step consists in the transfer of galactosyl units from galactinol to sucrose to form raffinose, the first RFO of the series. The enzyme catalyzing this reaction is known as raffinose synthase (galactinol:sucrose 6-galactosyltransferase). Stachyose is formed by the additional incorporation of a galactosyl unit to raffinose by a stachyose synthase (galactinol:raffinose 6-galactosyltransferase).

from BIOCYC source record: META_PWY-5337
Type: pathway
Taxonomic scope
conserved biosystem

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