A trisaccharide formed from the combination of three monomers: galactose, glucose, and fructose, and has a chemical formula of C18H32O16
Carbohydrates are one of the major classes of biomolecules. The simplest form of carbohydrates is a monosaccharide. The monosaccharides may combine by glycosidic bonds and form larger carbohydrates, such as oligosaccharides and polysaccharides. Oligosaccharides are carbohydrates made up of a small number of monosaccharide units and are relatively smaller than polysaccharides. An example of an oligosaccharide is raffinose.
Raffinose is a trisaccharide, meaning it is made up of three monomers of monosaccharides, namely galactose, glucose, and fructose. It has a chemical formula of C18H32O16. The galactose is connected to glucose by 1α→6 glycosidic linkage. When hydrolyzed with enzyme α-galactosidase, it yields D-galactose and sucrose. Raffinose occurs in legumes, whole grains, cabbage, brussel sprouts, broccoli, cotton seed, molasses of beet root, asparagus, etc.
Raffinose is just one of the members of the RFOs (raffinose family of oligosaccharides). Other members include stachyose (a tetrasaccharide), verbascose (a pentasaccharide), etc. Members of the RFOs are α-1, 6-galactosyl extensions of sucrose. They are formed from sucrose with the ensuing addition of galactose moieties donated from galactinol. RFOs are abundant in plants and serve as desiccation protectant in seeds, as transport sugar in phloem sap and as storage sugars.1
1 Sengupta,S., Mukherjee, S., Basak, P., and Majumder, A.L. (2015). Significance of galactinol and raffinose family oligosaccharide synthesis in plants. Front Plant Sci. 6: 656. doi: 10.3389/fpls.2015.00656