Oligosaccharides are carbohydrates formed by the union of a relatively small number of monosaccharides (from 3 to 10 units, while other authors also include disaccharides and sugars up to a maximum of 20 monosaccharide units). Among the most common monosaccharides we remember the glucose, fructose, galactose, mannose and ribose. Examples of oligosaccharides are instead given by maltotriose and fructo-oligosaccharides. The first derives from the digestion of starch and consists of three glucose monomers held together by bonds of the type α 1-4. Fructooligosaccharides (FOS), also of vegetable origin, are instead made up mainly of D-fructose units joined by β-glycosidic bonds (1-2).
Stachyose, verbascose and raffinose are other rather common oligosaccharides in the plant world; raffinose is a trisaccharide (glucose, fructose and galactose), while stachyose (glucose, galactose, galactose, fructose) and verbascose (galactose, galactose, glucose, fructose) are tetrasaccharides. These oligosaccharides are contained in legumes and are responsible for flatulence, as they are indigestible and unabsorbable by humans, but fermentable in the large intestine by the resident microbial flora. Other oligosaccharides, such as the aforementioned FOS and inulin, favor the growth of bacteria intestinal symbionts, useful for promoting the health of the whole organism; these oligosaccharides are defined as prebiotics.
So far we have talked about oligosaccharides of vegetable origin; in animals, including humans, these sugars are found mostly associated with fats and proteins, with which they form glycolipids and glycoproteins. These molecules, located mostly at the level of cell membranes, can act as a signal for recognition between cells, as receptors for hormones and neurotransmitters, or even play a role of antigens; this is the case, for example, of the antigenic glycoproteins of the AB0 system: blood groups A and B differ in the presence of two different oligosaccharides - glycolipids in the plasma membrane of red blood cells, group AB possesses both, while group 0 none of two.
