trisaccharide

A trisaccharide is a carbohydrate composed of three monosaccharide units linked by glycosidic bonds. Trisaccharides can be linear, with two linkages joining three sugar units, or branched, in which a central sugar bears two attachments. The specific monosaccharides involved (for example glucose, galactose, or fructose), the order, and the positions of the glycosidic bonds give rise to multiple structural isomers. Depending on the bonding pattern, trisaccharides may be reducing or non-reducing; if both anomeric carbons are engaged in glycosidic bonds, the molecule is non-reducing, whereas a free anomeric carbon on one end makes the molecule reducing.

Common examples include raffinose and maltotriose. Raffinose is a plant trisaccharide composed of galactose linked to glucose, which is linked to fructose; it is typically non-reducing because both anomeric carbons are involved in bonds. Maltotriose is a linear trio of glucose residues linked by α1→4 glycosidic bonds; this arrangement leaves the terminal glucose’s anomeric carbon free, making maltotriose a reducing trisaccharide.

Occurrence and significance vary by type. Raffinose occurs in seeds and some vegetables and is part of the raffinose family of oligosaccharides; such sugars can be indigestible for humans and are fermented by gut bacteria, sometimes contributing to gas formation. Maltotriose appears during starch digestion and can be further processed by enzymes.

Chemical properties of trisaccharides include hydrolysis to monosaccharides under acidic or enzymatic conditions. They are studied as intermediates in carbohydrate metabolism and as standards in analytical methods.