Disaccharides

Disaccharides are the most familiar class of oligosaccharide: on hydrolysis with either dilute acid or a specific enzyme, each disaccharide molecule splits into exactly two monosaccharide units, which may be identical to each other (as in maltose) or different (as in sucrose and lactose). The two monosaccharide units are held together by a glycosidic linkage -- an ether-oxide bridge (-O-) formed by the loss of one water molecule between an -OH group on one monosaccharide unit and an -OH group on the other. A key structural requirement is that AT LEAST ONE of the two monosaccharide units must contribute its own ANOMERIC hydroxyl group (the special -OH created at the ring-closure stereocentre, C-1 for an aldose or C-2 for fructose) to form this linkage; this requirement turns out to be exactly what determines whether the finished disaccharide keeps a free (reactive, 'reducing') hemiacetal/hemiketal group or not. Sucrose, maltose and lactose are the t …