9.3 Disaccharides

Figure 9.18: Disaccharide Formations

A disaccharide is a compound formed from two monosaccharides that have been covalently joined by an O-glycosidic bond (see figure 9.18). An O-glycosidic bond forms between the hydroxyl group of one sugar molecule and the anomeric carbon of another sugar molecule.

Figure 9.19: N-Glycosidic Bonds

N-glycosidic bonds, on the contrary, form between the anomeric carbon of a sugar and an amine. Figure 9.19 provides an example: the link between D-ribose and purines or pyrimidines in nucleic acids.

Figure 9.20: Commonly Found Disaccharides

Nonetheless, the commonly found disaccharides in figure 9.20 are:

1. Lactose

   This is found in milk; it is also a reducing disaccharide - the hydrolysis of which yields D-galactose and D-glucose.

   The anomeric carbon of lactose can be oxidized.

2. Sucrose

   This is otherwise known as table sugar and is a disaccharide of glucose and fructose. It is also a nonreducing sugar.

   Unlike lactose, sucrose does not contain any free anomeric carbons. The anomeric carbons of both monosaccharides are involved in the formation of a glycosidic bond.

3. Trehalose

   This is a disaccharide of D-glucose and is a major constituent of the hemolymph (i.e., the circulating fluid) of insects.

   Trehalose is an energy-storing compound and like sucrose, a nonreducing sugar.

9.3.1 Lactose intolerance

Lactose intolerance is an organism’s inability to digest lactose - this is caused by a deficiency of lactase in the intestines (i.e., intestinal lactase).

Figure 9.21: Catabolism of Lactose by Lactase

The activity of lactase gradually diminishes over time; most adult mammals have low levels of \(\beta\)-D-galactosidase (i.e., lactase). Figure 9.21 illustrates the general mechanism for the catabolism of lactose by lactase.