In this module:

Disaccharides and Glycosidic Bonds

Monosaccharides such as glucose can be linked together in condensation reactions. For example, sucrose (table sugar) is formed from one molecule of glucose and one of fructose, as shown below. Molecules composed of two monosaccharides are called disaccharides.

Click on the step numbers to see the steps in the formation of sucrose. Click on the mouse icon at left to clear the steps to see them again.

First, two monosaccharides are brought together such that two hydroxyl groups are close to each other.

1

In an enzyme-catalyzed reaction, a water molecule is eliminated, leaving a bond between C₁ of glucose and C₄ of fructose.

2

A glycosidic bond is left between the two monosaccharides.

3

Note that the glucose half of sucrose has the a configuration at C₁. Glycosidic bonds are labeled a or ß depending on the anomeric configuration of the C₁ involved in the glycosidic bond. Maltose, which links two glucose molecules, has an a glycosidic bond like sucrose. Lactose, the primary sugar in milk, links glucose and galactose in a ß glycosidic bond instead.

Can glycosidic bonds only be formed between C₁ and C₄, as in sucrose, maltose, and lactose?

Glycosidic bonds can also be formed between other carbons of monosaccharides. For example, several polymers of glucose involve glycosidic bonds between C₁ and C₆ in addition to bonds between C₁ and C₄. This fact makes polymers of monosaccharides potentially much more complex than polymers of amino acids (proteins) or nucleotides (DNA), as you will see shortly.