Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

The Molecules We Are Made Of
Sugars and Polysaccharides
Oligosaccharides

Disaccharides and other Oligosaccharides consisting of a small number of monomeric units are of great importance for PLANT AND ANIMAL METABOLISM. Examples include lactose (found in milk), sucrose (in green plants), and trehalose (in Fungi).

While the cyclic α- and β-forms of free sugars easily interconvert, the configuration at the anomeric carbon atom becomes "frozen" upon formation of a glycoside bond. Describing a glycoside bond requires specifying this configuration and the positions through which the rings are joined [see equation (2-9)]. Thus, lactose can be represented as a disaccharide containing a galactose unit in the β-pyranose cyclic form, linked via its anomeric carbon atom (C-1) to the 4-position of glucose (β-1,4 linkage):

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The systematic name of α-lactose—O-β-D-galactopyranosyl-(1—>4)-α-D-glucopyranose—fully describes the Stereochemistry of the compound, indicating the ring sizes and the mode of linkage. The abbreviated notation is as follows: β-D-Galp-(1—>4)-α-D-Glcp. Maltose is written as α-D-Glcp-(1—>4)-α-D-Glcp. Since pyranose rings and D-sugars are by far the most common in nature, the symbols D and p in such notations are usually omitted. Throughout this book (unless specified otherwise), we will consider only D-sugars. Sometimes, even after The sequence of sugar residues in a polysaccharide has been established, the mode of linkage remains ambiguous; in such cases, incomplete abbreviated formulas are used.

Note that in the drawing representing The Structure of lactose, the glucose ring is "flipped" relative to the galactose ring due to the presence of the β-1,4 linkage between them. In the representation of maltose, where the linkage is of the α-1,4 type, both rings are oriented in the same way [equation (2-9)].

In the lactose molecule, the glucose ring can open to form the aldehyde form, which (in solution) is in equilibrium with other cyclic forms. Therefore, the name "lactose" by itself does not imply that the glucose portion of the molecule has a fixed cyclic structure. Thus, the formula of lactose can be abbreviated as follows: βGall-(1—>4)-Glc. (Note that in crystalline form, lactose exists in either the α- or the β-configuration¹.) On the other hand, in sucrose and trehalose, the two rings are linked through their reducing groups. These molecules are locked in a single specific form.

¹ α-Lactose is more soluble and tastes sweeter than β-lactose. During prolonged storage of ice cream, β-lactose may crystallize, causing the product to become "gritty" or "sandy" in texture.