Oligosaccharides

Glycosidic bonds between monosaccharides give rise to oligosaccharides and polysaccharides. The simplest oligosaccharides, the disaccharides, include compounds such as sucrose and lactose, which are referred to as sugars (like the monosaccharides). Other common disaccharides include trehalose, maltose, gentiobiose, and cellobiose.

Four features distinguish disaccharides from each other:

> 1. The two specific sugar monomers and their stereoconfigurations

> 2. The carbons involved in the linkage

> 3. The order of the monomeric units, if they are different kinds

> 4. The anomeric configuration of the hydroxyl group on carbon 1 of each residue

Table 9.4 shows abbreviations for the common monosaccharides. The naming rules for oligosaccharides based on these abbreviations are as follows:

> 1. The sequence starts with the nonreducing end at the left using the abbreviations of Table 9.4

> 2. Anomeric and enantiomeric forms are designated by prefixes (e.g., , D-)

> 3. The ring configuration is designated by the suffixes p (pyranose) or f (furanose)

> 4. Numbers in parentheses between residue numbers are used to identify glycosidic bonds; e.g., (1->4) means a bond from carbon 1 of the residue on the left to carbon 4 of the residue on the right.

Thus, -D-Glcp(1->2)--D-Fruf corresponds to sucrose.

Oligosaccharides are also found as part of glycoproteins and play a role in cell recognition/identity. Oligosaccharides form the blood group antigens. In some cells, these antigens are attached as O-linked glycans to membrane proteins. Alternatively, the oligosaccharide may be linked to a lipid molecule to form a glycolipid. These oligosaccharides determine the blood group types in humans (Figure 9.29).

See also: Saccharides, Oligosaccharides as Cell Markers, Biosynthesis of Glycoconjugates (from Chapter 16), Diastereomers, Monosaccharide Nomenclature