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

The Molecules We Are Made Of
Sugars and Polysaccharides

Simple sugars (Monosaccharides) along with their oligomers and polymers (Polysaccharides) constitute one of the Major Classes of cellular components collectively known as CARBOHYDRATES. A typical monomeric unit is a-D-glucose (a-glucopyranose), which is the cyclic form of glucose:

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The reactive groups involved in forming bonds between monomers are Two Types of —OH groups: a hemiacetal hydroxyl group plus another hydroxyl group attached to any of the carbon atoms. The diversity in monomer types and the linkage positions between them give rise to a vast array of polysaccharide structures.

Sugar units in polysaccharides are linked via glycosidic (acetal) bonds [Equation (2-9)]. Equation (2-9) describes The formation of the disaccharide maltose from two glucose molecules. Further addition of glucose ultimately yields a polymer. The symbol a-1,4 indicates that in maltose the glycosidic bond connects the C-1 atom (the anomeric carbon) of one sugar ring to the C-4 atom of the other ring, with the configuration at the anomeric carbon being of the a-type (see the following section).

The Functional groups of polysaccharides include numerous free hydroxyl groups, which are sometimes involved in forming additional glycosidic bonds, resulting in branched chains. Furthermore, the presence of hydroxyl groups allows for the structural Modification of the sugar backbone even prior to polymerization. Consequently, alongside hydroxyl groups, a polysaccharide may contain —COOH, —NH₂, sulfate, —NHCOCH₃, and other functional groups.