![]() ![]() Building an oligosaccharide, such as a tetrasaccharide (four sugars) with an unlinked reducing end, using only a single sugar in one ring form, such as glucopyranose, the authors could construct 1792 distinct structures. In addition, each anomeric carbon is a stereogenic center, and therefore each glycosidic linkage can be constructed having either the α- or β-configuration. However, the possibility of making glycosidic bonds between the anomeric carbon of one sugar and any one of the unmodified hydroxyl groups in another mono- or oligosaccharide adds to the diversity, by allowing not only more linear products, but also branched products in which more than one hydroxyl group on a given sugar is used to make glycosidic bonds. If there were just one way to link monosaccharides, the choice among the dozen or so commonly used sugars would make the resulting polysaccharides more diverse than polynucleotides (four nucleotide choices for DNA and RNA) but less diverse than polypeptides (20 amino acid choices for mammalian proteins). The diversity arises not only from the choice of sugars but also from the way they are linked. This allows glycans to fill roles that vary from cell-surface interactions with proteins important in differentiation, recognition, and proliferation of cells, to interactions with other glycans that generate the mechanical properties of plant and microbial cell walls.ĭiverse structures can be created by simply linking different monosaccharides through glycosidic bonds, to make oligosaccharides or polysaccharides. The way in which assembly of both oligosaccharides and polysaccharides occurs produces structures of enormous diversity and widely varying properties. The latter are usually built on a core of repeating subunits of linked monosaccharides. The resulting glycans are called oligosaccharides (usually less than a dozen monosaccharides) or polysaccharides (usually more than a dozen monosaccharides). In the most common process, an initial sugar is linked to an aglycone (often a lipid or a protein) and this sugar is further elaborated by covalently joining other sugars through glycosidic linkages ( Chapter 2) between the anomeric carbon of the sugar being added and a hydroxyl oxygen of an existing sugar. Instead, they serve as building blocks for more complex molecules. Except in their roles as sources of energy for living organisms, sugars seldom occur in nature as monosaccharides.
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