Glycans are polymers of (arbitrarily) more than about ten monosaccharide residues linked glycosidically in branched or unbranched chains.
Glycans are polysaccharides. Usually, this word is used to refer to the carbohydrate moiety of a glycoprotein. There are 2 types: O-linked and N-linked. O-glycans consist of N-acetylgalactosamine, galactose, and neuraminic acid. N-glycans consist of N-acetyl glucosamine, fructose, mannose, galactose, and neuraminic acid. N-glycans are derived from a 14-sugar unit consisting of 14 residues (3 glucose, 9 mannose, and 2 N-acetylglucosamine). O-glycans, on the other hand, have one sugar added at a time. Examples of O-linked glycoproteins are:
Most glycoproteins are N-linked and have 5-15 sugar residues. O-linked glycoproteins have 1-4 sugar residues.
There is little doubt that glycans have many protective, stabilizing, organizational, and barrier functions. The glycocalyx that covers most cells can represent a substantial physical barrier. The glycans attached to matrix molecules such as collagens and proteoglycan are important for the maintenance of tissue structure, porosity, and integrity. Such molecules also contain binding sites for specific types of glycans, which in turn help with the overall organization of the matrix. The external location of glycans on most glycoproteins can provide a general shield, protecting the underlying polypeptide from recognition by proteases or antibodies
Glycoconjugates and glycans are assembled by glycosyltransferases in reactions specific to the sugar donor and acceptor. These enzymes generally reside in the golgi compartment, with their catalytic domain facing the lumen.
Recommended symbols and conventions for drawing glycan structures. The example used is a typical branched “biantennary” N-glycan with two types of outer termini. This symbolic system for representing monosaccharides is used throughout this book (the figure is also reproduced on the inside front cover). The monosaccharides assigned these symbols are those most commonly found in higher animal glycoconjugates. Unless otherwise indicated, all are assumed to be in the d-configuration, except for l-Fuc and l-IdoA; all glycosidically linked monosaccharides are assumed to be in the pyranose (p) form (six-membered ring); and all glycosidic linkages are assumed to originate from the C1 hydroxyl group except for the sialic acids, which are linked from the C2 hydroxyl group.
Diversity in nature
Available data indicate that considerable diversity of glycan structure and expression exists in nature. However, partly because of the inherent difficulties in studying glycan structure, relatively little is known about the details of this diversity (there are very few published reviews on this subject). For many taxa, there is essentially no information at all. Sufficient data indicate that, unlike the case with nucleic acids and the Genetic Code, there is no universal “glycan structure code.”