Glycoproteins carbohydrate structure

Many proteins found in nature are glycoproteins because they contain covalently linked oligo- and polysaccharide groups. The list of known glycoproteins includes structural proteins, enzymes, membrane receptors, transport proteins, and immunoglobulins, among others. In most cases, the precise function of the bound carbohydrate moiety is not understood. 

In order to investigate, and deduce, the complex carbohydrate structures of glycoproteins, it must be possible to release, and isolate, the oligosaccharide chain(s) from the glycoprotein. Following this, the structure is usually deduced by a combination of enzymic, chemical, and instrumental methods.34-36... 

Multiple-ion monitoring is, however, of considerable value in structural studies, but only if model compounds of known structure are available for comparison. Such an approach has been used in the study of the carbohydrate structures of glycoproteins from different tissues.50 Separation of glycopeptides obtained from various tissues was performed on columns of concanavalin A-Sepharose. Structural analysis by multiple-ion monitoring of partially methylated, alditol acetates derived from the various fractions indicated that the glycopeptides were separated according to the linkage pattern of mannose (see Fig. 1). 

The carbohydrate moiety of glycoproteins is added to a preformed protein molecule. Thus, the structure of the protein part is determined by the corresponding DNA structure, but the structure of the carbohydrate part is determined by the specificity of the enzymes (the structure of which is ultimately encoded in DNA as well) that catalyze the decoration of the protein once formed. The carbohydrate structures are generally quite complex and their formation is the product of several enzymes acting with several substrates. 

As an example of the carbohydrate component of a glycoprotein, the structure of one of the oligosaccharide chains of immunoglobulin G (IgG see p.300) is shown here. The oligosaccharide has an N-glycosidic link to the amide group of an asparagine residue in the Fc part of the protein. Its function is not known. 

O-type oligosaccharides form another important group of carbohydrate structures found as parts of glycoproteins. They are either linked to serine or threonine of the protein backbone. 

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