More than half of all eukaryotic proteins carry covalently attached oligosaccharide or polysaccharide chains. Glycoproteins differ from proteoglycans in that proteoglycans are preodominantly composed of glycosamino-glycan polysaccharides connected to extended core polypeptides, forming huge feathery molecules.

Glycoproteins are classified as N-linked or O-linked in two groups (Figure 9.28). In N-linked glycoproteins, the glycans are usually attached through N-acetylglucosamine or N-acetylgalactosamine to the side chain amino group in an asparagine residue. In O-linked glycoproteins, glycans are usually attached through an O-glycosidic bond between N-acetylgalactosamine and the hydroxyl group of a threonine or serine residue. See also Table 9.6.

N-linked Glycans

A common foundation for N-linked glycans is shown in here. Important N-linked glycans are found in ovalbumin and the immunoglobulins. Every immunoglobulin has carbohydrate attached to the constant domain of each heavy chain. Part of the recognition of immunoglobulins is due to the sequence of the oligosaccharide chains of the glycans.

A very important further use of N-linked oligosaccharides is in intracellular targeting in eukaryotic organisms. Proteins destined for certain organelles or for excretion from the cell are marked specifically by oligosaccharides during posttranslational processing to ensure they arrive at their proper destinations.

O-Linked Glycans

Mucins, which are found extensively in salivary secretions, contain many short O-linked glycans. These glycoproteins increase the viscosity of the fluids in which they are dissolved.

Some O-linked glycans appear to function in intracellular targeting and molecular and cellular identification. An example is found in the blood group antigens (Figure 9.29).

Antarctic fish contain a glycoprotein that serves as an "antifreeze", preventing the freezing of body fluids, even in extremely cold water.