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

The Molecules of Which We Are Made
Sugars and Polysaccharides
Peptidoglycans and Glycoproteins

Many simple Heteropolysaccharides are in fact Proteoglycans, in which the terminal polysaccharide unit is covalently attached via an O-glycosidic bond to a Serine residue in the protein [39–43]. Thus, the complete Structure of chondroitin sulfate is as follows:

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Dermatan sulfate and heparin are attached to Proteins in the same manner [43].

Small oligosaccharide groups are frequently attached to proteins located on The Cell surface, as well as to secreted proteins. The sugar chains are linked either via an O-glycosidic bond to the —OH groups of serine, Threonine, and hydroxylysine residues (in Collagen only) or via an N-glycosidic bond to the amide nitrogen of asparagine [43a]. Such Glycoproteins include Enzymes, Hormones, and structural proteins. In most cases, the function of this carbohydrate moiety remains unknown, but in mucins, which act as lubricants, the presence of numerous negatively charged sialic acid groups presumably gives rise to a rigid, extended conformation that significantly increases protein viscosity [41].

In certain mucins, the carbohydrate content exceeds 50%; for instance, in the ovine submandibular gland, up to 800 chains of the type [a-D-N-acetylneuraminic acid-(2—>6)-a-D-GlcNAc-] are attached per protein molecule via Serine and threonine residues [40]. Other glycoproteins contain only one or a few carbohydrate chains. Molecules of Ovalbumin (mol. wt. ∼44,500) from chicken eggs each possess a single carbohydrate chain. Furthermore, at least five different Oligosaccharides of the following type are found [39]:

Similar carbohydrate groups are present in such enzymes as bovine and porcine ribonucleases, a-amylase (Aspergillus oryzae), and bromelain (pineapple) [44]. Glycoproteins found in the Blood serum of Antarctic fish function as exceptionally effective antifreezes [45–47].