Glycosaminoglycans and glycoproteins

As constituents of proteoglycans (see p.346), the glycosaminoglycans—a group of acidic heteropolysaccharides—are important structural elements of the extracellular matrix. 

Many proteins on the surface of the plasma membrane, and the majority of secreted proteins, contain oligosaccharide residues that are post-translationally added to the endoplasmic reticulum and in the Golgi apparatus (see p.230). By contrast, cytoplasmic proteins are rarely glycosylated. Glycoproteins can contain more than 50% carbohydrate however, the proportion of protein is generally much greater. 

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. 

On the cell surface of certain glycoproteins, O-glycosidic links are found between the carbohydrate part and a serine or threonine residue, instead of N-glycosidic links to asparagine residues. This type of link is less common than the N-glycosidic one. 

Koolman, Color Atlas of Biochemistry, 2nd edition 2005 Thieme All rights reserved. Usage subject to terms and conditions of license. 

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Chapter 12 Metabolism of Monosaccharides and Disaccharides 135 Chapter 13 Pentose Phosphate Pathway and NADPH 143 Chapter 14 Glycosaminoglycans and Glycoproteins 155... 

R. L. Whistler and J. N. BeMiller, Methods in Carbohydrate Chemistry, Vol. VII General Methods, Glycosaminoglycans, and Glycoproteins, Academic Press, New York, 1976. 

It now seems reasonably certain that the continuous release of these hydrolytic enzymes from neopicistic cells is responsible for their invasive capability, for the selective routing by increased diffusion of nutrients towards the tumor cells, and perhaps even for sustaining the whole momentum of autonomous neoplastic proliferation (48,54). The effect of these hydrolytic enzymes on the matrix is to release a whole spectrum of glycosaminoglycan and glycoprotein breakdown products into the blood stream, the so-called "acute-phase reactants, the estimation of which forms the basis for the majority of serochemical tests for cancer. 

The glycosaminoglycan and glycoprotein composition of several subcellular fractions of rat brain and liver have been reported. Most of the hyaluronic acid and chondroitin sulphate is closely associated with a low density subfraction of smooth membranes. 

In Table VI are estimates of the metabolic turnover rates of some hexosamine-containing glyeolipids, glycosaminoglycans, and glycoproteins in mammalian systems. While not all the materials listed there behave as single metabolic pools, certain of these results have been interpreted as evidence that component hexosamines are metabolized simultaneously with the associated protein, and the A -acetyl and 0-sulfate residues (Schiller et of., 1656 Gross et al., 1660). 

Some alcohols, arylamines, and phenols are sulfated. The sulfate donor in these and other biologic sulfation reactions (eg, sulfation of steroids, glycosaminoglycans, glycolipids, and glycoproteins) is adenosine 3 -phos-phate-S -phosphosulfate (PAPS) (Chapter 24) this compound is called active sulfate. ... 

Current concepts suggest that every extracellular matrix contains one or more proteoglycans in addition to collagens and glycoproteins. Proteoglycans are often classified on the bases of the glycosaminoglycan chains that they bear and their tissue source. However, it is their core proteins that are the unique gene products (Hassell et al., 1986). 

The acetylated compound is a widespread unit of animal glycosaminoglycans in connective tissue (Chondroitin sulfate, dermatan sulfate) and component of many bacterial immunopolysaccharides and glycoproteins... 

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