Proteoglycan

Proteoglycans are a major component of the extracellular matrix in animal tissue. They are also located at the cell surface and intracellular secretory granules.

Proteoglycans form a class of particularly hard -glycosylated glycoproteins - macromolecules consisting of a protein and one or more covalently linked carbohydrate groups. In the case of the proteoglycans is in the side chains of glycosaminoglycans ( GAG). These are long, linear carbohydrate groups of repeating disaccharides.

Carbohydrates make up about 95 % of the molecular weight of proteoglycans, in contrast to glycoproteins whose protein content predominates the carbohydrate portion.

Function

Proteoglycans are a major component of animal extracellular matrix, the stabilization effect between the cells of an organism. Here they form large complexes, both to other proteoglycans, as well as to hyaluronic acid, as well as fibrous proteins such as collagen. They are also involved in the binding of cations and regulate the movement of molecules through the matrix. There is evidence that they can affect the activity and stability of proteins and signaling molecules within the matrix. The function of the proteoglycan can be partially attributed to the protein core or the attached GAG chain.

Building and Structure

Characteristic of the very heterogeneous family of proteoglycans covalently bound to the polyanionic glycosaminoglycan chains are Proteinzentralfilament ( GAG chains ). These are composed of repetitive disaccharide units that are covalently linked via a characteristic carbohydrate binding sequence to the Zentralfilament.

Smaller oligosaccharides are normally connected via the amide nitrogen of asparagine residues glycosidically to the core protein. Keratan sulfate and chondroitin sulfate chains are glycosidically bonded to the core protein via oligosaccharides that are covalently bonded to side chains, O- atom of serine or threonine residues. The central hyaluronic acid has bound up to 100 nuclear proteins ( up to 1000 disaccharide units ) that bind in turn, each about 50 keratan sulfate chains (with up to 250 disaccharide units) and approximately 100 chondroitin sulfate chains.

Proteoglycans are negatively charged and highly hydrated; they are also often referred to as acid mucopolysaccharides.

The binding region comprises the Tetrasaccharidsequenz:

The disaccharide units are composed of an N- acetylhexosamine (N -acetyl- glucosamine or N- acetylgalactosamine ) and a uronic acid (D -glucuronic acid or its C5 epimer, L -iduronic acid ) or galactose. These units may be modified with sulfamate and sulfate ester groups. A high heterogeneity among the glycosaminoglycans leads to both the variability in the degree of sulfation as well as the degree of epimerization.

Also possible is the formation of even larger macromolecules. Thus, hyaluronic acid as a keel several proteoglycans bind non-covalently in a feathery structure. With hyaluronic acid, although it is also a glycosaminoglycan, this comes in proteoglycans in itself but not before.

Grouping of proteoglycans

Proteoglycans are divided into different groups according to their glycosaminoglycan chains:

  • Heparan sulfate proteoglycans
  • Keratan sulfate proteoglycans
  • Chondroitin sulfate proteoglycans
  • Dermatan sulfate proteoglycans

A proteoglycan may also carry more than one type of glycosaminoglycan chains; for example has aggrecan KS and CS chains. In addition to the glycosaminoglycan chains, the center filaments also often carry N- or O- glycosidically linked oligosaccharides, as they also occur in other glycoproteins. Due to the size of the Proteinzentralfilaments be the proteoglycans of the ECM into two main groups, large and small proteoglycans, broken. Because common structural domains, the large proteoglycans are divided into Hyalektane (eg, aggrecan, neurocan, Brevican, versican and others), on the one hand bind hyaluronic acid and on the other hand contain the type lectin motif C, and in no hyaluronic acid -binding proteoglycans (z. example, perlecan, agrin and others). The small proteoglycans of the ECM include the leucine-rich proteoglycans (eg, decorin, biglycan, fibromodulin, lumican and others).

For details of the biological function of the proteoglycans made ​​to the article to the extracellular matrix (ECM ) referenced.

Swell

  • S. Ayad, R. P. Boot- Hanford, M. J. Humphries, K. E. Kadler, C. A. Shuttleworth: The Extracellular Matrix ( Facts Book ). Academic Press ( Harcourt Brace & Company, Publishers), Printed in Great Britain, pp. 14 ff ( 1998), ISBN 0-12-068911-1
  • Structural protein
  • Polysaccharide
  • Protein group
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