Proteoglycans

Proteoglycans are a major component of the animal extracellular matrix, consisting of glycosaminoglycans and proteins. They are degraded intracellularly by corresponding enzymes, the absence of which leads to intracellular storage of the substances (see also mucopolysaccharidoses, hereditary).

Proteoglycans are essential for the stabilization of the cell structures of an organism. Proteoglycans form very large molecular complexes with a molecular weight > 2 million Dalton. Proteoglycans store a considerable amount of water, which enables the diffusion of nutrients. Furthermore, the consistency of the skin (skin turgor) is decisively influenced.

Heparan sulfate glycosaminoglycans are responsible for the storage and release of growth factors such as FGFs and VEGFs.

In general, proteins that perform their function within cells are only rarely linked to carbohydrate side chains. Chemically, proteoglycans are a class of glycoproteins. They consist of about 95% carbohydrates (oligosaccharides) and 5% proteins. The core proteins bind up to > 100 polysaccharide molecules (glycosaminoglycan chains). The glycosaminoglycan chains consist of a sequence of repeating disaccharide units, usually a hexosamine and a glucuronic acid, which are linked to the core protein via o-glycosidic or n-glycosidic bonds. Depending on which glycosaminoglycan chains are predominantly bound, a distinction is made between:

• Chondroitin sulphate proteoglycans (e.g. decorin)
• Heparan sulphate proteoglycans (glypican, e.g. Syndecan - heparan sulphate/chondroitin sulphate)
• Ketaran sulphate proteoglycan (e.g. fibromodulin)
• Dermatan sulphate proteoglycans (biglycan)

Proteoglycans also form structural complexes with collagen fibrils. These are, for example, the main component of cartilage and mucus (in cartilage they make up approx. 25-35 % of the dry weight). The polysaccharide content in hyaline cartilage consists mainly of the glycosaminoglycans hyaluronic acid, chondroitin sulphates and keratan sulphates.

In the skin, the proteoglycans biglycan (chondroitin/dermatan sulphate), decorin (chondroitin/dermatan sulphate), fibromodulin (keratan sulphate), syndecane (chondroitin sulphate/heparan) and others play an important role.

The biosynthesis of proteoglycans begins in the rough endoplasmic reticulum with the synthesis of the polypeptide chain. Their glycosylation then takes place in the Golgi apparatus. After the exocytosis of the proteoglycans, proteoglycan aggregates are formed in the extracellular space, huge complexes with relative molecular masses of almost 108. Proteoglycans can bind large amounts of water because of the many polar and acidic groups in the molecule. This enables the diffusion of nutrients.

Soluble proteoglycan mixtures are used in cosmetic formulations (see below "Soluble proteoglycans")