Proteoglycans

Proteoglycans are complex proteins . High molecular weight compounds consisting of protein (the protein part accounts for 5-10% of the total mass) with a high degree of glycosylation (the carbohydrate part accounts for 90-95% of the total mass), the carbohydrate residues of which are long unbranched polysaccharide chains - glycosaminoglycans formed alternating residues of hexosamine and uronic acid (glucuronic, iduronic or galacturonic) or galactose . Glycosaminoglycan chains are often sulfonated ^[1] . Proteoglycans are a subclass of a large class of complex proteins called glycoproteins . Glycoproteins also contain polysaccharide chains of different lengths, covalently attached to the polypeptide base, but the carbohydrate component of glycoproteins is much smaller in mass than proteoglycans, and does not exceed 40% of the total mass.

Proteoglycans form an intermediate (main) substance of the connective tissue into which the fibrous elements of the connective tissue are immersed or coated. Proteoglycans play the role of interstitial layers and serve as a lubricant in the joints. Proteoglycans perform the function of binding extracellular water, as well as cations. For example, they can fix calcium ions in the foci of ossification.

Proteoglycans consist of core proteins ( English core protein ) with a molecular weight of 10 to 600 kDa, to which several glycosaminoglycan chains are covalently linked (up to 80 glycosidic units weighing up to 50 - 60 kDa); the number of protein-linked polysaccharide chains varies from one ( decorin fibroblasts ) to 100 and above ( cartilaginous aggrecan ). Such a structure causes a high molecular weight of proteoglycans (for aggrecan up to 5 M Da ).

The core proteins of proteoglycans, like other proteins, are synthesized by the ribosomes of the rough endoplasmic reticulum and transported to the Golgi apparatus , where they are glycosylated . At the first stage, “buildup” of the “binding tetrasaccharide” ( English proteoglycan linkage tetrasaccharide ) “ xylose - galactose - galactose - glucuronic acid ” (ß1-4Xyl ß1-3Gal ß1-3Gal GlcA) takes place on the serine residue of the core protein, followed by transfer of the corresponding glycosyl the glycosylaminoglycan chain specific for a given proteoglycan is built up.