Proteoglycan

[2] The point of attachment is a serine (Ser) residue to which the glycosaminoglycan is joined through a tetrasaccharide bridge (e.g. chondroitin sulfate-GlcA-Gal-Gal-Xyl-PROTEIN).

The chains are long, linear carbohydrate polymers that are negatively charged under physiological conditions due to the occurrence of sulfate and uronic acid groups.

Proteoglycans are a major component of the animal extracellular matrix, the "filler" substance existing between cells in an organism.

[5] They are also involved in binding cations (such as sodium, potassium and calcium) and water, and also regulating the movement of molecules through the matrix.

First, a special link tetrasaccharide is attached to a serine side chain on the core protein to serve as a primer for polysaccharide growth.

Mutations in the gene encoding the galactosyltransferase B4GALT7 result in a reduced substitution of the proteoglycans decorin and biglycan with glycosaminoglycan chains, and cause a spondylodysplastic form of Ehlers–Danlos syndrome.