Proteoglycans are proteins that are heavily glycosylated it's a organic compound composed of a saccharide joined to a peptide and forming the bottom substance of animal tissue (or) any of a category of glycoproteins of high relative molecular mass that are found particularly within the extracellular matrix of animal tissue and even have lubricator and support functions. The fundamental proteoglycan unit consists of a "core protein" with one or a lot of covalently connected glycosaminoglycan (GAG) chain(s). The purpose of attachment may be a Ser residue to that the glycosaminoglycan is joined through an oligosaccharide bridge.