Proteoglycan
Proteoglycans are a class of Glycoprotein. Proteoglycans have a specific structure, for example they have at least one Glycosaminoglycan (GAG)[1] chain covalently attached to their main protein, they also tend to have a much higher Carbohydrate content than ordinary glycoproteins. One of the main roles of proteoglycans involves adding structure to a variety of things. They can also act as lubricants[2].