Consisting of about 95% polysaccharide (glycosaminoglycans), 5% protein, proteoglycans serve as the ground substance (extracellular medium) of many types of connective tissue. What is the difference between a glycoprotein and a proteoglycan? Proteoglycans represent a special class of glycoproteins where, because they are so heavily glycosylated, the balance has shifted toward carbohydrate in the nomenclature. The protein does play an important role as the core of the structure, with covalently attached carbohydrate polymers. The carbohydrate chains are negatively charged at neutral pH due to sulphate and uronic acid groups in proteoglycans.

Proteoglycans are a major component of the extracellular matrix, especially within connective tissue.