Proteoglycans are glycoproteins that are very rich (over 50%) in carbohydrates.

The protein-carbohydrate, or proteoglycan, complex in cartilage is a filamentous structure built on a single long hyaluronic acid molecule, to which extended core proteins are attached noncovalently. The core proteins, in turn, have chondroitin sulfate and keratan sulfate chains covalently bound to them through serine side chains. In cartilage, this kind of structure binds collagen and helps hold the collagen fibers in a tight, strong network. The binding apparently involves electrostatic interactions between the sulfate and/or carboxylate groups of the proteoglycan complex and the basic side chains in collagen.