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Proteoglycans are a diverse group of glycosaminoglycan chains covalently attached to a core protein. They are major components of the extracellular matrix, particularly in connective tissues like cartilage, bone, and skin. The glycosaminoglycan chains are typically long, unbranched polysaccharides composed of repeating disaccharide units. These disaccharides consist of a hexosamine (either glucosamine or galactosamine) and a uronic acid (either glucuronic acid or iduronic acid). The presence of sulfate groups on these chains is a common feature, contributing to their highly negatively charged nature.

The core protein of a proteoglycan serves as a scaffold for attaching multiple glycosaminoglycan chains. The specific type of core protein and the length and composition of the attached glycosaminoglycan chains vary widely among different proteoglycans, leading to their diverse functions. These functions include providing structural support, regulating cell growth and differentiation, mediating cell adhesion, and acting as reservoirs for growth factors.

Examples of well-known proteoglycans include aggrecan, which is abundant in cartilage and plays a critical role in its compressive strength, and decorin, which is found in various connective tissues and can bind to collagen and growth factors. The synthesis and degradation of proteoglycans are tightly regulated processes involved in tissue homeostasis and remodeling. Dysregulation of proteoglycan metabolism is associated with various diseases, including osteoarthritis and certain developmental disorders.