proteoglykan

Proteoglycan is a glycoprotein that consists of a core protein with one or more covalently attached glycosaminoglycan (GAG) chains. The GAG chains are long, linear, highly sulfated polysaccharides such as chondroitin sulfate, dermatan sulfate, heparan sulfate, keratan sulfate, and, in some contexts, hyaluronic acid associates noncovalently as part of large aggregates.

The core protein provides a scaffold to which GAGs are attached at serine residues via a linker tetrasaccharide; the resulting proteoglycan may be secreted into the extracellular matrix or anchored to the cell surface as a membrane proteoglycan.

In tissues, proteoglycans contribute to hydration and resistance to compression by attracting water; they organize the extracellular matrix, regulate collagen fibrillogenesis, and form large aggregates (e.g., aggrecan–hyaluronan complexes in cartilage). They also bind and modulate growth factors, morphogens, and cytokines, thereby influencing cell signaling, proliferation, and wound healing. Membrane proteoglycans such as syndecans and glypicans participate in cell adhesion and receptor signaling.

Biosynthesis occurs in the Golgi apparatus, where the core protein is synthesized in the endoplasmic reticulum, then attached to GAG chains by specific transferases and sulfotransferases that determine chain length and sulfation.

Pathological changes in proteoglycan expression, sulfation, or degradation (e.g., by enzymes like heparanase) are associated with osteoarthritis, cancer progression, and fibrotic diseases.