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Glycosaminoglycans (GAGs) are repeating units of disaccharides that form long-chain sugars within proteoglycans. GAGs are negatively charged and remain surrounded by water molecules, creating swelling pressure within tissues. This allows tissues to withstand compressional forces, making GAGs crucial for structural support and resilience in the extracellular matrix.

Proteoglycan synthesis begins when galactosamine transferase transfers N-acetylgalactosamine from UDP-N-acetylgalactosamine to serine or threonine residues on the polypeptide in O-type glycosylation. As the protein moves through the Golgi lumen, glycosyltransferases add functional units such as galactose, sialic acid, N-acetylglucosamine, or fucose to complete the proteoglycan structure.

O-linked glycosylation is the most common type of protein glycosylation, where glycans attach to the oxygen atom of hydroxyl groups on serine or threonine residues. This process occurs later in protein processing as the protein passes through the Golgi cisternae, where some sugars are trimmed to form mature glycoproteins.

Proteoglycans are a subclass of O-linked glycoproteins in the extracellular matrix that attach to the extracellular side of the plasma membrane. They conjugate with proteins to form protective coats such as mucins on mucosal epithelial cell surfaces. The glycocalyx functions as a reservoir for sequestered growth factors surrounding the cell membrane.

The most commonly occurring glycosaminoglycans are heparan sulfate, chondroitin sulfate, and dermatan sulfate. GAGs are classified based on the number, composition, and degree of sulfation. These variations allow different proteoglycans to serve specialized functions in various tissues and extracellular matrices.

After synthesis and modification in the Golgi lumen, fully formed proteoglycans are released in secretory vesicles to the cytosol. These vesicles transport the proteoglycans from the Golgi to their final destinations, allowing them to reach the cell surface or extracellular matrix where they perform their structural and functional roles.

Abnormal O-linked glycosylation can cause autoimmune diseases such as immunoglobulin A nephropathy, systemic lupus erythematosus, and inflammatory bowel disease. Enhanced glycosylation and abnormal signaling result in some forms of diabetes, while abnormal mannose-mediated glycosylation causes congenital muscular dystrophies like Walker-Warburg syndrome and muscle-eye-brain disease.