Definition:

Glycosaminoglycans (GAGs) are long, unbranched polysaccharides made up of repeating disaccharide units.
The disaccharide units consist of an amino sugar (either N-acetylglucosamine or N-acetylgalactosamine) and an uronic acid (either glucuronic acid or iduronic acid).
Types of GAGs:

Hyaluronic Acid (HA): Unlike other GAGs, hyaluronic acid does not undergo sulfation and is not found attached to proteins. It has a structural role and is a major component of the extracellular matrix.
Chondroitin Sulfate (CS): Found in cartilage, tendons, and bones, chondroitin sulfate provides structural support and is commonly used as a dietary supplement.
Dermatan Sulfate (DS): Present in skin, blood vessels, and heart valves, dermatan sulfate plays a role in skin structure and elasticity.
Heparan Sulfate (HS): Found on cell surfaces and in the extracellular matrix, heparan sulfate is involved in cell signaling, anticoagulation, and other cellular processes.
Keratan Sulfate (KS): Located in the cornea, cartilage, and bone, keratan sulfate contributes to the structural integrity of these tissues.
Biological Functions:

Structural Support: GAGs contribute to the structural integrity of tissues and the extracellular matrix.
Cellular Signaling: They play essential roles in cell signaling, modulating the activities of growth factors, cytokines, and enzymes.
Lubrication: GAGs, especially hyaluronic acid, provide lubrication in joints and act as shock absorbers.
Clinical Applications:

Osteoarthritis Treatment: Chondroitin sulfate and glucosamine supplements are commonly used for managing symptoms of osteoarthritis.
Anticoagulant Medications: Heparin, a highly sulfated form of heparan sulfate, is used as an anticoagulant in medical settings.
Disease Associations:

Genetic Disorders: Mutations affecting enzymes involved in GAG metabolism can lead to various genetic disorders known as mucopolysaccharidoses (MPS).
Cancer: Altered expression of GAGs is observed in certain types of cancer and may influence tumor progression.
Glycosaminoglycans play critical roles in maintaining the structural and functional integrity of tissues and are involved in numerous physiological and pathological processes. Their diverse functions make them important components of the complex biological network.