Mucopolysaccharides

Mucopolysaccharides, more commonly called glycosaminoglycans (GAGs), are long, unbranched polysaccharides composed of repeating disaccharide units. Each disaccharide typically contains an amino sugar (such as N-acetylglucosamine or N-acetylgalactosamine) and an uronic acid (glucuronic or iduronic acid), and many units are sulfated. This combination gives a high negative charge, which attracts water and cations.

GAGs are often covalently attached to core proteins to form proteoglycans and are key components of the extracellular matrix and cell surfaces. The major sulfated GAGs are heparan sulfate, heparin, chondroitin sulfate, dermatan sulfate, and keratan sulfate; hyaluronic acid is a non-sulfated GAG that is not covalently linked to proteins. Together, GAGs provide structural integrity, hydration, and resistance to compression in connective tissues, contribute to the lubricating properties of synovial fluid, and regulate signaling by binding growth factors and cytokines.

Biosynthesis occurs in the Golgi apparatus, where sugar units are polymerized and sulfated; degradation occurs in lysosomes by specific glycosidases and sulfatases. Impairment of these enzymes leads to accumulation of GAGs and mucopolysaccharidoses, a family of lysosomal storage disorders characterized by skeletal abnormalities, organomegaly, and developmental issues.

The term mucopolysaccharide is historical; today the preferred term is glycosaminoglycan. GAGs are widespread in animal tissues and are essential components of cartilage, skin, blood vessels, and other connective tissues.