27.2: Glycosaminoglycans

Glycosaminoglycans (GAGs), also known as mucopolysaccharides, are long and linear polymers comprising of specific repeating disaccharides - the amino sugar that can be N-acetylglucosamine or N-acetylgalactosamine, and a uronic acid that is usually glucuronic acid or iduronic acid.

GAGS are found in the extracellular matrix of vertebrates, invertebrates, and bacteria. Due to their polar nature they attract water, and serve as excellent lubricants or shock absorbers in an animal body.

Hyaluronic acid is a major constituent of the synovial tissue and acts as a ground substance for various other connective tissues. It is widely used in eye drops as it helps maintain the eyeball's shape. It also helps in lubricating joints and binding cells together.

GAGs are classified into four groups: heparan sulfate, chondroitin and dermatan sulfate, keratan sulfate, and hyaluronic acid.

Heparan sulfate plays an important role in blood coagulation and inflammation response. Chondroitin is clinically prescribed to treat osteoarthritis and coronary artery disease. Keratan sulfate is one of the major components of the cornea. It helps in corneal development and maintains the transparency of the tissue. Dermatan sulfate is an important component of various organs and tissues and is vital in regulating blood coagulation, wound repair, and responding to infections. Hyaluronic acid helps in skin stretching and flexing, thereby reducing wrinkles.

Glycosaminoglycans or GAGs are linear polymers of specific repeating disaccharides - the amino sugar N-acetylglucosamine or N-acetylgalactosamine, and the other is usually a uronic acid that can be glucuronic acid or iduronic acid.

Four main groups of GAGs are hyaluronan, chondroitin, and dermatan sulfate, heparan sulfate, and keratan sulfate, based on the sugar, the type of linkage between the sugars, and the number and location of sulfate groups.

Because of the sugar-bound sulfate and carboxylic groups, GAGs are highly acidic, bearing a negative charge.

So, the GAGs present in the extracellular space of the connective tissue attract clouds of cations like sodium ions. As a result, water is sucked into the matrix. This creates a swelling or turgor pressure helping the matrix withstand compressive forces.

Most GAGs are covalently attached to a core protein forming proteoglycan. In the cartilage matrix, these proteoglycans are linked to hyaluronic acid, a non-sulfated GAG, forming proteoglycan aggregates, which provide load-bearing capacity and resistance to deformity.