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Glycosaminoglycans ( GAG ) or mucopolysaccharides are acidic polysaccharides made up of repeating disaccharides . The individual disaccharide units consist of a uronic acid (usually glucuronic acid , more rarely iduronic acid , uronic acid of idose ), which are 1-3-glycosidically linked to an amino sugar such as N- acetylglucosamine . The disaccharide units of the chains themselves are linked 1-4 glycosidically. Some of the glycosaminoglycans are further esterified with sulfuric acid or acetic acid .

Mucopolysaccharides are components of many biological macromolecules, for example in large numbers covalently bound to a protein in the form of proteoglycans . They form the framework of many fiber-forming substances and, thanks to their ability to absorb water, are extremely elastic .

Depending on the composition of the disaccharide units, a distinction is made between various subgroups of the glycosaminoglycans:

Hyaluronic acid (HA)

Structure of hyaluronic acid

Hyaluronic acid consists of β- (1 → 4) -glycosidically linked glucuronyl-β- (1 → 3) - N -acetylglucosamine disaccharide units, with up to 50,000 consecutive units. The sugar units are unsulphated. Hyaluronic acid exists as an amphipathic helix and is the only glycosaminoglycan not bound to a central protein filament. As a result of hydration, a hyaluronic acid molecule can occupy a space up to 10,000 times greater than the volume of the molecule itself, so that a viscous gel forms even at low concentrations. It thus serves as a biological lubricant . Hyaluronic acid is found in cartilage , synovial fluid , in the umbilical cord and in the vitreous humor of the eye. In the fibroblasts , hyaluronic acid is synthesized from D-glucose .

Heparin / heparan sulfate (HS)

Heparin consists of D-glucuronic acid, which is β- (1 → 4) -glycosidic, or L-iduronic acid, which is α- (1 → 4) -glycosidic, linked to a glucosamine. The bond between the different disaccharides is α- (1 → 4) -glycosidic. The amino function of glucosamine often has an acetyl group or a sulfate group. In addition, there are O sulfate groups. Heparan sulphate is structurally related to heparin, only it has fewer N and O sulphate residues and more N acetyl groups.

Chondroitin Sulphate / Dermatan Sulphate (CS / DS)

Structure of chondroitin sulfate

Chondroitin sulfate or chondritin sulfate consists of β- (1 → 4) -glycosidically linked glucuronyl-β- (1 → 3) - N -acetylgalactosamine disaccharides, whereby D-glucuronic acid is partially converted into L-iduronic acid. If more than 10% of the uronate is present as iduronate, it is called dermatan sulfate. The degree of sulfation of chondroitin sulfate and dermatan sulfate is approximately one sulfate residue per disaccharide. The sulfate in chondroitin sulfate occurs as 4-sulfate or 6-sulfate. With dermatan sulfate, glucuronate can also occur due to epimerization at C5.

The molar mass is between 10 and 50 kDa.

Chondroitin sulfate is found in connective tissue (especially in cartilage and synovia ), dermatan sulfate also in connective tissue (mainly in the skin ).

Keratan sulfate (KS)

The disaccharide units of keratan sulfate are β- (1 → 3) linked galactosyl-β- (1 → 4) -N-acetylglucosamine residues. Both sugars can show sulfation at the C6 atom. There are 0.8–1.5 sulfate groups per unit. Keratan sulfate is in cartilage, cornea and intervertebral discs ( Disci present).

Individual evidence

  1. Mucopolysaccharide in the Lexikon der Biochemie In: Wissenschaft-Online-Lexika .
  2. a b c d Thomas Kreutzig: Short textbook biochemistry. 12th, completely revised edition. Elsevier - Urban and Fischer, Munich et al. 2006, ISBN 3-437-41774-6 .