Glycosaminoglycan

Glycosaminoglycans (GAGs) or mucopolysaccharides are long unbranched polysaccharides consisting of a repeating disaccharide unit.

Production
Protein cores made in the rough endoplasmic reticulum are posttranslationally modified by glycosyltransferases in the Golgi apparatus, where GAG disaccharides are added to protein cores; the exception is hyaluronan, which is uniquely synthesized without a protein core and is "spun out" by enzymes at cell surfaces directly into the extracellular space.

Structure
This unit consists of an N-acetyl-hexosamine and a hexose or hexuronic acid, either or both of which may be sulfated.

The combination of the sulfate group and the carboxylate groups of the uronic acid residues gives them a very high density of negative charge.

Function
This family of carbohydrates is essential or important for the life of vertebrates and an assortment of lower animals.

GAGs form an important component of connective tissues. GAG chains may be covalently linked to a protein to form proteoglycans.

Some examples of glycoaminoglycan uses in nature include heparin as an anticoagulant, hyaluronate as a component in the synovial fluid lubricant in body joints, and chondroitins which can be found in connective tissues, cartilage and tendons.

Classification
Members of the glycosaminoglycan family vary in the type of hexosamine, hexose or hexuronic acid unit they contain (e.g. glucuronic acid, iduronic acid, galactose, galactosamine, glucosamine).

They also vary in the geometry of the glycosidic linkage.

Examples of GAGs include:

Abbreviations

 * GlcUA = β-D-glucuronic acid
 * GlcUA(2S) = 2-O-sulfo-β-D-glucuronic acid
 * IdoUA = α-L-iduronic acid
 * IdoUA(2S) = 2-O-sulfo-α-L-iduronic acid
 * Gal = β-D-galactose
 * Gal(6S) = 6-O-sulfo-β-D-galactose
 * GalNAc = β-D-N-acetylgalactosamine
 * GalNAc(4S) = β-D-N-acetylgalactosamine-4-O-sulfate
 * GalNAc(6S) = β-D-N-acetylgalactosamine-6-O-sulfate
 * GalNAc(4S,6S) = β-D-N-acetylgalactosamine-4-O, 6-O-sulfate
 * GlcNAc = α-D-N-acetylglucosamine
 * GlcNS = α-D-N-sulfoglucosamine
 * GlcNS(6S) = α-D-N-sulfoglucosamine-6-O-sulfate