Glycosaminoglycan

Glycosaminoglycans (GAGs) or mucopolysaccharides are long unbranched polysaccharides consisting of a repeating disaccharide unit. The repeating unit consists of a hexose (six-carbon sugar) or a hexuronic acid, linked to a hexosamine (six-carbon sugar containing nitrogen).

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 to yield proteoglycans; the exception is the GAG hyaluronan, which is uniquely synthesized without a protein core and is "spun out" by enzymes at the cell surface directly into the extracellular space.

Function
This family of carbohydrates is essential or important for life.

GAGs form an important component of connective tissues. GAG chains may be covalently linked to a protein to form proteoglycans. Water sticks to GAGs; this is where the resistance to pressure comes from. The density of sugar molecules and the net negative charges attract cations, for example, Na+, which, after the sodium binds, attracts water molecules.

Some examples of glycosaminoglycan uses in nature include heparin as an anticoagulant, hyaluronan 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