Introduction to chemistry of carbohydrates

1.5. Hetero polysaccharides

Hetero polysaccharides

These are polysaccharides containing more than one type of sugar residues

1. Glycosaminoglycans, (GAGs or mucopolysaccharides)

They are long, usually unbranched, composed of a repeating disaccharide unit

 * They are negatively charged heteroplolysaccharide chains (polyanions)

• The amino sugar is either D-glucosamine or D-galactosamine in which the amino group is usually acetylated, thus eliminating its positive charges.

• The amino sugar may also be sulphated on carbon 4, 6, or on a monoacetylated nitrogen.

• The acidic sugar is either D-glucuronic acid or its carbon 6 epimer, L-uronic acid. For example Hyaluronic acid, Heparin and Chondroitin sulphate.

Function of Glycosaminoglycans (GAGS)

1. They have the special ability to bind large amounts of water, there by producing the gel-like matrix that forms the basis of the body’s ground substance.

2. Since they are negatively charged, for example, in bone, glycosaminoglycans attract and tightly bind cations like ca⁺⁺, they also take-up Na⁺ and K⁺

3. GAGs stabilize and support cellular and fibrous components of tissue while helping maintain

the water and salt balance of the body.

4. Its essential components of the extra cellular matrix, GAGs’ play an important role in mediating cell-cell interactions

• Ground substance is a part of connective tissue, which is a gel like substance containing water, salt, proteins and polysaccharides.

An example of specialized ground substance is the synovial fluid, which serves as a lubricant in joints, and tendon sheaths.

Heparin

• contains a repeating unit of D-glucuronic and D-glucosamine, with sulfate groups on some of the hydroxyl and amino-groups

• It is an important anticoagulant, prevents the clotting of blood by inhibiting the conversion of prothrombin to thrombin. Thrombin is an enzyme that acts on the conversion of plasma fibrinogen into the fibrin.

• It is found in mast cells in lung, liver skin and intestinal mucosa.

Glycoproteins (Mucoproteins)

Glycoproteins are proteins to which oligosaccharides are covalently attached. They differ from the glycosaminoglycans in that the length of the glycoprotein’s carbohydrate chain is relatively short (usually two to ten sugar residues in length, although they can be longer), whereas it can be very long in the glycosaminoglycans.

The glycoprotein carbohydrate chains are often branched.

be negatively charged.

For example:

- Glycophorin, a glycoprotein found in human red cell membranes.

- Human gastric glycoprotein (mucin).

- Many protein hormones, receptors are glycoproteins

Proteoglycans

When glycosaminoglycans are attached to a protein molecule the compound is called proteoglycan [proteoglycans = Glycosaminoglycans + proteins].