Notes on Carbohydrates: Heteropolysaccharides and Glycosaminoglycans

Carbohydrates: General Aspects

Heteropolysaccharides

  • Heteropolysaccharides contain various groups in addition to carbohydrates.

    • Major types include:

    • Glycosaminoglycans (GAGs)

    • Glycoconjugates

    • Mucilages

Glycosaminoglycans (GAGs)

  • Composed of repeating disaccharide units that typically include:

    • An amino sugar (either N-acetylglucosamine or N-acetylgalactosamine)

    • A uronic acid (usually D-glucuronic acid or its 5-epimer L-iduronic acid)

  • Often contain sulfate groups attached to one or more of their hydroxyl groups.

Physical and Chemical Properties

  • The specific patterns of sulfated and non-sulfated sugar units enable GAGs to be recognized specifically by various ligands.

    • Definition: A ligand is an ion or molecule that forms a complex with another molecule.

  • Carboxyl and sulfate groups contribute to a strong negative charge on GAG molecules.

  • The negative charge results in:

    • A shell of water surrounding each molecule.

    • Repulsion between GAG molecules due to the uniform negative charge, which contributes to their viscosity and slipperiness.

Biological Relevance

  • GAGs are present in:

    • Cartilage

    • Skin

    • Blood vessels

    • Cornea

    • Tendons

    • Ligaments

    • Loose connective tissue

    • Heart valves

Proteoglycan Monomers

  • With the exception of hyaluronic acid, all GAGs combine with extracellular proteins through covalent bonds to form proteoglycan monomers, where polysaccharides can constitute 95% or more of their mass.

  • Notable GAGs include:

    • Hyaluronic acid

    • Chondroitin sulfate

    • Heparin

Hyaluronic Acid

  • Isolated from several tissues and even from bacteria.

  • Characteristics of hyaluronic acid include:

    • Jelly-like consistency.

    • High viscosity, which aids in joint lubrication.

    • Presence in the vitreous humor of the eye, providing a glassy appearance (derived from the Greek "hyalos" which means glassy).

  • Role in tissue:

    • Key component of the extracellular matrix of skin and connective tissue, serving as a cement substance that resists bacterial penetration.

  • Chemically:

    • High molecular weight substance consisting of alternating residues of N-acetylglucosamine and β-D-glucuronic acid.

    • May contain up to 50,000 repetitions of these disaccharide units:

    • Structure representation:

      • N-Acetyl-Glucosamine <-----> β-D-Glucuronic Acid

      • (Repeating unit)