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Sunday, 7 January 2018

Glycosaminoglycans ,GAGs ( mucopolysaccharides )

Glycosaminoglycans ,GAGs (mucopolysaccharides)


They are formed of repeating disaccharide units (acidic sugar –amino sugar) .The amino sugar is either D-glucosamine or D-galctosamine in which the amino group is usually acetylated.The amino sugar may also be sulfated at carbon 4 or 6 . GAGS often contain sulfate groups.The uronic acid and sulfate residues cause them to be very negatively charged . They are un-branched . Most of GAGs are present extracellularly except heparin . Most of them form the Structure components of connective tissue such as bone elastin and collagen . They act as lubricants for other tissues because they have the property of holding large quantities of water.

Glycosaminoglycans include :

Hyaluronic acid :

  • Structure : Repeated disaccharide units consists of :

  1. Glucuronic acid
  2. N-acetylglucosamine
  • Site :
  1. Cartilage
  2. Loose connective tissue
  3. Vitreous humor of the eye
  4. Embryonic tissue
  • functions:

  1. It is acomponent  of cell membrance and act as receptors .
  2. It participates in cell adhesion and cell – cell interaction
  3. It is present in basement membrane of the kindney and plays an important role in determining the charge selectiveness of glomerular filtration

Proteoglycans and Glycoproteins:

  • Introduction: 

proteoglycans and glycoproteins are proteins containing carbohydrates .They differ from each other in that they are present in different sites , contain different sugars and have different shape and size.

  • Proteoglycans: 

 These are chins of glycosminoglycans attached to protein molecule e.g. hyaluronic acid,keratin sulfate ,,.heprain.They  associated with structure elements of tissues as bone elastin and cartilage .The carbohydrate part is presented in very long unbranched chains (more than 50 monosaccharide molecules)attached to protein core

  • Glycoproteins (mucoproteins):
  • Structure :They consist of:

  1. protein core
  2. Carbohydrate chains which are branched short chain (from 2-15 monosaccharide units ) such chains are usually called oligosaccharide chains. They include:
  1. Hexoses : Galactose and mannose
  2. N- acetylglucosamine
  3. pentose :Arabinoe and xylose
  4. L – fucose
  5. Sialic acid
They contain no uronic acids or acids or sulfate groups
  • Functions :
  1. Glycoproteins are components of extracellular matrix
  2. They are components of mucins of gastrointestinal and urogental tracts where they act as protetive biologic lubricants .
  3. Glycoproteins are  components of cell member as :
  1. Blood group antigens (A,B,AB)
  2. Cell  surface recognition receptors : for hormones , other Cell and viruses.
  3. Glycophorin :This is glycoprotein present in human red cell membrane ..It has free polypeptide portions outside both the external and the internal (Cytoplasmic)surfaces 
  4. plasma proteins :globular proteins – except albumin – present in plasma are glycoprotein's .
  5. Most secreted enzyme and proteins(as hormones) are glycoprotein.

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