polysaccharides (glycans)

polysaccharides (glycans)

1. 10 or more monosaccharide units

2. homopolysaccharide

   • fuel storage
     • starch
       • fuel storage in molecule in plants and a major nutrient in animals
       • 98% amylose
         • a non-branching structure made up of D-glucose units linked by a(1->4) glycosidic bonds
       • 2% amylopectin
         • which consists of D-glucose units linked by a(1->4) glycosidic bonds and having a(1->6) glycosidic bond branch points every 24-30 units
     • glycogen
       • fuel storage molecule in animals
       • its structure is similair to amylopectin but is more branched with a(1->6) glycosidic bond branch points every 8-12 units
   • structural
     • cellulose
       • primary structural component of cell walls
       • it is a linear homopolymer of up to 15000 units of D-glucose linked by B(1->4) glycosidic bonds
     • chitin
       • main structural component of exoskeletons of invertebrates such as crustaceans, insects and spiders
       • its a linear homopolymer of N-acetyl-D-glucosamine units linked by B(1->4) glycosidic bonds

3. heteropolysaccharide

   • glucoseaminoglycans
     • mucopolysaccharides
     • consist mainly of unbranched polysaccharides of alternating uronic acid and hexosamine residues
     • hyaluronic acid
       • molecules of hyaluronic acid consists of 250-25000 disaccharide units of D-glucuronic acid and N-acetyl-D-glucosamine linked by B(1->3) glycosidic bonds
       • this mucopolysaccharide binds water to form a viscoelastic gel
       • important constituent of:
         • synovial fluid - present in joints where it acts as a lubricant
         • vitreous humor of the eye - gives shape to eye
         • ground substance - the gel-like matrix that embeds collagen and elastin fibers in the extracellular space of connective tissues
     • chondroitin-4-sulfate
       • an important structural component of cartilage. it is used as a dietry supplement for the treatment of osteoarthritis
     • keratin sulfate
       • found in cornea, cartilage and bone. acts as a cushion in joints to absorb mechanical shock
     • heparin
       • prevents blood clotting
   • yields a mixture of constituent monosaccharides and derived products upon enzymatic or acid hydrolysis
   • proteoglycans
     • mucoproteins
     • these are aggregates of proteins and glycosaminoglycans
     • are found in cartilage
     • are subunits consist of core protein to which glycosaminoglycans (keratin sulphate and chondroitin sulphate) are covalently linked
   • peptidoglycans
     • the cell wall of bacteria consist of peptidoglycan
       • its polysaccharide component consists of linear alternating units of B(1->4) linked N-acetylglucosamine and N-acetylmuramic acid
       • in gram positive staphylococcus aeureus, the tetrapeptides cross-linking polysaccharide chains via pentaglycine residues consisting of L-Ala-D-isoglutamyl-L-Lys-D-Ala
       • the D-amino acids confer resistance to proteases
       • penicillin binds to and inhibits enzymes involved in cross-linking peptidoglycan strands, resulting in bacterial lysis
       • penicillin resistant bacteria produce penicillase which inactivates penicillin by hydrolyzing it
   • glycoproteins
     • linked to proteins through O or N linkages
     • functions
       • protection
         • IgM and IgG (antibodies), mucins of mucous (mucus provides a physical barrier between a cell and its environment and its lubricating properties prevent mechanical damage caused by friction)
       • transport
         • transferrin (transports iron into cells)
       • host recognition
         • O-antigens coat the surfaces of pathogenic gram-negative bacteria and thus allows the host antibodies to recognize it
       • vision
         • bovine rhodopsin (a pigment present in the retina that is reponsible for both the formation of the photoreceptor cells and the first events in the perception of light)
       • growth promotion
         • granulocyte-macrophage colony-stimulating factor (GM-CSF) (stimulates stem cells to produce white blood cells such as neutrophils, eosinophils and basophils)
   • glycolipids