Carbohydrate Chemistry

Introduction 

         Carbohydrates have the general molecular formula (CH₂O)n, and thus were once thought to represent "hydrated carbon".

         All sugars are very soluble in water because of their many hydroxyl groups.

         They are called sugars because they are sweet in taste & crystalline in nature.

         Sugars are the most important source of energy for many cells.

Definition:

         Hydrates of carbon C_n(H₂O)_n

o       {exceptions:

         Acetic acid, C₂H₄O₂ & lactic Acid, C₃H₆O₃ Formaldehyde, CH₂Oare not carbohydrates but

         Deoxy Ribose C₆H₁₂O₅ is a carbohydrate

         carbohydrates are polyhydroxy aldehydes or ketones or compounds, which yields these on hydrolysis.

Classification:

• Classified as
  
  • Monosaccharides
    
  • Disaccharides
    
  • Oligosaccharides
    
  • Polysaccharides
    

• Monosaccharides
  

  • Simple sugars that cannot be hydrolyzed to yield another Carbohydrate.
    
  • Classification
    
    • Based on the functional group
      
      • They are classified as ALDOSES (-CHO) and KETOSES (=C=O)
        
    • Based on the number of Carbon atoms as
      
      • Triose (C3)
        
      • Tetrose(C4)
        
      • Pentose(C5)
        
      • Hexose(C6)
        
      • Heptose(C7)
        
      • Nonose (C9)
        
  • Examples
    
  • Trioses - C₃H₆O₃
    
    • Aldotriose
      
      • Glyceraldehyde
        
    • Ketotriose       
      
      • Dihydroxy acetone
        
  • Tetroses - C₄H₈O₄                                 
    

             Aldotetrose
    

            Erythrose
    

             Ketotetrose
    

            Erythrulose
    

  • Pentoses - C₅H₁₀O₅                               
    

             Aldopentose
    

            Ribose                        
    

             Ketopentose
    

            Ribulose
    

  • Hexoses -C₆H₁₂O₆                                 
    

             Aldohexose
    

            Glucose
    

            Mannose
    

            Galactose
    

             Ketohexose
    

            Fructose
    

  • Heptoses - C₇H₁₄O₇
    

             Sedoheptulose
    

  • Nonoses - C₉H₁₈O₉
    

             Sialic acid or Neuramic acid
    

• Disaccharides
  

  • Disaccharides are made up of two monosaccharides joined by Glycosidic bond.
    
  • The disaccharides on hydrolysis gives two monosaccharides
    
  • Examples
    
    • Biologically important Disaccharides are
      
      • Maltose
        
      • Sucrose
        
      • Lactose
        
    • Others include
      
      • Cellobiose
        
      • Trehalose
        
• Maltose C₃H₆O₃
  
  • Reducing in nature
    
  • the major degradation product of starch
    
  • composed of 2 glucose monomers in an alpha-(1,4) glycosidic bond.
    
• Lactose C₃H₆O₃
  
  • Milk Sugar
    
  • Reducing in nature
    
  • consists of galactose and glucose in a beta-(1,4) glycosidic bond.
    
• Sucrose C₃H₆O₃
  
  • Cane sugar and sugar beets
    
  • Non-Reducing in nature
    
  • composed of glucose and fructose through an alpha-(1,2)beta-glycosidic bond.
    

• Oligosaccharides
  

  • They are made up of 3 to 10 Monosaccharides joined by glycosidic bonds.
    
  • They on hydrolysis yields 3-10 monosaccharides
    
  • Examples
    
    • Maltotriose
      

              Glucose + Glucose + Glucose
      

              Degradation product of starch hydrolysis
      

    • Raffinose
      

              A trisaccharide that occurs in sugar beets and cotton seeds and cereals
      

    • Stachyose
      

              A tetrasaccharide found in the tubers of the Chinese artichoke
      

• Polisaccharides
  

  • Are madeup of more than 10 monosaccharides
    
  • They yields more than 10 Monosaccharides on hydrolysis.
    
  • Classified in to TWO types
    
    • Homopolysaccharides
      
    • Heteropolysaccharides
      

• Homopolysaccharides (Homoglycans)
  

  • Homopolysaccharides are polymer of a single type of Monosaccharide.
    
  • Classified in to
    
    • Storage Homopolysaccharides
      
    • Structural Homopolysaccharides
      

• Storage Homopolysaccharides
  

  • Examples
    
    • Starch
      
      • Polymer of Glucose
        
      • Source : Potato,tapioca,rice,wheat,&grains
        
      • Importance :Plant storage form of Energy
        
      • Gives blue colour with iodine
        
    • Amylose
      
      • Unbranched starch
        
      • Contains alpha 1-4 linkages
        
    • Amylopectin
      
      • Branched starch (at every 18-22 Glucose)
        
      • Contains both alpha 1-4 and alpha 1-6 linkages
        
    • Glycogen
      
      • Polymer of Glucose
        
      • Source : Liver (5%) and muscle
        
      • Branched (at every 8-10 Glucose)
        
      • Importance :Animal Storage form of Energy
        

• Structural Homopolysaccharides
  

  • Examples
    
    • Cellulose
      
      • Polymer of glucose
        
      • Contains beta 1-4 linkages
        
      • Considered as dietary fiber undigested by man
        
    • Chitin
      
      • Polymer of  N-Ac-Glucosamine
        
      • Present in the Exoskeleton of Insects
        

• Other homopolysaccharides
  

  • Inulin
    
    • Fructose
      
      • Source : various bulbs&tubers(onion,garlic)
        
      • Used for :Renal clearance studies.
        

• Hetroplysaccharides (Hetroglycans)
  

  • polymer of two or more types of sugar residues
    
  • Examples
    
•          Agar
  
•         Polymer of : Glu +Gal + other
  
•         Obtained from Sea weeds
  
  • A colloidal extract of algae; used especially in culture media and as a gelling agent in foods (undigested by bacteria, dissolves at 100 ^oC and upon cooling sets into a Gel)
    

• MUCOPOLYSACCHARIDES (MPS) or GLYCOSAMINOGLYCANS (GAG)
  

  • Are Carbohydrates containing Uronic Acid & Amino Sugars.
    
  • Examples
    

         Hyaluronic acid

•         A viscous mucopolysaccharide found in the connective tissue space and the synovial fluid of movable joints and the humours of the eye; a cementing and protective substance
  

         Heparin

• A polysaccharide produced in basophils (especially in the lung and liver) and that inhibit the activity of thrombin in coagulation of the blood.
  

         Chondroitin SO₄

•        Inground substance of connective tissues (cartilage, tendon, bone)
  

         Keratan SO₄

•         In cornea & tendons.