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S06L01 Carbohydrate chemistry

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S06L01 Carbohydrate chemistry Powered By Docstoc
					       Section 6:
Carbohydrate Metabolism

 1. Carbohydrate chemistry


                         10/14/2005
                Overview of catabolism
    FATS       POLYSACCHARIDES             PROTEINS
                                                           Stage 1

fatty acids,          glucose,             amino acids
  glycerol          other sugars
                                                           Stage 2

                    acetyl CoA

     H2O       O2                                 CoA
                                                           Stage 3
   oxidative           e–     v    Krebs
phosphorylation                    cycle      v
                       CO2

    ATP ADP + Pi
1                                            adapted from Fig. 14.12
                       Carbohydrates
       aka saccharides (suffix: -ose)
       elements: C, H, O
       common empirical formula: (CH2O)n n 3
       monosaccharides
         aka simple sugars
         carbon chain unbranched

         1 carbonyl (C=O) group
                    at end of chain: aldehyde
                    not at end:      ketone
         2 or more OH groups

                         # OH groups = n – 1
       oligosaccharides/polysaccharides
2          contain a few/many linked monosaccharides
            Monosaccharides: properties
   very polar (hydrophilic)
        functional groups
         OH: H-bond donor & acceptor
         =O: H-bond acceptor
   chemically reactive, especially the carbonyl group,
    which reacts with:
     OH groups, forming rings
                          oligosaccharides
                          polysaccharides
     oxidants & reductants

     amino groups on proteins (glycation)
3
              Stereoisomerism (chirality)
   stereoisomers
     differ only in arrangement of atoms or groups
     any C with 4 different groups attached (asymmetric or chiral C)

 enantiomers: a pair of molecules that are nonsuperimposable
                 mirror images of one another
                                                     H2C OH
 example: trioses
                                                         C O
    simplest monosaccharides
                                                     H2C OH
    DHA: achiral (no stereoisomerism)
                                                  dihydroxyacetone
                                                   DHA (a ketose)
    GA: D & L forms
                                            O      H        O     H
 in general,
                                                C              C
    # possible stereoisomers          HO            H H           OH
        n                                      C               C
     = 2 (n = # asymmetric Cs)
                                            H2C OH          H2C OH
    D forms more common
                                      L-glyceraldehyde D-glyceraldehyde
4                rotating L & D forms                GA (an aldose)
                               Glucose
   most important monosaccharide
      the major monosaccharide in most organisms

      main form in which carbohydrate transported in blood

   main roles: precursor of many biomolecules
                 energy source (fuel)
   aka dextrose                     O    H
    abbreviation: glc                   C    1    H2C OH
   6 carbons: a hexose               H C OH 2       C O
       carbon numbering starts at     HO C H          3     HO C H
        end nearest C=O                  H C OH        4       H C OH
 other hexoses:                         H C OH        5       H C OH
  fructose (frc)                          H2COH        6       H2COH
  galactose (gal)                      D-glucose             D-fructose
                                     (an aldohexose)       (a ketohexose)
5 mannose
      Glucose: ring formation (cyclization)
   carbonyl C is subject to nucleophilic attack by OH groups
    (arrow), forming a hemiacetal                    H2COH
                                                               6




   2 isomers form: O H                                    O
                                                           5

                        1
                            C                        H     H            H
    a: OH projects      2

                      H C OH
                                                     4

                                                           OH      H
                                                                         1



                                         H2COH      HO                  OH
     on side of         3

                     HO C H
                                                               3   2




                                         C OH              H       OH
     ring opposite      4              H H      H
                      H C OH
                                       C OH         a ring form
     from C6            5

                      H C OH
                                            H C
                                      HO C  C   O
     (trans to C6)      6                H  OH
                       H2COH
                                                         H2COH
    b: OH projects   D-glucose
                                                                   O OH
     on same side               open-chain form      H    H
     of ring as C6                                        OH       H H
                                                    HO
      (cis to C6)                                         H        OH

6                                                   b ring form
                 Glucose: ring structure
   cyclic forms of monosaccharides are usually drawn as
    Haworth projections, even though rings not actually planar
   tetrahedral geometry        H2COH
    around each ring atom            5
                                       O H         H CH OH
                               H H                        2
                                                HO            O
    causes 2 atoms to be         4

                                  OH H
                                                   1


                                                         H H
                                          OH                       H
    out-of-plane:            HO          3   2
                                                  HO
                                                             OH OH
                                  H    OH               H
    chair conformation                          "chair" conformation
                             a-D-glucopyranose
   6-atom cyclic forms are Haworthprojections    of a-isomer

    termed pyranoses after          
                                H2COH
    the heterocycle pyran
                                             O OH          O
                                 H   H
                                     OH      H H
                               HO                       pyran
                                     H    OH
7                               b-D-glucopyranose
                      Glucose isomers
   the a&bisomers are called anomers; the former carbonyl
     C (C1 in this case) is called the anomeric C
   anomer formation: aka anomerization
                                                       6
   anomeric C:                                     H2COH
    only one that has 2 of its bonds to O             5
                                                            O H
                                                   H H
   the reactions are:                 O    H     4             1
                                         1C           OH 2
                                                        3
                                                            H
                                                               OH
     rapid, reversible                   2
                                                 HO
                                        H C OH        H     OH
     & don’t need catalysts              3
                                      HO C H     a-D-glucopyranose
   forms in rapid-equilibrium           4
                                       H C OH
     with each other                     5          H2COH
                                       H C OH
       form          % at equil.         6
                                                           O OH
                                       H2COH       H   H
       open-chain    <1              open-chain        OH    H H
       a              36                form      HO
                                                       H     OH
       b              63                          b-D-glucopyranose
8
Glucose:
a & b ring forms
   ball & stick
    representations of
                                           1
      the a & b anomers

      galactose, an epimer

       of glc at C-4              a glc




         4                    4
                                           1


9              b gal               b glc
                      Fructose isomers
   like glc, fructose forms pyranoses
   it also forms 5-atom rings called furanoses
   furanoses are major forms in frc derivatives (e.g., sucrose)
   C2 is the anomeric carbon
                1
              H2C OH
                C O      OH                          OH    OH
                             OH
           HO C H H2C OH CH2                   H2C    O
                                                          1CH2
                       C     C                          C
              H C OH                             C
                      H H OH
                               O                 H H HO OH
             H C OH     C C                        C C
                      HO H                       HO H
              H2COH              O
           D-fructose
                                              a-D-fructofuranose

10                  open-chain form   furan
                            Oxidation
   carbohydrates with a free carbonyl group are easily
    oxidized, i.e., they are good reducing agents
   termed reducing saccharides/sugars
                                                             –
   oxidation reaction is      O    H                O     O
     used to measure [glc]        C                     C
                               H C OH                H C OH
     in blood & other
                              HO C H                HO C H
     fluids                            + 2 Cu2+                + 2 Cu+
                                H C OH                H C OH
      glc oxidase more
                                H C OH                H C OH
       reliable than Cu2+
                               H2COH                 H2COH
      color development D-glucose                 D-gluconate
       proportional to       glucose + O2          gluconate + H2O2
       [H2O2]                              glucose
                                           oxidase
11
        Glycation of protein amino groups
   carbonyl groups react slowly & reversibly with amines,
     e.g., protein side chains (abbreviation: Pr-NH2)
   the resulting aldimine rearranges very slowly &
     irreversibly to a ketimine
   function of long-lived proteins can be impaired
   probable factor                                       H
    in long-term        O    H          Pr N    H    Pr N
                           C                  C            CH2
    damage caused
                        H C OH             H C OH          C O
    by high blood
                       HO C H             HO C H       HO C H
    [glc] in diabetes            Pr NH2
                         H C OH             H C OH       H C OH
    mellitus
                       H C OH            H C OH          H C OH
                      H2COH              H2COH          H2COH
                     glucose        aldimine adduct   ketimine
12
            Next:

2. Glycosides & Polysaccharides

				
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