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Karbohidrat II Reaksi monosakarida Ikatan glikosida Fungsi

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					SATUAN ACARA DAN
JADUAL KULIAH BIOKIMIA
No.          Topik Perkuliahan    Tanggal       Pengajar
 1. Pendahuluan                  30-08-2005   Drs. Winarto
     Konsep dasar biokimia                    Hariadi, M.Si.
     Reaksi-reaksi biokimia
 2. Air dan Buffer               06-09-2005   Drs. Winarto
                                               Hariadi, M.Si.
 3. Karbohidrat I                13-09-2005   Dr. Ir. Arman
     Tinjauan umum                              Wijonarko,
     Monosakarida                                  M.Sc.
     Disakarida
     Polisakarida
 4. Karbohidrat II               20-09-2005   Dr. Ir. Arman
     Reaksi monosakarida                        Wijonarko,
     Ikatan glikosida                              M.Sc.
     Fungsi karbohidrat
SATUAN ACARA DAN
JADUAL KULIAH BIOKIMIA
No.              Topik Perkuliahan      Tanggal           Pengajar
  5. Asam Amino dan Protein I          27-09-2005     Dr. Ir. Arman
     Tinjauan umum                                  Wijonarko, M.Sc.
     Asam amino
     Biosintesis asam amino
  6. Asam Amino dan Protein II         04-10-2005     Dr. Ir. Arman
     Peptida                                        Wijonarko, M.Sc.
     Struktur protein
     Fungsi asam amino dan protein
     Biosintesis protein
  7. Lipida I                          11-10-2005   Drs. Winarto Hariadi,
     Tinjauan umum                                        M.Si.
     Asam lemak jenuh dan tak jenuh
     Reaksi asam lemak
  8. Lipida II                         18-10-2005   Drs. Winarto Hariadi,
     Fungsi asam lemak dan lipida                         M.Si.
     Biosintesis asam lemak
  9. UJIAN SISIPAN                     25-10-2005       Topik 1 s/d 8
SATUAN ACARA DAN
JADUAL KULIAH BIOKIMIA
No.           Topik Perkuliahan            Tanggal         Pengajar
10. Asam nukleat I                        15-11-2005   Ir. Sedyo Hartono,
    Tinjauan umum                                          M.P., Ph.D.
    Nukleosida dan nukleotida
11. Asam nukleat II                       15-11-2005   Ir. Sedyo Hartono,
    Struktur DNA dan RNA                                   M.P., Ph.D.
    Nukleosida dan nukleotida
12. Enzim I                               22-11-2005     Ir. Irfan D.
    Tinjauan umum                                       Prijambada,
    Klasifikasi enzim                                  M.Eng., Ph.D.
    Koenzim dan kofaktor
13. Enzim II                              29-11-2005     Ir. Irfan D.
    Mekanisme dan kinetika kerja enzim                  Prijambada,
    Penghambatan kerja enzim                           M.Eng., Ph.D.
SATUAN ACARA DAN
JADUAL KULIAH BIOKIMIA
No.          Topik Perkuliahan      Tanggal           Pengajar

14. Metabolisme I                 06-12-2005        Ir. Irfan D.
    Tinjauan umum                                  Prijambada,
    Jalur metabolisme                             M.Eng., Ph.D.


15. Metabolisme II                06-12-2005        Ir. Irfan D.
    Bioenergetika                                  Prijambada,
    Pengendalian metabolisme                      M.Eng., Ph.D.


16. UJIAN AKHIR                    Mengikuti       Topik 10 s/d 15
                                 jadual Fakultas
            KARBOHIDRAT II
            * Reaksi monosakarida
            * Ikatan glikosida
            * Fungsi karbohidrat
Irfan D. Prijambada, Ph.D.
Lab. Mikrobiologi Tanah dan Lingkungan,
Fakultas Pertanian UGM
 Monosakarida

 Memiliki atom karbon 3 sampai 7
 Setiap atom karbon memiliki gugus
  hidroksil, keton atau aldehida.
 Setiap molekul monosakarida memiliki
  1 gugus keton atau 1 gugus aldehida
 Gugus aldehida selalu berada di atom C
  pertama
 Gugus keton selalu berada di atom C kedua
   Monosakarida
Aldosa (mis: glukosa) memiliki Ketosas (mis: fruktosa) biasanya
gugus aldehida pada salah satu memiliki gugus keto pada atom
ujungnya.                      C2.

        H       O
            C                                   CH2OH

        H   C   OH                              C   O

       HO   C   H                         HO    C   H

        H   C   OH                          H   C   OH

        H   C   OH                          H   C   OH

            CH2OH                               CH2OH

        D-glucose                           D-fructose
   Notasi D vs L

Notasi D & L dilakukan           CH O                  CH O

karena adanya atom C     H C OH                 HO     C    H
dengan konfigurasi               CH2OH                 CH2OH
asimetris seperti pada
                       D-g li s eraldehi da    L-glis era ldehi da
gliseraldehida.
                                  CH O                 CH O
Penampilan dalam
                              H   C   OH       HO      C    H
bentuk gambar
bagian bawah disebut              CH2OH                CH2OH
Proyeksi Fischer.           D-glyceraldehyde   L-glis eraldehida
 Penamaan Gula

Untuk gula dengan
atom C asimetrik lebih      O   H         O    H
dari 1, notasi D atau L        C             C
ditentukan oleh atom       H – C – OH   HO – C – H
C asimetrik terjauh       HO – C – H     H – C – OH
dari gugus aldehida        H – C – OH   HO – C – H
atau keto.                 H – C – OH   HO – C – H
Gula yang ditemui di           CH2OH         CH2OH
alam adalah dalam          D-glukosa      L-glukosa

bentuk isomer D.
Gula dalam bentuk D          O    H            O    H
merupakan bayangan              C                 C
cermin dari gula dalam      H – C – OH       HO – C – H
bentuk L.                  HO – C – H         H – C – OH
Kedua gula tersebut         H – C – OH       HO – C – H
memiliki nama yang          H – C – OH       HO – C – H
sama, misalnya D-               CH2OH            CH2OH
glukosa & L-glukosa.
                             D-glukosa         L-glukosa

Stereoisomers lainnya memiliki names yang unik,
misalnya glukosa, manosa, galaktosa, dll.
Jumlah stereoisomer adalah 2n, dengan n adalah jumlah
pusat asimetrik.
Aldosa dengan 6-C memiliki 4 pusat asimetrik, oleh
karenanya memiliki 16 stereoisomer (8 gula berbentuk D
dan 8 gula berbentuk L).
Pembentukan hemiasetal & hemiketal

Aldehida dapat     H                                  H
bereaksi
dengan alkohol     C    O   +   R'   OH   R'     O    C     OH

membentuk          R                                  R
hemiasetal.      aldehida       alkohol        hemiasetal


Keton dapat        R                                  R
bereaksi           C    O   +   "R   OH   "R     O    C     OH
dengan alkohol
                   R'                                 R'
membentuk         keton         alkohol        hemiketal
hemiketal.
                                                    1
                                                        C HO
Pentosa dan
heksosa dapat                                   H       C    OH
                                                    2
membentuk struktur                             HO       C    H       D-glukosa
                                                    3
siklik melalui reaksi                                                (bentuk linier)
                                                H       C    OH
gugus keton atau                                    4
aldehida dengan                                 H
                                                    5
                                                        C    OH
gugus OH dari atom                                      C H 2O H
                                                    6
C asimetrik terjauh.
                                6 CH2OH                              6 CH2OH
Glukosa membentuk
                                5                                    5
hemiasetal intra-       H                  O        H        H                  O      OH
molekular sebagai               H                                     H
                            4              H        1            4               H     1
                                OH                                    OH
hasil reaksi aldehida                                                                  H
                        OH                          OH       OH
dari C1 & OH dari                   3       2                            3       2

atom C5, dinamakan               H         OH                         H         OH
cincin piranosa.                    a-D-glukosa                          b-D-glukosa

Penampilan dalam bentuk gula siklik disebut proyeksi Haworth.
            CH2OH
            1

            2C   O

      HO    C
            3
                 H
                         HOH2C 6          O        1 CH2OH
        H   C    OH                           HO
            4                5       H              2

        H   C    OH              H   4        3    OH
            5
                                     OH        H
            6 CH2OH

       D-fruktosa (linear)   a-D-fruktofuranosa

Fruktosa dapat membentuk
 Cincin piranosa, melalui reaksi antara gugus keto
  atom C2 dengan OH dari C6.
 Cincin furanosa, melalui reaksi antara gugus keto
  atom C2 dengan OH dari C5.
             6 C H OH                       6 C H OH
                  2                                 2
             5                              5            O
     H                   O     H    H                          OH
              H                              H
         4               H     1        4                H     1
              OH                             OH
     OH                        OH   OH                         H
                 3        2                     3         2
              H          OH                  H           OH
                 a-D-glukosa                    b -D-glukosa

Pembentukan cincin siklik glukosa menghasilkan pusat
asimetrik baru pada atom C1. Kedua stereoisomer disebut
anomer, a & b.
Proyeksi Haworth menunjukkan bentuk cincin dari gula
dengan perbedaan pada posisi OH di C1 anomerik :
    a (OH di bawah struktur cincin)
    b (OH di atas struktur cincin).
      H OH                          H OH
      4 6           H O                         H O
  HO            5                HO
    HO              2          H   HO                     OH
         3      H       OH 1                H     OH
            H             OH            H             H

     a-D-glukopiranosa              b-D-glukopiranosa

Karena sifat ikatan karbon yang berbentuk
tetrahedral, gula piranosa membentuk konfigurasi
“kursi" atau “perahu", tergantung dari gulanya.
Penggambaran konfigurasi kursi dari
glukopiranosa di atas lebih tepat dibandingkan
dengan proyeksi Haworth.
Turunan gula
                               CO O H            CH O

          CH2OH            H   C   OH        H   C   OH

     H    C    OH        HO    C   H       HO    C   H

     H    C    OH          H   C   OH        H   C   OH

     H    C    OH          H   C   OH        H   C   OH

          CH2OH                CH2OH             CO O H

         D-ribitol       Asam D-glukonat   Asam D-glukuronat

   Gula alkohol – tidak memiliki gugus aldehida atau ketone;
    misalnya ribitol.
   Gula asam –gugus aldehida pada atom C1, atau OH pada
    atom C6, dioksidasi membentuk asam karboksilat;
    misalnya asam glukonat, asam glukuronat.
Oksidasi gula aldehida
  H       O
      C                           CO O H
  H   C   OH                  H   C   OH

 HO   C   H
               Oksidator
                            HO    C   H

  H   C   OH                  H   C   OH

  H   C   OH                  H   C   OH

      CH2OH                       CH2 O H

  D-glucose                Asam D-glukonat
    Oksidasi gula aldehida
   Gula yang dapat dioksidasi adalah senyawa
    pereduksi. Gula yang demikian disebut
    sebagai gula pereduksi.
   Senyawa yang sering digunakan sebagai
    pengoksidasi adalah ion Cu+2, yang
    berwarna biru cerah, yang akan tereduksi
    menjadi ion Cu+, yang berwarna merah
    kusam. Hal ini menjadi dasar bagi pengujian
    Benedict yang digunakan untuk
    menentukan keberadaan glukosa dalam
    urin, suatu pengujian bagi diagnosa
    diabetes.
  Oksidasi gula aldehida
                   panas & alk . pH
Glukosa +   Cu++
        Gluconic acid + Cu2O (Cu2O is insol ppt)
               glukosa oksidase
Glukosa + O2
                           Asam glukonat + H2O2
                           (H2O2 nya diukur)
                   heksokinase
Glukosa + ATP

             Glukosa-6-P + ADP (G-6-Pnya diukur)
Turunan gula
           C H2OH                    CH 2OH

     H              O      H    H             O       H
           H                         H
           OH       H                OH       H

     OH                    OH   OH                O OH
           H        NH 2             H        N   C   CH 3
                                              H
          a-D-glukosamina       a-D-N-asetilglukosamina

 Gula amino - gugus amino menggantikan
 gugus hidroksil. Sebagai contoh glukosamina.
 Gugus amino dapat mengalami asetilasi,
 seperti pada N-asetilglukosamina.
 Ikatan Glikosida
Gugus hidroksil anomerik dan gugus hidroksil gula atau
senyawa yang lain dapat membentuk ikatan yang disebut
ikatan glikosida dengan membebaskan air :
   R-OH + HO-R'  R-O-R' + H2O
Misalnya methanol bereaksi dengan gugus OH anomerik dari
glukosa membentuk metil glukosida (metil-glukopiranosa).

   H OH                                               H OH
               H O                           H2O                  H O
HO                                                 HO
  HO                      H   +   CH3- O H           HO                    H
           H     OH                                           H     OH
       H             OH                                   H             OCH3
  a-D-glukopiranosa               metanol           Metil-a-D-glukopiranosa
                                6 CH2OH                                6 CH2OH

Disaccharides:          H
                                5          O           H       H
                                                                       5          O           H
                                H                                       H
Maltose, a cleavage     4       OH         H       1           4
                                                                        OH        H       1


product of starch       OH          3      2
                                                           O
                                                                            3         2
                                                                                              OH

(e.g., amylose), is a           H          OH
                                                        maltose         H         OH

disaccharide with an
a(1 4) glycosidic              6 CH 2OH

                                5
                                                                       6 CH 2OH

                                           O                           5          O           OH
link between C1 - C4        H
                                H
                                                               H
                                                                       H
OH of 2 glucoses.       4
                                OH         H       1       O       4
                                                                       OH         H       1

                                                       H
It is the a anomer      OH                                                                    H
                                    3          2                           3          2

                                H          OH                           H         OH
(C1 O points down).                                    cellobiose
Cellobiose, a product of cellulose breakdown, is the
otherwise equivalent b anomer (O on C1 points up).
The b(1 4) glycosidic linkage is represented as a
zig-zag, but one glucose is actually flipped over
Other disaccharides include:
   Sucrose, common table sugar, has a glycosidic
    bond linking the anomeric hydroxyls of glucose &
    fructose.
    Because the configuration at the anomeric C of
    glucose is a (O points down from ring), the linkage
    is a(12).
    The full name of sucrose is a-D-glucopyranosyl-
    (12)-b-D-fructopyranose.)
   Lactose, milk sugar, is composed of galactose &
    glucose, with b(14) linkage from the anomeric
    OH of galactose. Its full name is b-D-
         Polysaccharides
     CH2OH                         6CH OH                    CH2OH                    CH2OH                    CH2OH
                                      2
             O                     5      O      H                   O            H           O    H       H           O    H
H                     H       H                          H                H
     H                             H                         H                        H                        H
     OH      H    1               4 OH     H 1               OH      H                OH      H                OH      H
                          O                          O                        O                        O                    OH
OH                                         2
                                       3
     H       OH                    H       OH                H       OH               H       OH               H       OH
                                                             amylose


     Plants store glucose as amylose or amylopectin,
     glucose polymers collectively called starch. Glucose
     storage in polymeric form minimizes osmotic effects.
     Amylose is a glucose polymer with a(14) linkages.
     It adopts a helical conformation.
     The end of the polysaccharide with an anomeric C1
     not involved in a glycosidic bond is called the
     reducing end.
                   CH 2OH                            CH 2OH
          H                     O    H       H                    O     H                                            amylopectin
                   H                                 H
                   OH           H                    OH           H      1
                                         O
          OH
                                                                        O
                   H            OH                   H            OH

     CH 2OH                          CH 2OH                            6 CH 2                          CH 2OH                    CH 2OH
H              O        H       H                O        H       H    5        O        H       H              O    H       H            O    H
     H                               H                                   H                             H                         H
     OH        H                     OH          H                       OH     H    1               4 OH       H                OH       H
                                                                  4                          O                           O
                            O                                 O                                                                                OH
OH
                                                                         3       2
     H         OH                    H           OH                     H       OH                     H        OH               H        OH



     Amylopectin is a glucose polymer with mainly a(14)
     linkages, but it also has branches formed by a(16)
     linkages. Branches are generally longer than shown
     above.
     The branches produce a compact structure & provide
     multiple chain ends at which enzymatic cleavage can
                   CH2OH                             CH2OH
          H                     O                                 O
                                                                                                                               glycogen
                                     H       H                          H
                   H                                 H
                   OH           H                    OH           H      1
                                         O
          OH
                                                                        O
                   H            OH                   H            OH

     CH2OH                           CH2OH                             6 CH2                          CH2OH                    CH2OH
H              O        H       H                O        H       H    5       O        H       H             O    H       H           O    H
     H                               H                                   H                            H                        H
     OH        H                     OH          H                       OH    H    1               4 OH      H                OH      H
                                                                  4                         O                          O
                            O                                 O                                                                             OH
OH
                                                                         3      2
     H         OH                    H           OH                     H      OH                     H       OH               H       OH




     Glycogen, the glucose storage polymer in animals,
     is similar in structure to amylopectin. But glycogen
     has more a(16) branches.
     The highly branched structure permits rapid release
     of glucose from glycogen stores, e.g., in muscle
     during exercise. The ability to rapidly mobilize
     glucose is more essential to animals than to plants.
     CH2OH                         6CH OH                  CH2OH                    CH2OH                    CH2OH
                                      2
             O                     5      O                        O            H           O            H           O    OH
H                             H                        H
     H                             H                       H                        H                        H
     OH      H    1       O       4 OH   H 1       O       OH      H        O       OH      H        O       OH      H
OH                                                                      H                        H                        H
                      H                   2    H
                                    3
     H       OH                    H     OH                H       OH               H       OH               H       OH
                                                           cellulose

     Cellulose, a major constituent of plant cell walls,
     consists of long linear chains of glucose with
     b(14) linkages.
     Every other glucose is flipped over, due to the b
     linkages.
     van der Waals
     interactions, that cause
     This promotes intra-chain and inter-chain H-bonds
     cellulose chains to be
     and
     straight & rigid, and pack
     with a crystalline
     arrangement in thick           Schematic of arrangement of
     bundles called                 cellulose chains in a microfibril.
     CH2OH                         6CH OH                  CH2OH                    CH2OH                    CH2OH
                                      2
             O                     5      O                        O            H           O            H           O    OH
H                             H                        H
     H                             H                       H                        H                        H
     OH      H    1       O       4 OH   H 1       O       OH      H        O       OH      H        O       OH      H
OH                                                                      H                        H                        H
                      H                   2    H
                                    3
     H       OH                    H     OH                H       OH               H       OH               H       OH
                                                           cellulose

     Multisubunit Cellulose Synthase complexes in the
     plasma membrane spin out from the cell surface
     microfibrils consisting of 36 parallel, interacting
     cellulose chains.
     These microfibrils are very strong.
     The role of cellulose is to impart strength and rigidity
     to plant cell walls, which can withstand high
     hydrostatic pressure gradients. Osmotic swelling is
     prevented.
                                     CH2OH
          D-glucuronate             6
                             H         5    O
             6COO                    H
                              4                   1 O
                     O                       H
      H        5
              H               OH                 H
      4              H    1             3    2
              OH
                          H          H       NHCOCH3
               3      2       O
              H      OH           N-acetyl-D-glucosamine
     hyaluronate

Glycosaminoglycans (mucopolysaccharides) are
polymers of repeating disaccharides.
Within the disaccharides, the sugars tend to be
modified, with acidic groups, amino groups, sulfated
hydroxyl and amino groups, etc.
Glycosaminoglycans tend to be negatively
charged, because of the prevalence of acidic
                                    CH2OH
         D-glucuronate             6
                            H         5    O
            6COO                    H
                             4                   1 O
                    O                       H
      H       5
             H               OH                 H
     4              H    1             3    2
             OH
                         H          H       NHCOCH3
              3      2       O
             H      OH           N-acetyl-D-glucosamine
    hyaluronate

Hyaluronate is a glycosaminoglycan with a
repeating disaccharide consisting of 2 glucose
derivatives, glucuronate (glucuronic acid) & N-acetyl-
glucosamine.
The glycosidic linkages are b(13) & b(14).
                                      CH2OH
         D-glucuronate               6
                              H         5    O
            6COO                      H
                               4                   1 O
                      O                       H
      H       5
             H                 OH                 H
     4                H    1             3    2
             OH
                           H          H       NHCOCH3
              3       2        O
             H        OH           N-acetyl-D-glucosamine
    hyaluronate

Proteoglycans are glycosaminoglycans that are
covalently linked to specific core proteins.
Some proteoglycans of the extracellular matrix in
turn link non-covalently to hyaluronate via protein
domains called link modules.
                                       CH2OH
          D-glucuronate               6
                               H         5    O
             6COO                      H
                                4                   1 O
                       O                       H
      H        5
              H                 OH                 H
      4                H    1             3    2
              OH
                            H          H       NHCOCH3
               3       2        O
              H        OH           N-acetyl-D-glucosamine
     hyaluronate

For example, in cartilage multiple copies of the
aggrecan proteoglycan bind to an extended
hyaluronate backbone to form a large complex.
Versican, another proteoglycan that binds to
hyaluronate, is in the extracellular matrix of loose
connective tissues.
See web sites on aggrecan and aggrecan plus
        iduronate-2-sulfate       N-sulfo-glucosamine-6-sulfate
            H                             CH2OSO3

    H                O                H           O     H
            COO                          H
            OH       H            O       OH      H
                              H                             O

            H        OSO3                H       NHSO3
    heparin or heparan sulfate - examples of residues

Heparan sulfate is initially synthesized on a
membrane-embedded core protein as a polymer of
alternating        N-acetylglucosamine and
glucuronate residues.
Later, in segments of the polymer, glucuronate
residues may be converted to the sulfated sugar
iduronic acid, while N-acetylglucosamine residues
                                                   PDB 1RID

Heparin, a soluble
glycosaminoglycan found in
granules of mast cells, has a
structure similar to that of heparan
sulfates, but is more highly
sulfated.
When released into the blood, it
inhibits clot formation by interacting heparin: (IDS-SGN)5
with the protein antithrombin.               C O N S

Heparin has an extended helical
Charge repulsion by the many negatively charged
conformation.
groups may contribute to this conformation.
Heparin shown has 10 residues, alternating IDS
(iduronate-2-sulfate) & SGN (N-sulfo-glucosamine-6-
                          heparan sulfate
           core       glycosaminoglycan
           protein

                     transmembrane
                     a-helix
                                 cytosol

Some cell surface heparan sulfate
glycosaminoglycans remain covalently linked to
core proteins embedded in the plasma membrane.
Proteins involved in signaling & adhesion at the
cell surface recognize and bind segments of
heparan sulfate chains having particular patterns
of sulfation.
                             CH2OH                  C   O
                                    O    O    CH2  CH
Oligosaccharides H
that are covalently
                           H                      NH serine
                           OH       H                 residue
attached to           OH               O H
proteins or to             H      HN C CH3
membrane lipids
                       b-D-N-acetylglucosamine
may be linear or
branched chains.
O-linked oligosaccharide chains of glycoproteins
vary in complexity.
They link to a protein via a glycosidic bond between
a sugar residue & a serine or threonine OH.
O-linked oligosaccharides have roles in
recognition, interaction, and enzyme regulation.
               CH2OH                         C    O

          H             O       O     CH2    CH
               H                            NH    serine
               OH       H                         residue
          OH                O H
               H       HN   C   CH3

           b-D-N-acetylglucosamine

N-acetylglucosamine (GlcNAc) is a common O-
linked glycosylation of protein serine or threonine
residues.
Many cellular proteins, including enzymes &
transcription factors, are regulated by reversible
GlcNAc attachment.
Often attachment of GlcNAc to a protein OH
            CH2OH                O         HN

      H              O   HN      C   CH2    CH        Asn
            H
                     H                      C    O
            OH
                             H             HN
     OH
                                           HC    R     X
            H       HN   C    CH3
                                            C    O
                         O
          N-acetylglucosamine              HN

     Initial sugar in N-linked             HC    R   Ser or Thr
     glycoprotein oligosaccharide           C    O


N-linked oligosaccharides of glycoproteins tend to
be complex and branched. First N-
acetylglucosamine is linked to a protein via the
side-chain N of an asparagine residue in a
        NAN         NAN         NAN

        Gal         Gal         Gal

        NAG         NAG     NAG

              Man         Man         N-linked oligosaccharide

                Man                   Key:
                                      NAN = N-acetylneuraminate
                NAG                   Gal = galactose
                                      NAG = N-acetylglucosamine
                NAG        Fuc
                                      Man = mannose
                                      Fuc = fucose
                    Asn


Additional monosaccharides are added, and the N-
linked oligosaccharide chain is modified by removal
and addition of residues, to yield a characteristic
branched structure.
Many proteins secreted by cells have attached N-
linked oligosaccharide chains.
Genetic diseases have been attributed to deficiency
of particular enzymes involved in synthesizing or
modifying oligosaccharide chains of these
glycoproteins.
Such diseases, and gene knockout studies in mice,
have been used to define pathways of modification
of oligosaccharide chains of glycoproteins and
glycolipids.
Carbohydrate chains of plasma membrane
glycoproteins and glycolipids usually face the
outside of the cell.
Lectins are glycoproteins that recognize and
bind to specific oligosaccharides. A few
examples:
   Concanavalin A and wheat germ agglutinin
    are plant lectins that have been useful
    research tools.
   Mannan-binding lectin (MBL) is a
    glycoprotein found in blood plasma.
    It associates with cell surface carbohydrates
    of disease-causing microorganisms, promoting
    phagocytosis of these organisms as part of the
Selectins are integral        selectin
                                         lectin domain
proteins of mammalian
cell plasma membranes
with roles in cell-cell
recognition & binding.        outside
                                         transmembrane
A lectin-like domain is at               a-helix
the end of an extracellular   cytosol     cytoskeleton
segment that extends out                  binding domain
from the cell surface.
A cleavage site just outside the transmembrane a-
helix provides a mechanism for regulated release of
some lectins from the cell surface.
A cytosolic domain participates in regulated
interaction with the actin cytoskeleton.

				
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posted:10/16/2011
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