Karbohidrat II Reaksi monosakarida Ikatan glikosida Fungsi

KARBOHIDRAT
* Reaksi monosakarida
* Ikatan glikosida
* Fungsi karbohidrat
Prof. Dr. Ir. Chanif Mahdi, MS.
KARBOHIDRAT
Karbohidrat adalah golongan senyawa organik,
polihidroksi aldehid atau polihidroksi keton, atau
senyawa lainnya apabila dihidrolisa dapat
menghasilkan kedua senyawa tersebut.
Karbohidrat berasal dari kata karbon (C) dan
hidrat (H2O). Karena molekul karbohidrat selalu
mempunyai perbandingan antara hidrogen dan
oksigen = 2 : 1 . Oleh karena itu rumus umum
karbohidrat adalah C12 (H2O)11, tetapi tidak
semua senyawa yang mempunyai perbandingan
H : O = 2 : 1 adalah senyawa karbohidrat.
Contoh asam asetat mempunyai rumus C2H4O2
bukan termasuk karbohidrat.
Penggolongan Senyawa Karbohidrat
Berdasarkan susunan molekulnya, karbohidrat
dapat digolongkan menjadi tiga golongan
sebagai berikut :
1. Monosakarida
2. Disakarida / oligosakarida
3. Polisakarida
Monosakarida
Adalah golongan senyawa karbohidrat, yang
paling sederhana, yang tidak dapat dipecah lagi
menjadi gula yang lebih sederhana.
Berdasarkan gugus fungsionilnya,
monosakarida dapat digolongkan menjadi dua
golongan, masing- masing adalah aldose dan
ketose.
Berdasarkan jumlah atom C nya, monosakarida
dapat digolongkan menjadi empat golongan :
masing- masing adalah : Triose
(mengandung 3 atom C), Tetrose
(mengandung 4 atom C), Pentose
(mengandung 5 atom C), dan heksose
(mengandung 6 atom C.adapun secara
skematis penggolongan senyawa
karbohidrat seperti gambar skema berikut
:
Monosakarida
 Memiliki atom karbon 3 sampai 6
 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
Ketosas (mis: fruktosa) biasanya
Aldosa (mis: glukosa) memiliki
gugus aldehida pada salah satu memiliki gugus keto pada atom
C2.
ujungnya.
H
O
CH2OH
C
C
O
HO
C
H
OH
H
C
OH
OH
H
C
OH
H
C
OH
HO
C
H
H
C
H
C
CH2OH
CH2OH
D-glucose
D-fructose
Notasi D vs L
CH O
Notasi D & L dilakukan
karena adanya atom C
H C OH
dengan konfigurasi
CH2OH
asimetris seperti pada
D-gliseraldehida
gliseraldehida.
CH O
HO
H
C
OH
CH2OH
D-glyceraldehyde
H
CH2OH
L-gliseraldehida
CH O
Penampilan dalam
bentuk gambar
bagian bawah disebut
Proyeksi Fischer.
C
CH O
HO
C
H
CH2OH
L-gliseraldehida
Penamaan Gula
Untuk gula dengan
atom C asimetrik lebih
dari 1, notasi D atau L
ditentukan oleh atom
C asimetrik terjauh
dari gugus aldehida
atau keto.
Gula yang ditemui di
alam adalah dalam
bentuk isomer D.
O
H
C
H – C – OH
HO – C – H
H – C – OH
H – C – OH
CH2OH
D-glukosa
O
H
C
HO – C – H
H – C – OH
HO – C – H
HO – C – H
CH2OH
L-glukosa
Gula dalam bentuk D
merupakan bayangan
cermin dari gula dalam
bentuk L.
Kedua gula tersebut
memiliki nama yang
sama, misalnya Dglukosa & L-glukosa.
O
H
C
H – C –OH
HO – C –H
H – C – OH
H – C – OH
CH2OH
D-glukosa
O
H
C
HO – C – H
H – C – OH
HO – C – H
HO – C – H
CH2OH
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
bereaksi
dengan alkohol
membentuk
hemiasetal.
Keton dapat
bereaksi
dengan alkohol
membentuk
hemiketal.
H
C
H
O
+
R'
OH
R'
O
R
OH
R
aldehida
alkohol
hemiasetal
R
C
C
R
O
+
"R
OH
R'
keton
"R
O
C
R'
alkohol
hemiketal
OH
Pentosa dan
heksosa dapat
membentuk struktur
siklik melalui reaksi
gugus keton atau
aldehida dengan
gugus OH dari atom
C asimetrik terjauh.
Glukosa membentuk
hemiasetal intramolekular sebagai
hasil reaksi aldehida
dari C1 & OH dari
atom C5, dinamakan
cincin piranosa.
1
H
HO
H
H
2
3
4
5
6
CHO
C
OH
C
H
D-glukosa
C
OH
(bentuk linier)
C
OH
CH2OH
6 CH2OH
6 CH2OH
5
H
4
OH
H
OH
3
H
O
H
H
1
2
OH
a-D-glukosa
OH
5
H
4
OH
H
OH
3
H
O
OH
H
1
2
H
OH
b-D-glukosa
Penampilan dalam bentuk gula siklik disebut proyeksi Haworth.
CH2OH
1
HO
H
H
2C
O
C
H
C
OH
C
OH
3
4
5
HOH2C 6
6 CH2OH
D-fruktosa (linear)
H
5
H
1 CH2OH
O
4
OH
HO
2
3
OH
H
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 H2OH
5
H
4
OH
H
OH
3
H
2
O
H
H
1
2
OH
a-D-glukosa
OH
5
H
4
OH
H
OH
3
H
O
OH
H
1
2
H
OH
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
HO
HO
H O
HO
H
HO
5
3
H
H
2
H
OH 1
OH
a-D-glukopiranosa
H
OH
OH
H
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
CH O
COOH
CH2OH
H
C
OH
H
C
OH
HO
C
H
H
C
OH
HO
C
H
H
C
OH
H
C
OH
H
C
OH
H
C
OH
H
C
OH
H
C
OH
CH2OH
D-ribitol
CH2OH
Asam D-glukonat
COOH
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
HO
C
H
H
C
H
C
H
C
OH
HO
C
H
OH
H
C
OH
OH
H
C
OH
CH2OH
D-glucose
Oksidator
CH2 O H
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
Glukosa +
Cu++
panas & alk . pH
Gluconic acid + Cu2O (Cu2O is insol ppt)
glukosa oksidase
Glukosa + O2
Asam glukonat + H2O2
(H2O2 nya diukur)
Glukosa + ATP
heksokinase
Glukosa-6-P + ADP (G-6-Pnya diukur)
Turunan gula
CH 2OH
CH2OH
O
H
H
OH
H
H
OH
H
OH
OH
H
O
H
NH 2
a-D-glukosamina
H
H
O OH
OH
H
N
C
CH 3
H
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
H2O
H O
HO
HO
H
H
H
+
CH3- OH
H O
HO
HO
H
OH
H
OH
a-D-glukopiranosa
metanol
H
OH
OCH3
Metil-a-D-glukopiranosa
Disaccharides:
Maltose, a cleavage
product of starch
(e.g., amylose), is a
disaccharide with an
a(1 4) glycosidic
link between C1 - C4
OH of 2 glucoses.
It is the a anomer
(C1 O points down).
6 CH2OH
6 CH2OH
H
5
O
H
OH
4
OH
3
H
H
H
1
H
4
4
maltose
OH
H
H
1
OH
2
H
OH
2OH
H
H
1
O
4
5
O
H
OH
H
H
3
H
6 CH
O
H
OH
H
OH
3
OH
5
O
O
2
6 CH2OH
H
5
2
OH
3
cellobiose
H
OH
1
2
OH
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
H
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
H
O
H
OH
H
H
H
1
O
OH
6CH OH
2
5
O
H
4 OH
3
H
OH
H
H
H
H 1
O
H
OH
CH2OH
CH2OH
CH2OH
H
H
H
O
H
OH
H
O
O
H
H
O
H
OH
H
H
O
OH
2
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.
CH2OH
CH2OH
O
H
H
OH
H
H
OH
H
O
OH
CH2OH
H
H
OH
H
H
OH
H
H
OH
CH2OH
O
H
OH
O
H
OH
H
H
O
O
H
OH
H
H
OH
H
H
O
4
amylopectin
H
1
O
6 CH2
5
H
OH
3
H
CH2OH
O
H
2
OH
H
H
1
O
CH2OH
O
H
4 OH
H
H
H
H
O
OH
O
H
OH
H
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
O
H
H
OH
H
H
OH
H
O
OH
CH2OH
H
H
OH
H
H
OH
H
H
OH
CH2OH
O
H
OH
O
H
OH
H
H
O
O
H
OH
H
H
OH
H
H
O
4
glycogen
H
1
O
6 CH2
5
H
OH
3
H
CH2OH
O
H
2
OH
H
H
1
O
CH2OH
O
H
4 OH
H
H
H
H
O
OH
O
H
OH
H
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
H
O
H
OH
H
OH
H
1
O
H
H
OH
6CH OH
2
5
O
H
4 OH
3
H
H
H 1
2
OH
O
O
H
OH
CH2OH
CH2OH
CH2OH
H
H
O
O
H
OH
H
OH
O
H
O
H
OH
H
OH
OH
H
H
H
H
H
H
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 and inter-chain H-bonds
This
promotes
intra-chain
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
H
O
H
OH
H
OH
H
1
O
H
H
OH
6CH OH
2
5
O
H
4 OH
3
H
H
H 1
2
OH
O
O
H
OH
CH2OH
CH2OH
CH2OH
H
H
O
O
H
OH
H
OH
O
H
O
H
OH
H
OH
OH
H
H
H
H
H
H
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.
Tabel : Beberapa Uji Karbohidrat
-------------------------------------------------------------------------------------------Jenis KH
: Molisch
: Fehling/Benedict : Fermentasi
-------------------------------------------------------------------------------------------Glukose
:
+
:
+
:
+
Galaktosa
:
+
:
+
:
Fruktosa
:
+
:
:
+
Laktosa
:
+
:
+
:
Sukrose
:
+
:
:
+
Maltosa
:
+
:
+
:
+
Pati
:
+
:
:
Glikogen
:
+
:
:
+
CH 2OH
D-glucuronate
6

6COO
H
4
5
H
OH
3
H
H
2
OH
1
H
H
OH
O
O
H
4
O
H
5
3
H
2
1 O
H
NHCOCH 3
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
CH 2OH
D-glucuronate
6

6COO
H
4
5
H
OH
3
H
H
2
OH
1
H
H
OH
O
O
H
4
O
H
5
3
H
2
1 O
H
NHCOCH 3
N-acetyl-D-glucosamine
hyaluronate
Hyaluronate is a glycosaminoglycan with a
repeating disaccharide consisting of 2 glucose
derivatives, glucuronate (glucuronic acid) & N-acetylglucosamine.
The glycosidic linkages are b(13) & b(14).
CH 2OH
D-glucuronate
6COO
H
4
6

5
H
OH
3
H
H
2
OH
1
H
H
OH
O
O
H
4
O
H
5
3
H
2
1 O
H
NHCOCH 3
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.
CH 2OH
D-glucuronate
6COO
H
4
6

5
H
OH
3
H
H
2
OH
1
H
H
OH
O
O
H
4
O
H
5
3
H
2
1 O
H
NHCOCH 3
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
N-sulfo-glucosamine-6-sulfate
iduronate-2-sulfate
CH2OSO3
H
H
COO
OH
O
O
H
O
H
H
OH
H
H
H
H
OSO3
O
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-
core
protein
heparan sulfate
glycosaminoglycan
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
CH2
CH
O
O
Oligosaccharides H
H
NH serine
that are covalently
H
OH
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.
C
CH2OH
O
H
H
OH
O
CH2
CH
NH
H
O
serine
residue
O H
OH
H
HN
C
CH3
b-D-N-acetylglucosamine
N-acetylglucosamine (GlcNAc) is a common Olinked 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
O
H
H
OH
HN
C
HN
CH2
C
H
H
OH
H
HN
C
CH3
O
N-acetylglucosamine
Initial sugar in N-linked
glycoprotein oligosaccharide
Asn
CH
O
HN
HC
R
C
O
X
HN
HC
R
C
O
Ser or Thr
N-linked oligosaccharides of glycoproteins tend to
be complex and branched. First Nacetylglucosamine 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
Man
Key:
NAG
NAG
Asn
N-linked oligosaccharide
Fuc
NAN = N-acetylneuraminate
Gal = galactose
NAG = N-acetylglucosamine
Man = mannose
Fuc = fucose
Additional monosaccharides are added, and the Nlinked oligosaccharide chain is modified by removal
and addition of residues, to yield a characteristic
branched structure.
Many proteins secreted by cells have attached Nlinked 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
proteins of mammalian
cell plasma membranes
with roles in cell-cell
recognition & binding.
A lectin-like domain is at
the end of an extracellular
segment that extends out
from the cell surface.
selectin
lectin domain
outside
cytosol
transmembrane
a-helix
cytoskeleton
binding domain
A cleavage site just outside the transmembrane ahelix provides a mechanism for regulated release of
some lectins from the cell surface.
A cytosolic domain participates in regulated
interaction with the actin cytoskeleton.