KARBOHIDRAT * Reaksi monosakarida* Ikatan glikosida* Fungsi karbohidrat

http://114.141.57.234/wagiman/

KARBOHIDRAT :

Karbon dan hidrat

Senyawa yang rumus emphirisnya Cm(H2O)n

PEMBENTUKAN KARBOHIDRAT

Karbohidrat terjadi pada proses fotosintesis

mCO2 (g)+ nH2O (l) Cm(H2O)n(s) + mO2(g)klorofil

UV

PEMBAGIAN KARBOHIDRAT

Monosakarida glukosa,fruktosa, galaktosa (Karbohidrat paling sederhana) Disakarida maltosa, sukrosa dan laktosa (Karbohidrat yg terdiri dari 2 monosakarida) Oligosakarida : dekstrin (Karbohidrat yang terdiri dari beberapa monosakarida) Polisakarida amilum,glikogen dan selulosa (Karbohidrat yg terdiri dari banyak monosakarida)

Monosakarida

Memiliki atom karbon 3 sampai 7 Setiap atom karbon memiliki gugus

hidroksi dan keton atau aldehida. Setiap molekul monosakarida memiliki

1 gugus keton (ketosa) atau 1 gugus aldehida (aldosa)

Gugus aldehida selalu berada di atom C pertama

Gugus keton selalu berada di atom C kedua

Nama mono sakarida dengan akhiran osa

MonosakaridaAldosa (mis: glukosa) memiliki gugus aldehida pada salah satu ujungnya.

Ketosa (mis: fruktosa) biasanya memiliki gugus keton pada atom C2.

C

C OHH

C HHO

C OHH

C OHH

CH2OH

D-glucose

OH

C HHO

C OHH

C OHH

CH2OH

CH2OH

C O

D-fructose

Jika di sebelah kiri L

Notasi D vs L

Notasi D & L dilakukan karena adanya atom C dengan konfigurasi asimetris seperti pada gliseraldehida.

CH O

C

CH2OH

H OH

CH O

C

CH2OH

HO H

CH O

C

CH2OH

HO H

L-gliseraldehidaD-glyceraldehyde

L-gliseraldehida

CH O

C

CH2OH

H OH

D-gliseraldehida

Penampilan dalam bentuk gambar bagian bawah disebut Proyeksi Fischer.

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 keton.

Gula yang ditemui di alam adalah dalam bentuk isomer D.

O H O H C C H – C – OH HO – C – H HO – C – H H – C – OH H – C – OH HO – C – H H – C – OH HO – C – H CH2OH CH2OH

D-glukosa L-glukosa

Gula dalam bentuk D merupakan bayangan cermin dari gula dalam bentuk L. Kedua gula tersebut memiliki nama yang sama, misalnya D-glukosa & L-glukosa.

O H O H C C H – C – OH HO – C – H HO – C – H H – C – OH H – C – OH HO – C – H H – C – OH HO – C – H CH2OH CH2OH D-glukosa L-glukosa

Stereoisomers lainnya memiliki names yang unik, misalnya glukosa, manosa, galaktosa, dll. Jumlah stereoisomer adalah n2, 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.

C

R

R'

O

keton

aldehida

C

H

R

O

hemiasetal

O C

H

R

OHR'

alkohol

R' OH

hemiketal

O C

R

R'

OH" R

+

+ " R OH

alkohol

Pentosa dan heksosa dapat membentuk struktur siklik melalui reaksi gugus keton atau aldehida dengan gugus OH dari atom C asimetrik terjauh.Glukosa membentuk hemiasetal intra-molekular sebagai hasil reaksi aldehida dari C1 & OH dari atom C5, dinamakan cincin piranosa( segi 6). Penampilan dalam bentuk gula siklik disebut proyeksi Haworth

Perubahan FisherKe Hawort, gugusKiri di Fisher ditulisDi atas cincin dansebaliknya

H

OH

H

OHH

OH

CH2OH

H

H23

4

5

6

1

HC+

OH

C HHO

C OHH

C

O-H

H

CH OH

2

3

6

4

5

C

2

1

D - Glukosa

O- H+

Struktur FicherStruktur siklohemiasetal

H

CH O

OH

C HHO

C OHH

C

O

H

H

CH OH

2

3

6

4

5

C

2

1

α – D - Glukosa

O-H+

O-+

H O

OH

H

OHH

OH

CH2OH

H

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D- glukosa -D- glukosa

23

4

5

6

1 1

6

5

4

3 2

HC+

OH

C HHO

C OHH

C

O-H

H

CH OH

2

3

6

4

5

C

2

1

D - Glukosa

O- H+

Struktur Ficher Struktur siklohemiasetal

H

CH O

OH

C HHO

C OHH

C

O

H

H

CH OH

2

3

6

4

5

C

2

1

α – D - Glukosa

H

CH

OH

C HHO

C OHH

C

O

H

H

CH OH

2

3

6

4

5

C

2

1

β – D - Glukosa

O

H

CH O

OH

C HHO

C OHHO

C

H

H

CH OH

2

3

6

4

5

C

2

1

O

α. D galaktosaH OO

H

H

OHH

OH

CH2OH

H

OH

H

23

4

5

6

1

Monosakarida dapat membentuk Cincin piranosa, melalui reaksi antara gugus

aldehid atom C1 dengan OH dari C5. Cincin furanosa, melalui reaksi antara gugus keto

atom C2 dengan OH dari C5.

Pembentukan cincin siklik glukosa menghasilkan pusat asimetrik baru pada atom C1. Kedua stereoisomer disebut anomer, & .

Proyeksi Haworth menunjukkan bentuk cincin dari gula dengan perbedaan pada posisi OH di C1 anomerik :

(OH di bawah struktur cincin) (OH di atas struktur cincin).

H O

OH

H

OHH

OH

C H2OH

H

-D-glukosa

OH

H H O

OH

H

OHH

OH

C H2OH

H

H

OH

-D-glukosa

23

4

5

6

1 1

6

5

4

3 2

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.

O

H

HO

H

HO

H

OHOHH

H

OH

O

H

HO

H

HO

H

HOHH

OH

OH

-D-glukopiranosa -D-glukopiranosa

1

6

5

4

32

Turunan gula

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.

CH2OH

C

C

C

CH2OH

H OH

H OH

H OH

D-ribitol

COOH

C

C

C

C

H OH

HO H

H OH

Asam D-glukonat Asam D-glukuronat

CH2OH

OHH

CH O

C

C

C

C

H OH

HO H

H OH

COOH

OHH

Oksidasi gula aldehida

C

C OHH

C HHO

C OHH

C OHH

CH2OH

D-glucose

OH

Oksidator

Asam D-glukonat

CO O H

C

C

C

C

H OH

HO H

H OH

CH2O H

OHH

Gula yang dapat dioksidasi adalah senyawa pereduksi. Gula yang demikian disebut sebagai gula pereduksi.

Senyawa yang sering digunakan sebagai pengoksidasi adalah ion Cu+2,terdapat dalam reagen fehling , campuran fehling A (larutan CuSO4) dan fehling B Larutan KNa – tartrat. yang berwarna biru cerah, yang akan tereduksi menjadi ion Cu+, yang berwarna merah kusam. Hal ini menjadi dasar bagi pengujian fehling/Benedict yang digunakan untuk menentukan keberadaan glukosa dalam urin, suatu pengujian bagi diagnosa diabetes.

Oksidasi gula aldehida

ISOMERI OPTIS Cahaya bergetar ke segala arah, jika cahaya tsb

dilewatkan pada kaca nikol hanya akan memiliki satu arah dinamakan cahaya yang terpolarisasi/terkukup. Kaca nikol disebut polarisator. Jika suatu zat dilewati cahaya terpolarisasi arah bidangnya diputar zat tersebut dinamai zat optis aktif. Zat bersifat optis aktif minimal memiliki 1 atom C khiral/asimetris, ke 4 tangannya mengikat gugus yang berbeda.

MUTAROTASI

Glukosa yang baru dilarutkan jika diukur sudut putarnya terhadap bidang polarisasi cahaya : 112o tetapi setelah didiamkam beberapa lama sudut putarnya menjadi 52,6 o . Peristiwa perubahan besarnya sudut ini disebut mutarotasi.

Oksidasi gula aldehida

Glukosa + Cu++

Gluconic acid + Cu2O (Cu2O is insol ppt)

Glukosa + O2

Asam glukonat + H2O2 (H2O2 nya diukur)

Glukosa + ATP

Glukosa-6-P + ADP (G-6-Pnya diukur)

panas & alk . pH

glukosa oksidase

heksokinase

Sifat2 mono sakaridaglukosa fruktosa galaktosa

Putar kanan Putar kiri Putar kanan

Mereduksi fehling

Mereduksi fehling Mereduksi fehling

Mengalami fermentasi

mengalami fermentasi

Tidak mengalami fermentasi

Mengalami mutarotasi

Gula amino - gugus amino menggantikan gugus hidroksil. Sebagai contoh glukosamina. Gugus amino dapat mengalami asetilasi, seperti pada N-asetilglukosamina.

Turunan gula

H O

OH

H

OH

H

NH 2H

OH

CH2OH

H

-D-glukosamina

H O

OH

H

OH

H

NH

OH

CH2OH

H

-D-N-asetilglukosamina

C CH 3

O

H

Disakarida

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' + H2OMisalnya pada maltosa gabungan 2 molekul glukosa gugus OH no. 4 dengan OH no. 1 dari glukosa yang berbeda membentuk ikatan glukosida.

O

H

HO

H

HO

H

OHOHH

H

OH

-D-glukopiranosa

O

H

HO

H

HO

H

OCH3

OHHH

OH

Metil--D-glukopiranosa

CH3- OH+

metanol

H2O

Struktur disakarida

Cellobiose, a product of cellulose breakdown, is the otherwise equivalent anomer (O on C1 points up). The (1 4) glycosidic linkage is represented as a zig-zag, but one glucose is actually flipped over relative to the other.

H O

O H

H

O HH

O H

CH 2O H

HO H

O H

H

O HH

O H

CH 2O H

H

O

HH

1

23

5

4

6

1

23

4

5

6

m altose

H O

O H

H

O HH

O H

CH 2O H

HO O H

H

H

O HH

O H

CH 2O H

H

H

H

O1

23

4

5

6

1

23

4

5

6

cellobiose

DisakaridaMaltosa, pemecahan produk pati (amylose), sebagai disakarida dengan ikatan (1 4) glucosida ikatan antara C1 - C4 OH dari 2 molekul glukosa.

Sifat2 DisakaridaMaltose

( gula pati)Sukrosa

(gula tebu)Laktosa

(gula susu)Putar kanan Putar kanan Putar kanan

Mereduksi fehling Tidak mereduksi fehling

Mereduksi fehling

Mengalami fermentasi

Tidak mengalami fermentasi

Tidak mengalami fermentasi

Mengalami inversi

Sukrosa(putar kanan)

+ H2O Glukosa(putar kanan)

+ Fruktosa(putar kiri yg lebih kuat

Campuran berputar kekiriPerubahan arah disebut inversi

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 (O points down from ring), the linkage is (12).

The full name of sucrose is -D-glucopyranosyl-(12)--D-fructopyranose.)

Lactose, milk sugar, is composed of galactose & glucose, with (14) linkage from the anomeric OH of galactose. Its full name is -D-galactopyranosyl-(1 4)--D-glucopyranose

Polysaccharides

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 (14) 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.

H O

OHH

OHH

OH

CH 2 OH

HO H

H

OHH

OH

CH 2 OH

H

O

HH H O

OH

OHH

OH

CH 2 OH

HH H O

H

OHH

OH

CH 2 OH

H

OH

HH O

OH

OHH

OH

CH 2 OH

H

O

H1

6

5

4

3

1

2

a m y lo s e

Amylopectin is a glucose polymer with mainly (14) linkages, but it also has branches formed by (16) linkages. Branches are generally longer than shown above.The branches produce a compact structure & provide multiple chain ends at which enzymatic cleavage can occur.

H O

OHH

OHH

OH

CH2OH

HO H

H

OHH

OH

CH2OH

H

O

HH H O

OH

OHH

OH

CH2

HH H O

H

OHH

OH

CH2OH

H

OH

HH O

OH

OHH

OH

CH2OH

H

O

H

O

1 4

6

H O

H

OHH

OH

CH2OH

HH H O

H

OHH

OH

CH2OH

HH

O1

OH

3

4

5

2

amylopectin

Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin. But glycogen has more (16) 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.

H O

OHH

OHH

OH

CH 2OH

HO H

H

OHH

OH

CH 2OH

H

O

HH H O

OH

OHH

OH

CH 2

HH H O

H

OHH

OH

CH 2OH

H

OH

HH O

OH

OHH

OH

CH 2OH

H

O

H

O

1 4

6

H O

H

OHH

OH

CH 2OH

HH H O

H

OHH

OH

CH 2OH

HH

O1

OH

3

4

5

2

glycogen

Cellulose, a major constituent of plant cell walls, consists of long linear chains of glucose with (14) linkages.Every other glucose is flipped over, due to the linkages. This promotes intra-chain and inter-chain H-bonds and

c e l lu lo s e

H O

OHH

OHH

OH

CH 2 OH

HO

H

OHH

OH

CH 2 OH

HO

H H O

O H

OHH

OH

CH 2 OH

HH O

H

OHH

OH

CH 2 OH

H

H

OHH O

O H

OHH

OH

CH 2 OH

HO

H H H H

1

6

5

4

3

1

2

van der Waals interactions, that cause cellulose chains to be straight & rigid, and pack with a crystalline arrangement in thick bundles called microfibrils. Botany online website

Schematic of arrangement of cellulose chains in a microfibril.

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.Explore and compare structures of amylose & cellulose using Chime.

c e l lu lo s e

H O

OHH

OHH

OH

CH 2 OH

HO

H

OHH

OH

CH 2 OH

HO

H H O

O H

OHH

OH

CH 2 OH

HH O

H

OHH

OH

CH 2 OH

H

H

OHH O

O H

OHH

OH

CH 2 OH

HO

H H H H

1

6

5

4

3

1

2

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 groups.

H O

H

H

O HH

O H

COO

H

H O

O H H

H

NH COCH 3H

CH 2O H

H

OO

D -g lucuronate

O

1

23

4

5

61

23

4

5

6

N -acetyl-D -g lucosam inehyaluronate

Hyaluronate is a glycosaminoglycan with a repeating disaccharide consisting of 2 glucose derivatives, glucuronate (glucuronic acid) & N-acetyl-glucosamine.

The glycosidic linkages are (13) & (14).

H O

H

H

O HH

O H

COO

H

H O

O H H

H

NH COCH 3H

CH 2O H

H

OO

D -glucuronate

O

1

23

4

5

61

23

4

5

6

N -acetyl-D -g lucosam inehyaluronate

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.

H O

H

H

O HH

O H

COO

H

H O

O H H

H

NH COCH 3H

CH 2O H

H

OO

D -glucuronate

O

1

23

4

5

61

23

4

5

6

N -acetyl-D -g lucosam inehyaluronate

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 versican.

H O

H

H

O HH

O H

COO

H

H O

O H H

H

NH COCH 3H

CH 2O H

H

OO

D -glucuronate

O

1

23

4

5

61

23

4

5

6

N -acetyl-D -g lucosam inehyaluronate

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 may be deacetylated and/or sulfated.

H O

H

OSO3H

OH

H

COOO H

H

NHSO3H

OH

CH2OSO3

H

H

H

O

O

heparin or heparan sulfate - examples of residues

iduronate-2-sulfate N-sulfo-glucosamine-6-sulfate

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 with the protein antithrombin.

Heparin has an extended helical conformation.

heparin: (IDS-SGN)5

PDB 1RID

C  O  N  SCharge repulsion by the many negatively charged groups may contribute to this conformation.

Heparin shown has 10 residues, alternating IDS (iduronate-2-sulfate) & SGN (N-sulfo-glucosamine-6-sulfate).

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.

heparan sulfate glycosaminoglycan

cytosol

core protein

transmembrane -helix

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.

H O

OH

O

H

HNH

OH

CH2OH

H

C CH3

O

-D-N-acetylglucosamine

CH2 CH

C

NH

O

H

serine residue

Oligosaccharides that are covalently attached to proteins or to membrane lipids may be linear or branched chains.

H O

OH

O

H

HNH

OH

CH2OH

H

C CH3

O

-D-N-acetylglucosamine

CH2 CH

C

NH

O

H

serine residue

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 alternates with phosphorylation, with these 2 modifications having opposite regulatory effects (stimulation or inhibition).

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 particular 3-amino acid sequence.

H O

OH

HN

H

H

HNH

OH

CH2OH

H

C CH3

O

C CH2 CH

O HN

C

HN

O

HC

C

HN

HC

R

O

C

R

O

Asn

X

Ser or ThrN-acetylglucosamine

Initial sugar in N-linked glycoprotein oligosaccharide

Additional monosaccharides are added, and the N-linked oligosaccharide chain is modified by removal and addition of residues, to yield a characteristic branched structure.

NAN

Gal

NAG

Man

NAG

Gal

NAN

Man

Man

NAG

Gal

NAN

NAG

NAG

Asn

Fuc

N-linked oligosaccharide

Key:NAN = N-acetylneuraminateGal = galactoseNAG = N-acetylglucosamineMan = mannoseFuc = fucose

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.

They have roles in cell-cell interaction and signaling, and in forming a protective layer on the surface of some cells.

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 immune response.

A cleavage site just outside the transmembrane -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.

transmembrane -helix

lectin domain selectin

cytoskeleton binding domain

cytosol

outside

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.