Chapter 11

Carbohydrates

Carbohydrates are aldehydes or ketone with multiple hydroxyl groups

<Role of Carbohydrate>

• Energy stores, fuels, and metabolic intermediates

• Structural framework of DNA and RNA• Structural elements in the cell walls of

bacteria and plans• Linked to many proteins and lipids (cell-

cell communication)

Carbohydrates as mediators of cellular interactions

• Structural diversity in size and stereochemical configuration

• Built from monosaccharides• Attach to lipids and proteins

Glycomics: Studies on oligosaccharide structures and functions

11.1 Monosaccharides are aldehydes or ketones with multiple hydroxyl group

-Monosaccharides : the simplest carbohydrate.

Important fuel, building blocks for nuclearic acid

-The smallest monosaccharides:

Ketone group

Aldehyde group

Stereoisomer

- Enantiomer = mirror images of each other

※ Fischer projection -Horizontal bond : in frond of the page-Vertical bond : behind of the page -Clear and simple views of the stereochemistry at each carbon center.

Three carbontriose

Four carbontetrose

Five carbonpentose

Six carbonhexose

D- aldose

D- ketoseThree carbon

triose

Four carbontetrose

Five carbonpentose

Six carbonhexose

Pentoses and hexoses cyclize to form furanose and pyranose rings

-The predominant forms of ribose, glucose, fructose, and

many other sugars are cyclized into rings.

-Ring forms are energetically stable.

-Aldohexose glucose : C-1 aldehyde reacts with the C-5.

-Ketohexose fructose : C-2 keto group reacts with a

hydroxyl group of C-5 or C-6.

pyranose

- α-D-glucopyranose:

Hydroxyl group is on the

opposite side of the ring from

the CH2OH.

- β-D-glucopyranose:

Hydroxyl group is on the

same side of the ring from the

CH2OH.- C-1 carbon atom : anomeric carbon atom

- α and β form : anomer

- In case of glucose : 1/3 α anomer, 2/3 β

anomer

C-1

furanose

※ Haworth projection : The

approximate plane of the ring is

perpendicular to the plane of the

paper.

-Heavy line : projecting toward the

reader.

-Easy depiction of the

stereochemistry of sugars.

- Fructose forms both pyranose and furanose ring.

Pyranose and furanose rings can assume different conformations

-The six-membered pyranose ring is not planar.

-Pyranose rings adopt two classes of conformations

-Chair : substituents on the ring carbon atoms have

two orientation ; axial and equatorial.

-Boat : glucose boat form disfavored because it is

quite sterically hindered.

-The chair form is more stable owing to less steric

hindrance because the axial positions are occupied by

hydrogen atoms.

a : axiale : equatorial

-The five-membered furanose ring is also not planar.

-Envelope form : four atoms are nearly coplanar and

the fifth is about 0.5Å away from this plane.

Monosaccharides are joined to alcohols and amines through glycosidic bonds

-Monosaccharides react with alcohols and amines to form modified monosaccharide.

-Ex> D-glucose react with methanol.

-O-glucosidic bond : OCH3

-N-glucosidic bond : NR2

Ex) Nucleotide; ribose+adenine

- Methyl glycopyranoside differ in reactivity from the

parent monosaccharides.

-Unmodified glucose reacts with oxidizing agents such as Cu2+

because the open-chain form has a free aldehyde group that is

readily oxidized.(=reducing sugar)

-Methyl glucopyranoside do not react .(=non-reducing sugar)

-Glucose react with hemoglobin to form glycosylated hemoglobin.

diabete

Fehling’s solution

- Carbohydrates can be modified by the addition of

substituents. Such modified carbohydrates are often

expressed on cell surfaces.

Phosphorylated sugars are key intermediates in energy generation and biosyntheses

- Phosphorylation is a common modification of sugars.

Ex> The first step in the breakdown of glucose to obtain energy is its

conversion into glucose 6-phosphate.

-Phosphorylation makes sugars anionic(negative charge) to prevent

sugars from leave the cell spontaneously by crossing membrane

-Reactive intermediates in purine and pyrimidine biosyntesis

11.2 Complex carbohydrates are formed by the linkage of monosaccharides

-Glycosidic bond : the linkage of two or more

monosaccharides by O-glycosidic bond.

-Various glycosidic linkage is possible.

-Structurally diverse molecules.

Sucrose, lactose, and maltose are the common disaccharides

-A transport form of carbohydrates in plants.

-Obtained commercially from cane or beet.

-Glucose + fructose

-Can be cleaved by sucrase.

-Disaccharide of milk.

-Galactose + glucose. By β-1,4-glycosidic linkage

-Can be cleaved by lactase in human being.

-Can be cleaved by β-galactosidase in bacteria.

-Glucose + glucose. By α-1,4-glycosidic bond.

-Can be cleaved by maltase.

-Sucrase, lactase, maltase are located on the outer

surfaces of epithelial cells lining the small intestine.

Microvillus

Glycogen and starch are mobilizable stores of glucose

-Polysaccharides : the linkage of multiple

monosaccharides.

-Role : energy storage and maintaining the structural

integrity.

-Homopolymer : all of the monosaccharides are the

same.

-Glycogen : storage form of glucose.

large, branched polymer.

-Branch point : α-1,6-glycosidic bond.

present about once

in 10 unit.

-Starch : nutritional reservoir of carbohydrates in

plants.

-Two types of starch = amylose and amylopectin.

-Amylose : unbranched, α-1,4-linkage.

-Amylopectin : branched, one α-1,6-linkage per 30 α-

1,4-linkages.

Cellulose, the major structural polymer of plants, consists of linear chains of glucose units

-Cellulose : major polysaccharide of glucose found in plants. structural role rather than nutritional role. most abundant organic compound in the biosphere. β-1,4-linkage of glucose = long, straight chains. Optimal for fiber having a high tense strength

α-1,4-linkage (starch and glycogen) = helix conformation. Suitable for accessible store of sugar.

- Proteoglycan : proteins attached to a particular type of

polysaccharide called glycosaminoglycan (95% by weight). As

joint lubricants and structural components in connective tissue

-Many glycosaminoglycans are anionic polysaccharide chains

made of repeating disaccharide units containing glucosamine or

galactose.

-Sugars have a negatively charged carboxylate or sulfate.

※ Aggrecan : shock aborber.

- Large protein of 2397 amino

acids.

- Three globular domain.

-The first domain binds to

glycosaminoglycanname

(=hyaluron).

Cartilage key components: aggrecan, collagen- collagen: triple helix provides structure and tensile strength.

Specific enzymes are responsible for oligosaccharide assembly

- Glycosyltransferase : catalyze the formation of the

glycosidic bonds that link monosaccharide.

intermediates

-Carbohydrates are attached to proteins and lipids on the surfaces of the

red blood cells.

-The A and B antigens are formed by the addition of one extra

monosaccharide by type A transferase and type B transferase

respectively.

-Why are different blood types present in the human population? Arms

race!

ABO blood type

11.3 Carbohydrates can be attached to proteins to form glycoproteins

-A carbohydrate group can be covalently attached to a protein to form a glycoprotein.

-Many glycoproteins are components of cell membranes.

-Sugars are attached either to the amide nitrogen atom or to the oxygen atom

N-linkage O-linkage

Attach atom Amide nitrogen atom Oxygen atom

Residue Asparagine Serine or threonine

sequence Asn-X-Ser / Asn-X-Thr no

Carbohydrates can be linked to proteins through asparagine (N-linked) or through serine or threonine(O-linked) residues

- All N-linked oligosaccharides have in common a

pentasaccharides core consisting of three mannose

and two N-acetylglucosamine residue.

※ Erythropoietin (EPO)

- Glycoprotein hormone.

- Secreted by the kidneys and

stimulates the production of

red blood cells.

- 165 amino acids.

- Three N-glycosylation and one O-glycosylation.

(40% carbohydrate by weight)

- Glycosylation of EPO enhances the stability of

the protein in blood.

- Unglycosylated EPO has only about 10% of the

bioactivity of the glycosylated. Removed by

kidney

- recombinant human EPO for anemias, Drug-

testing

Protein glycosylation takes place in the lumen of the Endoplasmic Reticulum and in the Golgi complex

- Protein glycosylation takes

place inside the lumen of

the ER and in the Golgi

complex.

-N-linked glycosylation

begins in the ER and

continues in the Golgi.

-O-linked glycosylation

takes place in the Golgi.

20X

※ Dolichol phosphate : lipid molecule located in ER

membrane and containing about 20 isoprene units.

※ Phosphate group : the site of attachment of the large

activated oligosaccharide. And is transferred to Asn residue of

the growing polypeptide chain.

Activated sugar and complex enzyme are located on the ER

lumen;

- O-inked sugar units are fashioned. - N-linked sugar are modified in many different ways.

Golgi is Major Sorting Center

Errors in glycosylation can result in pathological conditions

Ex> I-cell disease. (=mucolipidosis)

-Lysosomal storage disease.

-A carbohydrate marker directs degradative enzymes from

Golgi to lysosome.

-Patients having I-cell disease cannot attach

sugar(mannose) to degradative enzymes. Enzymes cannot

be targeted to lysosome but secreted to blood.

-Inclusion body of undigested glycosaminoglycans and

glyco lipids in lysosome.

-Psychomotor retardation and skeletal deformities.

-Glycosylation is important for the

processing, stability, and targeting of

proteins.

Lysosome target marker

Oligosaccharides can be “sequenced”

Detachment of oligosaccharide from the protein by N-glycosidase F

MALDI-TOF or other mass spectrometric techniques

Cleavage of the oligosaccharide with enzymes of varying specificities

MALDI-TOF or other mass spectrometric techniques

Repeat and collect information

Glycosylation site?

• An enormous number of patterns in the composition

and structure of surface sugars are possible

because…

1). Different monosaccharides can be joined

2). C-1 linkage can have either α and β configuration

3). Branching

Carbohydrate = information rich

molecule.

11.4 Lectins are specific carbohydrate-binding proteins

Lectins promote interactions between cells

-The diverse carbohydrate structures displayed on cell surface → cell-cell and cell-environment interaction.

※ Lectin(from the Latin “to select”) -Bind to specific carbohydrate. -Function : cell-cell contact.-Contain two or more binding sites for carbohydrate.-C type lectin : Ca2+ requiring, found in animal.

※ Selectin : member of C type family. Bind to sites of

injury in the inflammatory response.

L selectin: Lymph-node vesselE selectin: endotheliumP selectin: activated blood platelet

Embryo implantation: L selection of embryo

-Plants also are rich in lectins. Potent insecticides

-The binding specificities of lectins from plants have

been well characterized.

-E.coli bacteria adhere to the gastrointestinal tract by lectins on

the E.coli surface

Influenza virus binds to sialic acid residues

-Some viruses gain entry into host cells by adhering to cell-surface carbohydrates.Ex> influenza virus recognizes sialic acid residues through “hamagglutinin” protein. (H)

After the virus penetrates the host cell, another viral protein, “neuraminidase”(N), cleaves the glycosidic bond to free the virus.

Tamiflu and Relenza are the inhibitor of neuraminidase

Ex> H5N1(Avian Influenza), H1N1(Swine Influenza)