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Isfahan University of Technology. Advance Biochemistry. Part 1: Carbohydrates (continue) Prepared by: Dr A. Riasi ( Isfahan University of Technology ) Reference: Lehninger Biochemistry. Polysaccharides. Polysaccharides or named glycans . The glycans are differ from each other in:

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Isfahan University of Technology

Advance Biochemistry

Part 1:

Carbohydrates (continue)

Prepared by:

Dr A. Riasi ( Isfahan University of Technology)


Lehninger Biochemistry


  • Polysaccharides or named glycans.

  • The glycans are differ from each other in:

    • The identity of their recurring monosaccharide units

    • In the length of their chains

    • In the types of bonds linking the units

    • In the degree of branching.


  • Homopolysaccharides are divided to two types:

    • Storage forms of monosaccharides:

      • Starch

      • Glycogen

    • Structural elements:

      • Cellulose

      • Chitin


  • Heteropolysaccharides provide extracellular support for organisms of all kingdoms.

  • In animal tissues, the extracellular space is occupied by several types of heteropolysaccharides.


  • Both storage polysaccharides (starch & glycogen) occur intracellularly as large clusters or granules.


  • Starch contains two types of glucose polymer:

    • Amylose

    • Amylopectin


a -1,6-glycosidic bond forms at approximately every 10 glucose units, making glycogen a highly branched molecule.


  • Concentration of starch in different feed materials.


  • In vitro digestion of starch from sorghum and maize genotypes varying in the ratio amylose:amylopectin


An electron micrograph of starch granules in the endosperm of sorghum showing the protein matrix with embedded protein bodies surrounding each granule. Indentations from the protein bodies can be seen on the starch granules.

A scanning electron micrograph of the endosperm of barley showing starch granules of various sizes embedded in a protein matrix containing numerous protein bodies.


  • Glycogen is the main storage polysaccharide of animal cells.

  • Glycogen is a polymer of (1-4) linked subunits of glucose, with (1-6) linked branches.


  • It has been calculated that hepatocytes store glycogen equivalent to a glucose concentration of 0.4 M.

  • However, the actual concentration of glycogen, which is insoluble and contributes little to the osmolarity of the cytosol, is about 0.01 µM.


  • Furthermore, with an intracellular glucose concentration of 0.4 M and an external concentration of about 5 mM, the free-energy change for glucose uptake into cells against this very high concentration gradient would be prohibitively large.


  • Dextrans are bacterial and yeast polysaccharides made up of (α1 → 6) linked poly-D-glucose;


  • Synthetic dextrans are used in several commercial products for example, Sephadex.


  • Some homopolysaccharides serve structural roles:

    • Cellulose

    • Chitin


  • Cellulose is found in the cell walls of plants, particularly in:

    • Stalks

    • Stems

    • Trunks

    • All the woody portions of the plant body


  • Wood-rot fungi and bacteria also produce cellulase


  • Chitin is a linear homopolysaccharide.

Special polysaccharides

  • Some polysaccharides have special properties and found in plant cell wall:

    • Pectin

    • Hemicellulose

Special polysaccharides


Monomer: D-galacturonic acid, L-rhamnose

Others: D-galactose, D-xylose,

D-arabinose short side chain)

Bonding: -1,4

Pectin hga
Pectin (HGA)

Special polysaccharides

Pectin rgi
Pectin (RGI)

Special polysaccharides

Pectin rgii
Pectin (RGII)

Special polysaccharides

Calcium pectate
Calcium Pectate

Special polysaccharides

Special polysaccharides

Guar gum

Monomer: galactose, mannose (galactomannan)

Bonding: -1,6/-1,4

Special polysaccharides

  • The folding of polysaccharides in three dimensions follows the same principles as those governing polypeptide structure.

  • Because polysaccharides have so many hydroxyl groups, hydrogen bonding has an especially important influence on their structure.

Special polysaccharides

  • There is free rotation about both C-O bonds linking the residues, but as in polypeptides rotation about each bond is limited by steric hindrance by substituent.

Special polysaccharides

  • The most stable three-dimensional structure for starch and glycogen is a tightly coiled helix, stabilized by interchain hydrogen bonds.

  • Each residue along the amylose chain forms a 60 angle with the preceding residue.

Special polysaccharides

  • For cellulose, the most stable conformation is that in which each chair is turned 180 relative to its neighbors, yielding a straight, extended chain.

  • All –OH groups are available for hydrogen bonding with neighboring chains.


  • Why the bacteria are not ruptured in solution with different osmotic pressures?

  • Heteropolysaccharides are the rigid component of bacterial cell walls.

  • The components are made of alternating (β1→4) linked N-acetylglucosamine and N-acetylmuramic acid residues.


  • How penicillin and related antibiotics kill bacteria?


  • Agar is a sulfated heteropolysaccharides made up of D-galactose and an L-galactose derivative ether-linked between C-3 and C-6.


  • Two major components of agar are:

    • Agarose

    • Agaropectin


  • The extracellular matrix of animal cells is composed of an interlocking meshwork of heteropolysaccharides and fibrous proteins such as collagen, elastin, fibronectin, and laminin.


  • These heteropolysaccharides are named glycosaminoglycans.

  • One of the two monosaccharides is always either N-acetylglucosamine or N-acetylgalactosamine.

  • The other is in most cases a uronic acid, usually D-glucuronic or L-iduronic acid.


  • Other glycosaminoglycans differ from hyaluronate in two respects:

    • They are generally much shorter polymers

    • They are covalently linked to specific proteins

    • One or two monosaccharide subunit is not the same with hyaluronate


  • Glycosaminoglycans which are attached to extracellular proteins named proteoglycans.


  • Glycoconjugates are carbohydrates which have some important biological roles.

  • Glycoconjugates are devided to three groups:

    • Proteoglycans: glycosaminoglycans + proteins

    • Glycoproteins: oligosaccharides + proteins

    • Glycolipids: oligosaccharides + membrane lipids


  • Proteoglycans are macromolecules found in:

    • The cell surface

    • The extracellular matrix


  • The glycosaminoglycan moiety commonly forms the greater fraction of the proteoglycan molecule, dominates the structure, and is often the main site of biological activity.


  • Glycoproteins are found on the outer face of the plasma membrane, in the extracellular matrix, inside the cells, and in the blood.


  • Glycoproteins have some properties as follow:

    • The carbohydrate moieties of glycoproteins are smaller and more structurally diverse than the glycosaminoglycans of proteoglycans.

    • The carbohydrate moiety of glycoproteins are rich in information and forming highly specific sites for recognition and high-affinity binding by other proteins.


  • The carbohydrate attachments are devided to two groups:

    • O-linked: A glycosidic bond between the anomeric carbon with the -OH of a Ser or Thr residue.

    • N-linked: A N-glycosyl link between the anomeric carbon and the amide nitrogen of an Asn residue.

Glycolipids and lipoglycans

  • Glycolipids and lipoglycans like glycoproteins, act as specific sites for recognition by carbohydrate- binding proteins.


  • Glycobiology is the study of structure and function of glycoconjugates.

  • It is one of the most active and exciting areas of biochemistry and cell biology.


  • As is becoming increasingly clear, cells use specific oligosaccharides to encode important information about intracellular targeting of proteins:

    • Cell-cell interaction

    • Tissue development

    • Extracellular signals


  • In glycobiology, lectins are proteins that read the sugar code and mediate many biological processes.

  • Lectins, found in all organisms, are proteins that bind carbohydrates with high affinity and specificity.


  • Lectins serve in a wide variety of cell-cell recognition, signaling, and adhesion processes and in intracellular targeting of newly synthesized proteins.

  • In the laboratory, purified lectins are useful reagents for detecting and separating glycoproteins with different oligosaccharide moieties.


  • Some peptide hormones that circulate in the blood have oligosaccharide moieties that strongly influence their circulatory half-life.


  • Luteinizing hormone (LH) and thyrotropin have N-linked oligosaccharides that end with the disaccharide, which is recognized by a lectin (receptor) of hepatocytes.

  • Receptor-hormone interaction mediates the uptake and destruction of luteinizing hormone and thyrotropin, reducing their concentration in the blood.


  • The residues of Neu5Ac (a sialic acid) situated at the ends of the oligosaccharide chains of many plasma glycoproteins protect those proteins from uptake and degradation in the liver.

  • For example, ceruloplasmin, a copper-containing serum glycoprotein, has several oligosaccharide chains ending in Neu5Ac.