Part 1: Carbohydrates (continue) Prepared by: Dr A. Riasi (
Isfahan University of Technology ) Reference: Lehninger
Biochemistry Advance Biochemistry Isfahan University of
Technology
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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. Polysaccharides
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Homopolysaccharides are divided to two types: Storage forms of
monosaccharides: Starch Glycogen Structural elements: Cellulose
Chitin Polysaccharides
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Heteropolysaccharides provide extracellular support for
organisms of all kingdoms. In animal tissues, the extracellular
space is occupied by several types of heteropolysaccharides.
Polysaccharides
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Both storage polysaccharides (starch & glycogen) occur
intracellularly as large clusters or granules.
Homopolysaccharides
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Starch contains two types of glucose polymer: Amylose
Amylopectin Homopolysaccharides
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a -1,6-glycosidic bond forms at approximately every 10 glucose
units, making glycogen a highly branched molecule.
In vitro digestion of starch from sorghum and maize genotypes
varying in the ratio amylose:amylopectin GrainStarch content (g/kg)
Amylose in starch (g/kg) Starch enzyme digestion (g/kg) Sorghum
Waxy isoline630240560 Non-waxy isoline640350330
Conventional660460300 Maize Cultivar 16380550 Cultivar 2663300350
Cultivar 3586570210 Barley HB24049460818 Richard592350301
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Homopolysaccharides A scanning electron micrograph of the
endosperm of barley showing starch granules of various sizes
embedded in a protein matrix containing numerous protein bodies. 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.
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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. Homopolysaccharides
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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 M.
Homopolysaccharides
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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.
Homopolysaccharides
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Dextrans are bacterial and yeast polysaccharides made up of (1
6) linked poly-D-glucose; Homopolysaccharides
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Synthetic dextrans are used in several commercial products for
example, Sephadex. Homopolysaccharides
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Some homopolysaccharides serve structural roles: Cellulose
Chitin Homopolysaccharides
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Cellulose is found in the cell walls of plants, particularly
in: Stalks Stems Trunks All the woody portions of the plant
body
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Homopolysaccharides
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Wood-rot fungi and bacteria also produce cellulase
Homopolysaccharides
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Chitin is a linear homopolysaccharide. Homopolysaccharides
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Some polysaccharides have special properties and found in plant
cell wall: Pectin Hemicellulose Special polysaccharides
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Pectin Monomer: D-galacturonic acid, L-rhamnose
Others:D-galactose, D-xylose, D-arabinose short side chain)
Bonding:-1,4 Special polysaccharides
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Pectin (HGA) Special polysaccharides
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Pectin (RGI) Special polysaccharides
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Pectin (RGII) Special polysaccharides
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Calcium Pectate Special polysaccharides
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Guar gum Monomer: galactose, mannose (galactomannan) Bonding:
-1,6/-1,4 Special polysaccharides
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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
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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
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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
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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. Special polysaccharides
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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 (14)
linked N-acetylglucosamine and N-acetylmuramic acid residues.
Heteropolysaccharides
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How penicillin and related antibiotics kill bacteria?
Heteropolysaccharides
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Agar is a sulfated heteropolysaccharides made up of D-galactose
and an L-galactose derivative ether- linked between C-3 and C-6.
Heteropolysaccharides
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Two major components of agar are: Agarose Agaropectin
Heteropolysaccharides
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The extracellular matrix of animal cells is composed of an
interlocking meshwork of heteropolysaccharides and fibrous proteins
such as collagen, elastin, fibronectin, and laminin.
Heteropolysaccharides
Slide 50
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.
Heteropolysaccharides
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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 Heteropolysaccharides
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Glycosaminoglycans which are attached to extracellular proteins
named proteoglycans. Heteropolysaccharides
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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 Glycoconjugates
Slide 59
Proteoglycans are macromolecules found in: The cell surface The
extracellular matrix Proteoglycans
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The glycosaminoglycan moiety commonly forms the greater
fraction of the proteoglycan molecule, dominates the structure, and
is often the main site of biological activity. Proteoglycans
Slide 61
Glycoproteins are found on the outer face of the plasma
membrane, in the extracellular matrix, inside the cells, and in the
blood. Glycoproteins
Slide 62
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. Glycoproteins
Slide 63
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.
Glycoproteins
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Glycolipids and lipoglycans like glycoproteins, act as specific
sites for recognition by carbohydrate- binding proteins.
Glycolipids and lipoglycans
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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. Glycobiology
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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 Glycobiology
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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. Glycobiology
Slide 69
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. Glycobiology
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Some peptide hormones that circulate in the blood have
oligosaccharide moieties that strongly influence their circulatory
half-life. Glycobiology
Slide 71
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.
Glycobiology
Slide 72
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. Glycobiology
