Transcript section 2 - Dr. Ahmad Riasi

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:
 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
Polysaccharides
 Homopolysaccharides are divided to two types:
 Storage forms of monosaccharides:
 Starch
 Glycogen
 Structural elements:
 Cellulose
 Chitin
Polysaccharides
 Heteropolysaccharides provide extracellular support
for organisms of all kingdoms.
 In animal tissues, the extracellular space is occupied
by several types of heteropolysaccharides.
Homopolysaccharides
 Both storage polysaccharides (starch & glycogen)
occur intracellularly as large clusters or granules.
Homopolysaccharides
 Starch contains two types of glucose polymer:
 Amylose
 Amylopectin
Homopolysaccharides
a -1,6-glycosidic bond forms at approximately every 10 glucose
units, making glycogen a highly branched molecule.
Homopolysaccharides
Homopolysaccharides
 Concentration of starch in different feed materials.
Feed
Starch
(%)
Feed
Starch
(%)
Legumes
Grasses
Corn silage
Corn
Ear corn
Oats
Barley
Wheat
2-5
1-3
25-35
70-75
55-60
40-45
55-65
60-70
Beet pulp
Brewers grains
Corn gluten F
Corn gluten M
Cottenseed
Distillers grains
Soyhulls
Wheat midds
0
4-10
20-30
15
1
3-5
5
25-30
Homopolysaccharides
 In vitro digestion of starch from sorghum and maize
genotypes varying in the ratio amylose:amylopectin
Grain
Starch content
(g/kg)
Amylose in starch Starch enzyme
(g/kg)
digestion (g/kg)
630
240
560
Non-waxy isoline
640
350
330
Conventional
660
460
300
Cultivar 1
638
0
550
Cultivar 2
663
300
350
Cultivar 3
586
570
210
HB240
494
60
818
Richard
592
350
301
Sorghum
Waxy isoline
Maize
Barley
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.
Homopolysaccharides
 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
 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.
Homopolysaccharides
 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
 Dextrans are bacterial and yeast polysaccharides
made up of (α1 → 6) linked poly-D-glucose;
Homopolysaccharides
 Synthetic dextrans are
used
in
several
commercial products for
example, Sephadex.
Homopolysaccharides
 Some homopolysaccharides serve structural roles:
 Cellulose
 Chitin
Homopolysaccharides
 Cellulose is found in the cell walls of plants,
particularly in:
 Stalks
 Stems
 Trunks
 All the woody portions of the plant body
Homopolysaccharides
Homopolysaccharides
Homopolysaccharides
Homopolysaccharides
 Wood-rot fungi and bacteria also produce cellulase
Homopolysaccharides
 Chitin is a linear homopolysaccharide.
Homopolysaccharides
Special polysaccharides
 Some polysaccharides have
special properties and found in
plant cell wall:
 Pectin
 Hemicellulose
Special polysaccharides
Monomer:
Others:
Bonding:
Pectin
D-galacturonic acid, L-rhamnose
D-galactose, D-xylose,
D-arabinose short side chain)
-1,4
Special polysaccharides
Pectin (HGA)
Special polysaccharides
Pectin (RGI)
Special polysaccharides
Pectin (RGII)
Special polysaccharides
Calcium Pectate
Special polysaccharides
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
important influence on their structure.
an
especially
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
Special polysaccharides
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
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.
Special polysaccharides
Heteropolysaccharides
 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.
Heteropolysaccharides
Heteropolysaccharides
 How penicillin and related antibiotics kill bacteria?
Heteropolysaccharides
 Agar is a sulfated heteropolysaccharides made up of
D-galactose and an L-galactose derivative etherlinked between C-3 and C-6.
Heteropolysaccharides
 Two major components of agar are:
 Agarose
 Agaropectin
Heteropolysaccharides
 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
 These
heteropolysaccharides
are
named
glycosaminoglycans.
 One of the two monosaccharides is always either Nacetylglucosamine or N-acetylgalactosamine.
 The other is in most cases a uronic acid, usually Dglucuronic or L-iduronic acid.
Heteropolysaccharides
Heteropolysaccharides
 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
 Glycosaminoglycans
which
are
attached
extracellular proteins named proteoglycans.
to
Heteropolysaccharides
Heteropolysaccharides
Heteropolysaccharides
Heteropolysaccharides
Glycoconjugates
 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
 Proteoglycans are macromolecules found in:
 The cell surface
 The extracellular matrix
Proteoglycans
 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
 Glycoproteins are found on
the outer face of the plasma
membrane, in the extracellular
matrix, inside the cells, and in
the blood.
Glycoproteins
 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
 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
Glycolipids and lipoglycans
 Glycolipids and lipoglycans
like glycoproteins, act as
specific sites for recognition
by carbohydrate- binding
proteins.
Glycobiology
 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
 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
 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
 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
 Some peptide hormones that circulate in the blood
have oligosaccharide moieties that strongly influence
their circulatory half-life.
Glycobiology
 Luteinizing hormone (LH) and thyrotropin have Nlinked 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
 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.