Cellulose is a linear polymer of β-(1-4)-D-glucopyranose units that is synthesized by the enzyme cellulose synthase. It is deposited and assembled on the outer surface of the plasma membrane by terminal complexes containing cellulose synthase. Hemicellulose polymers are also synthesized on the plasma membrane and associate with cellulose microfibrils in the cell wall. Pectin is synthesized intracellularly and contains a backbone of galacturonic acid residues with complex rhamnose and arabinose side chains.
2. Structure of cellulose
Cellulose is a linear polymer of β-(1-4)-D-glucopyranose
units .
10000 to 15000 glucose units joined by β-(1-4)-linkages
Represented as a series of glucopyranose rings in the
chair conformation
Most stable conformation is the chair turned 180 degree
relative to adjacent glucose residues yielding a straight
extended chain
5. Biosynthesis of Structural Polysaccharides
Cellulose is linear, unbranched
homoglycan of 10,000 to 15,000 D-
glucose units joined by β(1—>4).
It is synthesized by the enzyme
cellulose synthase.
6. Cellulose Biosynthesis
• NDP sugar (GDPG/UDPG) - function as monosaccharide
• Oligosaccharide (β-1-4 glucan)-is the primer
GDP glucose + (β-1-4 glucan)n
(or) UDP glucose
(β-1-4 glucan)n+1 + GDP or UDP
primer
Cellulose synthetase
7. • Cellulose synthetase molecules are folded as aggregates on
the surface of the plasmalemma
• Each enzyme molecule catalyse the transfer of a glucose
residue from GDP or UDPG to non reducing end
• The whole microfibril is elongated
8. Cellulose Synthetase
• Transmembrane protein accept nucleotide sugars at the
cytoplasmic surface and lays down cellulose polymers on the
external face
• UDPG or GDPG synthesized in the protoplast and then exported
to reach cellulose synthetase containing granules on the surface
of the plasmalemma
• Enzyme molecules are numerous
• Each molecule catalyse the transfer of a glucose residue from
GDP or UDPG to a non reducing end that is close
9. Biosynthesis of Structural Polysaccharides
Plant cell wall is made of cellulose microfibrils, which is consisted of
about 36 chains of cellulose, a polymer of b(14)glucose
Cellulose is synthesized by terminal complexes or rosettes,
consisting of cellulose synthase and associated enzymes.
Each rosette contains six multiple protein sub units.
Cellulose is synthesized from intracellular precursors but deposited
and assembled outside the plasma membrane.
10. Biosynthesis of Strl. Polysaccarides Contd..
UDP-Glc acts as the glucosyl donor
for cellulose synthesis.
UDP-Glc is generated from sucrose
catalyzed by sucrose synthase.
Sucrose +UDP UDP-Glc + Frc
Glucose is transferred from UDP-
glucose to a membrane lipid
(probably sitosterol) on the inner
face of the plasma membrane.
11. Biosynthesis of Strl. Polysaccarides Contd..
Intracellular cellulose synthase adds
several more glucose residues to the first
one, in (b14) linkage, forming a short
oligosacchairde chain attached to the
sitosterol (sitosterol dextrin).
12. Biosynthesis of Strl. Polysaccarides Contd..
Next, the whole sitosterol
dextrin flips across to the
outer face of the plasma
membrane, where most of
the polysaccharide chain is
removed by endo-1,4-b-
glucanase.
13. Biosynthesis of Strl. Polysaccarides Contd..
The dextrin primer (removed
from sitosterol by endo-1,4-β-
glucanase) is now (covalently)
attached to another form of
cellulose synthase.
Cellulose synthase spans the
membrane and uses cytosolic
UDP-Glc as the precursor for
extracellular cellulose synthesis.
A second form of cellulose
synthase extends the polymer to
500-15,000 glc units extruding it
onto the outer surface of the cell.
Each of the six globules of the
rosette synthesizes six cellulose
chains.
14. Biosynthesis of Strl. Polysaccarides Contd..
Parallel cellulose chains
crystallize to form microfibrils.
Each microfibril consists of
about 36 separate cellulose
chains lying side by side in a
parallel manner
The glucose associated with
UDP is a-linked.
Its configuration will be
converted by
glycosyltransferases so the
product (cellulose) is b-linked
15. Microfibril
• Intramolecular hydrogen bonding between the hydroxyl
group of C-3 of one glucose residue and the pyranose
ring oxygen atom of the next glucose residue.
• Within the microfibril, the adjacent cellulose molecules
are linked by intermolecular hydrogen bond between C-6
hydroxyl group of one molecule and the glycosidic bond
oxygen atom of adjacent cellulose molecule
16.
17. BIOSYNTHESIS OF CELL WALL
POLYSACCHARIDES
– Synthesis of sugar phosphates
– Synthesis of NDP-sugars
– Synthesis of polysaccharides
– Post-polymerisation modifications
(esterification, etherification) and
– Post-deposition modifications (Cross-linking)
18. NDP SUGARS
The synthesis of cell wall polysaccharides utilizes several
nucleotide sugars as substrates. Different nucleotide diphosphate sugars
(NDP sugars) are synthesized by their interconversions or from the
respective free sugars. Some of the nucleotide sugars are UDP-D-
glucose, ADP-D-glucose, UDP-D-galactose, UDP-D-xylose, etc. UDP- D-
glucose is formed from glucose-1-phosphate catalyzed by UDP-glucose
pyrophosphorylase.
19. Cross linking of polymers in the cell wall
Most of the polymers present in the cell wall are cross linked to one
another through a variety of bonds to form the molecular bag that
surrounds the plant cell.
a) Hydrogen bond Cellulose – Hemicellulose
b) Salt linkage Lys-Coo- of galacturonan
c) Glycosidic linkage xyloglucan – arabinogalactan
d) Covalent (C-C) cross links gala-ferulate-ferulate-gala
e) Ether linkage (biphenyl bridge) Tyrosine OH group of
extension
20. Cross linking of polymers in the cell wall
• Cellulose binds to xyloglucan and arabino-xylan via hydrogen bonds.
• The side chain amino group of lysine present in extensin, a
glycoprotein, is salt linked to galacturonan.
• Xyloglucan and arabinogalactan, and arabino galactan and
galacturonan are linked by glycosidic linkage.
• The phenolic hydroxyl group of tyrosine side chains in adjacent
extensin molecules combine oxidatively to form a biphenyl bridges.
• The C6 hydroxyl of galactose and the C3 hydroxyl of arabinose residues
both terminally located in side chains of rhamnogalacturonan can be
esterified with ferulate.
• Oxidative coupling produces a C-C bond bridging the pectin molecules.
21. Callose
• It exists in the cell walls of a wide variety of
higher plants.
• It plays important roles during a variety of
processes in plant development and/or in
response to multiple biotic and abiotic stresses.
• Callose is a b-1-3 glucan - deposited around sieve
plates and on the sides of sieve tube pores.
• A well documented response to chemical and
physical trauma is the appearance of callose
• Callose deposition can occur within minutes of
injury.
22. Callose biosynthesis
• enzyme that catalyses the synthesis of cellulose was
found also to catalyse the assembly of the b,1-3 glucan,
callose.
• proportion of each polysaccharide synthesized from the
common precursor and by the same enzyme may be
varied by effectors such as GTP, ATP, Ca2+ and Mg2+.
• physiological significance of this is that it is part of the
plant cell’s response to wounding.
23. Chitin
• Chitin polymer resembles cellulose both in the structure and conformation of
the single molecule and in the manner of its association into microfibrils
• In yeast and fungi it provides a thin anchoring layer for cell wall assembly.
• The monomeric unit, N-acetyl glucosamine (Glu N-Ac) is linked by a b, 1-4
linkage.
• Chitin is also found in the exoskelton of insects and crustacean.
• Hydrogen bonding between acetamido groups of adjacent chitin chains, in
addition to the inter- and intra –chain hydrogen bonding similar to that of
cellulose microfibrils makes the chitin, a tough molecule.
• Attempts have been made to harvest, extract and find commercial uses for
chitin or its derivatives.
• One such example is the treatment of crab shell to extract and convert chitin
to chitosan, the deacetylated form of the polymer.
• Chitosan is water soluble and can be used to coat fruits, producing a thin film
that provides protection against their deterioration.
26. PECTIN
Backbone of a-1-4 linked galacturonate residues
Regular backbone – interrupted in two ways.
Addition of Rhamnose Short, complex side chain
res. in place of GalUA link-xylose, gal. GluUA, Arabinose
Assembly of linear portion-4 transferases
2 transf
2-transferases – GalUA (UDP-GalUA Gal.UA of Rham.
2 transf
2 other transferases – UDP – rhamose Acidic / neutral residue.
Side chain partially assembled before attachment
CH3 group donated by S-adenosyl methionine.
27. HEMICELLULOSE
• Hemicelluloses are found in close association with cellulose in plant
cell walls.
• Compared to cellulose, the majority of the hemicelluloses are
relatively small molecules consisting of between 50 and 200
monosaccharide units.
• Most of the hemicelluloses are heteropolysaccharides consisting of
D-galactose, D-mannose, D-xylose, L-arabionse, D-glucose and
L-fucose.
• Hemicelluloses are divided into subgroups based on the predominant
monosaccharides present in them.
• Examples of hemicelluloses are xylan, xyloglucan, arabinogalactan and
glucomannan.
