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Carbohydrate structure and
function
PTT 103: Biochemistry
Department of Chemical Engineering Technology, UniMAP
Semester 1, 2013/2014
[email protected]
Intro to Polysaccharides
Functions
and
composition
•Polysaccharides used as storage forms of energy or as structural materials.
•Composed of large number of monosaccharide units connected by glycosidic
linkages
classification
•Classified on the basis of their main monosaccharide components and the
sequences and linkages between them, as well as the anomeric configuration of
linkages, the ring size (furanose or pyranose), the absolute configuration (D- or L) and any other substituents present.
•Divided into homopolysaccharides (e.g.Starch, glycogen, cellulose, and chitin) &
heteropolysaccharides (glycoaminoglycans or GAGs, murein).
size
•Molecular weights of polysaccharides have no fixed values; the size of molecules
depend on the metabolic state of the cell producing them.
•For e.g: when blood sugar is high, the liver synthesizes glycogen. When blood
sugar levels fall, the liver enzyme begin breaking down glycogen into glucose and
releasing it into the blood stream. If it continues, the process continues until
glycogen reserves are almost used up.
Classification of Polisacharides
HOMOPOLYSACCHARIDES
• Found in abundance in nature
• Important examples: starch, glycogen, cellulose,
and chitin
• Starch, glycogen, and cellulose all yield Dglucose when they are hydrolyzed
• Cellulose - primary component of plant cells
• Chitin – principal structural component of
exoskeletons of arthropods and cell walls of
many fungi; yield glucose derivative N-acetyl
glucosamine when it is hydrolyzed.
STARCH
A naturally abundant
nutrient carbohydrate,
found in the seeds, fruits,
tubers, roots, and stem
of plants, notably in corn,
potatoes, wheat, and rice,
Any of various
substances, such as
natural starch, used to
stiffen cloth, as in
laundering.
white amorphous
tasteless powder
Two polysaccharides
occur together in starch:
amylose and amylopectin
Amylose
• Long, unbranched chains of D-glucose residues.
• Linked by α-(1,4) glycosidic bonds
• Amylose has one reducing end in which ring can open to form a
free aldehyde group with reducing properties.
• The internal anomeric carbon- involved in acetal lingkages and
not free to act as reducing agents.
• Linear amylose forms long, tight helices; ideal for storage
functions.
• Iodine test: iodine inserts itself into these helices.
Amylopectin
• Branched polymer containing both α(1,4) and α(1,6) glycosidic
lingkages.
• The α (1,6) branch points may occur every 20-25 glucose
residues and prevent helix formation.
• Amylose
• amylopectin
Starch digestion
• Salivary enzyme α-amylase initiates hydrolysis of the glycosidic linkages.
In mouth
Small
intestines
• Pancreatic α-amylase randomly hydrolyses hydrolyses all the α(1,4) glycosidic bonds except
those next to the branch points
• Product of α-amylase are maltose, trisaccharide maltotriose and α-limit dextrins
(oligosaccharides contain 8 glucose units with 1 or more α(1,6) branch points
• Several enzymes secreted by cells that lined small intestines: convert to glucose
• Glucose absorbed into bloodstream and transported to the liver and the rest of the body
GLYCOGEN (Homopolysaccharide)
• Glycogen is the storage form of glucose in animals and humans
which is analogous to the starch in plants.
• Glycogen is synthesized and stored mainly in the liver and the
muscles.
• Structurally, glycogen is very similar to amylopectin with alpha
acetal linkages, however, it has even more branching (every fourth
glucose residue in the core of molecules) and more glucose units are
present than in amylopectin.
• In glycogen, the branches occur at intervals of 8-10 glucose units,
while in amylopectin the branches are separated by 12-20 glucose
units
• Various samples of glycogen have been measured at 1,700-600,000
units of glucose..
• The structure of glycogen consists of long polymer chains of glucose
units connected by an alpha acetal linkage.
• The branches are formed by linking C # 1 to a C # 6 through an
acetal linkages.
STRUCTURE OF GLYCOGEN
Cellulose
• Composed of D-glucopyranose residues linked by β(1,4) glycosidic
bonds.
• Most important structural polysaccharides of plants
• About 1 third of plants biomass comprise of cellulose. The most
abundant organic material..
• Cellulose has many uses as an anticake agent, emulsifier, stabilizer,
dispersing agent, thickener, and gelling agent but these are
generally subsidiary to its most important use of holding on to
water.
• Water cannot penetrate crystalline cellulose but dry amorphous
cellulose absorbs water becoming soft and flexible.
• Purified cellulose is used as the base material for a number of watersoluble derivatives e.g. Methyl cellulose, carbomethycellulose
• Pair of unbranched cellulose molecules contain
as many as 12,000 glucose units each.
• Held together by hydrogen bonding to form
sheetlike strips called microfibril.
• Each bundle of fibrils contains approx. 40 of
these pairs.
• The ability to digest cellulose is found only in
microbes that possess enzyme cellulase.
• Certain animal species (termites and cows) use
such organisms in their digestives tracts to
digest cellulose.
• The breakdown of cellulose makes glucose
available to both the microbes and their host.
• Many animals cannot digest cellulose however
Cellulose play a vital role as dietary fibre.
Cellulose as polymer of β-D-glucose
Cellulose in 3D
CHITIN (Homosaccharide)
• Chitin is a polymer that can be found in anything from
the shells of beetles to webs of spiders. It is present all
around us, in plant and animal creatures.
• The structure and function is similar to cellulose,
because the two are very molecularly similar.
• Cellulose contains a hydroxy group, and chitin contains
acetamide.
• Chitin is unusual because it is a "natural polymer," or a
combination of elements that exists naturally on earth.
• Usually, polymers are man-made. Crabs, beetles, worms
and mushrooms contain large amount of chitin.
• Chitin is a very firm material, and it help protect an
insect against harm and pressure
• The monomeric units is N-acetyl-glucosamine
(amino sugar) linked in unbranched chains by
β(1,4) glycosidic bonds.
• Microfibrils are formed from adjacent chitin
molecules that are strongly hydrogen bonded
together.
• Chitin occur in 3 types of microfibrils; α-chitin,
β-chitin and ɣ-chitin.
Structure of the chitin molecule, showing two of the Nacetylglucosamine units that repeat to form long chains in
beta-1,4 linkage.
CHITOSAN
• A spinoff of chitin that has been discovered by the
market is chitosan. This is a man-made molecule that is
often used to dye shirts and jeans in the clothing
industry.
• Chitosan can be used within the human body to regulate
diet programs, and researchers are looking into ways in
which it can sure diseases.
• Chitin, the polysaccharide polymer from which chitosan
is derived, is a cellulose-like polymer consisting mainly
of unbranched chains of N-acetyl-D-glucosamine.
Deacetylated chitin, or chitosan, is comprised of chains
of D-glucosamine. When ingested, chitosan can be
considered a dietary fiber.
CHEMICAL STRUCTURE OF CHITOSAN
http://www.pdrhealth.com/drug_info/nmdrugprofiles/nutsupdrugs/chi_0067.shtml
HETEROPOLYSACCHARIDES
• Are high-molecular-weight carbohydrate
polymers more than one kind of monosaccharide
• Important examples include glycosaminoglycans
(GAGs) – the principle components of
proteoglycans and murein, a major component
of bacterial cell walls.
Glycoaaminoglycans (GAGs)
• GAGs are linear polymers with disaccharides
repeating units. Many of their sugar residues are
amino derivatives.
• The repeating units contain hexuronic acid (a
uronic acid contain 6-C atoms) except for
keratan sulphate – contains galactose.
• Usually N-acetylglucosamine sulphate is also
present except in hyaluronic acid which contain
N-acetylglucosamine.
• Many disacharide units contain both carboxyl nd
sulfate functions groups.
• GAGs are classified according to their sugar
residues, the linkages between residues and the
presence and location of sulphate groups.
• 5 classes has been distinguished: hyaluronic
acid, chondroitin sulfate, dermatan sulfate,
heparin and keratan sulfate
THE SPECIFIC GAGs OF PHYSIOLOGICAL SIGNIFICANCE
Hyaluronic acid
Occurence : synovial fluid, ECM of loose
connective tissue
Hyaluronic acid is unique among the GAGs
because it does not contain any sulfate and is not
found covalently attached to proteins. It forms
non-covalently linked complexes with
proteoglycans in the ECM.
Hyaluronic acid polymers are very large (100 10,000 kD) and can displace a large volume of
water.
Hyaluronic acid (D-glucuronate + GlcNAc)
• (1,4)-O-β-D-Glucopyranosyluronic acid- (1,3)-2acetamido-2-deoxy-β-D-glucopyranose
Dermatan sulfate (L-iduronate + GlcNAc sulfate)
Occurence : skin, blood vessels, heart valves
(1,4)-O-α-L-Idopyranosyluronic acid-(1,3)-2-acetamido-2- deoxy-4-Osulfo-β-D-galactopyranose
Chondroitin sulfate (D-glucuronate + GalNAc
Occurence : cartilage, bone, heart valves ;
sulfate)
It is the most abundant GAG.
(1,4)-O-B-D-Glucopyranosyluronic acid-(1,3)-2-acetamido-2-deoxy-6O-sulfo-B-D-galactopyranose
Heparin and heparan sulfate (D-glucuronate sulfate +
N-sulfo-D-glucosamine)
Heparans have less sulfate groups than heparins
Occurence :
Heparin :component of intracellular granules of mast cells lining
the arteries of the lungs, liver and skin Heparan sulfate :
basement membranes, component of cell surfaces
Keratan sulfate ( Gal + GlcNAc sulfate)
Occurence : cornea, bone, cartilage ;
• Keratan sulfates are often aggregated with chondroitin
sulfates.
(1,3)-O-B-D-Galactopyranose-(1,4)-2-acetamido-2-deoxy-6-O-sulfo-B-Dglucopyranose.
MUREIN (Peptidoglycan)
• Peptidoglycan, also known as murein, is complex polymer that is
the major structural feature of the cell walls of all bacteria.
• The sugar component consists of alternating residues of β-(1,4)
linked N-acetylglucosamine and N-acetylmuramic acid residues.
• Attached to the N-acetylmuramic acid is a peptide chain of three to
five amino acids.
• The peptide chain can be cross-linked to the peptide chain of
another strand forming the 3D mesh-like layer.
• Some Archaea have a similar layer of pseudopeptidoglycan.
SUMMARY
• Monosaccharides, the simplest carbohydrates, are
classified as aldoses or ketoses.
• The cyclic hemiacetal and hemiketal forms of
monosacchs have either alfa or beta configuration at
their anomeric carbon.
• Monosacch derivatives include aldonic acids, uronic
acids, deoxy sugars, amino sugars, alfa & beta glycosides.
• Disaccharides simplest polysaccharides occuring as
hydrolysis products of larger molecules e.g.
Lactose,sucrose
• Oligosaccharides play important roles in determining
protein structure and in cell-surface recognition
phenomena. Oligosacchs with 3 or more sugar residues
are mostly found in plants.
Summary contd. -1
• POLOYSACCHARIDES consist of monosacchs
linked by glycosidic bonds.
• Cellulose and chitin are structural
polysacchs with beta(1-4) linkages that adopt
rigid and extended structures.
• The storage polysacchs starch and glycogen
consist of alfa-glycosidically linked glucose
residues
• Glycosaminoglycans are unbranched
polysacchs containing uronic acids and amino
sugars that are often sulfated
END NOTES
• The destiny of a nation depends on the manner
in which it feeds itself.
• We eat to live, NOT, live to eat.
• Lower your carbohydrate consumption, but
balance it with the right amount of protein and
fat.