Transcript lecture 2 - carbohydrates

LECTURE 3 -CARBOHYDRATES
CO 2:
Describe, Discuss, and
compare the classification,
structure and function of
carbohydrates and its
derivatives.
LECTURE 3 -CARBOHYDRATES
Section
Section
Section
Section
Section
TOPICS:
1. Role & Significance of
Carbohydrates
2. Monosacharides
3. Oligosacharides
4. Polysacharides
5. Glyconoconjugates
Sect.1. ROLE OF CARBOHYDRATES
As a major energy source for living
organisms
As a means of transporting energy
(glucose is a principal energy source in animal and plants)
( exp:
sucrose in plant tissues)
As a structural material
.
As a precursor for other biomolecules
( cellulose in plants, chitin in insects,
building blocks of nucleotides)
(purine, pyrimide)
Sect 1. SIGNIFICANCE OF CARBOHYDRATES
Carbohydrates are the most abundant
biomolecules in nature, having a direct link
between solar energy and the chemical bond
energy in living organisms.
Source of rapid energy production
Structural building blocks of cells
Components of several metabolic pathways
Recognition of cellular phenomena, such as cell
recognition and binding (e.g., by other cells,
hormones, and viruses)
CARBOHYDRATES
Carbohydrate : compounds contains H, C & O with the
comp : (CH2O)n (Hydrate of carbon)
Carbohydrates : Consist of sugar (saccharum)
Sugars : compound that contains alcohol & carbonyl
functional group
Carbonyl func.group : >C=o
Adehyde  aldose
Ketone  ketose
Examples:
Classification
Carbohydrate
Mono
saccharide
Glucose, fructose
Ribose (aldopentose)
Deoxy ribose
Oligo
saccharide
disaccharides
Glycoproteins
(bacterial cell
walls
Poly
saccharide
cellulose, chitin,
starch, glycogen,
glucoaminoglycans
Glyconoconjugates
glycoproteins
and
proteoglycans
Sect2.
MONOSACHARIDES
Sect. 2. Monosacharides
Sub sections :
2.1 Properties & classification
2.2 Stereoisomers
2.3 Cyclic structure
2.4 Important Reactions
2.5 Important monosach
2.6 glycoproteins and proteoglycans
2.7 Monosaccharide derivatives
2.1 Monosach Properties& classification
Colorless, crystalline solids
Soluble in water but insoluble in nonpolar
solvents
One of the carbon atoms is double-bonded to an
oxygen atom to form a carbonyl group; each of
the other carbon atoms has a hydroxyl group.
– Carbohydrates with an aldehyde (-CHO) functional
group are called aldoses e.g. glyceraldehyde (CH OH-CHOH-CHO)
Those with a keto group (-C=O) are ketoses
2
e.g.dihydroxyacetone (CH2OH-C=O-CH2OH)
– Classified according to the number of carbon atoms
they contain
Monosacharides : Exp. aldoses & ketoses
Aldotriose
Ketotriose
Aldotetrose
Ketotetrose
Aldopentoses
Ketopentose
Aldohexose
Ketohexose
2.2. MONOSACCHARIDES STEREOISOMERS
Isomers: same chemical formulas, different structures
Conformation : the spatial arrangement of substituent groups
chiral centers: asymmetric carbons, i.e carbon atom with four
different substituents
Enantiomers : mirror images Stereoisomers
The simplest aldose, glyceraldehyde, contains one chiral center
(the middle carbon atom) and has two different optical isomers, or
enantiomers
the projection in which the carbohydrate backbone is
drawn vertically with the carbonyl shown on the top.
D- and L- enantiomers
2.3 Cyclic structure of monosacharides
in aqueous solution, monosaccharides with five or more carbon atoms in the
backbone occur predominantly as cyclic (ring) structures in which the carbonyl group
has formed a covalent bond with the oxygen of a hydroxyl group along the chain.
The new chiral center in cyclic (c1) is called anomeric carbon
Pyranoses& Furanoses
Pyranoses: six-membered ring compounds ( resemble pyran )
Furanoses : fivemembered rings, (resemble furan)
The structure systematic names glucose & fructose become
HAWORTH STRUCTURES
An English chemist W.N.
Haworth gave a more accurate
picture of carbohydrate
structure. (Ref. P.205 of
textbook)
FISHER AND HAWORTH FORMS OF SUGAR
SUMMARY OF SUGAR STRUCTURES
ISOMERS- compounds that have the same chemical formula e.g.
fructose, glucose, mannose, and galactose are isomers of each
other having formula C6H12O6.
EPIMERS- refer to sugars whose configuration differ around one
specific carbon atom e.g. glucose and galactose are C-4 epimers
and glucose and mannose are C-2 epimers.
ENANTIOMERS- a special type of isomerism found in pairs of
structures that are mirror images of each other. The mirror images
are termed as enantiomers and the two members are designated as
D- and L- sugar. The vast majority of sugars in humans are Dsugars.
CYCLIZATION OF SUGARS- most monosaccharides with 5 or more
carbon atoms are predominately found in a ring form, where the
aldehyde or ketone group has reacted with an alcoholic group on the
same sugar group to form a hemiacetal or hemiketal ring.
Pyranose ring- if the ring has 5 carbons and 1 oxygen.
Furanose ring- if the ring is 5-membered (4 carbons and 1 oxygen
2.4.IMPORTANT REACTIONS IN MONOSACCHARIDES
Monosaccharides undergo the following reactions :
1.
2.
3.
4.
5.
6.
Mutarotation
Oxidation
Reduction
Isomerization
Esterification
Glycoside formation
IMPORTANT REACTIONS IN MONOSACCHARIDES
Details
1. Mutarotation –
alfa and beta forms of sugars are readily interconverted when
dissolved in water
2
Oxidation and reduction
in presence of oxidising agents, metal ions (Cu2+) and enzymes,
monosacchs undergo several oxidation reactions e.g. Oxidation of
aldehyde group (R-CHO) yields aldonic acid; of terminal CH2OH
(alcohol) yields uronic acid; and of both the aldehyde and CH2OH
gives aldaric acid. The carbonyl groups in both aldonic and uronic
can react with an OH group in the sam
3
REDUCTION
reduction of the aldehyde and ketone groups of monosacchs yield
sugar alcohols (alditols) Sugar alcohols e.g.sorbitol, are used
commercially in processing foods and pharmaceuticals. e molecule
to form a cyclic ester lactone.
IMPORTANT REACTIONS (Cont)
4.
ISOMERIZATION
Monosaccharides undergo several types of isomerization e.g. D-glucose in
alkaline solution for several hours containn D-mannose and D-fructose. The
conversion of glucose to mannose is termed s epimerization. (p.208-9 of
Textbook).
5
ESTERIFICATION
Free OH groups of carbohydrates react with acids to form esters. This
reaction an change the physical and chemical propteries of sugar.
6.
GLYCOSIDE FORMATIONHemiacetals and hemiketals reaction with alcohols to form the
corressponding aceta or ketal (p.210 of Text).On the contrary when a cyclic
hemiacetal or hemiketal form of monosaccharide reacts with alcohol, the
new linkage is called glycosidic linkage and the compound glycoside.
Alfa & beta GLYCOCIDIC BOND
REDUCING SUGARS
All monosacchs are reducing sugars.
They can be oxidised by weak oxidising
agent such as Benedict’s reagent
Benedict's reagent is a solution of copper
sulfate, sodium hydroxide, and tartaric
acid.
Aqueous glucose is mixed with Benedict's reagent and heated.
The reaction reduces the blue copper (II) ion to form a brick red
precipitate of copper (I) oxide. Because of this, glucose is
classified as a reducing sugar.
2.5 IMPORTANT MONOSACCHARIDES
GLUCOSE
FRUCTOSE
GALACTOSE
D-Glucose:
D-glucose (dextrose) is the primary fuel in living cells
especially in brain cells that have few or no mitochondria.
Cells such as eyeballs have limited oxygen supply and use
large amount of glucose to generate energy
Dietary sources include plant starch, and the disaccharides
lactose, maltose, and sucrose
DIABETES (diabetes mellitus)
Characterized by high blood glucose levels that splills
over into the urine
These high glucose levels impairs the insulin-stimulated
glucose entry into cells and starve the cells of insulin.
This leads to ketosis or high levels of ketone bodies
(acids) that hinders the buffering capacity of the blood in
the kidney, which controls blood pH (by excreting excess
H+ ions into the urine).
The H+ excretion is accompanied by the excretion
ammonia, sodium,potassium, and phosphate ions
causing severe dehydration
This leads to excessive thirst symptom of diabetes and
life-threatening decrease in blood volume.
Important monosaccharides. Cont
FRUCTOSE
– D-fructose (levulose) is often referred as fruit sugar
and is found in some vegetables and honey
– This molecule is an important member of ketose
member of sugars
– It is twice as sweet as sucrose (per gram basis) and is
used as sweeting agent in processed food products
– It is present in large amounts in male reproductive
tract and is synthesised in the seminal vesicles.
Important monosaccharides. Cont....
GALACTOSE
– is necessary to synthesize a variety of biomolecules
(lactose-in mammalary glands, glycolipids, certain
phospholipids, proteoglycans, and glycoproteins)
– Galactose and glucose are epimers at carbon 4 and
interconversion is catalysed by enzyme epimerase.
– Medical problems – galactosemia (genetic disorder)
where enzyme to metabolize galactose is missing;
accumulation of galactose in the body can cause liver
damage, cataracts, and severe mental retardation
2.7.MONOSACCHARIDE DERVATIVES
URONIC ACIDS – formed when terminal
CH2OH group of a mono sugar is oxidised
– Important acids in animals – D-glucuronic acid
and its epimer L-iduronic acid
– In liver cells glucuronic acid combines with
steroids, certain drugs, and bilirubin to
improve water solubility therby helping the
removal of waste products from the body
– These acids are abundant in the connective
tissue carbohydrate components.
Mono sugar derivatives
AMINO SUGARS –
– Sugars in which a hydroxyl group (common
on carbon 2) is replaced by an amino group
e.g. D-glucosamine and D-galactosamine
– common constituents of complex
carbohydrate molecule found attached to
cellular proteins and lipids
– Amino acids are often acetylated e.g. Nacetyl-glucosamine.
Mono sugar derivatives
DEOXYSUGARS
– monosaccharides in which an - H has replaced an –
OH group
– Important sugars: L-fucose (formed from D-mannose
by reduction reactions) and 2-deoxy-D-ribose
– L-fucose – found among carbohydrate components of
glycoproteins, such as those of the ABO blood group
determinates on the surface of red blood cells
– 2-deoxyribose is the pentose sugar component of
DNA.
GLYCOSIDES
• Monosaccharides can be linked by glycosidic bonds
(joining of 2 hydroxyl groups of sugars by splitting out
water molecule) to create larger structures.
• Disaccharides contain 2 monosaccharides e.g. lactose
(galactose+glucose); maltose (glucose+glucose);
sucrose (glucose+fructose)
• Oligosaccharides – 3 to 12 monosaccharides units
e.g. glycoproteins
• Polysaccharides – more than 12 monosaccharides
units e.g. glycogen (homopolysaccharide) having
hundreds of sugar units; glycosaminoglycans
(heteropolysaccharides) containing a number of different
monosaccharides species.
Section 3
DISACCHARIDES
AND
OLIGOSACCHARIDES
DISACCHARIDES AND
OLIGOSACCHARIDES
Cnfigurations: alfa or beta ( 1,4, glycosidic
bonds or linkages; other linkages 1,1; 1,2;
1,3; 1,6)
Digestion aided by enzymes. Defficiency
of any one enzyme causes unpleasant
symptoms (fermentation) in colon
produces gas [bloating of cramps].
Most common defficiency, an ancestoral
disorder, lactose intolerance caused by
reduced synthesis of lactase
Important sugars of Disaccharides
LACTOSE
(milk sugar) disaccharide found in milk; composed of one
molecule of galactose and glucose linked through beta(1,4)
glycosidic linkage; because of the hemiacetal group of the
glucose component, lactose is a reducing sugar
Lactose intolerance
Lactose (milk sugar) in infants is hydrolyzed by
intestinal enzyme lactase to its component
monosacch for absorption into the bloodstream
(galactose epimerized to glucose).
Most adult mammals have low levels of betagalactosidase. Hence, much of the lactose they
ingest moves to the colon, where bacterial
fermentation generates large quantities of CO2,
H2 and irritating organic acids.
These products cause painful digestive upset
known as lactose intolerance and is common
in the African and Asian decent.
MALTOSE ( malt sugar)
An intermediate product of starch hydrolysis; it is a disaccharide with an alfa(1,4)
glycosidic linkage between two D-glucose molecules; in solution the free
anomeric carbon undergoes mutarotation resulting in an equilibrium mixture of
alfa and beta – maltoses; it does not occur freely in nature
SUCROSE
common table sugar: cane sugar or beet sugar produced
in the leaves and stems of plants; it is a disaccharide
containing both alfa-glucose and beta-fructose residues
linked by alfa,beta(1,2)glycosidic bond.
CELLOBIOSE
degradation product of cellulose
containing two molecules of glucose linked
by a beta (1,4) glycosidic bond; it does not
occur freely in nature
OLIGOSACCHARIDE SUGARS
Oligosaccharides are small polymers often
found attached to polypeptides in
glycoproteins and some glycolipids.
They are attached to membrane and
secretory proteins found in endoplasmic
reticulum and Golgi complex of various
cells
Two classes: N-linked and O-linked
Section 4
POLYSACCHARIDES
4.1. Intro to Polysaccharides
4.2. Classification of Polisacharides
4.2.1. Homosacharides
4.2.2. Heteropolysacharides
4.1. Intro to Polysaccharides
Composed of large number of monosaccharide units
connected by glycosidic linkages
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. (http://www.lsbu.ac.uk/water/hypol.html)
Polysaccharides are more hydrophobic if they have a
greater number of internal hydrogen bonds, and as their
hydrophobicity increases there is less direct interaction
with water
Divided into homopolysaccharides (e.g.Starch, glycogen,
cellulose, and chitin) & heteropolysaccharides
(glycoaminoglycans or GAGs, murein).
4.2. Classification of Polisacharides
4.2.1.HOMOSACCHARIDES
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 (Homosaccharide)
A naturally abundant nutrient carbohydrate, (C6H10O5)n, found
chiefly in the seeds, fruits, tubers, roots, and stem pith of plants,
notably in corn, potatoes, wheat, and rice, and varying widely in
appearance according to source but commonly prepared as a white
amorphous tasteless powder.
Any of various substances, such as natural starch, used to stiffen
cloth, as in laundering.
Two polysaccharides occur together in starch: amylose and
amylopectin
Amylose – unbranched chains of D-glucose residues linked with
alfa(1,4,)glycosidic bonds
Amylopectin – a branched polymer containing both alfa(1,4,) and
alfa(1,6) glcosidic linkages; the alfa(1,6) branch points may occur
every 20-25 glucose residues to prevent helix formation
Starch digestion begins in the mouth; alfa-amylase in the saliva
initiates hydrolysis of the gycosidic linkages
GLYCOGEN (Homosaccharide)
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 and more glucose
units are present than in amylopectin.
Various samples of glycogen have been measured at 1,700600,000 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.
In glycogen, the branches occur at intervals of 8-10 glucose units,
while in amylopectin the branches are separated by 12-20 glucose
units.
STRUCTURE OF GLYCOGEN
CELLULOSE (Homosaccharide)
Cellulose is found in plants as microfibrils (2-20 nm diameter and
100 - 40 000 nm long). These form the structurally
strong framework in the cell walls.
Cellulose is mostly prepared from wood pulp
Cellulose is a linear polymer of β-(1 4)-D-glucopyranose units in 4C1
conformation. The fully equatorial conformation of β-linked
glucopyranose residues stabilizes the chair structure, minimizing its
flexibility
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
water-soluble derivatives e.g. Methyl cellulose, carbomethycellulose
Cellulose as polymer of β-D-glucose
Cellulose in 3D
CELLULOSE
CHITIN (Homosaccharide)
Chitin is a polymer that can be found in anything from
the shells of beetlesto webs of spiders. It is present all
around us, in plant and animal creatures.
It is sometimes considered to be a spinoff of 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
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
4.2.2.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.
GAGs - high-molecular-weight
carbohydrate polymers
Glycosaminoglycans forming the
proteoglycans are the most abundant
heteropolysaccharides in the body. They are
long unbranched molecules containing a
repeating disaccharide unit. Usually one sugar is
an uronic acid (either D-glucuronic or L-iduronic)
and the other is either GlcNAc or GalNAc. One
or both sugars contain sulfate groups (the only
exception is hyaluronic acid).
GAGs are highly negatively charged what is
essential for their function.
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)
Dermatan sulfate (L-iduronate + GlcNAc sulfate)
Occurence : skin, blood vessels, heart valves
Chondroitin sulfate (D-glucuronate +
GalNAc sulfate)
Occurence : cartilage, bone, heart valves ;
It is the most abundant GAG.
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.
MUREIN (Peptidoglycan)
Peptidoglycan, also known as murein, is a polymer consisting of
sugars and amino acids that forms a mesh-like layer outside the
plasma membrane of eubacteria.
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.
Peptidoglycan serves a structural role in the bacterial cell
wall, giving the wall shape and structural strength, as
well as counteracting the osmotic pressure of the
cytoplasm.
Peptidoglycan is also involved in binary fission during
bacterial cell reproduction.
The peptidoglycan layer is substantially thicker in Grampositive bacteria (20 to 80 nm) than in Gram-negative
bacteria (7 to 8 nm), with the attachment of the S-layer.
Peptidoglycan forms around 90% of the dry weight of
Gram-positive bacteria but only 10% of Gram-negative
strains.
In Gram-positive strains, it is important in attachment
roles and serotyping purposes.
SECTION 5
GLYCOCONJUGATES
5.1.
5.2.
5.3.
5.4.
Intro to glycoconjugates
glycoproteins
glycosylation
proteoglycans
5.1. Intro to glycoconjugates
They are compounds that result from covalent
linkages of carbohydrate molecules to both
proteins and lipids.
They have a profound effects on the functions of
individual cells as weell as cell-cell interactions
of multicellular organisms.
Two classes of carbohydrate-protein conjugate:
glycoproteins and proteoglycans.
The glycolipids (oligosaccharide-containing lipid
molecules) are found predominately on the outer
surface of plasma membrane.
5.2.glycoproteins
Glycoprotein carbohydrate chains are highly diverse.
They are formed by glycosylation and classified into
two groups:
1. N-linked oligosaccharides
2. O-linked oligosaccharides
The N-linked oligosaccharides have a minimum of 5
sugar residues
N-linked attached to polypeptides by an N-glycosidic
bond with a chain amide group of amino acid and
asparagine
O-linked oligosaccharides are generally short (1-4 sugar
residues)
O-linked are attached to polypeptides by the side chain
hydroxyl group of amino acids serine or threonine in
polypeptide chains or hydroxyl groups of membrane
lipids
5.3.glycosylation
Glycosylation is the process or result of addition of
saccharides to proteins and lipids
The process plays an important role in the synthesis of
membrane and secreted proteins
Majority of proteins synthesized in the rough ER undergo
glycosylation
It is an enzyme -directed site-specific process.
Two types of glycosylation exist: N-linked glycosylation
to the amide nitrogen of asparagine side chains and Olinked glycosylation to the hydroxy oxygen of serine and
threonine side chains.
Glycosylation may play a role in cell-cell adhesion (a
mechanism employed by cells of the immune system),
as well.
GLYCOPROTEIN FUNCTIONS
Types of glycoproteins: asparagine-linked carbohydrate;
mucin-type cabohydrate
Examples: glycophorin (membrane protein, source –
human RBC, % carbohydrate - 60); potato lectin (lectin,
carbohydrate binding proteins, source – potato, %
carbohydrate – 50)
Functions: Many glycoproteins have structural
functions: constituent of the cell wall; form connective
tissues such as collagen; found in gastrointestinal mucus
secretions; used as protective agents and lubricants
;found abundantly in the blood plasma.
The human blood groups A, B, AB, and O depend on the
oligosaccharide part of the glycoprotein on the surface
of erythrocyte cells. The terminal monosaccharide of the
glycoprotein at the nonreducing end determines blood
group.
BLOOD GROUPS
TYPE
A
B
AB
O
O
AB
TERMINAL SUGAR
N-acetylgalactosamine
a-D-galactose
both the above
neither of the above
is the “universal donor”
is the “universal
acceptor”
Oligosaccharides are antigeneic
determinants
Carbohydrates on cell surfaces are
immunochemical markers.
ABO blood group antigens are
oligosaccharide components of glycoproteins
and glycolipids on the surfaces of individual
cells, besides blood cells.
Individuals with type A cells have A antigens on
their cell surfaces and carry anti-B antibodies
Those with type B cells which bear B antigens,
carry anti-A antibodies
Those with type AB cells, which have both A and
B antigens, carry neither anti-A nor anti-B
antibodies
Type O individuals whose cells bear neither
antigen, carry both anti-A and anti-B antibodies.
Transfusion of type A blood group into a type B
individual, results in an anti-A antibody- A
antigen reaction.
This reaction clumps together (agglutinates) the
transfused erythrocytes, resulting in an often
fatal blockage of blood vessels
Functions of glycoproteins (elaborated)
http://www.cs.stedwards.edu/chem/Chemistry/CHEM43/CHEM43/Glycoproteins/Glycoproteins.HTML
Carbohydrates and proteins by themselves serve in a vast
number of biological functions,linking the two together
results in a macromolecule with an extremely large
number of functions.
Structural: Glycoproteins are found throughout
matrices. They act as receptors on cell surfaces that
bring other cells and proteins (collagen) together giving
strength and support to a matrix.
In nerve tissue glycoproteins are abundant in gray
matter and appear to be associated with synaptosomes,
axons, and microsomes.
Protection: Human lacrimal glands produce a
glycoprotein which protects the corneal epithelium from
desiccation and foreign particles. Human sweat glands
secrete glycoproteins which protect the skin from the
other excretory products that could harm the skin
Functions of glycoproteins (further elaborated-1)
Prothrombin, thrombin, and fibrinogen are all
glycoproteins that play an intricate role in the blood
clotting mechanism
In certain bacteria the slime layer that surrounds the
outermost components of cell walls are made up of
glycoproteins of high molecular weight. In addition to
forming these s-layers, glycoproteins also function as
bacterial flagella. These are made up of bundles of
glycoproteins protruding from the cell's surface. Their
rotation provides propulsion.
In plants, glycoproteins have roles in cell wall formation,
tissue differentiation, & embryogenesis.
Reproduction: Glycoproteins found on the surface of
spermatozoa appear to increase a sperm cell's attraction
for the egg by altering the electrophoretic mobility of the
plasma membrane.
Functions of glycoproteins (further elaborated-2)
Adhesion: Glycoproteins serve to adhere cells
to cells and cells to substratum.
Hormones: There are many glycoproteins that
function as hormones such as human chorionic
gonadotropin (HCG) which is present in human
pregnancy urine. Another example is
erythropoietin which regulates erythrocyte
production
Enzymes: Glycoprotein enzymes are of three
types. These are oxidoreductases, transferases,
and hydrolases.
Functions of glycoproteins (further elaborated-3)
Carriers: Glycoproteins can bind to certain molecules
and serve as vehicles of transport. They can bind to
vitamins, hormones, cations, and other substances.
Inhibitors: Many glycoproteins in blood plasma have
shown antiproteolytic activity. For example, the
glycoprotein a1-antichymotrypsin inhibits chymotrypsin.
Immunological: The interaction of blood group
substances with antibodies is determined by the
glycoproteins on erythrocytes. Many immunoglobulins
are actually glycoproteins.
B and T cells contain surface glycoproteins that attract
bacteria to these sites and bind them. In much the same
manner, glycoproteins can direct phagocytosis. Because
the HIV virus recognizes the receptor protein CD4, it
binds to helper T cells which contain it.
CELL MEMBRANE
The cell membrane is a fluid mosaic of lipids, proteins,
and carbohydrates.
Membrane carbohydrates are usually branched
oligosaccharides with fewer than 15 sugar units.
Some of these oligosaccharides are covalently bonded
to lipids, forming molecules called glycolipids.
Most are covalently bonded to proteins, which are
thereby glycoproteins.
Plants produce pectin, major component of cell wall.
The oligosaccharides on the external side of the plasma
membrane vary from species to species
The diversity of the molecules and their location on the
cell's surface enable oligosaccharides to function as
markers that distinguish one cell from another.
THE MOSAIC OF CELL’S MEMBRANE
CELL MEMBRANE MATRIX
The biological membrane is a collage of many
different proteins embedded in the fluid matrix
of the lipid bilayer.
The lipid bilayer is the main fabric of the
membrane, and its structure creates a semipermeable membrane.
The hydrophobic core impedes the transport of
hydrophilic structures, such as ions and polar
molecules but enable hydrophobic molecules,
which can dissolve in the membrane, cross it
with ease.
Protein layer of cell membrane
Proteins determine most of the
membrane's specific functions.
The plasma membrane and the
membranes of the various organelles each
have unique collections of proteins.
For example, to date more than 50 kinds
of proteins have been found in the plasma
membrane of red blood cells.
5.4. PROTEOGLYCANS
Proteoglycans represent a special class of
glycoproteins that are heavily glycosylated.
They consist of a core protein with one or more
covalently attached glycosaminoglycan chain(s).
These glycosaminoglycan (GAG) chains are long, linear
carbohydrate polymers that are negatively charged
under physiological conditions, due to the occurrence of
sulphate and uronic acid groups.
Proteoglycans can be categorised depending upon the
nature of their glycosaminoglycan chains. These chains
may be:
1. chondroitin sulfate and dematan sulfate
2. heparin and heparin sulfate
3. keratan sulfate
Proteoglycans can also be categorised by
size.
Example of large proteoglycan is
aggrecan.
Aggrecan, is the major proteoglycan in
cartilage, present in many adult tissues
including blood vessels and skin.
The small leucine rich repeat
proteoglycans (SLRPs) include decorin,
biglycan, fibromodulin and lumican.
SYNTHESIS OF PROTEOGLYCANS
The protein component of proteoglycans is
synthesized by ribosomes and translocated into
the lumen of the rough endoplasmic reticulum.
Glycosylation of the proteoglycan occurs in the
Golgi apparatus in multiple enzymatic steps.
First a special link tetrasaccharide is attached to
a serine side chain on the core protein to serve
as a primer for polysaccharide growth.
Then sugars are added one at the time by
glycosyl transferase.
The completed proteoglycan is then exported in
secretory vesicles to the extracellular matrix of
the cell.
Structure of proteoglycans
The GAGs extend perpendicular from the core protein in a
bottlebrush- like structure.
The linkage of GAGs such as (heparan sulfates and chondroitin
sulfates) to the protein core involves a specific trisaccharide linker
Some forms of keratan sulfates are linked to the protein core
through an N-asparaginyl bond.
The protein cores of proteoglycans are rich in Ser and Thr residues
which allows multiple GAG attachment.
FUNCTION OF PROTEOGLYCANS
Proteoglycans are a major component of the animal
extracellular matrix, the 'filler' substance existing
between cells in an organism.
Individual functions of proteoglycans can be attributed to
either the protein core or the attached GAG chain.
In extracellular matrix, they form large complexes, with
other proteoglycans, to hyaluronan and to fibrous matrix
proteins (such as collagen).
They are also involved in binding cations (such as
sodium, potassium and calcium) and water, and also
regulating the movement of molecules through the
matrix.
They can affect the activity and stability of proteins and
signalling molecules within the matrix.
ROLE OF PROTEOGLYCANS
http://www.cryst.bbk.ac.uk/pps97/assignments/projects/emilia/Proteoglycans.HTM
GAG dependent functions can be divided into two
classes: the biophysical and the biochemical.
The biophysical functions depend on the unique
properties of GAGs : the ability to fill the space, bind and
organize water molecules and repel negatively charged
molecules. Because of high viscosity and low
compressibility they are ideal for a lubricating fluid in the
joints. On the other hand their rigidity provides structural
integrity to the cells and allows the cell migration due to
providing the passageways between cells.
For example the large quantities of chondroitin sulfate
and keratan sulfate found on aggrecan play an
important role in the hydration of cartilage. They give
the cartilage its gel-like properties and resistance to
deformation.
Role of proteoglycans contd.
The other, more biochemical functions of GAGs
are mediated by specific binding of GAGs to
other macromolecules, mostly proteins.
Proteoglycans participate in cell and tissue
development and physiology.
Hurler’s syndrome: (refer Text) disease related
with proteoglycan metabolism where excessive
accumulation of dermatin sulfate due to
deficiency of a specific enzyme may cause
mental retardation, skeletal deformity ansd
death in early childhood.
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
Summary contd.-2
GLYCOCONJUGATES: Proteoglycans &
glycoproteins play important roles in information
transfer in living organisms.
Proteoglycans are enormous molecules
consisting of hyaluronate with attached core
proteins that bear numerous
glycosaminoglycans and oligosaccharides.
Found in the extracellular matrix of tissues
Bacterial cell walls are made up of
peptidoglycan, a network of polysaccharide
and poypeptide chains.
Summary contd.-3
Glycoproteins or Glycosylated proteins may
contain N-linked oligosacchs attached to Asn
(asparagine) or O-linked oligosacchs attached to
Ser or Thr (serine or threonine) or both
Different molecules of glycoproteins may contain
different sequences and locations of
oligosaccharides.
Occur in cells in both soluble and membrane
bound forms, and in extracellular fluids
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.