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Biochemistry
Dr .Anwar J almzaiel
Prosperities of carbohydrate
Physical properties:
They are colourless, crystalline compounds, readily soluble in water and sweet
in taste. Their solutions are optically active and exhibit the phenomenon of
mutarotation.
Chemical properties:
Involve the reaction of two functional groups, carbonyl group and hydroxyl
group.
a- Formation of asazones with phenyl hydrazine:
The reaction involves the C-containing the aldehyde or keto group and
the C atom immediately next to it
e.g. the formation of osazone by the reaction of D(+) glucose and 3
moles of phenyl hydrazine
Since the first two C are involved in the reactions, sugars which have the
same molecular structure in the remaining C give the same molecular
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structure in the remaining C give the same osazone (e.g. glucose,
mannose and fructose)
b- Reduction of aldehyde or ketone group to form alcohol called alditol e.g
c- Oxidation to produce sugar acids
1- Mild oxidizing agents (e.g. bromine water)
2- strong oxidizing agents (e.g. nitric acid)
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Dr .Anwar J almzaiel
Galactose form an insoluble saccaric acid called (mucic acid)
All saccaric acids are soluble except galactosaccharic acid form crystals,
this property can be used to identify lactose (glucose + galactose) by
using mucic acid
3- Formation of uronic acid by enzyme action in the living tissue e.g.
d- Action of dilute bases on monosaccharides
When glucose is exposed to dilute alkali for several hours the resulting
mixture contains both fructose and mannose. This is due to the
enolization of these sugars in the presence of alkali.
Dilute aqueous bases at room temperature causes rearrangement about
the anomeric C-atom and its adjacent C-atom (enolization, or
tautomerization) without affecting substituents at other C-atoms.
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e- Reducing action of sugars
The reducing sugars are able to function as reducing agents because free
or potentially free aldehyde or ketone groups are present in the
molecule
Biomedical importance of D-glucuronic acid:
In the body, glucoronic acid is formed from glucose in liver by uronic acid
pathway (un alternative pathway for glucose oxidation) it occurs as a
constituent of certain mucopolysaccarides
It conjugates with toxic substances, drugs, hormones, and even bilirubin
(catabolic product of Hb) and converts them to nontoxic substances, a
glucuronide, which is excreted in urines
Reduction of sugar to form sugar alcohols
The monosaccharaides may be reduced to their corresponding alcohols
by reducing agents such as Na-Amalgame e.g
D-glucose
The sugar alcohols are natural products.
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Biochemistry
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They are well crystalized compound, soluble in water and alcohol; they have a
sweet taste. In an alkaline medium, the aldehyde or keto group can reduce by
a number of substances such as Ag, Bi, Hg and Cu salts and ferricyanide.
It is important to realize that aketohexose is also a reducing sugar just like an
aldehyde because of the presence of a hydroxyl group α to the ketone group
which is responsible for this reaction (α-hydroxy ketones are very easily
oxidized). So Tollen’s, Fehling’s and Bendict are reagents or solutions do not
distinguish between ketoses and aldoses. Simple laboratory tests for sugars
usually employ the reduction of cupric salts (Benedict and Fehling solutions)
when Cu++ is reduced by reducing sugars such as glucose, the resulting Cu+ is
less soluble and Cu2O precipitates out of the alkaline solution as a red
precipitate. The reducing sugar in turn is oxidized, fragmented and
polymerized in the strongly alkaline Benedict’s or Fehling’s solution.
Cu+2
Cu+1
Cu2O
Glycosides
Are compounds formed from a condensation between monosaccharides or
monosaccharides residue, and the hydroxyl group of second compound that may
or not (in case of an a glycone) be another monosaccharide. The reaction
proceeds with the formation of anomeric α and β-glycosides.
Glycosides of medical importance
-Glycosides are formed in many drugs and species and in the constituent of
animal tissues; the glycane may be methanol, glycerol, sterol or phenol.
-The glycosides are important in medicine due to their action on the heart
(cardiac glycosides) all contain steroids as a glycone component, these include
derivatives of digitalis and sterophanthus such as Digoxin, a stimulator of
cardiac muscle contraction
Other glycosides include antibiotics such as sterptomycine
Disaccharides
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Biochemistry
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Are dimer made up of two (same or different) joined by glycoside bonds in two
ways
The glycosidic bond is an acetal link because it results from a reaction between
a hemiacetal group (formed from an aldehyde and OH- group) and another OH
group if the hemiacetal portion is glucose, the resulting compound is glucoside,
if galactose the resulting compound galactoside,etc
A simple example is the methyl glucoside formed when a solution of glucose in
boiling methyl alcohol is treated with 0.5 % HCl as a catalyst.
areducing disaccharides
e.g maltose it consist of 2 molecules of glucose [O-α – D-glycopyranosyl-(1
4)-β-D- gluco pyranose] it is reducing because it contain free anomeric carbon.
Lactose [O- β – D-galactopyranosyl-(1
4)- α -D- gluco pyranose] composed
of glucose and galgatose, it is found in the milk of mammals and contain
hemiacetal group, so it exists in both α, β forms undergo mutarotation and is
reducing sugar
b- Non reducing disaccharides
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Sucrose on hydrolysis, it yield equal amounts of D (+) glucose and D (-)
fructose, joined by an α-glycoside bond from glucose. Sucrose does not contain
a hemiacetal group so it is non reducing sugar. [O- β – D-fructofuranosyl-(1
2)- α -D- gluco pyranoside] or glucopyranose. There is no free anomeric- Catom so we cannot write α or β form but the type of linkage α or β can be
written.
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Dr .Anwar J almzaiel
Glycosides are formed by condensation between the hydroxyl group of the
anomeric carbon (the carbonyl carbon atom) of a monosaccharide and a
second compound that may or not (in the case of an aglycone) be another
monosaccharide. A glycone may be methanol, glycerol, a sterol, a phenol or a
base such as adenine.
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Biologically Important Sugar (Glucose) Derivatives
Deoxy sugars lack an oxygen atom
Deoxy sugars are those in which one hydroxyl group has been replaced by
hydrogen. An example is deoxy ribose in DNA.
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Amino sugars
The amino sugars include:
-D-glucosamine (a constituent of hyaluronic acid)
-D-galactosamine (also known as chondrosaamine, a constituent of
chonroition)
-D-mannosamine
Several antibiotics (e.g. erythromycin) contain amino sugar, which are
important for their antibiotic activity.
4. Sugar phosphates
Breakdown of sugar in animals involves formation of sugar phosphates. Glucose-6phosphate
is an example for a sugar phosphate (Fig. 5.9).
5. Aminosugars
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Those sugars in which an amino group is substituted for a hydroxyl group. Dglucosamine
is an example for an amino sugar). Amino sugars are components of
mucopolysaccharides, and antibiotics.
6. Glycosides
Polysaccharides
They are polymers of monosaccharides. They contain more than ten monosaccharide
units.The monosaccharides are joined together by glycosidic linkage.
Classification of Polysaccharides
Polysaccharides are classified on the basis of the type of monosaccharide present. The
two classes of polysaccharides are homo-polysaccharides and hetero-polysaccharides.
(a) Homopolysaccharides. They are entirely made up of one type of monosaccharides.
On
hydrolysis, they yield only one kind of monosaccharide.
(b) Heteropolysaccharides. They are made up of more than one type of
monosaccharides.
On hydrolysis they yield more than one type of monosaccharides.. D-(+) - glucose is
the most common monosaccharide found in nature. Most of it stored as
food for plants and animals, it is not stored as glucose, which is too soluble
in water, so it is converted into polymers of glucose which are less soluble.
The most important polysaccharides in nature are starch, glycogen and
cellulose, which are all made of same monomer (D-glucose). They differ in
the way the individual glucose molecules are bonded together.
Homopolysaccharide
They are entirely made up of one type of monosaccharides. On hydrolysis,
they yield only one kind of monosaccharide
1- Starch: the storage polysaccharide of higher plants and the major source
of carbohydrates in the human diet, it is mixture of 20% amylose and 80%
amylopectine
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Amylose: a liner polymer of D-glucose units in α (1-4) linkage
Amylopectins: branched chain polymer of D-glucose units in α1-4 and
1-6 linkages.
Amylopectine differ from amylose in that the glucose molecule of polymer
from an additional α-glycoside bond at C-6, this result in a branched-chain
structure for amylopectine, the branching occurs at one in every 20 to 25
glucose molecules in the chain.
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-Glucose residues (α-1-4 linkage) removed by the enzyme α-amylase
-α (1-6) branch point or linkage cleaved by the enzyme α (1-6) glucosidase
(debranching enzymes).
Starch is non reducing sugar because it have no reducing power, it contains
small amount of free anomers corresponding to the high number of the
glucose units.
Amylose
2- Glycogen: is the storage polysaccharide in animals and sometimes called
animal starch. It is branched polymer of glucose molecules joined by α-(1
4) linkage and α-(1
6) glycoside bonds. So glycogen has a similar
structure as amylopectin but it is more highly branched
The major source glycogen is the starch in the food we eat. Starch is
hydrolysed enzymes in the intestine to glucose. The excess glucose is
converted to glycogen and stored in the body and it hydrolysed to glucose as
needed by the body e.g during fasting or between meals
3- Cellulose: is found in plants, used to form cell wall and other structural
features of plants, it is like starch made up of D(+) glucose molecules
joined head to tail, but it differs in two ways
a1,4 glycoside bonds in cellulose are β-rather than α
bThe D(+) glucose molecules are all arranged in long chains, there are
very few branches
So the cellulose chains can be tightly packed and the arrangement allows many
hydrogen bonds to be formed between OH groups of adjacent chains so these
chains are quite strong and stable.
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Human cannot digest cellulose because they do not have the necessary
enzymes to hydrolyse the β-1-4 glycoside bond of it but many microorganisms
have these enzymes so they can digest cellulose. These microorganisms are
present in higher animals so they can digest it
4- Inulin: it is a polysaccharide of fructose with low molecular weight (5000).
It is used in assessing the glomerular filtration (G.F.R) in the study of
kidney function
5- Dextran
Structure It has structure similar to amylopectin. In the linear part, glucose
units are linked by α(1→6) glycosidic bond and α(1→3) glycosidic linkage is
present between glucose unit at branch points. It is polysaccharide present in
bacteria. Medical importance of dextran to maintain plasma volume dextran is
used in clinical medicine. Dental plaque is due to dextran synthesized from
sucrose by oral bacteria
Hetropolysaccharides of animals
They are also called as mucopolysaccharides and glycosaminoglycans.
Mucopolysaccharides consist of repeating disaccharide units. The disaccharide
consist of two types monosaccharides.The mucopolysaccharides are
component of connective tissue. Hence, they are often referred
as structural polysaccharides. The mucopolysaccharides are also found in
mucous secretions. The mucopolysaccharides combines with proteins like
collagen and elastin and forms extracellular medium or ground substance of
connective tissue. Mucopolysaccharides are also components of extracellular
matrix of bone, cartilage and tendons. The complex of mucopolysaccharide
and protein is called as proteoglycan. Mucopolysaccharides also function
as lubricants and shock absorbers.
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Hyaluronic acid, heparin, chondroitin sulphate, blood group substance and
sialic acids belong to this category
Hetropolysaccharides (mucopolysaccharides)
They are usually composed of amino sugar and uronic acid units as the
principle components, though some are chiefly made up of amino sugar and
monosaccharides units without the presence of uronic acid. The hexoamine
present is generally acetylated. They are essential components of connective
tissues. They are generally present either in free form or in combination with
proteins “mucoprotiens” and “glycopprotiens”.
Hyaluronic acids
A polysaccharide composed of equimolar portion of D-glucosamine acetate
and D-glucoronic acid residues, forms a viscous gel like material present in
connective tissues, eyes, synovial fluid, skin, joints. It functions as cementing
substances between the cells of connective tissue, because of its viscosity,
resists penetration by foreign matter, i.e. infection by bacteria.
It is attacked and liquefied by an enzyme (hyaluronidase) present in some
bacteria
Hyaluronic acid
Structure
The repeating disaccharide of hyaluronic acid consist of glucuronic acid and
Nacetylglucosamine. Functions
1. It is present in synovial fluid and function as lubricant.
2. It is also present in skin, loose connective tissue, umbilical cord and ovum.
3. It is present in vitreous body of eye.
Medical importance
1. As the age advances hyaluronic acid is replaced by-dermatan sulfate in
synovial fluid.
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Dermatan sulfate is not a good lubricant, hence age related pains develop in
old people.
2. In young people, vitreous is clear elastic gel in which hyaluronic acid is
associated with collagen. As the age advances the elasticity of vitreous is
reduced due to decreased association
between collagen and hyaluronic acid. As a result, vision is affected in older
people.
3. Hyaluronic acid of tumour cells has role in migration of these cells.
4. Hyaluronic acid is involved in wound healing (repair). In the initial phase of
wound healing (repair), hyaluronic acid concentration increases many fold at
the wound site. It allows rapid migration of the cells to the site of connective
tissue development.
5. Hyaluronic acid helps in scarless repair. If suitable levels of HA are
maintained during would healing scar formation is reduced or even prevented.
6. HA content of skin decreases as age advances this is the reason for increased
susceptibility of aged people for scar formation.
7. Pneumonia, meningitis and bacteremia causing pathogenic bacteria contains
hyaluronte lyase. Hydrolysis of HA by this enzyme facilitates invasion of host by
these bacteria.
2-Chondroitin sulfate A and B chondroitin-4-sulfate and chondroitin-6-sulfate
Structure
1. The repeating disaccharide unit of chondroitin sulfates consist of glucuronic
acid and N-acetyl galactosamine. N-acetyl galactosamine is sulfated.
2. In chondroitin-4-sulfate, 4th carbon atom of N-acetyl galactosamine is
sulfated where as in chondroitin-6-sulfate the 6th carbon is sulfated.
Functions
1. Chondroitin sulfates are components of cartilage, bone and tendons.
2. They are also present in the cornea and retina of the eye.
3. Chondroitin sulfate content decreases in cartilage as age advances.
3-Heparin
Structure
1. The repeating disaccharide unit of heparin consist of glucosamine and .
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either iduronic acid or glucuronic acid.
2. Majority of uronic acids are iduronic acids. Further amino groups of
glucosamine is sulfated
Functions
1. Heparin is a normal anti-coagulant present blood.
2. It is produced by mast cells present in the arteries, liver, lung and skin.
3. Unlike other glycosaminoglycans, heparin is an intracellular component.
Bacterial cell walls
The rigid component of bacterial cell walls is a hetropolymer of alternating (β14) linked N-acetylglucosamine and N-acetylmuramic acid residues. The linear
polymers lie side in the cell wall, cross-linked by short peptides, the exact
structure of which depends on the bacterial species.
The peptide cross -links weld the polysaccharide chains into a strong sheath
that envelops the entire cell and prevents cellular swelling and lysis due to the
osmotic entry of water.
The enzyme lysozyme kill bacteria by hydrolysing the (β1 4) glycosidic bond
between N-acetylglucosamine and N-acetylmuramic acid. Lysozyme present in
tears, presumably as a defense against bacterial infection of the eye. It also
produced by certain bacterial viruses to ensure their release from host
bacterial cell wall, an essential step of viral infection cycle
Penicillin and related antibiotics kill bacteria by preventing synthesis of the
cross-linking, leaving the cell wall too weak to resist osomatic lysis
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