1- Carbohydrates.pptx

Carbohydrates
Lecture 1,2&3
Noor Ullah
B.Sc MLT, M.Phil Biochemistry/ Mol. Biology
PhD Scholar Biochemistry
Lecturer MLT, KMU IPMS

Carbohydrates
• Carbohydrates- Carbon hydrates ( C- H20)
• Polyhydroxy aldehyde or ketones
• Many, but not all, carbohydrates have the empirical formula (C- H20)n
• Most abundant biomolecules on the earth
• Three major classes of carbohydrates:
• Monosaccharides, oligosaccharides, and polysaccharides
• (The word “saccharide” is derived from the Greek sakcharon, meaning
“sugar”).
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Khyber Medical University Institute of Paramedical Sciences

Monosaccharides
• Monosaccharides are the simplest carbohydrates
• They are colorless, crystalline solids that are freely soluble in water but insoluble
in nonpolar solvents.
• The simplest monosaccharides are the two three-carbon trioses: glyceraldehyde
and dihydroxyacetone.
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Monosaccharides
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Monosaccharides
• Monosaccharides with four, five, six, and seven carbon atoms in their backbones
are called, respectively, tetroses, pentoses, hexoses, and heptoses.
• The D-glucose and D-fructose are the most common monosaccharides in
nature.
• The aldopentoses, D-ribose and 2-deoxy-D-ribose are components of nucleotides
and nucleic acids.
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Monosaccharides
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Asymmetric or Chiral Centers
• Carbon to which four different atoms or groups are attached is called
asymmetric.
• All the monosaccharides except dihydroxyacetone contain one or more
asymmetric (chiral) carbon atoms and thus occur in optically active isomeric
forms- Enantiomers or Mirror images.
• To represent three-dimensional sugar structures on paper, we often use Fischer
projection formulas.
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Epimers
• Two sugars that differ only in the configuration around one carbon atom are
called epimers.
• D-glucose and D-mannose, which differ only at C-2 and D-glucose and D-
galactose (which differ at C-4) .
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Epimers
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Hemiacetals and hemiketals
• They are formed when an alcohol oxygen
atom adds to the carbonyl carbon of an
aldehyde or a ketone.
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Cyclization of glucose to its hemiacetal form
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Cyclization of Glucose
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Anomers and ring structure
• Isomeric forms of monosaccharides that differ only in their configuration about
the hemiacetal or hemiketal carbon atom are called anomers, and the carbonyl
carbon atom is called the anomeric carbon.
• Pyranose is a six-membered ring with Oxygen bridge between C no.1 and 5.
• Furanose is a five-membered ring with oxygen bridge between C no.1 and 4.
• The systematic names for the two ring forms of D-glucose are therefore alpha-D-
glucopyranose and beta-D-glucopyranose.
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Pyranose and Furanose ring structure
Pyranoses and furanoses. The pyranose forms of D-glucose and the furanose forms of D-fructose are shown
here as Haworth perspective formulas. The edges of the ring nearest the reader are represented by bold lines.
Pyran and furan are shown for comparison.
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Anomers—mutarotation
• Mutarotation- the change in the specific optical rotation representing the
interconversion of α and β forms of D-glucose to an equilibrium mixture
• The α and β anomers of glucose have different optical rotations
• The specific optical rotation of a freshly prepared glucose (α anomer) solution in
water is +112.2° which gradually changes and attains an equilibrium with a
constant value of +52.7°
• In the presence of alkali, the decrease in optical rotation is rapid
• The optical rotation of β-glucose is +18.7°
• α-D-Glucose (+112.2°) ↔Equilibrium mixture (+52.7°) ↔ β-D-Glucose(+18.7°)
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Reactions of monosaccharides- Tautomerism
• The process of shifting a hydrogen
atom from one carbon atom to
another to produce enediols is known
as tautomerization
• Sugars possessing anomeric carbon
atom undergo tautomerization in
alkaline solutions
• When glucose is kept in alkaline
solution for several hours, it undergoes
isomerization to form D-fructose and
D-mannose through a common
intermediate- enediol- two OH groups
are attached to the double bonded
carbon
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Reducing Sugars
• All sugars having a free aldehyde or keto group of
anomeric carbon are reducing
• All Monosaccharides such as Glucose, fructose,
glyceraldehyde, galactose etc. and disaccharides
lactose and maltose reduce cupric ions (Cu2+) of
copper sulphate to cuprous ions (Cu+), which form
a yellow precipitate of cuprous hydroxide or a red
precipitate of cuprous oxide
• Basis of Benedict’s test, Fehling’s test, Barfoed’s
test

Oxidation
• The terminal aldehyde (or keto) or the terminal alcohol or both the groups may
be oxidized
• Oxidation of aldehyde group (CHO→COOH) results in the formation of gluconic
acid
• Oxidation of terminal alcohol group(CH2OH → COOH) leads to the production of
glucuronic acid

Reduction
• When treated with reducing agents such as sodium amalgam, the aldehyde or
keto group of monosaccharide is reduced to corresponding alcohols
• D-Glucose → D-Sorbitol
• D-Galactose → D-Dulcitol
• D-Mannose → D-Mannitol
• D-Fructose → D-Mannitol + D-Sorbitol
• D-Ribose → D-Ribitol

Dehydration
• When treated with concentrated sulfuric acid,
monosaccharides undergo dehydration with
an elimination of 3 water molecules
• Thus hexoses give hydroxymethyl furfural
while pentoses give furfural on dehydration
• These furfurals can condense with phenolic
compounds (α -naphthol) to form colored
products
• Chemical basis of Molisch test

Glycosides
• When the hydroxyl group of one monosaccharide reacts with the hydroxyl group
of another monosaccharide, it forms glycosidic linkage.
• Glycosides are compounds in which a monosaccharide is attached at the
anomeric carbon to an alcohol residue of non-carbohydrate.
• The non-carbohydrate residue is called aglycon.
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Physiologically important glycosides
1. Glucovanillin (vanillin-D-glucoside) is a natural substance that imparts vanilla
flavor.
2. Cardiac glycosides (steroidal glycosides): Digoxin and digitoxin contain the
aglycone steroid and they stimulate muscle contraction.
3. Streptomycin, an antibiotic used in the treatment of tuberculosis is a glycoside.
4. Ouabain inhibits Na+ –K+ ATPase and blocks the active transport of Na+.
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Physiological important derivatives of monosaccharides

Ring structure- Haworth formula
• Cyclic structures of sugars are more accurately represented in Haworth
perspective formulas than in the Fischer projection commonly used for linear
sugar structures.
• In Haworth projections the six-membered ring is tilted to make its plane almost
perpendicular to that of the paper, with the bonds closest to the reader drawn
thicker than those farther away and numbering of the carbons is done in a
clockwise direction beginning with the anomeric carbon.
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Haworth projections
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Disaccharides
• Disaccharides consist of two monosaccharides joined covalently by an O-
glycosidic bond, which is formed when a hydroxyl group of one sugar molecule,
typically cyclic, reacts with the anomeric carbon of the other .
• Because maltose has a free OH group at C-1 of glucose, it is a reducing sugar
• Maltose, lactose, and sucrose
• Isomaltose, Cellobiose
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Disaccharides

Disaccharides
• In isomaltose, the glucose units are held
together by α (1 → 6) glycosidic linkage
• Cellobiose is a disaccharide, identical in
structure with maltose, except it has β(1
→ 4) glycosidic linkage
• Cellobiose is formed during the hydrolysis
of cellulose

Sucrose
• Sucrose (cane sugar) is the sugar of commerce, mostly produced by sugar cane
and sugar beets
• Sucrose is made up of α-D-glucose and β-D-fructose
• The two monosaccharides are held together by a glycosidic bond (α1 β2),
between C1 of α-glucose and C2 of β-fructose
• Non-reducing-the reducing groups of glucose and fructose are involved in
glycosidic bond
• It is sweeter than glucose, lactose and maltose
• Sucrase hydrolyses sucrose to glucose and fructose which are absorbed

Inversion of sucrose
• The process of change in optical rotation from dextrorotatory (+) to levorotatory
(–) is referred to as inversion
• Sucrose is dextrorotatory (+66.5°)
• Invert sugar- The hydrolyzed mixture of sucrose (glucose and fructose)
• Sucrase (invertase) or dilute acids hydrolyze sucrose to glucose and fructose
• When hydrolyzed, sucrose is first split into α-D-glucopyranose (+52.5°) and β-D-
fructofuranose- both are dextrorotatory
• However, β-D-fructofuranose is less stable and immediately gets converted to α-
D-fructopyranose which is strongly levorotatory (–92°)
• The overall effect is that dextro sucrose (+66.5°) on inversion is converted to
levo form (–28.2°).

Inversion of sucrose

Polysaccharides
• Most carbohydrates found in nature occur as polysaccharides
• Polysaccharides, also called glycans, 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, and in the degree of branching.
• Homopolysaccharides contain only a single monomeric species
• Heteropolysaccharides contain two or more different kinds
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Glycogen
branches every
8-12 glucose units
Amylopectin
branches about every
24-30 linear linkages
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Heteropolysaccharides
• These heteropolysaccharides, the glycosaminoglycans, are a family of linear
polymers composed of repeating disaccharide units. They are unique to animals
and bacteria and are not found in plants.
• Hyaluronic acid forms clear, highly viscous solutions that serve as lubricants in
the synovial fluid of joints and give the vitreous humor of the vertebrate eye its
jellylike consistency (the Greek hyalos means “glass”.
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Heteropolysaccharides
• Hyaluronan is also a component of the extracellular matrix of cartilage and
tendons, to which it contributes tensile strength and elasticity.
• Hyaluronidase, an enzyme secreted by some pathogenic bacteria, can hydrolyze
the glycosidic linkages of hyaluronan, rendering tissues more susceptible to
bacterial invasion.
• Hyaluronidase is also present in high concentration in testes, seminal fluid, and
in certain snake and insect venoms.
• Hyaluronidase of semen is assigned an important role in fertilization as this
enzyme clears the gel (hyaluronic acid) around the ovum allowing a better
penetration of sperm into the ovum.
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Chondroitin Sulfate
• Chondroitin sulfate (Greek chondros, “cartilage”) contributes to the tensile
strength of cartilage, tendons, ligaments, and the walls of the aorta.
• Dermatan sulfate (Greek derma, “skin”) contributes to the pliability of skin and is
also present in blood vessels and heart valves.
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Agar
• Agar is a complex mixture of polysaccharides.
• The remarkable gel-forming property of agarose makes it useful in the
biochemistry laboratory.
• Agarose gels are used as inert supports for the electrophoretic separation of
nucleic acids, an essential part of the DNA-sequencing process.
• Agar is also used to form a surface for the growth of bacterial colonies.
• Another commercial use of agar is for the capsules in which some vitamins and
drugs are packaged.
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Glycoproteins
• Proteins covalently bound to carbohydrates- glycoproteins
• The carbohydrate fraction varies from 1% to 90% by weight
• Mucoproteins- glycoprotein with > 4% carbohydrate contents
• Widely distributed in the cells and perform variety of functions
• Role as enzymes, hormones, transport proteins, structural proteins and
receptors
• The carbohydrates found in glycoproteins include mannose, galactose, N-acetyl-
glucosamine, N-acetylgalactosamine, xylose, L-fucose and N-acetylneuraminic
acid (NANA)

Antifreeze glycoproteins
• The Antarctic fish live below –2°C, a temperature at which the blood would
freeze
• These fish contain antifreeze glycoprotein which lower the freezing point of water
and interfere with the crystal formation of ice
• Antifreeze glycoproteins consist of 50 repeating units of the tripeptide- alanine-
alanine-threonine
• Each threonine residue is bound to β-galactosyl (1→3) α N-acetylgalactosamine

Blood group substances
• The blood group antigens on RBCs contain carbohydrates as glycoproteins or
glycolipids
• N-Acetylgalactosamine, galactose, fucose, sialic acid etc. are found in the blood
group antigens
• The carbohydrate content also plays a determinant role in blood grouping

Biomedical Importance of Carbohydrates
• Chief source of energy.
• Constituents of compound lipids and conjugated proteins.
• Certain carbohydrate derivatives are used as drugs like cardiac
glycosides/antibiotics.
• Lactose principal sugar of milk—in lactating mammary gland.
• Degradation products utilized for synthesis of other substances such as fatty
acids, cholesterol, amino acid, etc.
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• Constituents of mucopolysaccharides which form the ground substance of
mesenchymal tissues.
• Inherited deficiency of certain enzymes in metabolic pathways of different
carbohydrates can cause diseases, e.g. galactosemia, glycogen storage diseases
(GSDs), lactose intolerance, etc.
• Derangement of glucose metabolism is seen in diabetes mellitus.
• Seminal fluid is rich in fructose and sperms utilise fructose for energy. Fructose is
formed in the seminiferous tubular epithelial cells from glucose.
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• Various food preparations, such as baby are produced by hydrolysis of grains and
contain large amounts of maltose.
• From nutritional point of view they are thus easily digestible.
• In lactating mammary gland, the lactose is synthesized from glucose by the duct
epithelium and lactose present in breast milk is a good source of energy for the
newborn baby.
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• Glycosides are found in many drugs, spices and in the constituents of animal
tissues.
• Cardiac glycosides- digitalis
• Agar
• In human: Used as laxative in constipation. Like cellulose, it is not digested, hence
add bulk to the faces (“roughage” value) and helps in its propulsion.
• In microbiology: Agar is available in purified form. It dissolves in hot water and on
cooling it sets like gel. It is used in agar plate for culture of bacteria.
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