CARBOHYDRATES
Dr. Vidya.D
Asst. Professor,
College of Pharmacy,
Prince Sattam Bin Abdul Aziz University,
Kingdom of Saudi Arabia
PHL – 213
Biochemistry - I
Objectives
Identify the functions of carbohydrates.
Name the primary sources of carbohydrates.
Describe the classification of carbohydrates.
Facts
Primary source of energy for the body
Least expensive and most abundant of the energy
nutrients
Named for the chemical elements they are composed
of—carbon, hydrogen, and oxygen
Functions
 Provide energy
 Protein-sparing action
 Normal fat metabolism
 Provide fiber
Providing Energy
 Each gram of carbohydrate provides 4 calories.
 A body needs a constant energy supply.
 A half day’s supply of carbohydrates is stored in the
liver and muscles for use as needed.
 Stored form is called glycogen.
Protein-Sparing Action
The primary function of proteins is to build and
repair tissues.
When enough carbohydrates (at least 50–100 g/day)
are ingested, proteins are spared to be used for their
primary function.
Normal Fat Metabolism
Without an adequate supply of carbohydrates, fat is
not metabolized to meet energy requirements.
Ketones are produced as a byproduct of fat
metabolism.
Ketosis may result.
Providing Fiber
 Dietary fiber is found in grains, vegetables, and
fruits.
 Recommended intake is 20–35 g/day.
 Fiber lowers blood glucose levels; may prevent some
colon cancers; and helps prevent constipation,
hemorrhoids, and diverticular disease by softening
stool.
Food Sources
 Principal sources of
carbohydrates are
plant foods:

e.g. Cereal grains,
Vegetables, Fruits,
Nuts, Sugars
 The only substantial
animal source is milk.
Simple (or )
small
sugars
Monosaccharides
Aldoses
Isomers
Keto
group
C=O
Same
chemical
formula
Epimers
Enantiomers
Differ in
configuration
around one specific
carbon atom
Disaccharides
sucrose = glucose + fructose
Lactose = galactose + glucose
Maltose = glucose + glucose
Mirror images
of each other
Oligosaccharides
Complex (or ) large
sugars
Aldehyde
group
H-C=O
Ketoses
Polysaccharides
Homo-
Hetero-
Starch, glycogen,
cellulose
GAGs
Monosaccharides
Sweet In taste
Not hydrolysable
Have three to seven carbons
3 carbons=Triose
4 carbons=Tetrose
5 carbons=Pentose
6 carbons=Hexose
7 carbons=Heptose
Structure of monosaccharide
Fisher
projection
Haworth
projection
• The straight
chain
structural
formula
• Cyclic
formula or
ring
structure
X-ray
diffraction
analysis
• Boat and
chair form
Straight chain
Ring structure
Chair form
Isomerism
 The compounds possessing identical molecular
formula but different structures are called isomers.
Various types of isomerism
1. Structural isomerism
2. Stereoisomerism
Structural isomerism
 Same molecular formulae but differ from each other
by having different structures.
Stereoisomerism
 Same molecular formula and same structure but they
differ in configuration.
 That is arrangement of their atoms in space.
 Presence of asymmetric carbon atoms allow the
formation of stereoisomerism
Stereoisomerism
 The important types of stereoisomerism associated
with glucose are
D and L isomerism
Optical isomerism
Epimerism
α and βanomerism
D and L isomerism
Optical isomerism
 Optical activity is the capacity of a substance to
rotate the plane polarized light passing through it.
Clockwise direction
• Dextrorotatory(d) or (+)
Counterclockwise direction
• Levorotatory(l)or (-)
Optical isomerism
Epimerism
 Epimerism is the stereoisomerism if two
monosaccharides differ from each other in their
configuration around a single specific carbon(other
than anomeric) atom.
Epimerism
Anomerism
 These are isomers obtained from the change of
position of hydroxyl group attached to the anomeric
carbon e.g.  and  glucose are 2 anomers.
 Also  and  fructose are 2 anomers.
Anomerism
Mutarotaion
 Mutarotaion is defined as the change in the specific
optical rotation by the interconversion of α and β
forms of D glucose to an equilibrium mixture
 Types:
Aldoses
2. Ketoses
Aldoses contain Aldehydic group –CHO
Ketoses contain Ketonic group –CO1.
Name of the
sugar
Role IN the body
Example
Triose
Important in cellular respiration, in
the glycolysis step.
D-glyceraldehyde
L- glyceraldehye
Dihydroxyacetone
Pentoses
They form the backbone of
polysaccharides, Proteins, Lipids &
nucleic acids
Ribose
Ribulose
Hexoses
Glucose - Primary energy molecule
Fructose - energy molecule in semen
Galactose - dairy products, sugar
beets, gums and mucilage
Glucose, Fructose
Galactose, Mannose
Talose, Allose, Idose
Mannose - it forms part of
glycolipids & glycoproteins in several
tissues.
Derivatives of monosaccharides
1) Sugar phosphates
Metabolized as phosphate esters
2) Deoxy sugars
Hydrogen atoms replaces -OH group on C-2.
Important to structure of nucleic acids.
3) Amino sugars
Amino group (NH-) substituted for -OH group in monosaccharide.
4) Sugar alcohols
Replace carbonyl oxygen to form polyhydroxy alcohols
e.g. glycerol --> glyceraldehyde
Replace “-ose” with “-itol”.
Ribose --> ribitol
5) Sugar acids
Oxidation of carbonyl carbon or highest carbon.
glucose --> gluconate or glucuronate
Important in many polysaccharides.
6) Ascorbic acid
Derived from D-glucuronate.
Primates cannot do the conversion, so must be supplied in the diet.
Glucose
 Polyhydroxy aldehyde
 Dextrose=Dextrorotatory
 Grape sugar
 Blood Sugar - 110mg/1000mL in blood
 Energy source for the body
 Combines with others to form disaccharides
Importance of Glucose
 Most widely used Hexose
 An energy source
 A precursor forms: Cellulose, Glycogen, Starch etc.
 Hypoglycemia and Hyperglycemia
Importance of Fructose
 Found in foods and Drinks
 1 to 2 times sweeter than table sugar
 Used as artificial sweetener
 Anaerobic fermentation raw material for bacteria
and yeasts.
 Apricots, apples, grapes etc.
Fructose in body(from Sucrose)
Structure of Oligosaccharides
 Disaccharides
Disaccharides
Reducing
Maltose
Lactose
Isomaltose
Non-reducing
Sucrose
DISACCHARIDES
 These are glycosides formed by the condensation of
2 simple sugars.
 If the glycosidic linkage involves the carbonyl
groups of both sugars (
disaccharide is
) the resulting
 On the other hand, if the glycosidic linkage involves
the carbonyl group of only one of the 2 sugars (as in
maltose and lactose) the resulting disaccharide is
reducing.
POLYSACCHARIDES
 These are formed by the condensation of
n molecules of
monosaccharides with the removal of n-1 molecules of
water.
 Since condensation involves the carbonyl groups of the
sugars, leaving only one free carbonyl group at the end of
a big molecule, polysaccharides are non-reducing.
 They are of 2 types:
1.
2.
Homopolysaccharides (or) Homoglycans - composed of one type
of monosaccharide(e.g. Starch, Glycogen, cellulose).
Heteropolysaccharides (or ) Heteroglycans – composed of more
than one type of monosaccharide (e.g. glycosaminoglycans,
glycoproteins)
POLYSACCHARIDES
Often classified according to their biological role:
1) starch and glycogen - storage polysaccharides
Both are homoglycans.
Starch is storage form in plants and fungi.
Glycogen is storage form in animals.
Bacteria contain both.
Starch
Starch - mixture of amylose and amylopectin
amylose is an unbranched polymer of 100-1000 D-glucose in an a-(1
--> 4) glycosidic linkage.
amylopectin is a branched polymer a-(1--> 6) branches of residues
in an a-(1 --> 4) linkage; overall between 300-6000 glucose
residues, with branches once every 25 residues; side chains are 1525 residues long
α-amylase is an endoglycosidase found in human saliva but also
plants that randomly hydrolyzes the a (1--> 4) bond of amylose and
amylopectin.
β-amylase is an exoglycosidase found in higher plants that
hydrolyzes maltose residues from non-reducing ends of
amylopectin.
-1,6 linkage
between two glucose
units
- 1,4 linkage between
two glucose units
Glycogen
 Glycogen - branched polymer of glucose residues
with branches every 8-12 residues with branches
containing as many as 50,000 glucose residues
Cellulose & Chitin - structural
polysaccharides
Cellulose - straight chain homoglycan of glucose with b(1--> 4) linkages with alternating glucose molecules;
ranges in size from 300-15,000 glucose residues
 Extensive H-bonding within and between cellulose
chains.
 Makes bundles or fibrils ---> rigid.
Chitin - linear polymer of N-acetylglucosamine residues
 Alternating 180o with b - (1 --> 4) linkage.
 Lots of H-bonding between adjacent strands.
Heteroglycans (or) Hetropolysaccharides
(or) Glycoconjugates
Proteoglycans
 Complexes
of
polysaccharides
called
glycosaminoglycans & core proteins.
 Found in extracellular matrix of connective tissues.
 Glycosaminoglycans


are unbranched heteroglycans
made of disaccharide units (amino sugar, Dgalactosamine or D-glucosamine & alduronic acid).
e.g. hyaluronic acid Found in cartilage and synovial
fluid.
 Proteoglycan cartilage
Peptidoglycans
 Found in cell wall of bacteria.
 Composed of alternating residues of N-
acetylglucosamine and N-acetylmuramic acid
joined by b- (1--> 4) linkages.
Glycoproteins
 Proteins with oligosaccharides attached.
 Carbohydrate chains are from 1-30 residues in length.
 Examples: enzymes, hormones, structural proteins,
transport proteins.
 Found in eucaryotic cells.
 Can be attached to proteins with one of two
configurations:
O-linked - carbohydrate bonded to -OH of serine or threonine
N-linked - carbohydrate (usually N-acetylglucosamine) linked
to asparagines
ROLES OF CARBOHYDRATES IN BIOLOGY
Carbohydrates serve as information-rich molecules that
guide many biological processes.
Examples include:
1) Asialoglycoprotein receptor
 Present in liver cells; binds to asialoglycoproteins to
remove them from circulation
 Presence of sialoglycoprotein prevents glycoproteins
such as antibodies and peptide hormones from being
internalized
 Presence of sialic acid on terminal galactose on these
proteins mark the passage of time; when they are
removed (usually by the protein itself), the glycoproteins
are removed from circulation.
2) Lectins
Carbohydrate-binding proteins of plant origin.
Contain 2 or more binding sites for carbohydrate units ->
cross-link or agglutinate erythrocytes and other cells.
3) Many viruses and bacteria can gain entry into host cells via
carbohydrates displayed on cell surface.
Influenza virus contains a hemagglutinin protein that
recognizes sialic acid residues on cells lining respiratory tract.
Neisseria gonorrhoeae infects human genital or oral
epithelial cells because of recognition of cell surface
carbohydrates; other cells lack these carbohydrates.
4) Interaction of sperm with ovulated eggs
Contd..
 Ovulated eggs contain zona pellucida, an extracellular coat made of O-
linked oligosaccharides.
 Sperm cells have receptor for these carbohydrates.
 Binding of sperm to egg causes release of proteases and hyaluronidase,
which dissolve zona pellucida to allow sperm entry.
5) Selectins
 Carbohydrate-binding
adhesion proteins that mediate binding of
neutrophils and other leukocytes to sites of injury in the inflammatory
response.
6) Homing receptor of lymphocytes
 Homing is phenomenon in which lymphocytes tend to migrate to lymphoid
sites from which they were originally derived.
 Mediated by carbohydrates on lymphocyte surface and endothelial lining of
lymph nodes.