Carbohydrates

UNIT – III
COMPOSITION
&
METABOLISM
OF
CARBOHYDRATES
Ms. Kalaivani Sathish

INTRODUCTION
 Carbohydrates includes large group of compounds
commonly known as starches or sugars.
 Chemically, they are described as polyhydric alcohols,
having potentially active aldehyde and ketone groups.
 Carbohydrates are in general white soluble in water
with the exception of certain polysacchraides.
 Lower molecular weight and sweet taste.

Biological Significance
 Fuel: when carbohydrates oxidised it gives carbon
dioxide, water and energy.
 Catalysts: Promotes oxidation of food stuffs.
 Starting Materials: certain carbohydrates are
the starting materials for the biological synthesis if
componds such as fatty acids and amino acids.

Occurrence
 Distributed in plants and animals.
 In form of starch in plants & in glycogen form
in animals.
 Found in roots. Tubers, leaves, vegetables and
grains in plants.
 In animals muscles and liver it is present in a
huge amount.
 Starch and glycogen are polysaccharides having
high molecular weight.

Occurrence (Cont..........)
 Lower molecular weight carbohydrates are present
in nature and they comes under the group
monosaccharides and oligosaccharides. (Crystalline
substances, Sweeter in taste).
 Sucrose is obtained from sugarcane, nectar of
flowers.
 Glucosides are the derivatives of carbohydrates
which are used for the treatment of heart diseases.
 Glucose and fructose are simple sugars are widely
distributed in plans.
 Glucose is present in blood and other body fluids.
Lactose is present in milk.

Chemical Characteristics
 Carbon. Hydrogen. Oxygen are present.
 Molecular formula for glucose is C6 H12 O6.
 All simple sugars contains a potential aldehyde and ketone
group.

Classification
Monosaccharides
Disaccharides
Oligosaccharides
Polysaccharides

 On hydrolysis these molecules yields 2
molecules of monosaccharides.
 E.g., Sucrose, Lactose and Maltose
 C12 H22 O11

 On Hydrolysis they yields 2 to 10
monosaccharide units.
 Disaccharides also comes under this
category.
 Examples: Raffinose – Tri Saccharide
 Stachyose – Tetra Saccharide
 Verbascose – Penta Saccharide

Polysaccharides
 They yields more than ten molecules of
monosaccharides on hydrolysis.
 E.g., Starch, Glycogen, Dextrin, Cellulose and
Inulin.
 (C6 H10 O5)x

Mucopolysaccharides
 Mucopolysaccharides are long chains of sugar
molecules that are found throughout the body,
often in mucus and in fluid around the joints.
They are more commonly called
glycosaminoglycans.

Mucopolysaccharides
Importances
 It is an essential component of tissue structure.
 Hyaluronic acid, Heparin, dermatan sulfate and
keratan sulfate are important mucopolysachrides.
 Hyaluronic acid serves as a lubricant and shock
absorbent in joints.
 Heparin is an anticoagulants, which prevents blood
clotting.
 Dermatan sulafate is mostly present in skin.

DISACCHARIDES
 Sugars containing 2 molecules of
monosaccharides.
 On condensation between 2 monosaccharides
the union takes place between C- 1 of Second
monosaccharide and C – 4 of First
monosaccharide.
 The C- 1 of first monosaccharide is free and
has its potential alsehyde intact.
 Examples: Maltose, Lactose and Sucrose.

Maltose
 Maltose is composed of two glucose units.
 Maltose is the end product of digestion of
starch by the action of salivary amylase Ptyalin
in the mouth and Pancreatic amylase in the
intestines.

Lactose
 Also known as milk sugar occurs in the milk of
mammals.
 On hydrolysis by enzyme lactase it forms one
molecule of glucose and one molecule of
galactose.
 Lactose is present in the urine of pregnant
women and lactating mother.

Sucrose
 Sucrose is a cane sugar.
 Used as sugar in food.
 Obtained from sugarcane, beetroot, pineapple,
honey, fruits and certain palms.
 It is non – reducing sugar.
 It contains one molecule of fructose and one
molecules of glucose.

METABOLISM PATHWAYS
OF
GLUCOSE

INTRODUCTION
 The metabolism of glucose or glycogen may
be considered under the following sub
divisions.
 Glycolysis – The oxidation of glucose or
glycogen to pyruvate and lactate by the
Embden Myerhof Pathway.
 Glycogenesis – The synthesis of glycogen
from glucose.
 Glycogenolysis – Breakdown of liver
glycogen.

INTRODUCTION (Cont ...)
 Oxidation of Pyruvate to acetyl – CoA -
Oxidation of pyruvate, prior to entry into the
citric acid cycle.
 The Hexose Monophosphate Shunt
Pathway – This is an alternate aerobic
pathway for oxidation of glucose.
 Gluconeogenesis – Formation of glucose or
glycogen from non carbohydrate sources.

GLYCOLYSIS
 Glycolysis = “Glyco” – Glucose. “Lysis” – Split.
 Embden Meyerhof Pathway
 In this process we Starts with Glucose (6 C)
and it will ends with 2 X Pyruate (3 C).
 Two Phases of Glycolysis:
1. Phase – I. Energy Investment Phase
2. Phase – 2. Energy Generation Phase

 Glycolysis process is a 10 step process.
 First 5 Steps in the glycolysis Process is
Energy Investment Phase. Second 5 Steps of
the glycolysis process is energy generation
phase.
 In each phases of the glycolysis process 5
enzymes are involved.
 Totally 10 Enzymes are involved in the
glycolysis process.

 Out of 5 enzyme reactions in energy
investment phase 2 reactions are irreversible
and 3 are reversible.
 Out of 5 enzyme reactions in energy
generation phase 1 reaction is irreversible and
4 are reversible.

Enzymes Involved in Glycolysis
 Enzyme Investment
Phase
 Hexokinase
 Phosphohexose
Isomerase
 Phospho fructokinase
– 1
 Aldolase
 Triose Phosphate
Isomerase
 Energy Generation
Phase
 Glceraldhyde 3 –
Phosphate
Dehydrogenase
 Phospho Glycerate
Kinase
 Phospho Glycerate
Mutase
 Enolase
 Pyruvate Kinase

Four Stages
of
Embden Meyerhof Pathway
 Initial Phosphorylation
 Conversion to Trioses
 Oxidative Step
 Formulation of Pyruvic Acid and Lactic
Acid

Initial Phosphorylation
 Glucose enters into the glycolytic pathway by
phosphorylation to glucose – 6 – phosphate.
 Hexokinase enzyme catalyses the
phosphorylation.
 Glucose + ATP Glucose-6-P +
ADP

Conversion to Trioses
 First, Glucose-6-Phosphate is converted to
Fructose-6-Phosphate. (Enzyme –
Phosphohexose Isomerase).
 Secondly Fructose-6-Phosphate is
phosphorylated by ATP, to form Fructose-1-6-
diphosphate.
Fructose-6-P + ATP PFK Fructose-1-6 Di+ADP
Mg ++

Conversion to Trioses (Cont....)
 In the next step Fructose 1-6-diphosphate is split by an
enzyme aldolase, into 2 molecules of Triose
phosphates namely Glyceraldehyde-3-Phosphateand
Dihydroxyacetone Phosphate.
Fructose 1-6-Di-P
Glyceraldehyde – 3-P
Dihydroxyacetone - P

Conversion to Trioses (Cont....)
Fructose 1-6-Di-P
By the action of triose phosphate isomerase interconversion
of Glyceraldehyde 3-P and Dihydroxyacetone –P occurs.
(DHAP) can be changed into G3P, There will be 2
molecules of G3P for the oxidation in next step.
Glyceraldehyde – 3-P
Dihydroxyacetone - P

Oxidation
 Very imporatnt step because ATP is
generated.
 G3P is oxidised into 1-3 diphosphoglyceric
acid, this reaction is catalysed by an enzyme
Glyceraldehyde-3-Phosphate dehydrogenase.
 1-3 diphosphoglyceric acid is converted into 3
phosphoglyceric acid with the help of an
enzyme Phosphoglycerate kinase, during this
process 2 ADP Molecules are joined with 2
phosphate molecule to form 2 ATP Molecules.

Oxidation (Cont.....)
 At this stage the 3 Phosphoglycerate is
converted into 2-phosphoglyceric acid by
phosphoglyceromutase.
 In the next stage the 2-Phosphoglyceric acid is
dehydrated by enolase to form
phosphophenolpyruvate.

Formation of Pyruvic Acid
 From the high energy phosphate of
phosphophenolpyruvate 2 ADP is transferred
to produce 2 molecules of ATP. With the help
of an enzyme pyruvate kinase 2 Pyruvates are
formed from Phosphophenolpyruvate.

CONCLUSION
 Total amount of energy production during glycolysis
given below.
Reactions ATP Used ATP Produced
First Step (First Phase) 1 -
Third Step (First Phase) 1 -
Second Step (Second
Phase)
- 2
Fifth Step (Second
Phase)
2
For every glucose molecule entering glycolysis. 2 ATP & 2 NAD+ Used.
4 ATP & 4 NADH Is produced. (Net Gain is 2 ATP and 2 NADH)

Carbohydrates

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