Medical Biochemistry and Molecular
Biology
CARBOHYDRATE CHEMISTRY
Disaccharides
2
By
Ayman Elsamanoudy
Objectives (ILOs)
I.
to understand the concept and importances of
glycosidic bond.
II. to study the definition, classification of
disaccharides
III. to know the biological importance of
different disaccharides.
IV.To know the natural sources of disaccharides.
V. to understand how any disorder in
disaccharides digestion or absorption leads to
disease.
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Glycosides
 A glycoside is a compound formed by binding
of a sugar and the hydroxyl group of a second
compound which, may be sugar (glycon ) or may
be not a sugar (aglycon).
 Disaccharides
→ Sugar + Sugar.
 Methyl glycoside → Sugar + Methyl alcohol.
 Cardiac glycoside → Galactose + Steroid.
 Sugars are joined to alcohols and amines by
glycosidic bonds.
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Types of glycosidic bond
 1- O-glycosidic bonds :
 Monosaccharides can link to each other by
O-glycosidic bonds to form disaccharides,
oligosaccharides, and polysaccharides.
 2-N- glycosidic bond.
 The anomeric carbon atom of a sugar can
be linked to the nitrogen atom of an amine
by an N- glycosidic bond.
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 Naming of glycosidic bonds:

Glycosidic bonds between sugars are named
according :
to the numbers of the connected carbons &
 the position of the anomeric hydroxyl group of
the sugar.
So, If this anomeric hydroxyl group is in the α
configuration, the linkage is an α-bond.
 If it is in the β-configuration, the linkage is a βbond
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 Examples of glycosides:
 Disaccharides as maltose, lactose and sucrose.
 Polysaccharides
 Glycolipids.
 Glycoproteins.
 Nucleotides as ATP, CTP, GTP where the
aglycone is purine and pyrimidine bases (Nglycosides).
 Cardiac glycosides as digitalis which is used in
treatment of heart failure where the aglycone is a
steroid.
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Disaccharides
 Disaccharides consist of two sugars joined
by an O-glycosidic bond.
 The most abundant disaccharides are
sucrose, lactose and maltose.
 Other disaccharides include isomaltose,
cellobiose and trehalose.
 The disaccharides can be classified into :
Homodisaccharides
Heterodisaccharides.
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Other classification of disaccharides
(1)Reducing disaccharide
(2) Non-Reducing disaccharide
 Princible : presence of free aldehyde or
ketone group (a free reducing carbonyl
group) is the cause of reducing property.
REDUCING SUGARS
•When Benedicts or Fehling test is performed with the
disaccharides maltose and sucrose, the following result is
obtained:
Sucrose is a
non-reducing sugar
SUCROSE
RESULT
Maltose is a
reducing sugar
MALTOSE
RESULT
I-Hommodisaccharides
Maltose
Isomaltose
Celebiose
2 α-glucose
2β-D-glucose
(malt sugar )
structure
Type of
2α-glucose
α-1-4 glucosidic α 1-6 glucosidic β1  4 glucosidic
bond
bond
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bond
bond.
Free aldehyde
group
12
C1
C6
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Free active carbonyl group
Free active carbonyl group
CH 2OH
H
OH
CH 2OH
O
H
H
O
1
OH
H
H
OH
4 H
H
OH
H
H
OH
-Glucose
-Glucose
Cellobiose
B 1-4 glucosidic bond
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O
OH
H
Maltose
Isomaltose
Celebiose
free
free
free
Reducing
Reducing
Reducing
It is produced
by the hydrolysis
by the acid
from starch by
of some
hydrolysis of
the action of
polysaccharides
cellulose
amylase
such as dextran
(malt sugar )
Anomeric
Carbon
Reducing
property
Produced by
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enzymes
Maltose
(malt Isomaltose
Celebiose
sugar )
Composition
Type of bond
2α-glucose units 2 α-glucose
2β-D-glucose
β1  4
α-1-4 glycosidic α 1-6 glycosidic bond
bond
Anomeric C
Reducing
glycosidic bond.
free
free
free
Reducing
Reducing
Reducing
property
Produced by
It is produced by the hydrolysis of by
the
from starch by some polysaccharides hydrolysis
the
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action
of such as dextran
amylase enzymes
cellulose
acid
of
II- Heterodisaccharides: are formed of 2
different monosaccharide units
Composition
Sucrose
Lactose
α-D-glucose+ β–D-fructose
β-D-galactose and β-Dglucose
Type of bond
α-1-β-2 glucosidic bond OR
β 2- α-1 fructosidic bond
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a β (14)
galactosidic bond
Sucrose
Anomeric C no free aldehyde or
Lactose
free
ketone group
Reducing
is not a reducing
property
sugar
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Reducing
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Free active carbonyl group
CH2OH
OH
H
CH2OH
O
H
OH
H
H
OH
H
O
1
H
4
O
H
OH
H
H
OH
Glucose
-Galactose
and
Lactose
B 1-4 galactosidic bond
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OH ..
H ....
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CH2OH
H
O
H
-Glucose
OH
OH
H
H
H
No free
active
carbonyl
groups
O
H
OH
OH
H
Sucrose
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1
OH
CH 2OH O
-Fructose
H
2
CH 2OH
Sucrose
Effect of
Lactose
The hydrolysis of sucrose to glucose and hydrolysed
hydrolysis
by
the
fructose is catalysed by sucrase (also called intestinal lactase enzyme
invertase),
into
galactose
and
glucose
Present in
Milk sugar
Table sugar
Cane sugar,
It may appear in urine in
late
pregnancy
during lactation
beet sugar
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and
Sucrose
Lactose
Composition α-D-glucose + β–D-fructose
β-D-galactose and β-D-glucose
Type of bond α-1-β-2 glucosidic bond OR
a β (14)
galactosidic bond
β 2- α-1 fructosidic bond
Anomeric C no free aldehyde or ketone group
free
Reducing
Reducing
is not a reducing sugar
property
of The hydrolysis of sucrose to
glucose and fructose is catalysed
hydrolysis
by sucrase (also called invertase),
Effect
Present in
Table sugar
Cane sugar,
beet sugar
hydrolysed by the intestinal lactase
enzyme into galactose and glucose
Milk sugar
It may appear in urine in late
pregnancy and during lactation
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Comparison between sucrose and invert sugar
Sucrose
α-D-glucose + β–D-fructose
Composition
Invert sugar
Mix of α-D-glucopyranose &
β–D-fructofuranose
Type of bond
α-1-β-2 glycosidic bond
Anomeric C
no free aldehyde or ketone free
No bond
group
Reducing property
Specific rotation
is not a reducing sugar
Reducing
Sucrose has a specific rotation (glucose = +52.5° , fructose = -91°). So,
of
+ 66.5° i.e. dextrorotatory
mixture of its component monosaccharides has
a net negative rotation
Optical activity
Dextrorotatory
Levorotatory
Present in
Cane sugar, beet sugar
bee honey
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Digestion of lactose
•The intestinal villi secrete an enzyme called lactase (β-Dgalactosidase) to digest lactose, and produce glucose
and galactose, which can be absorbed.
 Lactose intolerance
 More than half of the world’s adults are lactose intolerance.
 Lactose intolerance is the inability to metabolize lactose,
because the lactase is absent in the intestinal system or its
availability is lowered.
Lactose intolerance
 In the absence of lactase, lactose remains
uncleaved and passes intact into the colon.
 The intestinal bacteria quickly switch over to
lactose metabolism, and produces large amounts
of gases (a mixture of hydrogen, carbon dioxide,
and methane).
 This, leads to appearance of abdominal
symptoms, including stomach cramps(colic),
bloating, and flatulence.
 Treatment for this disorder is simple to remove
lactose from diet.
Lactose intolerance
Most adult mammals have low levels of lactase
and so much of the lactose they might ingest
moves through their digestive tract to the
colon, where bacterial fermentation generates
large amounts of CO2, H2 and irritating
organic acids.
These products cause painful digestive upsets
known as lactose intolerance.
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Which of the following carbohydrates have at least 2 different
monosaccharid
A) Sucrose
B) Fructose
C) Glucose
D) Starch
The monosaccharide units are linked byα1 →4 glycosidic
linkage in
(A) Maltose
(B) Sucrose
(C) Cellulose
(D) Cellobiose
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A dissaccharide formed by 1,B2-glycosidic linkage
between their monosaccharide units is
(A) Lactose
(B) Maltose
(C) Trehalose
(D) Sucrose
What monosaccharides make up a sucrose (table sugar)
molecule?
Galactose and fructose (A
Galactose and maltose (A
Lactose and fructose (A
Glucose and fructose (A
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A carbohydrate found only in milk is
(A) Glucose
(B) Galactose
(C) Lactose
(D) Maltose
A carbohydrate, when hydrolyzed known commonly as
invert sugar, is
(A) Fructose
(B) Sucrose
(C) Glucose
(D) Lactose
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Short Questions
1. Glycosides(def,types,examples).
2. Enumerate homodissacharides and differentiate
between them
3. Enumerate heterodissacharides and differentiate
between them.
4. Enumerate reducing disacharides and mention the type
of bond in each of them.
5. Enumerate disacharides that contain α glycosidic bond
and draw the structural formula of each of them.
6. Enumerate disacharides that contain β glycosidic bond
and draw the structural formula of each of them
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7.Compare between maltose and cellibiose
8.Compare between maltose and isomaltose.
9.Compare between sucrose and lactose
10.Discuss in short;lactose intolerance.
11.Compare between sucrose and invert sugar.
12. Compare between sucrose and maltose.
13. Enumerate disacharides that contain α-D glucose.
14. Enumerate disacharides that contain β-D glucose.
14. Enumerate disacharides that contain glucosidic
bond.
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 15. Mention disacharide that contain galactosidic
bond and draw the structural formula of it.
 16. Mention disacharide that contain fructosidic
bondbond and draw the structural formula of it.
 17. Explain the role of free active carbonyl group in
determing the property of disacharides ,give
examples.
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GREAT
THANKS
Ayman Elsamanoudy
10/21/2014
Ahmed A.Albadry
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