MONOSACCHARIDES
DR AMINA TARIQ
BIOCHEMISTRY
# Carbons
Category Name
Relevant examples
3
Triose
4
Tetrose
Erythrose
5
Pentose
Ribose, Ribulose,
Xylulose
6
Hexose
Glucose, Galactose,
Mannose, Fructose
7
Heptose
Sedoheptulose
9
Nonose
Glyceraldehyde,
Dihydroxyacetone
Neuraminic acid, also
called sialic acid
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Three common sugars share the same molecular
formula: C6H12O6. Because of their six carbon atoms,
each is a hexose.
They are:
 glucose, "blood sugar", the immediate source of
energy for cellular respiration
 galactose, a sugar in milk (and yogurt), and
 fructose, a sugar found in honey.
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The simplest monosaccharides of biological interest
have n=3 (trioses):
Glyceraldehyde and Dihydroxyacetone
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GLYCERALDEHYDE
It is the smallest sugar
It has got three carbons
Its carbon 2 is asymmetric carbon
All monosaccharides having more than 3 carbons will
have two or more asymmetric carbon atoms.
For D and L designation all monosaccharides are
compared with glyceraldehyde.
Thus it is called a reference sugar.
CHO
CHO
H
C
OH
HO
L-glyceraldehyde
CHO
H
C
OH
CH2OH
D-glyceraldehyde
H
CH2OH
CH2OH
D-glyceraldehyde
C
CHO
HO
C
H
CH2OH
L-glyceraldehyde
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For sugars with more
than one chiral center,
D or L refers to the
asymmetric C farthest
from the aldehyde or
keto group.
O
H
C
H – C – OH
HO – C – H
H – C – OH
H – C – OH
CH2OH
D-glucose
O
H
C
HO – C – H
H – C – OH
HO – C – H
HO – C – H
CH2OH
L-glucose
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DIHYDROXYACETONE
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It is the only exception of a monosaccharide
that does not possess any asymmetric carbon.
MONOSACCHARIDES
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Monosaccharides are carbohydrates that cannot be
hydrolyzed to obtain smaller molecules of
carbohydrate.
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White crystalline solids , very soluble in water, have
sweet taste.
Glucose
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Glucose is by far the most common carbohydrate
and classified as a monosaccharide,
an aldose,
a hexose,
and is a reducing sugar.
Principal MS utilized for energy purposes.
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It is also known as dextrose, because it is
dextrorotatory.
Glucose units combine and give rise to:
Glycogen
Starch
Cellulose
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Glucose is also called blood sugar as it circulates in
the blood at a concentration of 65-110 mg/mL of
blood.
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Normally trace amounts in urine
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Increased amounts in diabetes mellitus.
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Glucose is initially synthesized by chlorophyll in plants
using carbon dioxide from the air and sunlight as an
energy source. Glucose is further converted to
starch for storage.
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an aqueous sugar solution contains only 0.02% of the
glucose in the chain form, the majority of the
structure is in the cyclic chair form.
Fructose
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Fructose is classified as a:
monosaccharide,
the most important ketose sugar,
a hexose,
and is a reducing sugar.
Fructose, along with glucose are the monosaccharides
found in disaccharide, sucrose.
Sweetest of all sugars.
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Fruit juices. Honey.
Hydrolysis of cane sugar and of inulin.
Can be changed to glucose in the liver and so used in
the body.
Main source of energy for the spermatozoa
Levorotatory (in sucrose dextrorotatory)
Hereditary fructose intolerance
Galactose
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Galactose is classified as a:
monosaccharide,
an aldose,
a hexose,
and is a reducing sugar.
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Hydrolysis of lactose
Galactose is more commonly found in the
disaccharide, lactose or milk sugar.
It is found as the monosaccharide in peas.
Constituent of glycolipids and glycoproteins.
C-4 epimer of glucose
Galactose is part of nerve and brain
biochemicals, so milk is essential to infants.
Pentoses
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Xylulose- uronic acid pathway
Arabinose and xylose- glycoproteins
Ribose - RNA
Deoxyribose - DNA
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All are classified as:
monosaccharides,
aldoses,
Pentoses,
and are reducing sugars.
Ribose
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Ribose and its related compound, deoxyribose, are
the building blocks of the backbone chains in nucleic
acids, better known as DNA and RNA.
Ribose is used in RNA and deoxyribose is used in
DNA.
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The presence or absence of the -OH group on
carbon (#2) is an important distinction between
ribose and deoxyribose.
Ribose has an alcohol at carbon # 2, while
deoxyribose does not have the alcohol group.
Deoxy hexoses also present in milk and blood group
substances.
Disaccharides
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Three common disaccharides:
Sucrose — common table sugar = glucose +
fructose
Lactose — major sugar in milk = glucose +
galactose
Maltose — product of starch digestion = glucose +
glucose
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The resulting linkage between the sugars is called a
glycosidic bond. The molecular formula of each of
these disaccharides isC12H22O11 = 2 C6H12O6 − H2O
Sucrose
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Sucrose is made from glucose and fructose units
Sucrose or table sugar
Cane, beet sugar, Pineapple
Has no free anomeric carbon
Is a non-reducing sugar
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Dextrorotatory
Invert sugar
No osazone crystals
In sucrase deficiency, malabsorption leads to diarrhea
and flatulence
Maltose
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Glucose+ Glucose
Digestion by amylase or hydrolysis of starch.
Reducing sugar
Baby and invalid foods
Lactose
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Glucose + Galactose
Milk sugar
May occur in urine during pregnancy
In lactase deficiency, malabsorption leads to diarrhea
and flatulence
Certain bacteria can ferment lactose to lactic acid souring of milk. ( lactobacillus)
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LACTULOSE -
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galactose and fructose
Osmolar laxative
Relieves Constipation
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