1. Dental Biochemistry 1- (5)
Chemistry and digestion of
carbohydrates
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2. DEFINITION
Carbohydrates are organic substances
composed of carbon, hydrogen and
oxygen.
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3. Function of carbohydrates:
• Carbohydrates are the main sources of
energy in the body. Brain cells and RBCs are
almost wholly dependent on carbohydrates as
the energy source. Energy production from
carbohydrates will be 4 Kcal/g.
• Storage form of energy (starch and glycogen).
• Excess carbohydrate is converted to fat.
• Glycoproteins and glycolipids are components
of cell membranes and receptors,
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4. • Structural unit of many organisms: Cellulose of
plants; exoskeleton of insects, cell wall of
microorganisms, mucopolysaccharides as ground
substance in higher organisms.
• Important part of nucleic acids and free
nucleotides and coenzyme.
• Major antigens are carbohydrates in nature, e.g.,
blood group substance.
• Has a biological role as a part of hormones and
their receptors and enzymes.
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5. Nomenclature
• Molecules having only one actual or potential sugar
group [(CH2O)n] are called monosaccharides (e.g.
C6H12O6); they cannot be further hydrolyzed into
smaller units.
• When two monosaccharides are combined
together with elimination of a water molecule, it is
called a disaccharide (e.g. C12H22O11).
• Trisaccharides contain three sugar groups. Further
addition of sugar groups will correspondingly
produce tetrasaccharides, pentasaccharides and so
on, commonly known as oligosaccharides.
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6. • When more than 10 sugar units are combined, they
are generally named as polysaccharides.
• Polysaccharides having only one type of
monosaccharide units are called
homopolysaccharides and those having different
monosaccharide units are called
heteropolysaccharides.
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7. Monosaccharides
They have the common formula
(CH2O)n, where n = 3 or some
larger number
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8. CLASSIFICATION OF MONOSACCHARIDES
Can be carried out by one of two methods:
1) According to the number of carbon atoms
:
Trioses, Tetroses, Pentoses,
Hexoses, Heptoses, Octoses.
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9. • 2) According to the characteristic
carbonyl group:
Aldehyde group or ketone group
a) Aldo sugars: Aldoses :
Monosaccharides containing aldehyde group e.g.
glucose, ribose, erythrose and glyceraldehyde.
b) Keto sugars: Ketoses :
Monosaccharides containing ketone group e.g.
fructose, ribulose and dihydroxyacetone.
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11. STEREOISOMERS
• Compounds having same structural formula, but
differ in spatial configuration are known as
stereoisomers.
• All monosaccharides can be considered as
molecules derived from glyceraldehyde.
• Depending on the configuration of Hand OH
around the reference carbon atom, the two
mirror forms are designated as Land D forms.
• All naturally occurring sugars are D sugars.
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13. Epimerism of Aldoses
• When sugars are different from one another,
only in configuration with regard to a single
carbon atom (other than the reference carbon
atom), they are called epimers.
• For example, glucose and mannose
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14. Anomerism of Sugars
• This is explained by the fact that D-glucose has
two anomers, alpha and beta varieties.
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15. Monosaccharides of
physiologic importance
1-Pentoses:
• They are sugars containing 5 carbon
atoms. Ribose is a constituent of RNA
while deoxyribose is seen in DNA.
• Ribose is also seen in co-enzymes
such as ATP and NAD.
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17. Hexoses
Glucose, Mannose and Galactose
• They are the common aldohexoses.
• Glucose is the sugar in human blood. It is the
major source of energy.
• Mannose is a constituent of many
glycoproteins. Mannose was isolated from
plant mannans; hence the name.
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18. b) D-Fructose :
( Fruit sugar = Levulose). It is found in fruits, honey
and obtained from sucrose and inulin by hydrolysis.
c)D-Galactose :
It is obtained from hydrolysis of lactose (milk sugar). .
It is a constituent of galactolipids, glycoprotein.
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20. DlSACCHARIDES
• When two monosaccharides are
combined together by glycosidic
linkage, a disaccharide is formed.
The important disaccharides are
sucrose, maltose, isomaltose and
lactose.
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21. 1. Sucrose
• It is the sweetening agent known as cane sugar. It is
present in sugarcane and various fruits. Hydrolysis
of sucrose will produce one molecule of glucose
and one molecule of fructose.
• ii. The enzyme producing hydrolysis of sucrose is
called sucrase.
• Sucrose is not
a reducing sugar.
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22. 2. Lactose
• Reducing disaccharide.
• Capable of forming osazone.
• It is only found in milk.
• By acid or lactase enzyme in the intestine,
it yields D-galactose and D-glucose
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23. 3. Maltose
• Maltose contains two glucose residues with
alpha1,4 linkage.
• It is found in germinating cereals and malt.
• Intermediate product of the action of amylases on
starch. It is reducing disaccharide
• Hydrolysis: By acid or by maltase enzyme into
two D-glucose units.
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24. 4. Isomaltose
• It is also a reducing sugar. It contains 2 glucose
units combined in alpha -1, 6 linkage.
• Partial hydrolysis of glycogen and starch
produces isomaltose.
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25. POLYSACCHARIDES
• These are polymerized products of many
monosaccharide units.
• They may be homoglycans composed of
single kind of monosaccharides, e.g. starch.
glycogen and cellulose.
• Heteroglycans are composed of two or more
different monosaccharides.
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26. 1. Starch
• It is the reserve carbohydrate of plant as in
potatoes, rice, wheat. 10-20% is soluble part called
amylose. The insoluble part is called amylopectin.
• Amylose is made up of glucose units with alpha-
1,4 glycosidic linkages to form an unbranched long
chain.
• Amylopectin is also made up of glucose units, but
is highly branched with molecular weigh more
than 1 million. The branching points are made by
alpha-1,6 linkage (similar to isomaltose) .
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27. 2. Glycogen
• It is the reserve carbohydrate in animals. It is
stored in liver and muscle. About 5% of weight of
liver is made up by glycogen.
• Glycogen is composed of glucose units joined by
alpha-1 ,4 and alpha-1 ,6 glycosidic linkages.
3. Cellulose
• It is the chief carbohydrate in plants.
• The enzyme act on hydrolysis of cellulose is absent
in animal and human digestive system, and hence
cellulose cannot be digested.
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28. 4. Inulin
• It is the reserve carbohydrate present in onion,
garlic, etc.
• 5. Chitin
• It is present in exoskeletons of crustacea and
insects
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29. B- Heteroglycan:
These are polysaccharides containing more than one
type of sugar residues.
1.Agar:
• It is prepared from sea weeds and contains
galactose, glucose and other sugars.
• It is used as a supporting medium for
electrophoresis.
• 2. Mucopolysaccharides:
• Mucopolysaccharides or glycosaminoglycans (GAG)
are carbohydrates containing uronic acid and amino
sugars.
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30. 3. Hyaluronic acid:
• It is present in connective tissues, tendons,
4. Heparin:
• It is an anticoagulant
• 5. Keratan sulphate:
• It is found in cornea and tendons.
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31. Glycoproteins and Mucoproteins
• When the carbohydrate chains are attached to a
polypeptide chain it is called a proteoglycan.
• If the carbohydrate content is less than 10%, it is
generally named as a glycoprotein.
• If the carbohydrate content is more than 10% it is a
mucoprotein.
• They are seen in almost all tissues and cell
membranes.
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32. Digestion of carbohydrates
• Cooking helps in breaking of glycosidic
linkages in polysaccharides and thus makes
the digestion process easier.
• In the diet carbohydrates are available as
polysaccharides (starch, glycogen), and to a
minor extent, as disaccharides (sucrose and
lactose).
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33. • This process of digestion starts in mouth by
the salivary alpha-amylase. However, the
time available for digestion in the mouth is
limited. The gastric hydrochloric acid will
inhibit the action of salivary amylase.
• In the pancreatic juice another alpha-amylase
is available which will hydrolyze the alpha-1,4
glycosidic linkages randomly, so as to produce
smaller subunits like maltose, isomaltose,
dextrin and oligosaccharides.
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34. • The intestinal juice (succus entericus)
and brush border of intestinal cells
contain enzymes, which will hydrolyze
disaccharides into component
monosaccharaides.
• These enzymes are sucrase, maltase,
isomaltase and lactase.
• The monosaccharides are then
absorbed.
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35. Lactose intolerance
• This is produced by the deficiency of lactase.
This enzyme hydrolyses lactose to glucose and
galactose.
• In this condition, lactose accumulate in the
gut produce irritant diarrhea.
• If milk is withdrawn temporarily, the diarrhea
will be limited. Curd is also an effective
treatment, because the lactobacilli present in
curd contains the enzyme lactase.
• Lactase activity is high during infancy and it
decreases to adult levels by 5-7 years of age.
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36. Absorption of carbohydrates
• Only monosaccharides are absorbed by the
intestine. Minute quantities of disaccharides that
may be absorbed, are immediately eliminated
through kidneys.
• The duodenum and upper jejunum absorb the
bulk of the dietary sugars.
• Insulin is not required for the uptake of glucose by
intestinal cells.
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37. • However, different sugars have different
mechanisms of absorption.
1. galactose and glucose are transported into the
mucosal cells by an active, energy-requiring
process that involves a specific transport protein
and requires a concurrent uptake of sodium ions
(sodium-dependent monosaccharide
transporter SGluT).
2. Fructose uptake requires a sodium-independent
monosaccharide transporter (GLUT-5) for its
absorption.
3. All three monosaccharides are transported from
the intestinal mucosal cell into the portal circulation
by yet another transporter, GLUT-2.
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