Biochemistry for Dental Students

1. Nursing - 5
Introduction to carbohydrates
and Glycolysis

2. Function of carbohydrates
• Carbohydrates are the most abundant organic
molecules in nature
• providing a significant fraction of the energy in the
diet of most organisms.
• Acting as a storage form of energy in the body.
• Serving as cell membrane components that
mediate some forms of intercellular
communication.
• Serve as a structural component of many
organisms, including the cell walls of bacteria, the
exoskeleton of many insects, and the fibrous
cellulose of plants.
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3. CLASSIFICATION
Carbohydrates are classified into three main groups:
• 1) Monosaccharides (simple sugars) : They consist of a
single polyhydroxyaldehyde(glucose) or ketone units
(fructose). They can not be hydrolysed into simpler
units. The sugar unit is sometimes referred to as
glycan.
• 2) Oligosaccharides (oligo= few) : contain from two to
ten monosaccharide units joined in glycosidic bonds.
e.g. disaccharides (2 units) e.g. maltose and sucrose,
trisaccharides(3 units).....etc.
• 3) Polysaccharides (poly = many): Also known as glycans.
They are composed of more than ten monosaccharide
units e.g. starch, glycogen, cellulose.....etc.3

4. Monosaccharides
• They have the common formula
(CH2O)n, where n = 3 or some larger
number
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5. • Isomers, epimers and enantiomers:
• Compounds that have the same chemical formula but have
different structures are called isomers. Eg. fructose,
glucose, mannose, and galactose are all isomers.
• two monosaccharides differ in configuration around only
one specific carbon atom, they are defined as epimers of
each other. Eg. glucose and galactose are C-4 epimers.
• A special type of isomerism is found in the pairs of
structures that are mirror images of each other.
• These mirror images are called enantiomers, and the two
members of the pair are designated as a D- and an L-sugar.
• The vast majority of the sugars in humans are D-sugars.
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6. 6

7. Galactose
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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.
b) Keto sugars: Ketoses :
Monosaccharides containing ketone group e.g. fructose,
ribulose.
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10. Complex carbohydrates
• Carbohydrates can be attached by glycosidic bonds
to non-carbohydrate structures, including:
• purines and pyrimidines (found in nucleic acids),
• aromatic rings (such as those found in steroids
and bilirubin),
• proteins (found in glycoproteins and glycos-
aminoglycans)
• lipids (found in glycolipids).
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11. A. Digestion of carbohydrates begins in
the mouth
• The major dietary polysaccharides are of animal
(glycogen) and plant origin (starch, composed of
amylose and amylopectin).
• During mastication, salivary α-amylase acts briefly
on dietary starch in a random manner, breaking
some α(1- 4) bonds.
• Because branched amylopectin and glycogen also
contain α(1- 6) bonds, the digest resulting from the
action of α-amylase contains a mixture of smaller,
branched oligosaccharide molecules.
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13. • Carbohydrate digestion halts temporarily in the
stomach, because the high acidity inactivates
the salivary α-amylase.
• Note: There are both α(1- 4) and β(1- 4)-
endoglucosidases in nature, but humans do
not produce and secrete the latter in digestive
juices. Therefore,
• they are unable to digest cellulose—a
carbohydrate of plant origin containing β(1- 4)
glycosidic bonds between glucose residues.
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14. B. Further digestion of carbohydrates
by pancreatic enzymes occurs in the
small intestine
• When the acidic stomach contents reach
the small intestine, they are neutralized by
bicarbonate secreted by the pancreas, and
pancreatic α-amylase continues the
process of starch digestion.
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15. • C- Digestion by intestinal enzymes:
• The final digestive processes occur at the mucosal
lining of upper jejunum by enzymes synthesized
by the intestinal mucosal cells.
• Brush borders intestinal enzymes are:
• 1- Oligo 1,6 glucosidase releases glucose residues
from branched oligosaccharides.
• 2- disaccharidasess
• - Sucrase converts sucrose to glucose and
fructose.
• - Lactase converts lactose to glucose and
galactose.
• - Maltase converts maltose to two glucose
molecules15

16. bymonosaccharidesD. Absorption of
intestinal mucosal cells
• The duodenum and upper jejunum absorb the bulk
of the dietary sugars.
• Insulin is not required for the uptake of glucose by
intestinal cells.
• 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.
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17. 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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18. Glucose metabolism
Importance of Glucose:
1. Glucose is the preferred source of energy for most
of the body tissues. Brain cells derive energy
mainly from glucose.
2. When glucose metabolism is deranged, life-
threatenung conditions may occur. A minimum
amount of glucose is always required for normal
functioning.
3. Normal fasting plasma glucose level is 70 to 110
mg/dl. After a heavy carbohydrate meal, in a
normal person, this level is below 150 mg/dl.
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19. • Glucose is used in:
• Oxidation:
• The pathways for oxidation of glucose are classified
into two main groups:
• A. The major pathways which are mainly for
energy production:
• 1. Glycolysis.
• 2. Citric acid cycle (CAC).
• B. The minor pathways for oxidation which are
not for energy production:
• 1. Hexose monophosphate shunt (HMS).
• 2. Uronic acid pathway.
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20. • II. Conversion to biologically active
substances as:
• 1. Galactose: which is essential for formation
of lactose, glycolipids, mucopolysaccharides,
...etc.
• 2. Fructose: needed for nutrition of sperms.
• 3. Amino sugars.
• 4. Non-essential amino acids.
• 5. Fatty acids.
• 6. Ribose-5-P.
• 7. Glucuronic acid.
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21. • III. Storage of glucose:
• 1. As glycogen in the liver and muscles mainly.
• 2. As triglycerides (TG), mainly in adipose
tissues.
• IV. Excretion of glucose in urine:
• When blood glucose level exceeds a certain
limit (renal sugar threshold), it will pass to
urine. This will occur when blood glucose level
is above 180 mg/dl and this is known as
glucosuria.
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22. Glycolysis
(Embden Meyerhof Pathway)
Definition:
• It is degradation of glucose to generate ATP and
to provide intermediates for other synthetic and
metabolic pathways. Aerobically it ends with
pyruvate while anaerobically lactate is the end
product.
Site of glycolysis:
• It occurs in the cell cytosol of all tissues.
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23. Glycolysis
(Embden-Meyerhof Pathway)
Importance of the pathway
• In this pathway glucose is converted to pyruvate
(aerobic condition) or lactate (anaeroboic
condition), along with production of a small
quantity of energy.
• Glycolysis is the only source of energy in
erythrocytes.
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24. • In strenuous exercise, when muscle tissue
lacks enough oxygen, anaerobic glycolysis
forms the major source of energy for muscles.
• The glycolytic pathway may be considered as
the preliminary step before complete
oxidation.
• The glycolytic pathway also provides carbon
skeleton for synthesis of certain nonessential
amino acids as well as glycerol part of fat.
• Most of the reactions of the glycolytic
pathway are reversible, which are also used
for gluconeogenesis.
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25. • Steps of glycolysis:
• Conversion of glucose to G-6-P, is irreversible
(non-equilibrium reaction). It is catalyzed by
glucokinase (in liver and pancreas) or by
hexokinase (in all other tissues).
• Aldolase A: (F1,6 diphosphate aldolase) Occurs
in most tissues.
Two molecules of ATP are liberated in each
step since every molecule of glucose gives rise
to two trioses.
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27. 27

28. Energy production
1. Under aerobic conditions:
Glucose ------→ 2 Pyruvate + 8 ATP.
For tissues that have mitochondria,
glycolysis is considered a preparatory
step for complete oxidation via citric acid
cycle since pyruvate is transported into
the mitochondria to provide oxaloacetate
or active acetate where it is oxidized by
citric acid cycle for more energy.
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29. • The hydrogens of NADH produced by
glycolysis are transported to the mitochondria
mainly to be oxidized by electron transport
chain (ETC).
• 2. Under anaerobic conditions:
Glucose ------→ 2 lactate + 2 ATP
Pyruvate is reduced to lactate in a reversible
reaction catalyzed by lactate dehydrogenase
(LDH).29

30. • This occurs in muscles during severe exercise
(hypoxic conditions) and in tissues that lack
mitochondria as RBCs and lens.
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31. Cori’s cycle or lactic acid cycle
• In an actively contracting muscle, only about
8% of the pyruvate is utilized by the citric acid
cycle, and the remaining molecules are
therefore reduced to lactate.
• The lactic acid thus, generated should not be
allowed to accumulate in the muscle tissues.
• The muscle cramps, often associated with
strenuous muscular exercise, are through to
be due to lactate accumulation.
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33. glycolysisRegulation of
Hormonal regulation of glycolysis:
• a) Glucagon : is secreted in hypoglycemia or in
carbohydrate deficiency. It affects liver cells mainly
as follows:
It acts as inhibitors for glycolytic key enzymes
(glucokinase,PFK-1, pyruvate kinase).
• b) Insulin: It is secreted in hyperglycemia and after
carbohydrates feeding, it causes:
Stimulation of glycolytic key enzymes.
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34. Inhibitors of glycolysis:
1- 2- deoxyglucose inhibits
hexokinase.
2- Mercury and iodoacetate inhibit
glyceraldehyde-3-P dehydrogenase.
3- Fluoride inhibits enolase by
removal of Mg2+ as Mg fluoride.
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