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Chapter 23 Metabolic Pathways
for Carbohydrates
23.4 Digestion of Carbohydrates
23.5 Glycolysis: Oxidation of Glucose
23.6 Pathways for Pyruvate
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Stage 1: Digestion of
Carbohydrates
„
„
„
„
In the mouth, salivary amylase hydrolyzes
α-glycosidic bonds in polysaccharides to give
smaller polysaccharides (dextrins), maltose, and
some glucose.
In the small intestine, pancreatic amylase
hydrolyzes dextrins to maltose and glucose.
The disaccharides maltose, lactose, and sucrose
are hydrolyzed to monosaccharides.
The monosaccharides enter the bloodstream for
transport to the cells.
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Digestion of Carbohydrates
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Glycolysis
„
In Stage 2, the
metabolic pathway
called glycolysis
degrades glucose (6C)
to pyruvate (3C).
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Glycolysis: Energy-Investment
In reactions 1-5 of glycolysis:
„ Energy is used to add phosphate groups to
glucose and fructose.
„ Glucose is
converted
to two
three-carbon
molecules.
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Glycolysis: Energy Investment
4
1
3
5
5
2
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Glycolysis: Energy-Production
„
In reactions 7
and 10, the
hydrolysis of
phosphates in
the triose
phosphates
generates four
ATP
molecules.
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Glycolysis:
Reactions 6-10
9
6
8
10
7
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Glycolysis: Overall Reaction
„
„
„
Glycolysis generates 2 ATP and 2 NADH.
Two ATP are used in energy-investment to add
phosphate groups to glucose and fructose-6phosphate.
Four ATP are formed in energy-generation by
direct transfers of phosphate groups to four ADP.
Glucose + 2ADP + 2Pi + 2NAD+
2Pyruvate + 2ATP + 2NADH + 4H+
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Regulation of Glycolysis
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„
„
Reaction 1 Hexokinase is inhibited by high
levels of glucose-6-phosphate, which prevents
the phosphorylation of glucose.
Reaction 3 Phosphofructokinase, an allosteric
enzyme, is inhibited by high levels of ATP and
activated by high levels of ADP and AMP.
Reaction 10 Pyruvate kinase, another allosteric
enzyme is inhibited by high levels of ATP or
acetyl CoA.
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Learning Check
In glycolysis, what compounds provide
phosphate groups for the production of ATP?
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Solution
In glycolysis, what compounds provide
phosphate groups for the production of ATP?
In reaction 7, phosphate groups from two 1,3bisphosphoglycerate molecules are used to form
two ATP.
In reaction 10, phosphate groups from two
phosphoenolpyruvate molecules are used to
form two more ATP.
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23.5 The Fate of Pyruvate
„
When oxygen is present in the cell, (aerobic
conditions), pyruvate from glycolysis is
decarboxylated to produce acetyl CoA and CO2.
O
||
CH3—C—COO- + HS—CoA + NAD+
pyruvate
O
||
CH3—C—S—CoA + CO2 + NADH + H+
acetyl CoA
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Lactate Formation
When oxygen is not available (anaerobic
conditions), pyruvate is reduced to lactate, which
replenishes NAD+ to continue glycolysis.
O
lactate
||
dehydrogenase
CH3—C—COO- + NADH + H+
pyruvate
OH
|
CH3—CH—COO- + NAD+
lactate
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Lactate in Muscles
Under anaerobic conditions (strenuous exercise):
„ Oxygen in the muscles is depleted.
„ Lactate accumulates in the muscles.
„ Muscles tire and become painful.
„ Rest is needed to repay the oxygen debt and to
reform pyruvate in the liver.
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Pathways for Pyruvate
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Learning Check
Match the following terms with the descriptions:
1) Catabolic reactions
2) Coenzymes
3) Glycolysis
4) Lactate
A. Produced during anaerobic conditions.
B. Reaction series that converts glucose to pyruvate.
C. Metabolic reactions that break down large
molecules to smaller molecules + energy.
D. Substances that remove or add H atoms in
oxidation and reduction reactions.
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Solution
Match the following terms with the descriptions:
1) Catabolic reactions
2) Coenzymes
3) Glycolysis
4) Lactate
A. 4 Produced during anaerobic conditions.
B. 3 Reaction series that converts glucose to pyruvate.
C. 1 Metabolic reactions that break down large
molecules to smaller molecules + energy.
D. 2 Substances that remove or add H atoms in
oxidation and reduction reactions.
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23.7 Regulation of Glucose Metabolism
and energy Production
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„
High glucose
levels and insulin
promote
glycolysis.
Low glucose
levels and
glucagon
promote
gluconeogenesis.
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23.10 Glycogen Metabolism:
Glycogenesis and Glycogenolysis
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Glycogenesis
Glycogenesis:
„ Stores glucose by converting glucose to
glycogen.
„ Operates when high levels of glucose-6phosphate are formed in the first reaction of
glycolysis.
„ Does not operate when energy stores
(glycogen) are full, which means that
additional glucose is converted to body fat.
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Diagram of Glycogenesis
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Formation of Glucose-6-Phosphate
„
Glucose is converted to glucose-6-phosphate
using ATP.
P O CH2
O
OH
OH
OH
OH
Glucose-6-phosphate
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Formation of Glucose-1-Phosphate
„
Glucose-6-phosphate is converted
to glucose-1-phosphate.
P O CH2
H O CH2
O
O
OH
OH
OH
OH
OH
Glucose-6-phosphate
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O P
OH
OH
Glucose-1-phosphate
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UDP-Glucose
„
UTP converts glucose-1-phosphate to
UDP-glucose and pyrophosphate
(PPi).
O
CH2OH
H
O
OH
O
O P O
OH
OH
-
O
O
O
P O CH2
-
O
UDP-glucose
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OH
N
N
O
OH
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Glycogenesis: Glycogen
„
The glucose in UDP-glucose adds to glycogen.
UDP-Glucose + glycogen
„
glycogen-glucose + UDP
The UDP reacts with ATP to regenerate UTP.
UDP + ATP
UTP + ADP
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Glycogenolysis
„
Glycogenolysis
is the break
down of
glycogen to
glucose.
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Glycogenolysis
Glycogenolysis:
Is activated by glucagon (low blood glucose).
„ Bonds glucose to phosphate to form glucose-1-phosphate.
Glycogen-glucose + Pi
Glycogen + glucose-1-phosphate
„
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Isomerization of Glucose-1phosphate
„
The glucose-1-phosphate isomerizes to glucose6-phosphate, which enters glycolysis for energy
production.
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Glucose-6-phosphate
Glucose-6-phosphate:
„ Is not utilized by brain and skeletal muscle
because they lack glucose-6-phosphatase.
„ Hydrolyzes to glucose in the liver and kidney,
where glucose-6-phosphatase is available providing
free glucose for the brain and skeletal muscle.
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Learning Check
Match each description with:
1) Glycogenesis
2) Glycogenolysis
A. Activated by low levels of blood glucose.
B. Converts glucose-1-phosphate to glucose-6phosphate.
C. Activated by high levels of glucose-6-phosphate.
D. Glucose + UTP
UDP-glucose + PPi
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Solution
Match each description with:
1) Glycogenesis
2) Glycogenolysis
A. 2 Activated by low levels of blood glucose.
B. 2 Converts glucose-1-phosphate to glucose-6phosphate.
C. 1 Activated by high levels of glucose-6-phosphate.
D. 1 Glucose + UTP
UDP-glucose + PPi
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Utilization of Glucose
Glucose:
„ Is the primary
energy source for the
brain, skeletal
muscle, and red
blood cells.
„ Deficiency can
impair the brain and
nervous system.
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23.11 Gluconeogenesis:
Glucose Synthesis
Gluconeogenesis is:
„ The synthesis of
glucose from
carbon atoms of
noncarbohydrate
compounds.
„ Required when
glycogen stores are
depleted.
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Gluconeogenesis: Glucose
Synthesis
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Carbon atoms for gluconeogenesis from lactate,
some amino acids, and glycerol are converted to
pyruvate or other intermediates.
Seven reactions are the reverse of glycolysis and
use the same enzymes.
Three reactions are not reversible.
Reaction 1
Hexokinase
Reaction 3
Phosphofructokinase
Reaction 10
Pyruvate kinase
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Gluconeogenesis: Pyruvate to
Phosphoenolpyruvate
„
„
Pyruvate adds a carbon to form oxaloacetate by
two reactions that replace the reverse of reaction
10 of glycolysis.
Then a carbon is removed and a phosphate added
to form phosphoenolpyruvate.
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Phosphoenolpyruvate to Fructose1,6-bisphosphate
„
Phosphoenolpyruvate is converted to fructose1,6-bisphosphate using the same enzymes in
glycolysis.
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Glucose Formation
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A loss of a phosphate from fructose-1,6bisphosphate forms fructose-6-phosphate and Pi.
A reversible reaction converts fructose-6phosphate to glucose-6-phosphate.
The removal of phosphate from glucose-6phosphate forms glucose.
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Cori Cycle
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When anaerobic conditions occur in active
muscle, glycolysis produces lactate.
The lactate moves through the blood stream to the
liver, where it is oxidized back to pyruvate.
Gluconeogenesis converts pyruvate to glucose,
which is carried back to the muscles.
The Cori cycle is the flow of lactate and glucose
between the muscles and the liver.
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Pathways for Glucose
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Learning Check
Identify each process as:
1) glycolysis
2) glycogenesis
3) glycogenolysis
4) gluconeogenesis
A. The synthesis of glucose from
noncarbohydrates.
B. The breakdown of glycogen into glucose.
C. The oxidation of glucose to two pyruvate.
D. The synthesis of glycogen from glucose.
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Solution
Identify each process as:
1) glycolysis
2) glycogenesis
3) glycogenolysis
4) gluconeogenesis
A. 4 The synthesis of glucose from
noncarbohydrates.
B. 3 The breakdown of glycogen into glucose.
C. 1 The oxidation of glucose to two pyruvate.
D. 2 The synthesis of glycogen from glucose.
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Tutorials:
„
Glycolysis URLs
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http://instruct1.cit.cornell.edu/courses/biomi290/ASM/glycolysis.dcr
http://programs.northlandcollege.edu/biology/Biology1111/animations/glycolysis.html
http://www.tcd.ie/Biochemistry/IUBMB-Nicholson/swf/glycolysis.swf
„
„
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