Biochemistry I
Overview of Gluconeogenesis,
and Pentose Phosphate Pathway
Cori Cycle
Gluconeogenesis
Chapter 16 – part 3
Covered on Exam 3
(includes material from Chapter 20, p.589-590)
Dr. Ray
Control of Glycolysis
1
• The flux through the glycolytic pathway must
be adjusted in response to conditions both
inside and outside the cell. The rate of
conversion of glucose into pyruvate is
regulated to meet two major cellular needs:
2
3
1) the production of ATP, generated by the
degradation of glucose
4
2) the provision of building blocks for
synthetic reactions, such as the formation of
fatty acids.
5
In metabolic pathways, enzymes
catalyzing essentially irreversible
reactions are potential sites of control.
1. Which glycolytic enzymes are
likely to be sites of control?
The ________________ reactions at
steps
__________ , which is
near equilib in cellular concentrations)
6
7
8
9
10
Energetics of Glycolysis
• Most of the decrease in free energy in glycolysis takes place in the
three essentially irreversible steps catalyzed by hexokinase,
phosphofructokinase, and pyruvate kinase.
step ? 1
3
10
• The energy released in the anaerobic conversion of glucose into
two molecules of pyruvate is only a portion of the total energy
captured during complete aerobic glucose catabolism.
2. Where does most of the rest of the energy capture occur?
3. If glucose needed to be made, which steps would require
the most amount of energy to overcome?
http://higheredbcs.wiley.com/legacy/college/boyer/0471661791/animations/animations.htm
 Introduction to Metabolism  Traits of Metabolism
Gluconeogenesis
 Focus on comparing Gluconeogenesis to Glycolysis
• The synthesis of glucose from non-carbohydrate precursors
(such as pyruvate and lactate) is called gluconeogenesis.
1. Why is this metabolic pathway important?
because the _______ depends on glucose as its primary fuel
and _______________ use only glucose as a fuel
Red Blood Cells do not have mitochondria, so they do not do aerobic metabolism.
All ATP is made only from glycolysis.
• The daily glucose requirement of the brain in a typical adult human being is
about 120 g, which accounts for most of the 160 g of glucose needed daily by the
whole body.
• The amount of glucose present in body fluids is about 20 g, and that readily
available from glycogen, a storage form of glucose, is approximately 190 g.
•Thus, the direct glucose reserves are
sufficient to meet glucose needs for
about a day. During a longer period of
not eating, glucose must be formed
from non-carbohydrate sources,
through gluconeogenesis.
http://higheredbcs.wiley.com/legacy/college/boyer/0471661791/
animations/animations.htm
Hexokinase
1
2
Phosphofructo
kinase
3
4
5
6
7
8
9
Pyruvate
kinase
Text Fig 16.2
10
Glycolysis and
Gluconeogenesis
• Under typical cellular
conditions, most of the
decrease in free energy in
glycolysis takes place in the
three essentially irreversible
steps catalyzed by:
(1) hexokinase
(3) phosphofructokinase
(10) pyruvate kinase
• In gluconeogenesis, these
three irreversible steps are
by-passed by other reactions,
in order to overcome the
energetic barrier.
• Gluconeogenesis is
energetically costly.
http://higheredbcs.wiley.com/legacy/college/boyer/
0471661791/animations/animations.htm
 Gluconeogenesis  Energetics
Gluconeogenesis is not the Reverse of Glycolysis
DG’ for the formation of pyruvate from glucose (in glycolysis) is about -84 kJ/mol
1
3
10
Unique Reactions of Gluconeogenesis:
In gluconeogenesis, the three irreversible steps above are by-passed by other
unique reactions, in order to overcome the energetic barrier.
Replace step 10
(with 2 steps)
Replace step 3:
Replace step 1:
Energetics of Gluconeogenesis
in glycolysis
(step 3 - kinase)
DGo’ = -14.2 kJ/mol
DGo’ = -16.3 kJ/mol
in gluconeogenesis
(step 8 - phosphatase)
Exergonic hydrolysis catalyzed by Fructose 1,6-bisphosphatase
Both kinase and
phosphatase reactions
are exergonic.
Kinase uses ATP for
phosphate source and
for energy source .
What is the phosphoryl transfer potential
of Pi (inorganic phosphate?
Phosphatases hydrolyze
phosphorylated alcohols
(ROP), which have some
small amount of free
energy of hydrolysis.
Gluconeogenesis
- NOT a reversal of glycolysis
The three exergonic (irreversible)
steps of glycolysis in cell:
phosphorylate
(1) Hexokinase
(2) Phosphofructokinase
(3) Pyruvate kinase Substrate level
phosphorylation
are replaced by other favorable
reactions:
dephosphorylate
(1) Glucose-6-phosphatase
(2) Fructose-1,6-bisphosphatase
Pyruvate  Oxaloacetate  PEP
(3a) Pyruvate carboxylase
(3b) Phosphoenolpyruvate
carboxykinase
(reactions 3a & 3b) know
overview only, NO details
Gluconeogenesis vs. Reversal of Glycolysis
The stoichiometry of gluconeogenesis is:
Gluconeogenesis costs 6 ATP equivalents
In contrast, the stoichiometry for the reversal of glycolysis is:
Glycolysis produces 2 ATP
DGo’ = - 84 kJ/mol
Overall, both pathways are highly exergonic (spontaneous)!
Recall that one ATP equivalent changes the equilibrium constant by a
factor of about 108. Hence, the input of four additional high-energy bonds
in Gluconeogenesis changes the equilibrium by a factor of about 1032.
This is a clear example of the coupling of reactions: ATP hydrolysis is used
to power an energetically unfavorable reaction.
Non-carbohydrate Precursors
of Gluconeogenesis
• Lactate – formed by lactate dehydrogenase in
active skeletal muscle when rate of glycolysis
exceed rate of oxidative metabolism (because
of insufficient levels of oxygen).
• Amino acids – from proteins in the diet and
during starvation from breakdown of proteins
in skeletal muscle.
• Glycerol – from hydrolysis of triacylglycerols
in fat cells. Glycerol can enter the glycolytic
or gluconeogenic pathways through DHAP:
Phosphorylate at C3
Oxidize at C2
Comparison of Glycolysis & Gluconeogenesis
Glycolysis makes 2 ATP
(1) Hexokinase (- ATP)
- is used
+ is made
Gluconeogenesis costs 6 ATP
(11) Glucose-6-phosphatase
(3) Phosphofructokinase (- ATP) (9) Fructose-1,6-bisphosphatase
(10) Pyruvate kinase (+ ATP)
(1) Pyruvate carboxylase (- ATP) and
(2) Phosphoenolpyruvate carboxykinase (- GTP)
(7) Phosphoglycerate kinase (+ATP)
(5) Phosphoglycerate kinase (-ATP)
happens twice
In most tissues gluconeogenesis stops after formation of glucose-6-phosphate,
which can be stored as glycogen.
One advantage to ending gluconeogenesis at glucose 6-phosphate is that, unlike
free glucose, the molecule cannot diffuse out of the cell. To keep glucose inside
the cell, the generation of free glucose is controlled in two ways:
1. Enzyme responsible for the conversion of glucose 6-phosphate into glucose,
glucose 6-phosphatase, is regulated.
2. Enzyme is present only in tissues whose metabolic duty is to maintain
blood-glucose homeostasis. Tissues that release glucose into the blood are
the liver and to lesser extent the kidney.
Gluconeogenesis
Glycolysis and Gluconeogenesis are coordinated, in a tissuespecific fashion, to ensure that the glucose-dependent energy
needs of ALL cells are met. In a particular cell, when one
pathway is upregulated, the other is downregulated.
Reciprocal regulation
Cori Cycle (Figure 16.33)
• The main site of gluconegenesis is
in the liver, with a small amount
occurring in the kidneys.
• Gluconeogenesis in these two organs
helps maintain glucose levels in the
blood so that the brain, red blood
cells and muscles can extract
sufficient glucose from blood to
satisfy their energy needs.
http://www.wiley.com/college/fob/quiz/quiz21/21-5.html
The Cori Cycle
CORI CYCLE - Lactate formed by active muscle travels through the
blood and is converted into glucose by the liver. This cycle shifts
part of the metabolic burden of active muscle to the liver.
Cori Cycle (Figure 16.33)
Lactate Dehydrogenase (LDH) Isozymes:
H4, H3M1, H2M2, H1M3, M4
H = heart (higher affinity for lactate) takes
in lactate, oxidizes it to pyruvate, then uses
pyruvate via aerobic metabolism (citric
acid cycle, electron transport chain)
M = skeletal muscle and liver, have lower
affinity for lactate
In the liver, lactate is oxidized to pyruvate by lactate dehydrogenase. This
pyruvate undergoes gluconeogenesis to produce free glucose, which is released
into the blood. The liver restores the level of glucose necessary for active
muscle cells to continue anaerobic glycolysis for immediate energy needs.
Carbohydrate Metabolism and the Liver
Workbook, Chapter 16 Self-Test, Q27:
1. Which of the following statements about the Cori cycle and its
physiologic consequences are true?
A)
B)
C)
D)
E)
It involves the synthesis of glucose in muscle.
It involves release of lactate by muscle.
It involves lactate synthesis in the liver.
It involves ATP synthesis in muscle.
It involves release of glucose by the liver.
Anaerobic
glycolysis in
muscle
2. How much energy is extracted when
one glucose is converted to 2 pyruvate?
3. What is the energy cost of producing one
glucose from 2 pyruvate?
Gluconeogenesis
in liver
Other Metabolic Pathways Associated with Glycolysis
• The entry point of
other important dietary
sugars into glycolysis
varies with the
monosaccharide and
the organ.
• Common entry points
are: Glucose-6-P
Fructose-6-P
DHAP & GAP
GLYCOGEN
Glycogen Energy Storage
Pentose Phosphate Pathway
PFK
Biosynthesis
NADPH
and Growth
ribose-5-phosphate
FRUCTOSE
hexokinase
O
C
H
Phosphorylation
H
HO
OH
H
H
OH
H
OH
CH2OH
Glucose
hexokinase
O
C
H
Isomerization
CH2OH
O
H
Phosphorylation CH2 O
O
OH
H
HO
H
OH
H
OH
H
OH
H
OH O
H
OH O
H
OH O
CH2 O P OG6P
CH2 O P O-
HO
O-
OF6P
O OCH2OH
OO
C
CH2 O P O-
H
H
OH O
CH2 O P O-
OFBP
O
CH2 O P O-
P O-
H
HO
PFK
O
H
DHAP
OGAP
Multiple Fates of Glucose-6-Phosphate (G6P)
from Introduction to Chapter 20 (p. 589 – 590)
Figure 21.3
1. Used as fuel in anaerobic or
aerobic glycolysis
2. Converted to free gluocose by liver
and released into the blood to
raise blood sugar when too low
3. Processed by Pentose Phosphate
Pathway, which produces:
• 5C Ribose (for DNA/RNA
synthesis) and
• NADPH (for reductive
biosynthesis of biomolecules)
First stage
of PPP:
http://www.wiley.com/college/fob/quiz/quiz15/15-1.html
Pentose Phosphate Pathway – Needed for Biosynthesis
Anabolism is REDUCTIVE so biosynthesis and growth require the reduced
cofactor __________ , which is produced by the pentose phosphate pathway.
• In PPP glucose-6-phosphate is converted into
ribose-5-phosphate (needed for nucleotide biosynthesis
to form RNA & DNA), with production of two NADPH
for metabolic biosynthesis.
• Excess Ribose-5-P formed is completely converted into
the glycolytic intermediates fructose-6-P and
glyceraldehyde-3-P.
H
O
H
OH
H
OH
H
OH
2 NADP+
2 NADPH
+ CO2
CH 2OH
Ribose-5-phosphate
PHASE 2
(nonixidative)
2 Fructose-6-P + glyceraldehyde-3-P
(which feed into glycolysis)
Pentose Phosphate Pathway
• The pentose phosphate pathway (occurs in cytosol) meets the need of
all organisms for a source of NADPH to use in reductive biosynthesis.
Synthetic pathways requiring NADPH: Detoxification processes need NADPH:
• Fatty acid biosynthesis
• Reduction of oxidized glutathione
• Cholesterol biosynthesis
• Cytochrome P450 monooxygenases
• Neurotransmitter biosynthesis
• Nucleotide biosynthesis (and NAD+, FAD, acetyl CoA)
Liver exports glucose
NADPH & ribose-5-P
Glucose-6-P
Glycolysis:
• convert glucose into pyruvate
• produce 2 ATP & 2 NADH
Pyruvate
Glycogen (energy storage,
when energy not needed)
Gluconeogenesis:
• converts pyruvate into glucose
• utilizes 4 ATP, 2 GTP & 2 NADH
Metabolism and the Liver
Workbook, Chapter 20 Self-Test, Q13:
Liver synthesizes fatty acids and lipids for export to other tissues.
Would you expect the Pentose Phosphate Pathway (PPP) to have
a low or a high activity in this organ? Explain.
HIGH
Biosynthesis of fatty acids requires reducing
equivalents in the form of NADPH. In all organs that carry out
reductive biosynthesis, the PPP supplies a large portion of the
required NADPH.
What types of reactions do the following enzymes catalyze?
(A) phosphatase – hydrolyzes phosphate from phosphorylated alcohol
so transfers phosphate to H2O
(B) kinase – transfers phosphate from ATP to alcohol
(C) ATPase – hydrolyzes ATP and uses energy released to create a
concentration gradient or drive a DG + reaction
Carbohydrate Metabolism and the Liver
Workbook, Chapter 16 Self-Test, Q27:
1. Which of the following statements about the Cori cycle and its
physiologic consequences are true?
A)
B)
C)
D)
E)
It involves the synthesis of glucose in muscle.
It involves release of lactate by muscle.
It involves lactate synthesis in the liver.
It involves ATP synthesis in muscle.
It involves release of glucose by the liver.
Ans: B, D, E
Anaerobic
glycolysis in
muscle
2. How much energy is extracted when
one glucose is converted to 2 pyruvate?
2 ATP + 2 NADH
3. What is the energy cost of producing one
glucose from 2 pyruvate?
4 ATP + 2 GTP + 2 NADH
Gluconeogenesis
in liver