essential biochemistry -...
TRANSCRIPT
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Lecture Notes for
Chapter 13Glucose Metabolism
Essential BiochemistryThird Edition
Charlotte W. Pratt | Kathleen Cornely
Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.
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Glucose Metabolism
• Glycolysis
• “Glyco”-”Lysis”: breakdown of glucose monomer
• Gluconeogenesis
• “New” “genesis” of “glucose”
• Used when supply of glycogen is exhasted
• Glycogen Synthesis and Degradation
• Storing glucose long-term and recovering it later
• The Pentose Phosphate Pathway
• Used to generate pentoses (5-C sugars)
• Starts with glucose
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GLYCOLYSIS
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Glycolysis Overview
• Net reaction:
• Glycolysis occurs in 10 steps.
– Steps 1-5 = energy investment
– Steps 6-10 = energy payoff
• Glucose (a six-carbon molecule) is broken down
into two 3-carbon molecules.
• Electron carriers are reduced.
• Occurs in the cytosol.
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10 Steps of
Glycolysis
Let’s look at
each step…
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First 5 Steps of Glycolysis
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Add ℗
Change
form
Add ℗
split
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Step 1: Hexokinase Reaction
• Kinases are enzymes that phosphorylate molecules.
• ATP is invested; ATP hydrolysis drives the reaction.
• Reaction is irreversible (note the one-way arrow)
• Blocks glucose from transport out of cell
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Step 2: Phosphoglucose Isomerase
Reaction
• Conversion of a hexose to a pentose
• Reaction is near equilibrium.
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Step 3: Phosphofructokinase Reaction
• Reaction is irreversible (one-way arrow).
• This is the flux-control point or rate determining reaction
• Another ATP is invested; ATP hydrolysis drives the reaction.
• Energy supplied by ℗ destabilizes the molecules – more apt
to split in step 4
• Must convert to ketose before 2nd ℗ added – or ring unable
to open
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Fructose-2,6-bisphosphate is the most
potent activator of phosphofructokinase
in mammals.
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Other molecules activate or inhibit
phosphofructokinase.
Regulation in Bacteria Regulation in Mammals
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Remember: Sugars can be in cyclic
or linear forms.
Cleavage of fructose-1,6-
bisphosphate is easiest to
understand using the linear
form of the molecule.
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Step 4: Aldolase Reaction• The aldolase reaction is the reverse of an aldol
condensation.
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Rapid consumption of products pulls this reaction forward.
ΔG0’ = +22.8 kJ · mol-1
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Aldolase
Mechanism
in Detail
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Aldolase
Mechanism
in Detail
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Aldolase Mechanism in Detail
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Aldolase
Mechanism
in Detail
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Aldolase
Mechanism
in Detail
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Step 5: Triose Phosphate Isomerase
Reaction
• Converts DHAP to GAP (and vice versa)
• Result: 2 GAP’s proceed through remainder of glycolysis
• Even though ΔG>0, reaction proceeds forward because GAP is
quickly consumed.
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Triose phosphate isomerase is a
catalytically “perfect” enzyme.
Loop closure
stabilizes
transition state
Transition State
Analog (orange)
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Last 5 Steps of Glycolysis
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First 5 Steps of Glycolysis
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Step 6: GAP Dehydrogenase
Reaction
• Notice: phosphate does not come from ATP.
• NAD+ is reduced to NADH.
• Reaction is both a phosphorylation and an oxidation-
reduction reaction.
• Reaction is inhibited by AsO43-, which competes with PO4
2-
for binding the enzyme.
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GAP Dehydrogenase Mechanism
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Step 7: Phosphoglycerate Kinase
Reaction
• ATP is formed.
• Since reaction occurs twice, 2 ATP have been recouped
from the energetic investment.
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ΔG0’ = -12.1kJ · mol-1
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Step 8: Phosphoglycerate Mutase
Reaction
• Phosphate gets moved to C-2.
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Isomerization of 3-phosphoglycerate
occurs via an active site His residue.
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Step 9: Enolase Reaction
• Enolase catalyzes a dehydration reaction.
• H2O is produced.
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Step 10: Pyruvate Kinase Reaction
• Final step of glycolysis
• ATP formed: energetic payoff nets 2 ATP
• Reaction is irreversible.
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This reaction occurs in two parts
ΔG0’ = -16kJ · mol-1
ΔG0’ = -46kJ · mol-1
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Last 5 Steps of Glycolysis
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Graphical Representation of the Free
Energy Changes of Glycolysis
Recall:
Steps 1, 3 and 10 are irreversible
(forward only) reactions.
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Near equilibrium (ΔG ≒0 ) accommodate flux
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The enzymes which catalyze
irreversible reactions in glycolosis
are
1. Hexokinase, aldolase, pyruvate kinase
2. Phosphofructokinase, glyceraldehyde-3-
phosphate dehydrogenase,enolase
3. Phosphoglucose isomerase, triose phosphate
isomerase, enolase
4. Hexokinase, phosphofructokinase, pyruvate
kinase
5. Aldolase, enolase, pyruvate kinase
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What happens to pyruvate?
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During exercise pyruvate can be
temporarily converted to lactate.
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Further breakdown of pyruvate to CO2
and H2O is much more highly favored
than lactate.
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Organisms such as yeast can regenerate
NAD+ by converting pyruvate to ethanol.
Anaerobic Conditions
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Pyruvate can still be further oxidized.
• Decarboxylation results in each three-carbon molecule
being broken down into two-carbon fragments.
• Acetyl group gets picked up by CoA.
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Pyruvate is a precursor of oxaloacetate.
• Oxaloacetate is a metabolite used in:
– The citric acid cycle
– Gluconeogenesis
• Oxaloacetate is also an intermediate in amino
acid synthesis.© 2014 John Wiley & Sons, Inc. All rights reserved.
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Pyruvate carboxylase uses biotin as
a cofactor.
• Biotin is covalently attached to a Lys residue in
the enzyme.
• Biotin carries CO2 in an unusual mechanism →© 2014 John Wiley & Sons, Inc. All rights reserved.
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Pyruvate
Carboxylase
Mechanism
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GLUCONEOGENESIS
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13-2
Gluconeogenesis
• Many glycolytic
enzymes are used.
• Four new enzymes
– Pyruvate carboxylase
– Phosphoenolpyruvate
carboxykinase
– Fructose
bisphosphatase
– Glucose-6-phosphatase
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Pyruvate is converted to
phosphoenolpyruvate in 2 steps.
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Four gluconeogenic enzymes plus some
glycolytic enzymes convert pyruvate to
glucose
3
4
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If glycolysis and gluconeogenesis
occurred simultaneously, there
would be a net consumption of ATP!
• Goal of producing ATP would be futile!
• Instead, glycolysis and gluconeogenesis
are regulated based on the cell’s needs.
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Gluconeogenesis is regulated at the
fructose bisphosphatase step.
• A single compound can control flux through two
opposing pathways in a reciprocal manner.
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GLYCOGEN SYNTHESIS AND
DEGRADATION
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13-3 Glycogen Synthesis and
Degradation
• Dietary glucose and the glucose produced by
gluconeogenesis are stored in the liver and other tissues
as glycogen.
• Glucose units can be removed from the glycogen polymer
by phosphorolysis.
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Glycogen is composed of monomers
of glucose-1-phosphate made
through an isomerization reaction.
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Glycogen synthesis
consumes the free
energy of UTP.
• Hydrolysis of inorganic
pyrophosphatase
drives the reaction.
• UDP-glucose is the
major intermediate.
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Glycogen synthase adds glucose to
extend the glycogen polymer.
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C1
C4
C6
transglycosylase
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Glycogenolysis
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Only gluconeogenic tissues
have this enzyme
Ex: liver
available to
the body
enter glycolysis at step 2,
skip the ATP consumption in Step 1
break down glycogen
only for their own needs
Ex: muscle
hormonal control
Net gain of ATP is higher!
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THE PENTOSE PHOSPHATE
PATHWAY
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13-4 The Pentose Phosphate Pathway
• An oxidative pathway for producing NADPH and
converting glucose to ribose.
• Degradative enzyme → NADH
Biosynthetic enzyme → NADPH
• The interconversion of ribose and
intermediates of glycolysis and
gluconeogenesis.
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Oxidative reactions of the pentose
phosphate pathway produce NADPH.
© 2014 John Wiley & Sons, Inc. All rights reserved.
• Step 1
From:Glycosis
Glycogen
phosphorolysis
Gluconeogenesis
蠶豆症:Glucose-6-Phosphate Dehydrogenase deficiency
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Production of 6-phosphogluconate
can also occur in the absence of an
enzyme.
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The third step of the pentose
phosphate pathway involves oxidative
decarboxylation.
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Ribose-5-phospate is a precursor of
the ribose unit of nucleotides.
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Isomerization and interconversion
reactions generate a variety of
monosaccharides
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Ribonucleotide reductase converts
ribose to deoxyribose.
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Net Reaction for the
Pentose Phosphate Pathway
• Ribose derivative is produced.
• 2 NADPH molecules are formed.
• Pathway is active in rapidly dividing cells.
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Summary of
Glucose
Metabolism
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