essential biochemistry -...

Lecture Notes for

Chapter 13Glucose Metabolism

Essential BiochemistryThird Edition

Charlotte W. Pratt | Kathleen Cornely

Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.

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

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

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

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

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

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

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

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

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.

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.

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

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.

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

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!

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.

Oxidative reactions of the pentose

phosphate pathway produce NADPH.

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• Step 1

From：Glycosis

Glycogen

phosphorolysis

Gluconeogenesis

蠶豆症：Glucose-6-Phosphate Dehydrogenase deficiency

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

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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