Essential Biochemistry - 國防醫學院數位學習系統lms.ndmctsgh.edu.tw/sysdata/47/9247/doc/6b6a0b25... · Glycolysis Overview • Net reaction: • Glycolysis occurs in 10

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

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

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.


10 Steps of

Glycolysis

Let’s look at

each step…


First 5 Steps of Glycolysis


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


Step 2: Phosphoglucose Isomerase

Reaction

• Conversion of a hexose to a pentose

• Reaction is near equilibrium.


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.


Other molecules activate or inhibit

phosphofructokinase.

Regulation in Bacteria Regulation in Mammals


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.


Step 4: Aldolase Reaction• The aldolase reaction is the reverse of an aldol

condensation.


Rapid consumption of products pulls this reaction forward.

ΔG0’ = +22.8 kJ · mol-1

Aldolase

Mechanism

in Detail


Aldolase

Mechanism

in Detail


Aldolase Mechanism in Detail


Aldolase

Mechanism

in Detail


Aldolase

Mechanism

in Detail


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.


Triose phosphate isomerase is a

catalytically “perfect” enzyme.

Loop closure

stabilizes

transition state

Transition State

Analog (orange)


Last 5 Steps of Glycolysis


First 5 Steps of Glycolysis


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.


GAP Dehydrogenase Mechanism


Step 7: Phosphoglycerate Kinase

Reaction

• ATP is formed.

• Since reaction occurs twice, 2 ATP have been recouped

from the energetic investment.


ΔG0’ = -12.1kJ · mol-1

Step 8: Phosphoglycerate Mutase

Reaction

• Phosphate gets moved to C-2.


Isomerization of 3-phosphoglycerate

occurs via an active site His residue.


Step 9: Enolase Reaction

• Enolase catalyzes a dehydration reaction.

• H2O is produced.


Step 10: Pyruvate Kinase Reaction

• Final step of glycolysis

• ATP formed: energetic payoff nets 2 ATP

• Reaction is irreversible.


This reaction occurs in two parts

ΔG0’ = -16kJ · mol-1

ΔG0’ = -46kJ · mol-1

Last 5 Steps of Glycolysis


Graphical Representation of the Free

Energy Changes of Glycolysis

Recall:

Steps 1, 3 and 10 are irreversible

(forward only) reactions.


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?


During exercise pyruvate can be

temporarily converted to lactate.


Further breakdown of pyruvate to CO2

and H2O is much more highly favored

than lactate.


Organisms such as yeast can regenerate

NAD+ by converting pyruvate to ethanol.

Anaerobic Conditions


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.


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


GLUCONEOGENESIS


13-2

Gluconeogenesis

• Many glycolytic

enzymes are used.

• Four new enzymes

– Pyruvate carboxylase

– Phosphoenolpyruvate

carboxykinase

– Fructose

bisphosphatase

– Glucose-6-phosphatase


Pyruvate is converted to

phosphoenolpyruvate in 2 steps.


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.


Gluconeogenesis is regulated at the

fructose bisphosphatase step.

• A single compound can control flux through two

opposing pathways in a reciprocal manner.


GLYCOGEN SYNTHESIS AND

DEGRADATION


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.


Glycogen synthesis

consumes the free

energy of UTP.

• Hydrolysis of inorganic

pyrophosphatase

drives the reaction.

• UDP-glucose is the

major intermediate.


Glycogen synthase adds glucose to

extend the glycogen polymer.


C1

C4

C6

transglycosylase

Glycogenolysis


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


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.


• 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.


The third step of the pentose

phosphate pathway involves oxidative

decarboxylation.


Ribose-5-phospate is a precursor of

the ribose unit of nucleotides.


Isomerization and interconversion

reactions generate a variety of

monosaccharides

Ribonucleotide reductase converts

ribose to deoxyribose.


Net Reaction for the

Pentose Phosphate Pathway

• Ribose derivative is produced.

• 2 NADPH molecules are formed.

• Pathway is active in rapidly dividing cells.


Summary of

Glucose

Metabolism


Documents

Essential Biochemistry - 國防醫學院數位學習系統lms.ndmctsgh.edu.tw/sysdata/47/9247/doc/6b6a0b25... · Glycolysis Overview • Net reaction: • Glycolysis occurs in 10