CAMPBELL BIOLOGY IN FOCUS
Urry • Cain • Wasserman • Minorsky • Jackson • Reece
6
An Introduction
to Metabolism
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
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Figure 6.1
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Concept 6.1: An organism’s metabolism
transforms matter and energy
 Metabolism - totality of an organism’s chemical
reactions
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Figure 6.UN01
Enzyme 1
Starting
molecule
A
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Enzyme 2
C
B
Reaction 1
Enzyme 3
Reaction 2
Reaction 3
D
Product
 Catabolic pathways release energy by breaking
down complex molecules
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 Anabolic pathways consume energy to build
complex molecules from simpler ones
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Figure 6.2
A diver has more potential
energy on the platform.
Climbing up converts the kinetic
energy of muscle movement
to potential energy.
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Diving converts
potential energy to
kinetic energy.
A diver has less potential
energy in the water.
Figure 6.3
Heat
Chemical
energy
(a) First law of thermodynamics
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(b) Second law of thermodynamics
Figure 6.4
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Free-Energy Change (G), Stability, and
Equilibrium
 A living system’s free energy is energy that can
do work when temperature and pressure are
uniform
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 The change in free energy (∆G) is the difference
between the free energy of the final state and the
free energy of the initial state
∆G = Gfinal state – Ginitial state
 Only processes with a negative ∆G are spontaneous
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Figure 6.5
• More free energy (higher G)
• Less stable
• Greater work capacity
In a spontaneous change
• The free energy of the
system decreases (G  0)
• The system becomes more
stable
• The released free energy can
be harnessed to do work
• Less free energy (lower G)
• More stable
• Less work capacity
(a) Gravitational
motion
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(b) Diffusion
(c) Chemical
reaction
Exergonic and Endergonic Reactions in Metabolism
 An exergonic reaction
 net release of free energy
 spontaneous
 ∆G is negative
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Figure 6.6a
(a) Exergonic reaction: energy released, spontaneous
Free energy
Reactants
Amount of
energy
released
(G  0)
Energy
Products
Progress of the reaction
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Figure 6.6b
(b) Endergonic reaction: energy required,
nonspontaneous
Free energy
Products
Energy
Reactants
Progress of the reaction
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Amount of
energy
required
(G  0)
 An endergonic reaction
 absorbs free energy
 Nonspontaneous
 ∆G is positive
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The Structure and Hydrolysis of ATP
 ATP (adenosine triphosphate)
 composed of
 ribose (sugar),
 adenine (nitrogenous base)
 three phosphate groups
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Figure 6.8
Adenine
Phosphate groups
Ribose
(a) The structure of ATP
Adenosine triphosphate (ATP)
Energy
Inorganic
phosphate
Adenosine diphosphate (ADP)
(b) The hydrolysis of ATP
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Figure 6.8a
Adenine
Phosphate groups
(a) The structure of ATP
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Ribose
Figure 6.8b
Adenosine triphosphate (ATP)
Energy
Inorganic
phosphate
Adenosine diphosphate (ADP)
(b) The hydrolysis of ATP
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Figure 6.10
Transport protein
Solute
Solute transported
(a) Transport work: ATP phosphorylates transport proteins.
Vesicle
Motor protein
Cytoskeletal track
Protein and
vesicle moved
(b) Mechanical work: ATP binds noncovalently to motor proteins
and then is hydrolyzed.
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Concept 6.4: Enzymes speed up metabolic reactions
by lowering energy barriers
 An enzyme is a catalytic protein
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Figure 6.UN02
Sucrase
Sucrose
(C12H22O11)
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Glucose
(C6H12O6)
Fructose
(C6H12O6)
Figure 6.12
A
B
C
D
Free energy
Transition state
A
B
C
D
EA
Reactants
A
B
G  0
C
D
Products
Progress of the reaction
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How Enzymes Speed Up Reactions
 Enzymes catalyze reactions by lowering the
activation energy (EA) barrier
 Enzymes do not affect the change in free energy
(∆G); instead, they hasten reactions that would
occur eventually
Animation: How Enzymes Work
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Figure 6.13
Free energy
Course of
reaction
without
enzyme
EA
without
enzyme
EA with
enzyme
is lower
Reactants
G is unaffected
by enzyme
Course of
reaction
with enzyme
Products
Progress of the reaction
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Substrate Specificity of Enzymes
 The reactant that an enzyme acts on is called the
substrate
 enzyme-substrate complex enzyme bound to
substrate
 active site - region on enzyme where substrate binds
 Enzyme specificity results from complementary fit
between active site and substrate
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Figure 6.14
Substrate
Active site
Enzyme
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Enzyme-substrate
complex
Figure 6.15-1
1 Substrates enter
active site.
Substrates
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2 Substrates are
held in active site by
weak interactions.
Enzyme-substrate
complex
Figure 6.15-2
1 Substrates enter
active site.
Substrates
2 Substrates are
held in active site by
weak interactions.
Enzyme-substrate
complex
3 Substrates are
converted to
products.
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Figure 6.15-3
2 Substrates are
held in active site by
weak interactions.
1 Substrates enter
active site.
Substrates
Enzyme-substrate
complex
4 Products are
released.
Products
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3 Substrates are
converted to
products.
Figure 6.15-4
2 Substrates are
held in active site by
weak interactions.
1 Substrates enter
active site.
Substrates
Enzyme-substrate
complex
5 Active
site is
available
for new
substrates.
Enzyme
4 Products are
released.
Products
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3 Substrates are
converted to
products.
Effects of Local Conditions on Enzyme Activity
 An enzyme’s activity can be affected by
temperature, pH and Chemicals that influence the
enzyme
 Each enzyme has an optimal temperature & pH
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Figure 6.16
Rate of reaction
Optimal temperature for
typical human enzyme
(37C)
0
Optimal temperature for
enzyme of thermophilic
(heat-tolerant)
bacteria (77C)
40
80
60
Temperature (C)
(a) Optimal temperature for two enzymes
20
Rate of reaction
Optimal pH for pepsin
(stomach
enzyme)
0
1
2
3
5
pH
(b) Optimal pH for two enzymes
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4
120
100
Optimal pH for trypsin
(intestinal
enzyme)
6
7
8
9
10
Cofactors
 Cofactors are organic or inorganic nonprotein
enzyme helpers
 organic cofactors are called coenzymes
 Some coenzymes are vitamins
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Enzyme Inhibitors
 Competitive inhibitors bind to active site
competing with substrate
 Noncompetitive inhibitors bind to another part of
an enzyme, causing the enzyme to change shape
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Figure 6.17
(a) Normal binding
(b) Competitive inhibition
(c) Noncompetitive
inhibition
Substrate
Active site
Competitive
inhibitor
Enzyme
Noncompetitive
inhibitor
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Allosteric Regulation of Enzymes
 Allosteric regulation - regulatory molecule binds to
protein at one site and affects protein’s function at
another site
 may either inhibit or stimulate enzyme
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Figure 6.18
(a) Allosteric activators and inhibitors
Allosteric enzyme
with four subunits
Active site
(one of four)
Regulatory
site (one
Activator
of four)
Active form
Substrate
Stabilized
active form
Oscillation
Nonfunctional
active site
Inactive
form
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Inhibitor
(b) Cooperativity: another type of allosteric
activation
Stabilized
inactive form
Inactive form
Stabilized
active form
Feedback Inhibition
 In feedback inhibition, the end product of a
metabolic pathway shuts down the pathway
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Figure 6.19
Active site available
Isoleucine
used up by
cell
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Intermediate A
Feedback
inhibition
Enzyme 2
Intermediate B
Enzyme 3
Isoleucine
binds to
allosteric
site.
Intermediate C
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)
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Tomorrow…
…will include class time to
work on Chapter 6 WS, so
your book might be
helpful….
...(AP Bio Book, not Lord of
the Flies…
…Unless it discusses enzymes…
I don’t remember, but I suppose it might…
Golding was an unusual guy after all…)
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