Transcript Chapter 8

Chapter 8
An Introduction to Metabolism
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures prepared by
Dr. Jorge L. Alonso
Florida International
University
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: The Energy of Life
•
The living cell is a
miniature chemical
factory where
thousands of
reactions occur
•
The cell extracts
energy and applies
energy to perform
work
Energy is the capacity to cause change (perform an
activity, or do work = force x motion)
Potential Energy (possible as opposed to actual) an object
possesses by virtue of its position or chemical composition
(bonds).
C6H12O6 (l) + 6 O2 (g)
6 CO2(g) + 6 H2O(g)
Electromagnetic
(chemical bond)
energy
Energy (-E):
work + heat
Kinetic Energy an object possesses by virtue of its motion.
1
KE =  mv2
2
•
Potential energy is energy that
matter possesses because of its
location or structure
•
Kinetic energy is energy
associated with motion
Forms of Energy
•
Energy is the capacity to cause change (perform an activity, or do work
= force x motion)
•
Energy exists in various forms, some of which can perform useful work
•
Energy can be converted from one form to another
•
Laws of Thermodynamics govern energy transformations
Potential Energy
Electromagnetism
• light,
Kinetic (motion & heat)
• electricity
• chemical bonds
Nuclear
Gravity
Animation: Energy Concepts
Some organisms even convert food energy into light, as in bioluminescence
Bioluminescent fungi
Bioluminescent coral
food energy to electricity , as in electric eels
Concept 8.1: An organism’s metabolism
(1) transforms matter and energy,
(2) subject to the laws of thermodynamics
• Metabolism is the totality of an organism’s chemical reactions
Enzyme 1
A
Reaction 1
Starting
Molecule
Enzyme 2
B
Enzyme 3
C
Reaction 2
D
Reaction 3
Product
(matter + energy)
• A metabolic pathway begins with a specific molecule and
ends with a product
• Each step is catalyzed by a specific enzyme
Metabolism is the totality of an
organism’s chemical reactions
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Catabolic pathways release
energy by breaking down
complex molecules into
simpler compounds
o
x
y
d
a
t
i
o
n
Catabolism
vs.
Anabolism
Anabolic pathways
consume energy to build
complex molecules from
simpler ones
r
e
d
u
c
t
i
o
n
Catabolism
vs.
Anabolism
Can you detect
which one has more
catabolic as opposed
to anabolic reactions
taking place?
Anabolic
Steroids
Anabolic
Catabolic
The First Law of Thermodynamics
• All the energy of the universe is constant:
– Energy can be transferred and transformed, but it cannot
be created or destroyed
• The first law is also called the “Law of Conservation of Energy”
Heat
Potential Energy
Heat
Electromagnetism
• light
Kinetic (motion )
and Heat
Disorganized
Energy
(dissipates to
surroundings
and cannot be
captured for
further
conversions)
• electricity
• chemical bonds
Nuclear
Heat
Gravity
Heat
The 2nd Law:
Heat is released in all
energy transformations
Heat is no longer
available for further
transformations
The Second Law of Thermodynamics
According to the second law of thermodynamics:

During every energy transformation, some energy is lost as heat (-ΔH),
and this is heat no longer transformable and is thus unusable, it
dissipates into the surroundings causing increase of kinetic energy
(disorder) in the surrounding environment.

Entropy (ΔS) is a measure of the amount of disorder in a system.
Entropy increases when molecules are moving, which is caused by an
increase in heat.

“Every energy transformation increases the entropy (disorder) of
the universe”
Food (chemical) Energy

Motion (KE) +
Heat
The 2nd Law:
Heat is released in all
energy transformations
Heat is no longer
available for further
transformations
Heat causes an increase
in the entropy (disorder)
of the universe
Thermodynamic language: open, closed, and
isolated systems, surroundings, and the universe
•
System: object under study
•
Surroundings: everything that
surrounds the system
•
Universe = system + surrounding
•
Isolated system, no exchange of
matter or energy occurring
between system and
surroundings.
•
A closed system, allow the
exchange of heat, but no matter is
exchanged
•
In an open system, energy and
matter can be transferred between
the system and its surroundings
•
Organisms are open systems
The Second Law of Thermodynamics, again
According to the second law of
thermodynamics:


“Every energy transformation
increases the entropy (disorder)
of the universe”
The entropy of the universe is
always increasing = sum of the
entropy of the system + entropy
of the surroundings = + ∆S
Create Order
(- ∆Ssys)
(∆Ssurr)
+
= +∆Suniv
(∆Ssys)
By causing lots of disorder
+
(+∆Ssurr)
= +∆Suniv
The entropy of
the universe is
always
increasing =
ENTROPIC
DOOM !
Is entropy increasing or
decreasing as embryos develop
into human adults? Does this
violate the Second Law?
Entropy (disorder)
may decrease - (∆Ssys)
in the development
and growth of an
organism, but….
+∆Ssurr
…. the universe’s
total entropy
increases
• The evolution of more complex organisms does
not violate the second law of thermodynamics
• More complex organisms transform more energy
and thus produce more heat (entropy)
More order
- ∆Ssys
Does Evolution violate the Second Law?
Less order
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- ∆Ssys
Spontaneity (1) tendency for things to occur by
themselves, without apparent external cause;
(2) occurring from natural inclination or impulse and
not from external cause
• Spontaneous processes occur without energy
input; they can happen quickly or slowly
• For a process to occur
without energy input, it
must increase the
entropy of the universe
• Living cells unavoidably convert
organized (concentrated) forms of energy
into heat, which is disorganized and
dissipates into surroundings.
Biological Order and Disorder
• Cells create ordered structures from less
ordered materials
• Organisms also replace ordered forms of
matter and energy with less ordered forms
• Energy flows into an ecosystem in the form of
light and exits in the form of heat
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 8.2: The Gibbs free-energy change (ΔG) of a
reaction tells us whether or not the reaction occurs
spontaneously
There are three things that natural processes do spontaneously:
1. Release energy (as opposed to absorb it): -H
2. Increase entropy (disorder), rather than decrease it (order): + S
The Heat released in energy transformations, causes an increase in
temperature (T), which increases the Entropy of the universe.
•
The change in free energy (∆G) during a process is related to the change in
enthalpy (∆H), change in entropy (∆S), and temperature in Kelvin (T):
∆G = ∆H – T∆S
(-) = (-) - T(+)
•
Only processes with a negative ∆G are spontaneous.
•
Spontaneous processes can be harnessed to perform work
Free-Energy Change, G
• A living system’s free energy is energy that
can do work when temperature and pressure
are uniform, as in a living cell
•
Biologists want to know which reactions occur spontaneously and which
require input of energy
•
To do so, they need to determine energy changes that occur in chemical
reactions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Free Energy (+∆G ), Instability  Spontaneity (-∆G )
 Equilibrium, Stability, and Work
•
Free energy (+G) is a measure of a system’s instability, its tendency to change to
a more stable state
•
During a spontaneous change, free energy decreases (-∆G ) and the stability of a
system increases
•
Equilibrium is a state of maximum stability
•
A process is spontaneous and can perform work only when it is moving toward
equilibrium
• 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
- ∆G
- ∆G
• Less free energy (lower G)
• More stable
• Less work capacity
Gravitational motion
Diffusion
Chemical reaction
Equilibrium and Metabolism
•
•
Closed and open hydroelectric
systems can serve as analogies
for metabolism
Reactions in a closed or isolated
systems eventually reach
equilibrium and then do no work
•
Cells are not in equilibrium; they
are open systems experiencing a
constant flow of materials
•
A defining feature of life is that
metabolism is never at equilibrium
•
∆G < 0
∆G = 0
An isolated hydroelectric system
An open
Hydroelectric
system
∆G < 0
∆G < 0
∆G < 0
∆G < 0
A catabolic pathway in a cell
releases free energy in a series of
reactions
A multistep open hydroelectric system
Exergonic & Endergonic
Reactions in Metabolism
• An endergonic
reaction absorbs
free energy from
its surroundings
and is
nonspontaneous
Free energy
Amount of
energy
released
(∆G < 0)
Energy
Products
Progress of the reaction
Exergonic reaction: energy released
Products
Free energy
• An exergonic
reaction proceeds
with a net release
of free energy and
is spontaneous
Reactants
Amount of
energy
required
(∆G > 0)
Energy
Reactants
Progress of the reaction
Endergonic reaction: energy required
Concept 8.3: ATP powers cellular work by coupling
exergonic reactions to endergonic reactions
• A cell does three main kinds of work:
– Chemical: build chemicals for growing and
reproducing cells
– Transport: to move materials
– Mechanical: locomotion
Energy Coupling, the use of an exergonic process
to drive an endergonic one. Most energy coupling
in cells is mediated by ATP
Overall, the coupled
reactions are exergonic
The Structure of ATP
• ATP (adenosine triphosphate) is the cell’s energy
shuttle
• ATP is composed of (1) ribose (a sugar), (2) adenine
(a nitrogenous base), and (3) three phosphate groups
Adenine
Phosphate groups
Ribose
The Hydrolysis of ATP
• The bonds between the
phosphate groups of ATP’s tail
can be broken by hydrolysis
• Energy is released from ATP
when the terminal phosphate
bond is broken
• This release of energy comes
from the chemical change to a
state of lower free energy, not
from the phosphate bonds
themselves
How ATP
Performs Work
•
In the cell, the energy
from the exergonic
reaction of ATP
hydrolysis can be used
to drive an endergonic
reaction. Overall, the
coupled reactions are
exergonic
•
ATP drives endergonic
reactions by Substrate –
Level phosphorylation,
transferring a phosphate
group to some other
molecule, such as a
reactant
•
The recipient molecule is
now phosphorylated
In Transport Work
ATP drives
endergonic
reactions by
phosphorylation,
transferring a
phosphate group to
some other
molecule, such as a
reactant which
ATP
becomes
phosphorylated
In Mechanical
Work the whole
ATP molecule binds
to proteins and then
it becomes
hydrolyzed and
released.
Membrane protein
P
Solute
Pi
Solute transported
Transport work: ATP phosphorylates
transport proteins
ADP
+
Pi
Vesicle
Cytoskeletal track
ATP
Motor protein
Protein moved
Mechanical work: ATP binds noncovalently
to motor proteins, then is hydrolyzed
The Regeneration of ATP
• The chemical potential energy temporarily stored in ATP drives
most cellular work
• How does the organism regenerate more ATP?
• The energy to phosphorylate ADP comes from catabolic
reactions in the cell
ATP + H2O
Energy from
catabolism (exergonic,
energy-releasing
processes)
ADP + P i
Energy for cellular
work (endergonic,
energy-consuming
processes)
Concept 8.4: Enzymes speed up metabolic
reactions by lowering energy barriers
• Enzymes are
proteins that
catalyze
reactions in
cells
• A catalyst is a
chemical agent
that speeds up
a reaction
without being
consumed by
the reaction
Glucose (C6H12O6)
Sucrose (C12H22O11)
Sucrase
Fructose (C6H12O6)
Animation: How
Enzymes Work
The Activation Energy Barrier
The initial
energy
needed to
start a
chemical
reaction is
called the
activation
energy (EA)
often supplied
in the form of
heat from the
surroundings
How do
Enzymes
Catalyze
reactions ?
by lowering
the EA
Barrier
A
B
•
C
D
Transition state
Chemical reactions
involves bond breaking
and bond forming
AB + CD  AC + BD
-H
-G
A
B
C
D
EA with
enzyme
is lower
EA
Reactants
A
Course of
reaction
with enzyme
B
∆G < O
C
D
Products
Progress of the reaction
Other characteristics of enzymes:
1.
they are substrate specific
2.
they are affected by temperature and pH
3.
4.
they may be aided by other chemicals
(cofactors / coenzymes)
they may be inhibited by
others chemicals
(competitively or noncompetitively)
1. Substrate Specificity
Glu
Fru
active site
Enzyme-Substrate
Complex
•
Induced fit of a substrate
brings chemical groups of
the active site into
positions that enhance
their ability to catalyze the
reaction
Catalysis in the Enzyme’s Active Site
•
The active site can lower an EA barrier by
–
Orienting substrates correctly
–
Straining substrate bonds
–
Providing a favorable microenvironment
–
Covalently bonding to the substrate
Substrate
Active site
Enzyme
Enzyme-substrate
complex
Fig. 8-17
1 Substrates enter active site; enzyme
changes shape such that its active site
enfolds the substrates (induced fit).
2 Substrates held in
active site by weak
interactions, such as
hydrogen bonds and
ionic bonds.
Substrates
Enzyme-substrate
complex
6 Active
site is
available
for two new
substrate
molecules.
Enzyme
5 Products are
released.
4 Substrates are
converted to
products.
Products
3 Active site can lower EA
and speed up a reaction.
Effects of Local Conditions on Enzyme Activity
• An enzyme’s activity can be affected by
 General environmental factors, such as temperature and
pH cause enzymes to become denatured (loose their
active shape)
 Chemicals that specifically influence the enzyme
1. Enzyme helpers: Cofactors and Coenzymes
2. Inhibitors
Effect of environmental factores (temp. and pH):
Effect of chemicals:
Effects of Temperature and pH on Enzyme Activity
• Each enzyme has
an optimal pH in
which it can
function
Rate of reaction
Optimal temperature for
enzyme of thermophilic
(heat-tolerant)
bacteria
40
60
80
Temperature (ºC)
(a) Optimal temperature for two enzymes
0
20
Optimal pH for pepsin
(stomach enzyme)
100
Optimal pH
for trypsin
(intestinal
enzyme)
Rate of reaction
• Each enzyme has
an optimal
temperature in
which it can
function
Optimal temperature for
typical human enzyme
4
5
pH
(b) Optimal pH for two enzymes
0
1
2
3
6
7
8
9
10
Cofactors and Coenzymes: Enzyme helpers
• Cofactors are
enzyme helpers that
may be inorganic
(such as a metal in
ionic form) or organic.
• Coenzymes are
organic cofactors.
• Coenzymes include
vitamins
Enzyme Inhibitors affect the normal functioning of
enzymes, they include toxins, poisons,
pesticides, and antibiotics
• Competitive inhibitors bind to the active site of an
enzyme, competing with the substrate
• Noncompetitive inhibitors bind to another part of an
enzyme, causing the enzyme to change shape and making
the active site less effective
Concept 8.5: Regulation of enzyme activity helps
control metabolism
• Chemical chaos
would result if a
cell’s metabolic
pathways were not
tightly regulated
• A cell does this by:
1. regulating the
activity of
enzymes or
2. by switching on
or off the genes
that encode
specific enzymes
DNA
Allosteric Regulation of Enzymes
• Allosteric: change in shape and activity of an enzyme by a
regulatory substance (activator or inhibitor)
• Most allosterically regulated enzymes are made of several
polypeptide subunits
• Allosteric regulation: the binding of a regulatory molecule to a
protein at one site that affects the protein’s function at another
site
Allosteric Activation and Inhibition
• Allosteric regulation may either inhibit or stimulate an
enzyme’s activity
• Allosteric enzymes
have both active and
inactive forms
• The binding of an
activator stabilizes
the active form of
the enzyme
• The binding of an
inhibitor stabilizes
the inactive form of
the enzyme
• Cooperativity is a form of allosteric regulation
that can amplify enzyme activity
• In cooperativity, binding by a substrate to one
active site stabilizes favorable conformational
changes at all other subunits
Substrate
Inactive form
Stabilized active
form
Cooperativity: another type of allosteric activation
Identification of
Allosteric
Regulators
• Allosteric regulators
are attractive drug
candidates for
enzyme regulation
• Inhibition of
proteolytic enzymes
called caspases may
help management of
inappropriate
inflammatory
responses
EXPERIMENT
Caspase 1
Active
site
Substrate
SH
Known active form
SH
Active form can
bind substrate
SH Allosteric
binding site
Allosteric
Known inactive form inhibitor
S–S
Hypothesis: allosteric
inhibitor locks enzyme
in inactive form
RESULTS
Caspase 1
Active form
Inhibitor
Allosterically
Inactive form
inhibited form
Feedback Inhibition
Initial substrate
(threonine)
Active site
available
• In feedback
inhibition, the end
product of a
metabolic pathway
shuts down the
pathway
• Feedback inhibition
prevents a cell from
wasting chemical
resources by
synthesizing more
product than is
needed
Isoleucine
used up by
cell
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Intermediate A
Feedback
inhibition
Isoleucine
binds to
allosteric
site
Enzyme 2
Active site of
enzyme 1 no
longer binds Intermediate B
threonine;
pathway is
Enzyme 3
switched off.
Intermediate C
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)
Specific Localization of Enzymes Within the Cell
•
Structures within the cell help bring order to metabolic pathways
•
Cellular Respiration occurs in three different parts of the cell (1) Glycolysis
in the cytoplasm, (2) the Citric Acid Cycle in the matrix of the mitochondria,
and (3) Oxidative Phosphorylation in the inner membrane of the mitochondria
Mitochondria
(1)
• Some enzymes act as structural components
of membranes
Energy carrying Hydrogens from carbohydrates + Oxygen  H2O
Phosphorylation of ADP + Pi  ATP
Oxidative Phosphorylation: this last step of cellular respiration is catalyzed by an
enzyme system that is embedide in the inner mmbrane of the mitochondia
You should now be able to:
1. Distinguish between the following pairs of
terms: catabolic and anabolic pathways;
kinetic and potential energy; open and closed
systems; exergonic and endergonic reactions
2. In your own words, explain the second law of
thermodynamics and explain why it is not
violated by living organisms
3. Explain in general terms how cells obtain the
energy to do cellular work
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
4. Explain how ATP performs cellular work
5. Explain why an investment of activation
energy is necessary to initiate a spontaneous
reaction
6. Describe the mechanisms by which enzymes
lower activation energy
7. Describe how allosteric regulators may inhibit
or stimulate the activity of an enzyme
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings