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CAMPBELL
BIOLOGY
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
8
An Introduction
to Metabolism
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
© 2014 Pearson Education, Inc.
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
• Some organisms even convert energy to
light, as in bioluminescence
© 2014 Pearson Education, Inc.
Figure 8.1
An organism’s metabolism transforms
matter and energy, subject to the laws of
thermodynamics
• Metabolism is the totality of an organism’s
chemical reactions
• Metabolism is an emergent property of life
that arises from interactions between
molecules within the cell
© 2014 Pearson Education, Inc.
The chemistry of life is organized into
metabolic pathways
• A metabolic pathway begins with a specific
molecule, which is then altered in a series of
defined steps to form a specific product.
• A specific enzyme catalyzes each step of the
pathway.
© 2014 Pearson Education, Inc.
Two types of Metabolic Pathways
1. Catabolic pathways release energy by
breaking down complex molecules into simpler
compounds
– Cellular respiration, the breakdown of glucose
in the presence of oxygen, is an example of a
pathway of catabolism
2. Anabolic pathways consume energy to build
complex molecules from simpler ones
– The synthesis of protein from amino acids is
an example of anabolism
• Bioenergetics is the study of how organisms
manage their energy resources
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Forms of Energy
• Energy is the capacity to cause change
• Energy exists in various forms, some of which can
perform work. Two important forms of energy are
1. Kinetic energy is the energy associated with the
relative motion of objects.
– Objects in motion can perform work by imparting motion to
other matter.
– Heat or thermal energy is kinetic energy associated with the
random movement of atoms or molecules.
2. Potential energy is the energy that matter possesses
because of its location or structure.
– Water behind a dam possesses potential energy
because of its altitude above sea level.
– Molecules possess potential energy because of the
arrangement of their atoms.
• Chemical energy is a form of potential energy stored in
molecules because of the arrangement of their atoms.
© 2011 Pearson Education, Inc.
Energy Conversion
• Energy can be converted from one form to another.
• For example, as a boy climbs stairs to a diving platform,
he is releasing chemical energy stored in his cells from
the food he ate for lunch.
• The kinetic energy of his muscle movement is converted
to potential energy as he climbs.
• As the boy dives, the potential energy is converted back
to kinetic energy.
• Kinetic energy is transferred to the water as the boy
enters it.
• Some energy is converted to heat due to friction.
© 2011 Pearson Education, Inc.
Figure 8.2 A diver has more potential
1 energy on the platform
than in the water.
2 Climbing up converts the kinetic
energy of muscle movement
to potential energy.
3 Diving converts
potential energy to
kinetic energy.
4 A diver has less potential
energy in the water
than on the platform.
The Laws of Energy Transformation
• Thermodynamics is the study of energy
transformations
• A closed system, such as that
approximated by liquid in a thermos, is
isolated from its surroundings
• In an open system, energy and matter can
be transferred between the system and its
surroundings
• Organisms are living in open systems
© 2014 Pearson Education, Inc.
The First Law of Thermodynamics
• According to the first law of
thermodynamics, 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 principle of
conservation of energy
– Plants do not produce energy; they transform
light energy to chemical energy.
• During every transfer or transformation of energy,
some energy is converted to heat, which is the
energy associated with the random movement of
atoms and molecules.
© 2014 Pearson Education, Inc.
The Second Law of Thermodynamics
• During every energy transfer or transformation,
some energy is unusable, and is often lost as heat
• According to the second law of thermodynamics:
– Every energy transfer or transformation
increases the entropy (disorder) of the
universe due to the loss of usable energy.
By combination of both first and second law’s
quantity of energy will same but quality of
energy will not same in the universe.
© 2014 Pearson Education, Inc.
Figure 8.3
Entropy and Heat
Heat
Chemical energy
(a) First law of thermodynamics
(b) Second law of thermodynamics
Much of the increased entropy of the universe takes the form of
increasing heat, which is the energy of random molecular motion.
In most energy transformations, some energy is converted at least
partly to heat.
Living cells unavoidably convert usable forms of energy to
unusable heat.
Only a small portion of the chemical energy from the cheetah’s
food is transferred to movement; it is lost as heat.
The word spontaneous describes a process
that can occur without an input of energy
• For a process to occur spontaneously, without outside help in the
form of energy input, it must increase the entropy of the universe.
• Spontaneous processes need not occur quickly.
• Some spontaneous processes are instantaneous, such as an
explosion. Some are very slow, such as the rusting of an old car.
• A process that cannot occur on its own is said to be
nonspontaneous; it will happen only if energy is added to the
system.
• Water flows downhill spontaneously but moves uphill only with an
input of energy, such as when a machine pumps the water against
gravity.
• Here is another way to state the second law of thermodynamics:
• For a process to occur spontaneously, it must increase the entropy
of the universe.
© 2014 Pearson Education, Inc.
Biological Order and Disorder
• The structure of a multicellular
body is organized and
complex.
– However, an organism also
takes in organized forms of
matter and energy from its
surroundings and replaces
them with less ordered
(unusable) forms.
• For example, an animal
consumes organic molecules
as food and catabolizes them
to low-energy carbon dioxide
and water.
• This increase in organization
does not violate the second
law of thermodynamics.
• The entropy of a particular
system, such as an organism,
may decrease as long as the
total entropy of the universe—
the system plus its
surroundings—increases.
Figure 8.4
Living systems increase the entropy of
their surroundings,
even though they create ordered
structures from less ordered starting
materials.
The free-energy change of a reaction tells
us whether or not the reaction occurs
spontaneously
• 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
• A living system’s free energy is energy that
can do work when temperature and
pressure are uniform, as in a living cell
© 2014 Pearson Education, Inc.
How can we determine which reactions occur
spontaneously and which ones require an
input of energy?
•
The concept of free energy (symbolized by the letter G) is useful for
measuring the spontaneity of a system.
• Free energy is the portion of a system’s energy that can
perform work when temperature and pressure are uniform
throughout the system, as in a living cell.
• The change in free energy, ∆G, can be calculated for any
specific chemical reaction with the formula
• ∆G = ∆H – T∆S
– In this formula:
– ∆H symbolizes the change in the system’s total energy
– ∆S is the change in the system’s entropy
– T is the absolute temperature in Kelvin (K) units (K = °C +
273).
•
For a process to occur spontaneously, the system must give up total
energy ([H] must decrease), give up order (TS must increase), or both.
• ∆G must have a negative value (∆G < 0) in order for a process
to be spontaneous. Remember spontaneous processes do not
require energy input from the outside
Free Energy, Stability, and Equilibrium
• Free energy is a measure of a system’s
instability, its tendency to change to a more
stable state
• During a spontaneous change, free energy
decreases 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
© 2014 Pearson Education, Inc.
Free energy, stability, work capacity, and
spontaneous change
• 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
(b) Diffusion
Figure 8.5
(c) Chemical reaction
Free energy, stability, work capacity, and spontaneous change
•
Gravitational Motion is an unstable system. The diver
is unstable on top of the platform. Once he jumps and
gravity pulls him down into the water:
He is more stable, has less free energy/work capacity
This process will occur spontaneously.
• Diffusion is an unstable system. The concentrated dye
is unstable (more likely to disperse) than when the dye is
spread randomly in a solution.The dye molecules will
randomly disperse:
The dye becomes more stable, has less free
energy/work capacity
This process will occur spontaneously.
• Chemical reactions are unstable system. The
polysaccharide molecule is unstable (more likely to break
down) than when the polysaccharide is broken down into
monosaccharides. The polysaccharide will be broke
down:
The polysaccharide becomes more stable, has less free
energy/work capacity.
This process will occur spontaneously.
Maximum Stability=Equilibrium
• Another term for a state of maximum stability is
equilibrium.
• In a chemical reaction at equilibrium, the rates of forward
and backward reactions are equal, and there is no
change in the relative concentrations of products or
reactants.
• At equilibrium, ∆G = 0, and the system can do no work.
• A process is spontaneous and can perform work only
when it is moving toward equilibrium.
• Movements away from equilibrium are nonspontaneous
and require the addition of energy from an outside
energy source (the surroundings).
Free Energy and Metabolism
• The concept of free energy can be applied
to the chemistry of life’s processes
Exergonic and Endergonic Reactions in Metabolism
An exergonic reaction proceeds with a net
release of free energy and is spontaneous.
An endergonic reaction absorbs free energy from its
surroundings and is nonspontaneous
© 2014 Pearson Education, Inc.
Exergonic Reactions in Metabolism
• An exergonic
reaction proceeds
with a net release of
free energy and is
spontaneous,
• ∆G is – (negative)
Figure 8.6
The magnitude of ∆G for an exergonic reaction
represents the maximum amount of work
the reaction can perform
Cellular Respiration is an example of
Exergonic Reaction
• For the overall reaction of cellular respiration,
C6H12O6 + 6O2  6CO2 + 6H2O
∆G = −686 kcal/mol.
• For each mole (180 g) of glucose broken down
by respiration under standard conditions (1 M of
each reactant and product, 25°C, pH 7), 686
kcal of energy are made available to do work in
the cell.
• The products (6CO2 and 6 H2O) have 686 kcal
less free energy per mole than the reactants.
© 2014 Pearson Education, Inc.
Endergonic Reactions in Metabolism
• An endergonic
reaction absorbs free
energy from its
surroundings and is
nonspontaneous
• An endergonic
reaction stores free
energy in molecules,
∆G is + (positive)
Figure 8.6
The magnitude of ∆G for an endergonic reaction
represents the amount of energy required
to drive the reaction.
Photosynthesis is an example of
Endergonic Reaction
• If cellular respiration releases 686
kcal/mol, then photosynthesis, the reverse
reaction, must require an equivalent
investment of energy.
6CO2 + 6H2O  C6H12O6 + 6O2
∆G = +686 kcal/mol
• Photosynthesis is strongly endergonic,
powered by the absorption of light energy.
© 2014 Pearson Education, Inc.
Equilibrium and Metabolism
• Reactions in a closed system eventually reach
equilibrium and then do no work
• Cells are not in equilibrium; they are open systems
experiencing a constant flow of materials. A cell
continues to do work throughout its life.
• A defining feature of life is that metabolism is
never at equilibrium
• A catabolic pathway in a cell releases free
energy in a series of reactions
• Closed and open hydroelectric systems can
serve as analogies
Figure 8.7
∆G < 0
Equilibrium and work in
an isolated hydroelectric
system
∆G = 0
Figure 8.8
• Catabolic pathways release energy by breaking down complex molecules
•
into simpler compounds
Cellular respiration, the breakdown of glucose in the presence of oxygen, is an
example of a pathway of catabolism:
C6H12O6 + 6O2
6CO2 + 6H2O
Catabolic pathways in a cell release free energy in a series of
reactions.
Equilibrium is never reached because the products of one step in the
series
become reactants for the next reaction.
Finally, waste products are expelled from the cell.
What keeps the sequence of reactions going?
Glucose and Oxygen
Fig. 8.8
Carbon Dioxide and Water
• The huge free energy difference between glucose and
oxygen at the top of the energy “hill” and CO2 and H2O
at the bottom of the “hill”.
• As long as cells have a steady supply of glucose (or
other fuel sources) and waste products can be
eliminated, a metabolic pathway will never reach
equilibrium and the cell will be able to do the work
necessary to live.
ATP powers cellular work by coupling
exergonic reactions to endergonic
reactions
• A cell does three main kinds of work:
1. Chemical
2. Transport
3. Mechanical
• To do work, cells manage energy resources by
energy coupling, the use of an exergonic
process to drive an endergonic one
• Most energy coupling in cells is mediated by
ATP
© 2014 Pearson Education, Inc.
The Structure and Hydrolysis of ATP
• ATP (adenosine triphosphate) is the cell’s energy
shuttle
• ATP is composed of ribose (a sugar), adenine (a
purine- nitrogenous base), and three phosphate
groups
• The bonds between the phosphate groups on ATP
can be broken by hydrolysis.
• When the terminal phosphate bond is broken, a
molecule of inorganic phosphate (HOPO32-,
abbreviated Pi here) leaves ATP, which then
becomes adenosine diphosphate (ADP) and energy
is released. This release of energy comes from the
chemical change to a state of lower free energy, not
from the phosphate bonds themselves
•
Hydrolysis of ATP-
ATP + H2O  ADP + Pi + Energy
This reaction is exergonic and In cells,
ΔG = -7.3 kcal.
© 2014 Pearson Education, Inc.
Figure 8.9
Adenine
Phosphate groups
Ribose
(a) The structure of ATP
Adenosine triphosphate (ATP)
Energy
Inorganic
phosphate
Adenosine diphosphate (ADP)
(b) The hydrolysis of ATP
How ATP Performs Work
• The three types of cellular work (mechanical,
transport, and chemical) are powered by the
hydrolysis of ATP
• In the cell, the energy from the hydrolysis of ATP
is directly coupled to endergonic processes by the
transfer of the phosphate group to another
molecule.
– This recipient molecule is now phosphorylated
(phosphorylated intermediate); it is more reactive
(less stable) than the original unphosphorylated
molecules.
• The key to coupling exergonic and endergonic
reactions is the formation of a phosphorylated
intermediate.
Figure 8.10
(a) Glutamic acid
conversion
to glutamine
NH3
Glutamic
acid
(b) Conversion
reaction
coupled
with ATP
hydrolysis
NH2
Glu
Glu
∆GGlu = +3.4 kcal/mol
Glutamine
Ammonia
NH3
P
1
Glu
ATP
2
ADP
Glu
Glu
Phosphorylated
intermediate
Glutamic
acid
NH2
ADP
Glutamine
∆GGlu = +3.4 kcal/mol
(c) Free-energy
change for
coupled
reaction
NH3
Glu
∆GGlu = +3.4 kcal/mol
+ ∆GATP = −7.3 kcal/mol
ATP
NH2
Glu
ADP
Pi
∆GATP = −7.3 kcal/mol
Net ∆G = −3.9 kcal/mol
How ATP Performs Work
I. Glutamine Synthesis
Pi
II. Transport and Mechanical Work
Fig. 8.10
The Regeneration of ATP an Endergonic Process
Fig. 8.11
• ATP is a renewable resource that is regenerated by addition of
a phosphate group to adenosine diphosphate (ADP)
• The energy to phosphorylate ADP comes from catabolic
reactions in the cell
• The ATP cycle is a revolving door through which energy
passes during its transfer from catabolic to anabolic pathways
• The chemical potential energy temporarily stored in ATP drives
most cellular work
© 2014 Pearson Education, Inc.
Enzymes speed up metabolic reactions by
lowering energy barriers
Sucrase
Sucrose
(C12H22O11)
•
•
•
•
Glucose
(C6H12O6)
Fructose
(C6H12O6)
An enzyme is bio catalyst is a chemical agent that
speeds up a reaction without being consumed by the
reaction
An enzyme is a catalytic protein
Hydrolysis of sucrose by the enzyme sucrase is an
example of an enzyme-catalyzed reaction
No sucrase = years to hydrolyze
With sucrase = seconds to hydrolyze
© 2014 Pearson Education, Inc.
The Activation Energy Barrier
• Every chemical reaction between
molecules involves bond
breaking and bond forming
Figure 8.13
– To reach a state at which bonds
can break and reform, reactant
molecules must absorb energy
from their surroundings. When
the new bonds of the product
molecules form, energy is
released as heat as the
molecules assume stable
shapes with lower energy.
• The initial energy needed to start
a chemical reaction is called the
free energy of activation, or
activation energy (EA)
• Activation energy is often
supplied in the form of heat from
the surroundings
AB + CD
AC + BD
How Enzymes Lower the EA Barrier
Enzymes catalyze
reactions by lowering
the EA barrier.
Activation energy
is the amount of
energy necessary
to push the
reactants over an
energy barrier so
that the “downhill”
part of the reaction
can begin.
Enzymes do not affect
the change in free
energy (∆G); instead,
they hasten reactions
that would occur
eventually.
© 2014 Pearson Education, Inc.
Course of
reaction
without
enzyme
Free energy
Figure 8.14
EA
without
enzyme
Reactants
Course of
reaction
with enzyme
EA with
enzyme
is lower
∆G is unaffected
by enzyme
Products
Progress of the reaction
Enzymes are substrate specific
• The reactant that an enzyme acts on is the
substrate.
• The enzyme binds to a substrate, or substrates,
forming an enzyme-substrate complex.
• While the enzyme and substrate are bound, the
catalytic action of the enzyme converts the
substrate to the product or products.
• The reaction catalyzed by each enzyme is very
specific.
• What accounts for enzyme-substrate
recognition? The specificity of an enzyme
results from its three-dimensional shape, which
is a consequence of its amino acid sequence.
Active Site
Figure 8.15
•
•
•
•
The active site of an enzyme is typically a pocket or groove on the surface
of the protein where catalysis occurs.
– The active site is usually formed by only a few amino acids.
– The rest of the protein molecule provides a framework that determines
the configuration of the active site.
The specificity of an enzyme is due to the fit between the active site and
the substrate.
The active site is not a rigid receptacle for the substrate.
– As the substrate enters the active site, interactions between the
chemical groups on the substrate and those on the R groups (side
chains) of the amino acids of the protein cause the enzyme to change
shape slightly.
This change leads to an induced fit that brings the chemical groups of the
active site into position to catalyze the reaction.
Catalysis in the Enzyme’s Active Site
• In an enzymatic reaction, the substrate binds to
the active site of the enzyme
• The active site can lower an EA barrier by
– Orienting substrates correctly
• In reactions involving more than one reactant, the active site
brings substrates together in the correct orientation for the
reaction to proceed.
– Straining substrate bonds
• As the active site binds the substrate, the enzyme may stretch
the substrate molecules toward their transition-state form,
stressing and bending critical chemical bonds that must be
broken during the reaction.
– Providing a favorable microenvironment
• An active site may be a pocket of low pH, facilitating H+ transfer
to the substrate as a key step in catalyzing the reaction.
– Covalently bonding to the substrate
• Enzymes may briefly bind covalently to substrates.
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
3 Active site can lower EA
and speed up a reaction.
6 Active
site is
available
for two new
substrate
molecules.
Enzyme
Figure 8.16
5 Products are
released.
4 Substrates are
converted to
products.
Products
Copyright © 2014 Pearson Education, Inc.
Effects of Local Conditions on Enzyme
Activity
An enzyme’s activity can be affected by
– General environmental factors, such as
temperature and pH
– Chemicals that specifically influence the
enzyme
Each enzyme works best at certain optimal
conditions, which favor the most active 3-D
conformation for the enzyme molecule.
A cell’s physical and chemical
environment affects enzyme activity
• Temperature has a major impact on reaction rate.
• Each enzyme has an optimal temperature that allows
the greatest number of molecular collisions and the
fastest conversion of the reactants to product
molecules.
– Most human enzymes have optimal temperatures of about
37°C.
– The thermophilic bacteria (heat-tolerant) that live in hot springs
contain enzymes with optimal temperatures of 77°C or higher.
• Each enzyme has an optimal pH.
• This optimal pH falls between 6 and 8 for most enzymes.
– However, digestive enzymes-pepsin in the stomach are designed to
work best at pH 2.
– Intestinal enzymes-trypsin have an optimal pH of 8.
© 2014 Pearson Education, Inc.
Figure 8.17
Rate of reaction
Optimal temperature for
Optimal temperature for
typical human enzyme (37°C) enzyme of thermophilic
(heat-tolerant)
bacteria (77°C)
60
80
Temperature (°C)
(a) Optimal temperature for two enzymes
0
20
40
Rate of reaction
Optimal pH for pepsin
(stomach
enzyme)
0
5
pH
(b) Optimal pH for two enzymes
1
2
3
4
120
100
Optimal pH for trypsin
(intestinal
enzyme)
6
7
8
9
10
Cofactors
• Many enzymes require non-protein helpers,
called cofactors, for catalytic activity.
• Cofactors bind permanently or reversibly to
the enzyme.
– Some inorganic cofactors are zinc, iron,
and copper in ionic form.
• Organic cofactors are called coenzymes.
– Most vitamins are coenzymes or the raw
materials from which coenzymes are
made.
Competitive Enzyme Inhibitors
• Certain chemicals
selectively inhibit the
action of specific
enzymes.
• If inhibitors attach to the
enzyme by covalent
bonds, inhibition may be
irreversible.
• If inhibitors bind by weak
bonds, inhibition may be
reversible.
• Some reversible
inhibitors resemble the
substrate and compete
for binding to the active
site. These molecules are
called competitive
inhibitors.
• Competitive inhibition can
be overcome by
increasing the
concentration of the
substrate.
© 2014 Pearson Education, Inc.
Figure 8.18
Examples of Enzyme inhibitors include
toxins, poisons, pesticides, and
antibiotics
1. Toxins and poisons are often irreversible
enzyme inhibitors.
– Sarin is a nerve gas that binds covalently to
the R group on the amino acid serine. Serine
is found in the active site of
acetylcholinesterase, an important nervous
system enzyme.
2. DDT acts as a pesticide by inhibiting key
enzymes in the nervous system of insects.
3. Penicillin blocks the active site of an enzyme
that many bacteria use to make their cell walls.
Non-Competitive Enzyme Inhibitors
• Non-competitive
inhibitors effect
enzymatic reactions
by binding to another
part of the molecule.
• Binding by the
inhibitor causes the
enzyme to change
shape, rendering the
active site less
effective at
catalyzing the
reaction.
Figure 8.18
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The Evolution of Enzymes
• Enzymes are proteins encoded by genes
• Changes (mutations) in genes lead to changes
in amino acid composition of an enzyme
• Altered amino acids in enzymes may alter their
substrate specificity
• Under new environmental conditions a novel
form of an enzyme might be favored
© 2014 Pearson Education, Inc.
Two changed amino acids were
found near the active site.
Active site
Figure 8.19
Two changed amino acids
were found in the active site.
Two changed amino acids
were found on the surface.
Mimicking evolution of an enzyme with a new function
Regulation of enzyme activity helps
control metabolism
• Metabolic control often depends on allosteric
regulation.
• Many molecules that naturally regulate enzyme
activity behave like reversible noncompetitive
inhibitors.
• In allosteric regulation, a protein’s function at
one site is affected by the binding of a regulatory
molecule to a separate site.
• This regulation may result in either inhibition or
stimulation of an enzyme’s activity.
• Chemical chaos would result if a cell’s metabolic
pathways were not tightly regulated
• A cell does this by switching on or off the genes
that encode specific enzymes or by regulating the
activity of enzymes
Allosteric Regulation
• Most allosterically regulated
enzymes are constructed of
two or more polypeptide
chains.
• Each subunit has its own
active site.
• The complex oscillates
between two shapes, one
catalytically active and the
other inactive.
• In the simplest case of
allosteric regulation, an
activating or inhibiting
regulatory molecule binds to a
regulatory site (sometimes
called an allosteric site), often
located where subunits join.
• The binding of an activator
stabilizes the conformation that
has functional active sites,
whereas the binding of an
inhibitor stabilizes the inactive
form of the enzyme.
Figure 8.20
Allosteric Regulation
• By binding to key enzymes, the reactants and products
of ATP hydrolysis play a major role in balancing the flow
of traffic between anabolic and catabolic pathways.
• For example, ATP binds to several catabolic enzymes
allosterically, inhibiting their activity by lowering their
affinity for substrate.
– If ATP production exceeds demand, the excess ATP will
allosterically inhibit catabolic enzymes.
• ADP functions as an activator of the same enzymes.
– If ATP production lags behind its use, ADP will accumulate ,
activate the exzymes that speed up catabolism.
• ATP and ADP also affect key enzymes in anabolic
pathways.
Co-operativity
Figure 8.20
•
•
•
In enzymes with multiple catalytic subunits, binding by a substrate to one active site
by induced fit triggers the same favorable conformational changes at all other
subunits.
This mechanism, called cooperativity, amplifies the response of enzymes to
substrates, priming the enzyme to accept additional substrates.
The vertebrate oxygen-transport protein hemoglobin is a classic example of
cooperativity.
– Hemoglobin is made up of four subunits, each with an oxygen-binding site.
– The binding of an oxygen molecule to each binding site increases the affinity for
oxygen of the remaining binding sites.
– Under conditions of low oxygen, as in oxygen-deprived tissues, hemoglobin is
less likely to bind oxygen and releases it where it is needed.
– When oxygen is at higher levels, as in the lungs or gills, the protein has a
greater affinity for oxygen as more binding sites are filled.
© 2014 Pearson Education, Inc.
Metabolic Regulation
Feedback Inhibition
• 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
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Figure. 8.21
Specific Localization of Enzymes
Within the Cell
• Structures within the cell help bring order
to metabolic pathways
• Some enzymes act as structural
components of membranes
• In eukaryotic cells, some enzymes reside
in specific organelles; for example,
enzymes for cellular respiration are
located in mitochondria
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Metabolic Regulation
Cellular Localization
•
•
•
•
•
The cell is compartmentalized: The
organization of cellular structures
helps bring order to metabolic
pathways.
A team of enzymes for several
steps of a metabolic pathway may
be assembled as a multienzyme
complex.
The product from the first reaction
becomes the substrate for an
adjacent enzyme in the complex
until the final product is released.
Some enzymes and enzyme
complexes have fixed locations
within the cells as structural
components of particular
membranes.
Others are confined within
membrane-enclosed eukaryotic
organelles.
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The matrix
contains
Enzymes
involved in one
stage of Cellular
respiration
The cristae inner
membranes
contain enzymes
needed for
another stage of
Cellular
respiration
Figure 8.22