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Transcript of 1308 Energy
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Energy and Metabolism
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The Energy of Life
The living cell generates thousands of
different reactions
Metabolism
Is the totality of an organisms chemical
reactions
Arises from interactions between
molecules
An organisms metabolism transforms matter
and energy, subject to the laws of
thermodynamics
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Metabolic Pathways
Biochemical pathways are the organizational units of metabolism
Metabolism is the total of all chemical reactions carried out by anorganism
A metabolic pathway has many steps that begin with a specificmolecule and end with a product, each catalyzed by a specific enzyme
Reactions that join small molecules together to form larger, morecomplex molecules are called anabolic.
Reactions that break large molecules down into smaller subunits arecalled catabolic.
Enzyme 1 Enzyme 2 Enzyme 3
A B C D
Reaction 1 Reaction 2 Reaction 3
Starting
molecule
Product
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Metabolic Pathway A sequence of chemical reactions, where the product of
one reaction serves as a substrate for the next, is called ametabolic pathway or biochemical pathway
Most metabolic pathways take place in specific regions ofthe cell.
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Bioenergetics
Bioenergetics is the study of how organismsmanage their energy resources via
metabolic pathways
Catabolic pathways release energy bybreaking down complex molecules into
simpler compounds
Anabolic pathways consume energy to build
complex molecules from simpler ones
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Energy
Energy is the capacity to do work or ability tocause change. Any change in the universe
requires energy. Energy comes in 2 forms:
Potential energy is stored energy. Nochange is currently taking place
Kinetic energy is currently causing
change. This always involves some type
of motion.
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Form s of Energy
Kinetic energy is theenergy associated withmotion
Potential energy
Is stored in the
location of matter Includes chemical
energy stored inmolecular structure
Energy can be convertedfrom one form to another
On the platform, a diver
has more potential energy.
Diving converts potential
energy to kinetic energy.
Climbing up converts kinetic
energy of muscle movement
to potential energy.
In the water, a diver has
less potential energy.
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Laws of Energy Transformation
Thermodynamics is the study of energychanges.
Two fundamental laws govern all energy
changes in the universe. These 2 laws aresimply called the first and second laws of
thermodynamics:
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The First Law o f Thermodynam ics
According to the first law of thermodynamics
Energy cannot be created or destroyed
Energy can be transferred and transformed
For example, the chemical (potential) energy
in food will be converted to the kinetic
energy of the cheetahs movement
Chemical
energy
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Second Law of Thermodynamics The disorder (entropy) in the universe is continuously increasing.
Energy transformations proceed spontaneously to convert matter from a
more ordered, less stable form, to a less ordered, more stable form Spontaneous changes that do not require outside energy increase the
entropy, or disorder, of the universe
For a process to occur without energy input, it must increase the entropy of
the universe
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During each conversion, some of the energy dissipates into theenvironment as heat.
During every energy transfer or transformation, some energy isunusable, often lost as heat
Heat is defined as the measure of the random motion of molecules
Living cells unavoidably convert organized forms of energy to heat
According to the second law of thermodynamics, every energytransfer or transformation increases the entropy (disorder) of theuniverse
Second Law of Thermodynamics
For example, disorder is added to the cheetahssurroundings in the form of heat and
the small molecules that are the by-products of metabolism.
Heat co2
H2O
+
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B iolog ical Order and Diso rder
Cells create ordered structures from lessordered materials
Organisms also replace ordered forms of
matter and energy with less ordered forms
The evolution of more complex organisms does
not violate the second law of thermodynamics
Entropy (disorder) may decrease in an
organism, but the universes total entropyincreases
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B iolog ical Order and Diso rder
Living systems
Increase the entropy of the universe Use energy to maintain order
A living systems free energy is energy that can
do work under cellular conditions Organisms live at the expense of free energy
50m
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Free Energy
Free energyis the portion of a systems energy that is able
to perform work when temperature and pressure is uniformthroughout the system, as in a living cell
Free energy also refers to the amount of energy actuallyavailable to break and subsequently form other chemicalbonds
Gibbs free energy (G) in a cell, the amount of energycontained in a molecules chemical bonds (T&P constant)
Change in free energy -G
Endergonic - any reaction that requires an input ofenergy
Exergonic - any reaction that releases free energy
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Exergonic reactions Reactants have more free energy than the products
Involve a net release of energy and/or an increase inentropy
Occur spontaneously (without a net input of energy)
Reactants
Products
Energy
Progress of the reaction
Amount of
energy
released
(G
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Endergonic Reactions
Reactants have less free energy than the products
Involve a net input of energy and/or a decrease in
entropy
Do not occur spontaneously
Energy
Products
Amount of
energy
released
(G>0)
Reactants
Progress of the reaction
Freeenergy
(b) Endergonic reaction: energy required
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Reactant Reactant
Product
Product
ExergonicEndergonic
Energy isreleased.
Energymust be
supplied.Energy
supplied
Energ
y
released
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Equ i libr ium and Metabo l ism
Reactions in a closed system eventually reachequilibrium and then do no work
Cells are not in equilibrium; they are open
systems experiencing a constant flow ofmaterials
A catabolic pathway in a cell releases free
energy in a series of reactions
Closed and open hydroelectric systems can
serve as analogies
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An Analogy For Cellular Respiration
Glucose Catabolism
(c) A multistep open hydroelectric system.Cellular respiration isanalogous to this system: Glucoce is brocken down in a series
of exergonic reactions that power the work of the cell. The product
of each reaction becomes the reactant for the next, so no reaction
reaches equilibrium.
G< 0
G< 0
G< 0
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Energy Coupling
Living organisms have the ability to coupleexergonic and endergonic reactions:
Energy released by exergonic reactions is
captured and used to make ATP from ADP
and Pi
ATP can be broken back down to ADP and
Pi, releasing energy to power the cells
endergonic reactions.
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The Structure and Hydrolysis of ATP
ATP (adenosine triphosphate) Is the cells energy shuttle
Provides energy for cellular functions
O O O O CH2
H
OH OH
H
N
H H
O
NC
HC
N CC
N
NH2Adenine
RibosePhosphate groups
O
O O
O
O
O
-
- - -
CH
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Cellular Work
A cell does three main kinds of work Mechanical
Transport
Chemical Energy coupling is a key feature in the way cells
manage their energy resources to do this work
ATP powers cellular work by coupling exergonic
reactions to endergonic reactions
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Energy Coupling - ATP / ADP Cycle
Releasing the third phosphate from ATP to make ADP generates
energy (exergonic):
Linking the phosphates together requires energy - so making ATP from
ADP and a third phosphate requires energy (endergonic),
Catabolic pathways drive the regeneration of ATP from ADP and
phosphate
ATP synthesis from
ADP + P irequires energy
ATP
ADP + P i
Energy for cellular work
(endergonic, energy-
consuming processes)
Energy from catabolism
(exergonic, energy yielding
processes)
ATP hydrolysis toADP + P iyields energy
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How ATP Performs Work
ATP drives endergonic reactions byphosphorylation, transferring a phosphate
group to some other molecule, such as a
reactant
The recipient molecule is now phosphorylated
The three types of cellular work (mechanical,
transport, and chemical) are powered by the
hydrolysis of ATP
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How ATP Performs Work ATP drives endergonic reactions by phosphorylation, transferring a
phosphate to other molecules - hydrolysis of ATP
(c) Chemical work: ATP phosphorylates key reactants
P
Membrane
protein
Motor protein
P i
Protein moved
(a) Mechanical work: ATP phosphorylates motor proteins
ATP
(b) Transport work: ATP phosphorylates transport proteins
Solute
P P i
transportedSolute
Glu GluNH3
NH2P i
P i
+ +
Reactants: Glutamic acid
and ammoniaProduct (glutamine)
made
ADP+
P
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Activation Energy
All reactions, both endergonic and exergonic, require an input of energy
to get started. This energy is called activation energy
The activation energy, EA Is the initial amount of energy needed to start a chemical reaction
Activation energy is needed to bring the reactants close together and
weaken existing bonds to initiate a chemical reaction.
Is often supplied in the form of heat from the surroundings in a system.
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Freeenergy
Progress of the reaction
G < O
EAA B
C D
Reactants
A
C D
B
Transition state
A B
C D
Products
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Reaction Rates
In most cases, molecules do not have enough
kinetic energy to reach the transition state whenthey collide.
Therefore, most collisions are non-productive, and
the reaction proceeds very slowly if at all.
What can be done to speed up these reactions?
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Increasing Reaction Rates
Add Energy (Heat) - molecules move faster so they collide
more frequently and with greater force. Add a catalysta catalyst reduces the energy needed to
reach the activation state, without being changed itself.
Proteins that function as catalysts are called enzymes.
Reactant
Product
CatalyzedUncatalyzed
Product
Reactant
Activationenergy
Activationenergy
Ene
rgy
supplied
Energy
released
Copyright TheMcGraw-HillCompanies, Inc. Permissionrequiredfor reproductionor display.
Activation Energy and Catalysis
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Enzymes Lower the EABarrier
An enzyme catalyzes reactions by lowering
the EAbarrier
Progress of the reaction
Products
Course of
reaction
without
enzyme
Reactants
Course ofreaction
with enzyme
EA
withoutenzyme
EA with
enzyme
is lower
Gis unaffectedby enzyme
Free
energy
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Substrate Specificity of Enzymes Almost all enzymes are globular proteins with one or more active sites on their
surface.
The substrate is the reactant an enzyme acts on Reactants bind to the active siteto form an enzyme-substrate complex.
The 3-D shape of the active site and the substrates must match, like a lock andkey
Binding of the substrates causes the enzyme to adjust its shape slightly, leadingto a better induced fit.
Induced fit of a substrate brings chemical groups of the active site into positions
that enhance their ability to catalyze the chemical reaction When this happens, the substrates are brought close together and existing
bonds are stressed. This reduces the amount of energy needed to reach thetransition state.
Substate
Active site
Enzyme
Enzyme- substrate
complex
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The Catalytic Cycle Of An Enzyme
Substrates
Products
Enzyme
Enzyme-substrate
complex
1 Substrates enter active site; enzyme
changes shape so its active site
embraces the substrates (induced fit).
2 Substrates held in
active site by weak
interactions, such as
hydrogen bonds and
ionic bonds.
3 Active site (and R groups of
its amino acids) can lower EA
and speed up a reaction by
acting as a template for
substrate orientation,
stressing the substrate bonds
and stabilizing the
transition state,
providing a favorablemicroenvironment,
participating directly in the
catalytic reaction.
4 Substrates are
Converted intoProducts.
5 Products are
Released.
6Active site
Is available for
two new substrate
Mole.
Figure 8.17
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1 The substrate, sucrose, consists
of glucose and fructose bonded together.
Bond
Enzyme
Active site
The substrate binds to the enzyme,
forming an enzyme-substrate
complex.
2
H2O
The binding of the substrate
and enzyme places stress onthe glucose-fructose bond,
and the bond breaks.3
Glucose Fructose
Products are
released, and the
enzyme is free to
bind other
substrates.
4
The Catalytic Cycle Of An Enzyme
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Factors Affecting Enzyme Activity
Temperature - rate of an enzyme-catalyzed
reaction increases with temperature, but only
up to an optimum temperature.
pH - ionic interactions also hold enzymes
together.
Inhibitors and Activators
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Effects o f Temperatu re and pH
Each enzyme has an optimal temperature in
which it can function
Optimal temperature for
enzyme of thermophilic
Rateofreaction
0 20 40 80 100Temperature (C)
(a) Optimal temperature for two enzymes
Optimal temperature for
typical human enzyme
(heat-tolerant)bacteria
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Effects o f Temperatu re and pH
Each enzyme has an optimal pH in which
it can function
Figure 8.18
Rateofreaction
(b) Optimal pH for two enzymes
Optimal pH for pepsin
(stomach enzyme)Optimal pH
for trypsin
(intestinalenzyme)
10 2 3 4 5 6 7 8 9
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Factors Affecting Enzyme Activity
Inhibitor - substance that binds to an enzyme
and decreases its activityfeedback
Competitive inhibitors - compete with the
substrate for the same active site
Noncompetitive inhibitors - bind to the
enzyme in a location other than the active
site
Allosteric sites - specific binding sites
acting as on/off switches
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Enzyme Inh ibi tors
Competitive inhibitors bind to the active site of an enzyme,
competing with the substrate
(b) Competitive inhibition
A competitiveinhibitor mimics the
substrate, competing
for the active site.
Competitive
inhibitor
A substrate can
bind normally to the
active site of an
enzyme.
Substrate
Active site
Enzyme
(a) Normal binding
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Enzyme Inh ibi tors
Noncompetitive inhibitors bind to anotherpart of an enzyme, changing the function
A noncompetitive
inhibitor binds to the
enzyme away from
the active site, altering
the conformation of
the enzyme so that its
active site no longer
functions.
Noncompetitive inhibitor
(c) Noncompetitive inhibition
R l ti Of E A ti it H l
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Regulation Of Enzyme Activity Helps
Control Metabolism
Chemical chaos would result if a cells metabolicpathways were not tightly regulated
To regulate metabolic pathways, the cell switches
on or off the genes that encode specific enzymes
Allosteric regulation is the term used to describe
any case in which a proteins function at one site is
affected by binding of a regulatory molecule at
another site Enzymes change shape when regulatory
molecules bind to specific sites, affecting function
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A l los ter ic Act ivation and Inh ibi t ion
Most allosterically regulated enzymes aremade from polypeptide subunits
Each enzyme has active and inactive forms
The binding of an activator stabilizes the activeform of the enzyme
The binding of an inhibitor stabilizes theinactive form of the enzyme
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Allosteric Regulation of Enzymes Allosteric regulation may either inhibit or stimulate an
enzymes activity
Stabilized inactiveform
Allosteric activater
stabilizes active fromAllosteric enyzme
with four subunitsActive site
(one of four)
Regulatory
site (oneof four)
Active form
Activator
Stabilized active form
Allosteric activater
stabilizes inactive form
InhibitorInactive formNon-
functional
active
site
(a) Allosteric activators and inhibitors.In the cell, activators and inhibitors
dissociate when at low concentrations. The enzyme can then oscillate again.
Oscillation
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Cooperativity
Is a form of allosteric regulation that can amplify
enzyme activity
Binding of one substrate molecule to
active site of one subunit locks
all subunits in active conformation.
Substrate
Inactive form Stabilized active form
(b)Cooperativity: another type of allosteric activation.Note that the
inactive form shown on the left oscillates back and forth with the active
form when the active form is not stabilized by substrate.
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Factors Affecting Enzyme Activity
Activators - substances that bind to allosteric
sites and keep the enzymes in their active
configurations - increases enzyme activity
Cofactors - chemical components that
facilitate enzyme activity
Coenzymes - organic molecules that
function as cofactors
R l ti f Bi h i l P th
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Regulation of Biochemical Pathways
Biochemical pathways must be coordinated
and regulated to operate efficiently.
Advantageous for cell to temporarily shut
down biochemical pathways when their
products are not needed
Feedback Inhibition
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Feedback Inh ibi t ion
In feedback inhibition theend product of a metabolic
pathway shuts down the
pathway
When the cell producesincreasing quantities of a
particular product, it
automatically inhibits its
ability to produce more
Active siteavailable
Isoleucine
used up by
cell
Feedback
inhibition
Isoleucine
binds toallosteric
site
Active site of
enzyme 1 no
longer binds
threonine;
pathway is
switched off
Initial substrate
(threonine)
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Intermediate A
Intermediate B
Intermediate C
Intermediate D
Enzyme 2
Enzyme 3
Enzyme 4
Enzyme 5
End product
(isoleucine)