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Figure 8-01
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This new material we begin today will be on exam #2
Material for exam #1 (March 4) will include these textbook chapters:
Chapter 3-water
Chapter 4-Carbon and the Molecular Diversity of Life (especially functional groups)
Chapter 6-Cell Structure and Function
Chapter 7-Cell Membranes
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An Introduction to Metabolism
I. Metabolism, Energy and Life• The chemistry of life is organized into metabolic pathways• Organisms transform energy• The energy transformations of life are subject to two laws of
thermodynamics• Organisms live at the expense of free energy• ATP powers cellular work by coupling exergonic to endergonic
reactionsII. Enzymes• Enzymes speed up metabolic reactions by lowering energy barriers• Enzymes are substrate-specific• The active site is an enzyme’s catalytic centerIII. The Control of Metabolism• Metabolic control often demands allosteric regulation• The location of enzymes within a cell helps to order metabolism
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Cell Energetics
• Energy definition-capacity to do work. In terms of cell, what are some types of work that have to be done to stay alive?
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Figure 6.2x1 Kinetic and potential energy: dam
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Important forms of kinetic and potential
energy for living organisms
• Kinetic-sunlight; heat
• Potential-chemical bond energy (glucose, ATP, etc.)
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Cell Energetics are Governed by the Laws of Thermodynamics
• First Law of Thermodynamics (law of Conservation of Energy)
• Energy cannot be created or destroyed but it can be changed in form.
• Energy transformation is permissible (life depends on this happening)-sunlightchemical
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LE 8-2On the platform,the diver hasmore potentialenergy.
Diving convertspotentialenergy to kinetic energy.
Climbing up convertskinetic energy ofmuscle movement topotential energy.
In the water, the diver has lesspotential energy.
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Second Law of Thermodynamics
• All matter tends to spontaneously move to the greatest possible state of stability (bonding)
• All matter tends to spontaneously move from areas of higher concentration to lower concentration (diffusion)
• All matter tends to spontaneously move from states of higher free energy (less stable, more concentrated) to states of lower free energy (more stable, less concentrated)
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Entropy-Another way to look at the 2nd law of Thermodynamics
• We’ve defined the 2nd law previously in terms of stability and free energy.
• Another way is to understand the 2nd Law is to use the concept of entropy.
• Entropy is the measure of the disorganization of a system
• All systems spontaneously assume the state of greatest entropy.
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Cells, entropy, and the 2nd Law of Thermodynamics
• Cells are very organized (low entropy)• How can a cell exist in the face of the 2nd Law (maintain
organization)? Expend energy. • It’s work to stay alive!• The key is that the cell decreases its entropy at the
expense of increasing the entropy of its surroundings. • Cells take ordered (high energy molecules from the
environment and return unordered waste products). • The cell is an open system (can exchange energy with
its environment). • Can a closed system maintain a state of low entropy?
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LE 8-3
Chemical energy
Heat CO2
First law of thermodynamics Second law of thermodynamics
H2O
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What does the 2nd Law of Thermodynamics say about Life?
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• Life is improbable.• Life cannot violate the principle that entropy increases.• Living things live at the expense of their environment
(living things are able to maintain a state of decreased entropy because the entropy of the entire universe (system + surroundings) is increasing
• Life can only exist if energy is constantly being expended.
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What form of energy does the cell use to maintain its organization?
• ATP
• ATP cycle
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LE 8-8
Phosphate groups
Ribose
Adenine
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LE 8-9
Adenosine triphosphate (ATP)
Energy
P P P
PPP i
Adenosine diphosphate (ADP)Inorganic phosphate
H2O
+ +
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ATP hydrolysis “releases” energy so that cellular work can be done
• bond breaking versus bond formation
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Why does ATP hydrolysis “release” energy?
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ATP and cellular work
• How does ATP hydrolysis power cellular work?
• Phosphorylation and dephosphorylation of proteins
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LE 8-11
NH2
Glu
P i
P i
P i
P i
Glu NH3
P
P
P
ATPADP
Motor protein
Mechanical work: ATP phosphorylates motor proteins
Protein moved
Membraneprotein
Solute
Transport work: ATP phosphorylates transport proteins
Solute transported
Chemical work: ATP phosphorylates key reactants
Reactants: Glutamic acidand ammonia
Product (glutamine)made
+ +
+
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How is ATP produced?
• Cellular respiration (cash versus check analogy)
• Difference in autotrophs and heterotrophs
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LE 8-UN141
Enzyme 1
A B
Reaction 1
Enzyme 2
C
Reaction 2
Enzyme 3
D
Reaction 3
ProductStarting
molecule
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Metabolism and Spontaneous reactions
• Previously stated that catabolic reactions were spontaneous and anabolic were not spontaneous.
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Metabolism
• Cell respiration is one example of a metabolic pathway. The chemistry of life is organized into metabolic pathways (complex and regulated by enzymes). (street analogy).
• 2 types of metabolic pathways• a. catabolic-degradative, energy releasing
(downhill), oxidative, spontaneous• b. anabolic-synthetic, energy requiring (up hill),
reduction, not spontaneous
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Figure 6.1 The complexity of metabolism
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Spontaneous reactions
• What are they?
• They happen on their own without an input of energy
• They are a result of the 2nd law of Thermodynamics
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LE 8-5
Gravitational motion Diffusion Chemical reaction
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What determines spontaneity of a chemical reaction?
• A +B C+D (substrate products)• Spontaneous-decrease in free energy (downhill), not
spontaneous-increase free energy (uphill)• What is free energy? Energy available to do work. G=H-TS• Maximum amount of free energy that can be harvested
from a reaction is the free energy change of the reaction ( G).
G=H-T S G-energy available to do work (keep cells alive) H-total energy S-energy unavailable to do work
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How is G determined for a reaction? FE of products –FE of reactants.
• a. If G is negative (the free energy of the reactants is > fe of products), the reaction is spontaneous (downhill).
• b. If G is positive (the fe of the reactants is < fe of products), the reaction is not spontaneous (uphill).
• c. If G is 0-no free energy difference. The reaction is at equilibrium. No work can be done from that reaction.
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Figure 6.5 The relationship of free energy to stability, work capacity, and spontaneous change
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LE 8-6a
Reactants
EnergyProducts
Progress of the reaction
Amount ofenergy
released(G < 0)
Fre
e en
erg
y
Exergonic reaction: energy released
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LE 8-6b
ReactantsEnergy
Products
Progress of the reaction
Amount ofenergy
required(G > 0)
Fre
e en
erg
y
Endergonic reaction: energy required
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Exergonic and Endergonic reactions
• Reactions can also be classified based on free energy changes. Endergonic and exergonic reactions.
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Differences in Exergonic and Endergonic Reactions
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Metabolic reactions and equilibrium
• What would be the problem for a cell if its metabolic reactions (especially catabolic ones) were allowed to reach equilibrium?
• Cellular metabolism is normally not allowed to reach equilibrium.
• Metabolism generally involves multi-step pathways where the product of one reaction becomes the substrate of the next reaction.
• This strategy only works because cells are open systems
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LE 8-7a
G = 0
A closed hydroelectric system
G < 0
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LE 8-7b
An open hydroelectric system
G < 0
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LE 8-7c
A multistep open hydroelectric system
G < 0G < 0
G < 0
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Relationship between catabolic and anabolic reactions
• Coupling endergonic and exergonic reactions
• example.
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LE 8-12
Pi
ADP
Energy for cellular work
(endergonic, energy-
consuming processes)
Energy from catabolism
(exergonic, energy-
yielding processes)
ATP
+
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LE 8-10
Endergonic reaction: G is positive, reactionis not spontaneous
Exergonic reaction: G is negative, reactionis spontaneous
G = +3.4 kcal/mol
G = –7.3 kcal/mol
G = –3.9 kcal/mol
NH2
NH3Glu Glu
Glutamicacid
Coupled reactions: Overall G is negative;together, reactions are spontaneous
Ammonia Glutamine
ATP H2O ADP P i
+
+ +
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LE 8-13
SucroseC12H22O11
GlucoseC6H12O6
FructoseC6H12O6
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Enzymes
• Characteristics
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LE 8-17
Enzyme-substratecomplex
Substrates
Enzyme
Products
Substrates enter active site; enzymechanges shape so its active siteembraces the substrates (induced fit).
Substrates held inactive site by weakinteractions, such ashydrogen bonds andionic bonds.
Active site (and R groups ofits amino acids) can lower EA
and speed up a reaction by• acting as a template for substrate orientation,• stressing the substrates and stabilizing the transition state,• providing a favorable microenvironment,• participating directly in the catalytic reaction.
Substrates areconverted intoproducts.
Products arereleased.
Activesite is
availablefor two new
substratemolecules.
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LE 8-16
Substrate
Active site
Enzyme Enzyme-substratecomplex
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Catalysis
• Properties of a catalyst
• How do enzymes increase the rate of a chemical reaction
• Why don’t enzymes alter the of a reaction?
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LE 8-14
Transition state
C D
A B
EA
Products
C D
A B
G < O
Progress of the reaction
Reactants
C D
A B
Fre
e en
erg
y
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LE 8-15
Course ofreactionwithoutenzyme
EA
without enzyme
G is unaffectedby enzyme
Progress of the reaction
Fre
e en
erg
y
EA withenzymeis lower
Course ofreactionwith enzyme
Reactants
Products
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Factors influencing enzyme activity
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Factors influencing enzyme activity
• Temperature
• pH
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LE 8-18a
Optimal temperature fortypical human enzyme
Optimal temperature forenzyme of thermophilic (heat-tolerant bacteria
Temperature (°C)
Optimal temperature for two enzymes
0 20 40 60 80 100
Rat
e o
f re
acti
on
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LE 8-18b
Optimal pH for pepsin(stomach enzyme)
Optimal pHfor trypsin(intestinalenzyme)
pH
Optimal pH for two enzymes
0
Rat
e o
f re
acti
on
1 2 3 4 5 6 7 8 9 10
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Factors influencing enzyme activity
Inhibitors:
A. nonreversible
B. Reversible
1. Competitive inhibition
2. Noncompetitive inhibition
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LE 8-19a
Substrate
Active site
Enzyme
Normal binding
A substrate canbind normally to the
active site of anenzyme.
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LE 8-19b
Competitiveinhibitor
Competitive inhibition
A competitiveinhibitor mimics the
substrate, competingfor the active site.
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LE 8-19c
Noncompetitive inhibitor
Noncompetitive inhibition
A noncompetitiveinhibitor binds to the
enzyme away from theactive site, altering the
conformation of theenzyme so that its
active site no longerfunctions.
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A practical application of noncompetitive inhibition
• Feedback Inhibition
• Purpose of feedback inhibition
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LE 8-UN159
OML
N
S
R
Q
P–
–
–
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LE 8-21
Active siteavailable
Initial substrate(threonine)
Threoninein active site
Enzyme 1(threoninedeaminase)
Enzyme 2
Intermediate A
Isoleucineused up bycell
Feedbackinhibition Active site of
enzyme 1 can’tbindtheoninepathway off
Isoleucinebinds toallostericsite
Enzyme 3
Intermediate B
Enzyme 4
Intermediate C
Enzyme 5
Intermediate D
End product(isoleucine)