1. 6
An Introduction to Metabolism

2. 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
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3. Figure 6.1 What causes these breaking waves to glow?
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4. Concept 6.1: An organism’s metabolism transforms matter and energy
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
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5. Metabolic Pathways
A metabolic pathway begins with a specific molecule and ends with a product
Each step is catalyzed by a specific enzyme
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6. Enzyme 1 Enzyme 2 Enzyme 3 A B C D Reaction 1 Reaction 2 Reaction 3
Figure 6.UN01
Enzyme 1
Enzyme 2
Enzyme 3 A B C D
Reaction 1
Reaction 2
Reaction 3
Starting
molecule
Product
Figure 6.UN01 In-text figure, metabolic pathway, p. 122
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7. Catabolic pathways release energy by breaking down complex molecules into simpler compounds
One example of catabolism is cellular respiration, the breakdown of glucose and other organic fuels to carbon dioxide and water
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8. The synthesis of proteins from amino acids is an example of anabolism
Anabolic pathways consume energy to build complex molecules from simpler ones
The synthesis of proteins from amino acids is an example of anabolism
Bioenergetics is the study of how energy flows through living organisms
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9. Forms of Energy Energy is the capacity to cause change
Energy exists in various forms, some of which can perform work
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10. Kinetic energy is energy associated with motion
Thermal energy is kinetic energy associated with random movement of atoms or molecules
Heat is thermal energy in transfer from one object to another
Light is another type of energy that can be harnessed to perform work
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11. Energy can be converted from one form to another
Potential energy is energy that matter possesses because of its location or structure
Chemical energy is potential energy available for release in a chemical reaction
Energy can be converted from one form to another
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12. Animation: Energy Concepts
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13. A diver has more potential energy on the platform. Diving converts
Figure 6.2
A diver has more potential
energy on the platform.
Diving converts
potential energy to
kinetic energy.
Figure 6.2 Transformations between potential and kinetic energy
Climbing up converts the kinetic
energy of muscle movement
to potential energy.
A diver has less
potential energy in
the water.
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14. The Laws of Energy Transformation
Thermodynamics is the study of energy transformations
In an open system, energy and matter can be transferred between the system and its surroundings
In an isolated system, exchange with the surroundings cannot occur
Organisms are open systems
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15. The First Law of Thermodynamics
According to the first law of thermodynamics, the energy of the universe is constant
Energy can be transferred and or transformed, but it cannot be created or destroyed
The first law is also called the principle of conservation of energy
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16. (a) First law of thermodynamics (b) Second law of thermodynamics
Figure 6.3
Heat
CO2
H2O
Chemical
energy
Figure 6.3 The two laws of thermodynamics
(a) First law of thermodynamics
(b) Second law of thermodynamics
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17. (a) First law of thermodynamics
Figure 6.3-1
Chemical
energy
Figure The two laws of thermodynamics (part 1: conservation of energy)
(a) First law of thermodynamics
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18. The Second Law of Thermodynamics
During every energy transfer or transformation, some energy is lost as heat
According to the second law of thermodynamics
Every energy transfer or transformation increases the entropy of the universe
Entropy is a measure of disorder, or randomness
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19. (b) Second law of thermodynamics
Figure 6.3-2
Heat
CO2
H2O
Figure The two laws of thermodynamics (part 2: entropy)
(b) Second law of thermodynamics
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20. Living cells unavoidably convert organized forms of energy to heat
Spontaneous processes occur without energy input; they can happen quickly or slowly
For a process to occur spontaneously, it must increase the entropy of the universe
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21. 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
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22. Figure 6.4 Order as a characteristic of life
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23. Figure 6.4-1 Order as a characteristic of life (part 1: biscuit star)
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24. Figure 6.4-2 Order as a characteristic of life (part 2: agave plant)
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25. The evolution of more complex organisms does not violate the second law of thermodynamics
Entropy (disorder) may decrease in a system, but the universe’s total entropy increases
Organisms are islands of low entropy in an increasingly random universe
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26. Concept 6.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously
Biologists measure changes in free energy to help them understand the chemical reactions of life
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27. 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, as in a living cell
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28. ∆G = Gfinal state – Ginitial state
The change in free energy (∆G) during a chemical reaction 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
Spontaneous processes can be harnessed to perform work
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29. 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
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30. • More free energy (higher G) • Less stable • Greater work capacity
Figure 6.5
• More free energy (higher G)
• Less stable
• Greater work capacity
In a spontaneous change
• The free energy of the system decreases (DG < 0)
• The system becomes more stable
• The released free energy can be harnessed to do work
Figure 6.5 The relationship of free energy to stability, work capacity, and spontaneous change
• Less free energy (lower G)
• More stable
• Less work capacity
(a) Gravitational motion
(b) Diffusion
(c) Chemical reaction
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31. • More free energy (higher G) • Less stable • Greater work capacity
Figure 6.5-1
• More free energy (higher G)
• Less stable
• Greater work capacity
In a spontaneous change
• The free energy of the system decreases (DG < 0)
• The system becomes more stable
• The released free energy can be harnessed to do work
Figure The relationship of free energy to stability, work capacity, and spontaneous change (part 1: free energy and spontaneity)
• Less free energy (lower G)
• More stable
• Less work capacity
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32. (a) Gravitational motion (b) Diffusion (c) Chemical reaction
Figure 6.5-2
Figure The relationship of free energy to stability, work capacity, and spontaneous change (part 2: gravitational motion, diffusion, and chemical reaction)
(a) Gravitational motion
(b) Diffusion
(c) Chemical reaction
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33. At equilibrium, forward and reverse reactions occur at the same rate; it is a state of maximum stability
A process is spontaneous and can perform work only when it is moving toward equilibrium
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34. Free Energy and Metabolism
The concept of free energy can be applied to the chemistry of life’s processes
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35. Exergonic and Endergonic Reactions in Metabolism
An exergonic reaction proceeds with a net release of free energy and is spontaneous; ∆G is negative
The magnitude of ∆G represents the maximum amount of work the reaction can perform
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36. (a) Exergonic reaction: energy released, spontaneous
Figure 6.6
(a) Exergonic reaction: energy released, spontaneous
Reactants
Amount of
energy
released
(DG < 0)
Free energy
Energy
Products
Progress of the reaction
(b) Endergonic reaction: energy required, nonspontaneous
Products
Figure 6.6 Free energy changes (∆G) in exergonic and endergonic reactions
Amount of
energy
required
(DG > 0)
Free energy
Energy
Reactants
Progress of the reaction
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37. (a) Exergonic reaction: energy released, spontaneous
Figure 6.6-1
(a) Exergonic reaction: energy released, spontaneous
Reactants
Amount of
energy
released
(DG < 0)
Free energy
Energy
Products
Figure Free energy changes (∆G) in exergonic and endergonic reactions (part 1: exergonic)
Progress of the reaction
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38. An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous; ∆G is positive
The magnitude of ∆G is the quantity of energy required to drive the reaction
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39. Energy (b) Endergonic reaction: energy required, nonspontaneous
Figure 6.6-2
(b) Endergonic reaction: energy required, nonspontaneous
Products
Amount of
energy
required
(DG > 0)
Free energy
Energy
Reactants
Figure Free energy changes (∆G) in exergonic and endergonic reactions (part 2: endergonic)
Progress of the reaction
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40. Equilibrium and Metabolism
Hydroelectric systems can serve as analogies for chemical reactions in living systems
Reactions in an isolated system eventually reach equilibrium and can then do no work
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41. (a) An isolated hydroelectric system
Figure 6.7
DG < 0
DG = 0
(a) An isolated hydroelectric system
(b) An open hydroelectric system
DG < 0
Figure 6.7 Equilibrium and work in isolated and open systems
DG < 0
DG < 0
DG < 0
(c) A multistep open hydroelectric system
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42. (a) An isolated hydroelectric system
Figure 6.7-1
DG < 0
DG = 0
Figure Equilibrium and work in isolated and open systems (part 1: isolated system)
(a) An isolated hydroelectric system
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43. Cells are not in equilibrium; they are open systems experiencing a constant flow of materials
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44. (b) An open hydroelectric system
Figure 6.7-2
(b) An open hydroelectric system
DG < 0
Figure Equilibrium and work in isolated and open systems (part 2: open system)
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45. A catabolic pathway in a cell releases free energy in a series of reactions
The product of each reaction is the reactant for the next, preventing the system from reaching equilibrium
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46. (c) A multistep open hydroelectric system
Figure 6.7-3
DG < 0
DG < 0
DG < 0
Figure Equilibrium and work in isolated and open systems (part 3: multistep open system)
(c) A multistep open hydroelectric system
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47. Concept 6.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions
A cell does three main kinds of work
Chemical
Transport
Mechanical
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48. Most energy coupling in cells is mediated by ATP
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
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49. The Structure and Hydrolysis of ATP
ATP (adenosine triphosphate) is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups
In addition to its role in energy coupling, ATP is also used to make RNA
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50. Video: ATP Space-filling Model
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51. Video: ATP Stick Model
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52. (a) The structure of ATP
Figure 6.8
Adenine
Triphosphate group
(3 phosphate groups)
Ribose
(a) The structure of ATP P P P
Adenosine triphosphate (ATP)
Figure 6.8 The structure and hydrolysis of adenosine triphosphate (ATP)
H2O P P P i
Energy
Inorganic
Adenosine diphosphate (ADP)
phosphate
(b) The hydrolysis of ATP
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53. (a) The structure of ATP
Figure 6.8-1
Adenine
Triphosphate group
(3 phosphate groups)
Ribose
Figure The structure and hydrolysis of adenosine triphosphate (ATP) (part 1: structure)
(a) The structure of ATP
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54. Energy is released from ATP when the terminal phosphate bond is broken
The bonds between the phosphate groups of ATP 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
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55. Adenosine triphosphate (ATP)
Figure 6.8-2 P P P
Adenosine triphosphate (ATP)
H2O
Figure The structure and hydrolysis of adenosine triphosphate (ATP) (part 2: hydrolysis) P P P
Energy i
Inorganic
Adenosine diphosphate (ADP)
phosphate
(b) The hydrolysis of ATP
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56. ATP hydrolysis releases a lot of energy due to the repulsive force of the three negatively charged phosphate groups
The triphosphate tail of ATP is the chemical equivalent of a compressed spring
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57. How the Hydrolysis of 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 exergonic reaction of ATP hydrolysis can be used to drive endergonic reactions
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58. The recipient molecule is now called a phosphorylated intermediate
ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant
The recipient molecule is now called a phosphorylated intermediate
Overall, the coupled reactions are exergonic
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59. Phosphorylated intermediate
Figure 6.9
NH3
NH2
DGGlu = +3.4 kcal/mol
Glu
Glu
Glutamic acid Ammonia
Glutamine
(a) Glutamic acid conversion to glutamine
NH3 P
NH2
ADP
ADP P
ATP i
Glu
Glu
Glu
Glutamic acid
Phosphorylated
intermediate
Glutamine
(b) Conversion reaction coupled with ATP hydrolysis
DGGlu = +3.4 kcal/mol
Figure 6.9 How ATP drives chemical work: energy coupling using ATP hydrolysis
NH3
NH2
ATP
ADP P
Glu
Glu i
DGGlu = +3.4 kcal/mol
DGATP = 7.3 kcal/mol
+ DGATP = 7.3 kcal/mol
Net DG = 3.9 kcal/mol
(c) Free-energy change for coupled reaction
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60. (a) Glutamic acid conversion to glutamine
Figure 6.9-1
NH3
NH2
DGGlu = +3.4 kcal/mol
Glu
Glu
Glutamic acid
Ammonia
Glutamine
(a) Glutamic acid conversion to glutamine
Figure How ATP drives chemical work: energy coupling using ATP hydrolysis (part 1: a nonspontaneous reaction)
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61. Phosphorylated intermediate Phosphorylated intermediate
Figure 6.9-2 P
ADP
ATP
Glu
Glu
Glutamic acid
Phosphorylated
intermediate
NH3 P
ADP
NH2
ADP P i
Glu
Glu
Figure How ATP drives chemical work: energy coupling using ATP hydrolysis (part 2: phosphorylation)
Phosphorylated
intermediate
Glutamine
(b) Conversion reaction coupled with ATP hydrolysis
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62. (c) Free-energy change for coupled reaction
Figure 6.9-3
DGGlu = +3.4 kcal/mol
NH3
NH2
ATP
ADP P
Glu
Glu i
DGGlu = +3.4 kcal/mol
DGATP = 7.3 kcal/mol
+ DGATP = 7.3 kcal/mol
Figure How ATP drives chemical work: energy coupling using ATP hydrolysis (part 3: coupled free energy change)
Net DG = 3.9 kcal/mol
(c) Free-energy change for coupled reaction
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63. Transport and mechanical work in the cell are powered by ATP hydrolysis
ATP hydrolysis leads to a change in a protein’s shape and often its ability to bind to another molecule
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64. (a) Transport work: ATP phosphorylates transport proteins.
Figure 6.10
Transport protein
Solute
ATP
ADP P i P P i
Solute transported
(a) Transport work: ATP phosphorylates transport proteins.
Vesicle
Cytoskeletal track
Figure 6.10 How ATP drives transport and mechanical work
ATP
ADP P
ATP i
Motor protein
Protein and vesicle moved
(b) Mechanical work: ATP binds noncovalently to motor proteins and then is hydrolyzed.
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65. The Regeneration of ATP
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
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66. catabolism (exergonic, energy-releasing processes) Energy for cellular
Figure 6.11
ATP
H2O
Energy from
catabolism (exergonic,
energy-releasing
processes)
Energy for cellular
work (endergonic,
energy-consuming
processes)
Figure 6.11 The ATP cycle
ADP P i
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67. Concept 6.4: Enzymes speed up metabolic reactions by lowering energy barriers
A 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
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68. (C12H22O11) (C6H12O6) (C6H12O6) Sucrase + H2O + O H O OH Sucrose
Figure 6.UN02
Sucrase +
H2O + O
H O OH
Sucrose
(C12H22O11)
Glucose
(C6H12O6)
Fructose
(C6H12O6)
Figure 6.UN02 In-text figure, sucrose hydrolysis, p. 131
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69. The Activation Energy Barrier
Every chemical reaction between molecules involves bond breaking and bond forming
The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA)
Activation energy often occurs in the form of heat that reactant molecules absorb from the surroundings
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70. Progress of the reaction
Figure 6.12 A B C D
Transition state A B
Free energy E A C D
Reactants A B
Figure 6.12 Energy profile of an exergonic reaction
DG < 0 C D
Products
Progress of the reaction
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71. How Enzymes Speed Up Reactions
Instead of relying on heat, organisms carry out catalysis to speed up reactions
A catalyst (for example, an enzyme) can speed up a reaction by lowering the EA barrier without itself being consumed
Enzymes do not affect the change in free energy (∆G); instead, they hasten reactions that would occur eventually
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72. Animation: How Enzymes Work
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73. Progress of the reaction
Figure 6.13
Course of
reaction
without
enzyme EA
without
enzyme
EA with
enzyme
is lower
Reactants
Free energy
Course of
reaction
with enzyme
DG is unaffected
by enzyme
Figure 6.13 The effect of an enzyme on activation energy
Products
Progress of the reaction
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74. Substrate Specificity of Enzymes
Enzymes are very specific for the reactions they catalyze
The reactant that an enzyme acts on is called the enzyme’s substrate
The enzyme binds to its substrate, forming an enzyme-substrate complex
The active site is the region on the enzyme where the substrate binds
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75. Enzymes change shape due to chemical interactions with the substrate
Enzyme specificity results from the complementary fit between the shape of the enzyme’s active site and the shape of the substrate
Enzymes change shape due to chemical interactions with the substrate
This induced fit of the enzyme to the substrate brings chemical groups of the active site together
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76. Video: Enzyme Induced Fit
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77. Substrate Active site Enzyme Enzyme-substrate complex Figure 6.14
Figure 6.14 Induced fit between an enzyme and its substrate
Enzyme
Enzyme-substrate
complex
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78. 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
Straining substrate bonds
Providing a favorable microenvironment
Covalently bonding to the substrate
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79. Substrates enter Substrates are active site. held in active site by
Figure 6.15-s1
Substrates enter
active site.
Substrates are
held in active site by
weak interactions.
Substrates
Enzyme-substrate
complex
Figure 6.15-s1 The active site and catalytic cycle of an enzyme (step 1)
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80. Substrates enter Substrates are active site. held in active site by
Figure 6.15-s2
Substrates enter
active site.
Substrates are
held in active site by
weak interactions.
Substrates
Enzyme-substrate
complex
Figure 6.15-s2 The active site and catalytic cycle of an enzyme (step 2)
Substrates are
converted to
products.
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81. Substrates enter Substrates are active site. held in active site by
Figure 6.15-s3
Substrates enter
active site.
Substrates are
held in active site by
weak interactions.
Substrates
Enzyme-substrate
complex
Figure 6.15-s3 The active site and catalytic cycle of an enzyme (step 3)
Products are
released.
Substrates are
converted to
products.
Products
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82. Substrates enter Substrates are active site. held in active site by
Figure 6.15-s4
Substrates enter
active site.
Substrates are
held in active site by
weak interactions.
Substrates
Enzyme-substrate
complex
Active site
is available
for new
substrates.
Figure 6.15-s4 The active site and catalytic cycle of an enzyme (step 4)
Enzyme
Products are
released.
Substrates are
converted to
products.
Products
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83. The rate of enzyme catalysis can usually be sped up by increasing the substrate concentration in a solution
When all enzyme molecules in a solution are bonded with substrate, the enzyme is saturated
At enzyme saturation, reaction speed can only be increased by adding more enzyme
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84. 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
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85. Effects of Temperature and pH
Each enzyme has an optimal temperature and pH at which its reaction rate is the greatest
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86. Optimal temperature for typical human enzyme (37C)
Figure 6.16
Optimal temperature for
typical human enzyme (37C)
Optimal temperature for
enzyme of thermophilic
(heat-loving)
bacteria (75C)
Rate of reaction 20 40 60 80
100
120
Temperature (C)
(a) Optimal temperature for two enzymes
Optimal pH for pepsin
(stomach
enzyme)
Optimal pH for trypsin
(intestinal
enzyme)
Figure 6.16 Environmental factors affecting enzyme activity
Rate of reaction 1 2 3 4 5 6 7 8 9 10 pH
(b) Optimal pH for two enzymes
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87. Optimal temperature for typical human enzyme (37C)
Figure
Optimal temperature for
typical human enzyme (37C)
Optimal temperature for
enzyme of thermophilic
(heat-loving)
bacteria (75C)
Rate of reaction
Figure Environmental factors affecting enzyme activity (part 1: temperature) 20 40 60 80
100
120
Temperature (C)
(a) Optimal temperature for two enzymes
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88. (b) Optimal pH for two enzymes
Figure
Optimal pH for pepsin
(stomach
enzyme)
Optimal pH for trypsin
(intestinal
enzyme)
Rate of reaction
Figure Environmental factors affecting enzyme activity (part 2: pH) 1 2 3 4 5 6 7 8 9 10 pH
(b) Optimal pH for two enzymes
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89. Figure
Figure Environmental factors affecting enzyme activity (part 3: cyanobacteria)
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90. Cofactors Cofactors are nonprotein enzyme helpers
Cofactors may be inorganic (such as a metal in ionic form) or organic
An organic cofactor is called a coenzyme
Most vitamins act as coenzymes or as the raw materials from which coenzymes are made
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91. Enzyme Inhibitors
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
Examples of inhibitors include toxins, poisons, pesticides, and antibiotics
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92. (b) Competitive inhibition (c) Noncompetitive inhibition
Figure 6.17
(a) Normal binding
(b) Competitive inhibition
(c) Noncompetitive inhibition
Substrate
Active site
Competitive
inhibitor
Enzyme
Figure 6.17 Inhibition of enzyme activity
Noncompetitive
inhibitor
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93. The Evolution of Enzymes
Most 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 activity or substrate specificity
Under new environmental conditions a novel form of an enzyme might be favored
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94. Concept 6.5: 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 switching on or off the genes that encode specific enzymes or by regulating the activity of enzymes
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95. Allosteric Regulation of Enzymes
Allosteric regulation may either inhibit or stimulate an enzyme’s activity
Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site
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96. Allosteric Activation and Inhibition
Most allosterically regulated enzymes are made from polypeptide subunits
Each enzyme has 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
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97. Active site (one of four)
Figure 6.18
(a) Allosteric activators and inhibitors
(b) Cooperativity: another type of allosteric activation
Allosteric enzyme
with four subunits
Active site
(one of four)
Substrate
Regulatory site (one of four)
Activator
Inactive form
Stabilized active form
Active form
Stabilized active form
Oscillation
Non-
functional
active site
Figure 6.18 Allosteric regulation of enzyme activity
Inhibitor
Inactive form
Stabilized inactive form
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98. (a) Allosteric activators and inhibitors
Figure
(a) Allosteric activators and inhibitors
Allosteric enzyme
with four subunits
Active site
(one of four)
Regulatory
site (one
of four)
Activator
Active form
Stabilized
active form
Oscillation
Non-
functional
active site
Figure Allosteric regulation of enzyme activity (part 1: activation and inhibition)
Inhibitor
Inactive form
Stabilized
inactive form
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99. Cooperativity is a form of allosteric regulation that can amplify enzyme activity
One substrate molecule primes an enzyme to act on additional substrate molecules more readily
Cooperativity is allosteric because binding by a substrate to one active site affects catalysis in a different active site
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100. (b) Cooperativity: another type of allosteric activation
Figure
(b) Cooperativity: another type of allosteric activation
Substrate
Figure Allosteric regulation of enzyme activity (part 2: cooperativity)
Inactive form
Stabilized active form
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101. 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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101

102. Enzyme 1 (threonine deaminase)
Figure 6.19
Active site available
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Isoleucine
used up by
cell
Intermediate A
Feedback
inhibition
Enzyme 2
Intermediate B
Enzyme 3
Intermediate C
Isoleucine
binds to
allosteric
site.
Figure 6.19 Feedback inhibition in isoleucine synthesis
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)
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103. Organization 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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103

104. 1 mm Mitochondrion The matrix contains enzymes in solution that
Figure 6.20
Mitochondrion
The matrix contains
enzymes in solution that
are involved in one stage
of cellular respiration.
Figure 6.20 Organelles and structural order in metabolism
Enzymes for another
stage of cellular
respiration are
embedded in the
inner membrane.
1 mm
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105. 1 mm Mitochondrion The matrix contains enzymes in solution that
Figure
Mitochondrion
The matrix contains
enzymes in solution that
are involved in one stage
of cellular respiration.
Figure Organelles and structural order in metabolism (part 1: TEM)
Enzymes for another
stage of cellular
respiration are
embedded in the
inner membrane.
1 mm
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106. Figure 6.UN03a
Figure 6.UN03a Skills exercise: making a line graph and calculating a slope (part 1)
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107. Figure 6.UN03b
Figure 6.UN03b Skills exercise: making a line graph and calculating a slope (part 2)
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108. DG is unaffected by enzyme Course of reaction without EA enzyme
Figure 6.UN04
Course of
reaction
without
enzyme EA
without
enzyme
EA with
enzyme
is lower
Reactants
Free energy
DG is unaffected
by enzyme
Course of
reaction
with enzyme
Figure 6.UN04 Summary of key concepts: enzymes and activation energy
Products
Progress of the reaction
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109. Figure 6.UN05
Figure 6.UN05 Test your understanding, question 12 (kinetic and potential energy)
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