1. An Introduction to Metabolism6
An Introduction to Metabolism

2. Do now: (think back to most recent lab)
Write down the reaction for the decomposition of hydrogen peroxide. Identify the reactants and the products.
What was the enzyme used to facilitate this reaction? Why was this enzyme necessary?
What was the purpose of the guaiacol?
What effect do you think pH would have on the enzyme?

3. 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
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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
Energy is the capacity to cause change
Catabolic pathways release energy by breaking down complex molecules into simpler compounds
Ex: cellular respiration, hydrolysis
Anabolic pathways consume energy to build complex molecules from simpler ones
Dehydration synthesis of lipids, carbohydrates, proteins
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5. 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
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
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6. Free Energy and Metabolism
The concept of free energy can be applied to the chemistry of life’s processes
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7. An exergonic reaction proceeds with a net release of free energy and is spontaneous; ∆G is negative
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8. An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous; ∆G is positive
In biological systems the energy released from exergonic reactions are used to drive endergonic reactions
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9. 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 is often supplied in the form of thermal energy that the reactant molecules absorb from their surroundings
Why is obtaining thermal energy not an effective strategy for biological reactions?
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10. Progress of the reaction
Figure 6.12 A B C D
Transition state A B EA
Free energy C D
Reactants A B
Figure 6.12 Energy profile of an exergonic reaction
G  0 C D
Products
Progress of the reaction 10

11. Enzymes
A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction
An enzyme is biological catalyst. (proteins)
What reaction is shown below?
Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction
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12. How Enzymes Speed Up Reactions
Enzymes catalyze reactions by lowering the EA barrier
Enzymes do not affect the change in free energy (∆G); instead, they hasten reactions that would occur eventually
Animation: How Enzymes Work
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13. Progress of the reaction
Figure 6.13
Reactants
Free energy
G is unaffected
by enzyme
Figure 6.13 The effect of an enzyme on activation energy
Products
Progress of the reaction 13

14. 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
G is unaffected
by enzyme
Figure 6.13 The effect of an enzyme on activation energy
Products
Progress of the reaction 14

15. Label: substrate, enzyme, active site, enzyme substrate complex
Figure 6.14 Induced fit between an enzyme and its substrate
Enzyme
Enzyme-substrate
complex 15

16. Substrate Specificity of Enzymes
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
Enzyme specificity results from the complementary fit between the shape of its active site and the substrate shape
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17. Enzymes change shape due to chemical interactions with the substrate
What is the difference between the lock and key model vs the induced fit model?
This induced fit of the enzyme to the substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction
Video: Enzyme Induced Fit
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18. In case you were curious….
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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19. Substrates are 2 Substrates enter 1 held in active site by
Figure 2
Substrates are
held in active site by
weak interactions. 1
Substrates enter
active site.
Substrates
Enzyme-substrate
complex
Figure The active site and catalytic cycle of an enzyme (steps 1-2) 19

20. Substrates are 2 Substrates enter 1 held in active site by
Figure 2
Substrates are
held in active site by
weak interactions. 1
Substrates enter
active site.
Substrates
Enzyme-substrate
complex
Figure The active site and catalytic cycle of an enzyme (step 3) 3
Substrates are
converted to
products. 20

21. 2 Substrates are held in active site by weak interactions. 1
Figure 2
Substrates are
held in active site by
weak interactions. 1
Substrates enter
active site.
Substrates
Enzyme-substrate
complex
Figure The active site and catalytic cycle of an enzyme (step 4) 4
Products are
released. 3
Substrates are
converted to
products.
Products 21

22. 2 Substrates are held in active site by weak interactions. 1
Figure 2
Substrates are
held in active site by
weak interactions. 1
Substrates enter
active site.
Substrates
Enzyme-substrate
complex
Active
site is
available
for new
substrates. 5
Figure The active site and catalytic cycle of an enzyme (step 5)
Enzyme
Products are
released. 4 3
Substrates are
converted to
products.
Products 22

23. 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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24. Each enzyme has an optimal temperature in which it can function
Optimal temperature for
typical human enzyme
(37C)
Optimal temperature for
enzyme of thermophilic
(heat-tolerant)
bacteria (77C)
Rate of reaction
Figure 6.16a Environmental factors affecting enzyme activity (part 1: temperature) 20 40 60 80
100
120
Temperature (C)
(a) Optimal temperature for two enzymes 24

25. Each enzyme has an optimal pH in which it can function
Optimal pH for pepsin
(stomach
enzyme)
Optimal pH for trypsin
(intestinal
enzyme)
Rate of reaction
Figure 6.16b 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 25

26. Activating an enzyme Cofactors Inorganic
Ex: heme in hemoglobin
Coenzymes- special cofactors which are organic.
include vitamins
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27. Inhibiting an enzyme (a) Normal binding (b) Competitive inhibition
(c) Noncompetitive
inhibition
Substrate
Active site
Competitive
inhibitor
Enzyme
Figure 6.17 Inhibition of enzyme activity
Noncompetitive
inhibitor 27

28. 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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29. Regulation of Enzymes
Sometimes a cell needs a reaction to occur, sometimes it needs it to stop.
Analogous to when a car needs to break, and when it needs to accelerate. How does it regulate?
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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30. Allosteric Activation and Inhibition
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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31. Cooperativity
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
Is this positive or negative feedback?
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32. Enzyme Feedback– Positive or Negative?
feedback inhibition, the end product of a metabolic pathway shuts down the pathway
Advantage??
prevents a cell from wasting chemical resources by synthesizing more product than is needed

33. 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, enzymes for cellular respiration are located in mitochondria
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34. some enzymes reside in specific organelles
Figure 6.20
Mitochondria
The matrix contains
enzymes in solution
that are involved in
one stage of cellular
respiration.
some enzymes reside in specific organelles
Enzymes for another
stage of cellular
respiration are
embedded in the
inner membrane.
Figure 6.20 Organelles and structural order in metabolism
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