1. Concept 8.5: 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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2. 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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3. 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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4. Figure 8.20 Allosteric regulation of enzyme activity
Allosteric enyzme
with four subunits
Active site
(one of four)
Regulatory
site (one
of four)
Activator
Active form
Stabilized active form
Oscillation
Non-
functional
active
site
Inhibitor
Inactive form
Stabilized inactive
form
Figure 8.20 Allosteric regulation of enzyme activity
(a) Allosteric activators and inhibitors
Substrate
Inactive form
Stabilized active
form
(b) Cooperativity: another type of allosteric activation

5. Stabilized active form
Fig. 8-20a
Allosteric enzyme
with four subunits
Active site
(one of four)
Regulatory
site (one
of four)
Activator
Active form
Stabilized active form
Oscillation
Figure 8.20a Allosteric regulation of enzyme activity
Non-
functional
active
site
Inhibitor
Inactive form
Stabilized inactive
form
(a) Allosteric activators and inhibitors

6. Cooperativity is a form of allosteric regulation that can amplify enzyme activity
In cooperativity, binding by a substrate to one active site stabilizes favorable conformational changes at all other subunits
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7. (b) Cooperativity: another type of allosteric activation
Fig. 8-20b
Substrate
Inactive form
Stabilized active
form
Figure 8.20b Allosteric regulation of enzyme activity
(b) Cooperativity: another type of allosteric activation

8. Identification of Allosteric Regulators
Allosteric regulators are attractive drug candidates for enzyme regulation
Inhibition of proteolytic enzymes called caspases may help management of inappropriate inflammatory responses
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9. Hypothesis: allosteric inhibitor locks enzyme in inactive form
Fig. 8-21
EXPERIMENT
Caspase 1
Active
site
Substrate SH SH
Known active form
Active form can
bind substrate
Allosteric
binding site SH
S–S
Allosteric
inhibitor
Known inactive form
Hypothesis: allosteric
inhibitor locks enzyme
in inactive form
Figure 8.21 Are there allosteric inhibitors of caspase enzymes?
RESULTS
Caspase 1
Inhibitor
Active form
Allosterically
inhibited form
Inactive form

10. EXPERIMENT Allosteric binding site Allosteric inhibitor Caspase 1
Fig. 8-21a
EXPERIMENT
Caspase 1
Active
site
Substrate SH SH
Known active form
Active form can
bind substrate
Figure 8.21 Are there allosteric inhibitors of caspase enzymes?
Allosteric
binding site SH
S–S
Allosteric
inhibitor
Known inactive form
Hypothesis: allosteric
inhibitor locks enzyme
in inactive form

11. RESULTS Caspase 1 Inhibitor Active form Allosterically inhibited form
Fig. 8-21b
RESULTS
Caspase 1
Inhibitor
Figure 8.21 Are there allosteric inhibitors of caspase enzymes?
Active form
Allosterically
inhibited form
Inactive form

12. 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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13. Fig. 8-22
Initial substrate
(threonine)
Active site
available
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Isoleucine
used up by
cell
Intermediate A
Feedback
inhibition
Enzyme 2
Active site of
enzyme 1 no
longer binds
threonine;
pathway is
switched off.
Intermediate B
Enzyme 3
Intermediate C
Figure 8.22 Feedback inhibition in isoleucine synthesis
Isoleucine
binds to
allosteric
site
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)

14. 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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15. Fig. 8-23
Mitochondria
Figure 8.23 Organelles and structural order in metabolism
1 µm

16. Progress of the reaction
Fig. 8-UN2
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
Products
Progress of the reaction

17. Fig. 8-UN3

18. Fig. 8-UN4

19. Fig. 8-UN5

20. You should now be able to:
Distinguish between the following pairs of terms: catabolic and anabolic pathways; kinetic and potential energy; open and closed systems; exergonic and endergonic reactions
In your own words, explain the second law of thermodynamics and explain why it is not violated by living organisms
Explain in general terms how cells obtain the energy to do cellular work
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21. Explain how ATP performs cellular work
Explain why an investment of activation energy is necessary to initiate a spontaneous reaction
Describe the mechanisms by which enzymes lower activation energy
Describe how allosteric regulators may inhibit or stimulate the activity of an enzyme
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings