BCOR 011 Lecture 12 9/28/2005

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BCOR 011 Lecture 12 9/28/2005 ENZYME
S
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Last time -?G reaction can go
spontaneous But when will it go? And at what
rate?
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Thermodynamics
Whether a reaction will occur
Kinetics
WHEN a reaction will occur
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What governs WHEN a reaction will occur?
The tower of blocks falling is favorable but
when will it happen? Oxidation of carbohydrate
polymers (starch) to carbon dioxide and water is
favorable but when will it happen? Gasoline
burning to carbon dioxide and water is
favorable but when will it happen ?
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For a Reaction to occur need to Destabilize
Existing State to INPUT ENERGY
Destabilization energy input Activation Energy
Potential net usable energy
Potential net usable energy
Now
In Transition
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Need to INPUT ENERGY to Destabilize Existing State
Regain Activation Energy Invested
net usable energy released
Potential net usable energy
In Transition
After
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What does activation energy represent? For a
Reaction to Occur
- reactants must find each other, - meet in
proper orientation - and hit with sufficient
force
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Productive Collision
Many Non-productive Collisions
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Needs of Typical chemical reactions
- need large number of molecular collisions
- need collide violently enough to break
pre-existing bonds (not bounce)
- need high concentration to find each other at
significant rate
HEAT !
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• The energy profile for an exergonic reaction

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EA
Temp 1
Temp 2
Molecules with sufficient Energy (lt5)
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ENZYMES make reactions easier to occur at
reasonable temperature by LOWERING the
ACTIVATION ENERGY EA of the reaction
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Activation Energy
Energy necessary to overcome the status quo
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CATALYSTS
promote a specific reaction
But are NOT consumed in the process
Key concepts Promotes - does not alter
what would normally occur thermodynamically
Specificity - promotes only one reaction,
only between specific reactants to give specific
products Reusable - regenerated in the
process
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ENZYMES are biological CATALYSTS

- usually PROTEINS
- sometimes RNA or RNA/protein complexes
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Enzymes work as catalysts by providing an easy
path to the same point
Hard path
Easy path
HOW?
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How do Enzymes do it?
1. Enzymes have BINDING AFFINITY for their
reactants Substrates
Brings substrates in close proximity conc
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Charged
Stabilized Interactions
Nonpolar
Polar
Have a very Specific 3-D Shape With a Specific
Arrangement of Functional Groups Flexible
OH
HO
HO

Enzymes act as a Specific Platform
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ENZYMES Bind ONLY specific things Bind them
ONLY in a Specific 3-D Orientation
HO
OH
OH
OH
HO
-
HO

SPECIFICITY is the Key to Enzyme Action
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2. Enzymes ORIENT Substrates always in
productive orientation
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Productive Collision
ONLY Productive Collisions
Many Non-productive Collisions
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With just a little nudge, cant help but react
HO
OH
OH
OH
HO
-
HO

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3. Enzymes cause BOND STRAIN - destabilize
existing bonds nutcracker effect
3a. Physical Strain
3b.Chemical Strain
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• The active site
• Is the region on the enzyme where the substrate
  binds

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• Induced fit of a substrate

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Enzyme-substrate interactions
Fischer Lock key
Koshland Induced fit
3a. Physical bond strain Draw an quarter - an
anvil
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• The catalytic cycle of an enzyme

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3b. Chemical Bond Strain tease the bond to fall
apart
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Chemical Bond Strain
Stabilize a Fictitious state
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Cofactors
Non-polypeptide things at the active site that
help enzymes do their job

• Cofactors
• Are nonprotein enzyme helpers, eg Zn
• Coenzymes
• Are organic cofactors

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4. Enzymes partake in reactions but are
not consumed in them
Converts MANY As into Bs
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H
OH-
H
Partakes but start and end with the same enzyme
config
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Lysozyme
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Lysozyme kills bacteria Works at pH 4-5 Why?
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SUMMARY
Enzymes 1. Bring reactants (substrates) in
close proximity 2. Align substrates in proper
orientation 3. Can act as a Lever a press or
an anvil small shape change translates to large
force 4. Release products when reaction
done rebind more substrates 5. Many small
steps, each easily achieved rather than one
huge leap
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Enzymes carry out reactions in a series of small
steps rather than one energetic event
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Reaction rates Example H2O2-gt H2O
O2 uncatalyzed months Fe 30,000x
faster Catalase 100,000,000 x faster Enzyme
kinetics- kinetikos moving
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An enzyme catalyzed rxn Can be saturated
Vmax
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The lower the Km the better the enzyme recognizes
substrate finds it at low conc
mpg
The higher the Vmax the more substrate an
enzyme can process per min
(if substrate around)
top speed
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Things that affect protein structure often
affect enzyme activity
temperature
0 20 40 60 80 100
º C
pH
0 1 2 3 4 5 6 7 8
9 10
pH
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Enzyme regulation Activity controlled Continual
ly adjusted
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Principal Ways of Regulating Enzymes
Competitive Inhibition Allosteric
Inhibition Covalent Modification
(phosphorylation)
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S2
S1
Competitive Inhibitors bind to active site
unproductively and block true
substrates access
HO
OH
OH
-
I
HO
OH
OH
HO
HO

S I bind to same site
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Competitive inhibition
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Allosteric Inhibitors
other site
Distorts the conformation of the enzyme
Negative allosteric regulator
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Allosteric inhibition
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Positive allosteric regulators
Helps enzyme work better promotes/stabilizes an
active conformation
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Allosteric activation
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• Allosteric regulators change the shape
• conformation of the enzyme

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A frequent regulatory modification of
enzymes
Phosphorylation
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Phosphorylase kinase
inactive
P active
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• Summary
• enzymes are catalysts
• Lower activation energy EA
• Mechanism of action
• Enzyme kinetics- Vmax, Km
• Regulation of enzyme activity
• - competitive, allosteric
• phosphorylation