enzymes (if you don’t have the energy, we can help!)
TRANSCRIPT
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Enzymes
(If you don’t have the energy, we can help!)
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Enzymes
• Biological catalysts
• Made primarily from proteins
• Bind to substrate
• Not used up
• Speed up reaction
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CatalysisUncatalyzed
G
time
R
P
Transition
EA
time
R
P
G
Catalyzed
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Catalysts
• Increase rate of reaction
• Provide activation energy– Energy is conserved– Activation energy has to come from
somewhere
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Sources of Activation energy
• Binding energy– Solvent released from active site when
substrate binds increases ΔSsurr
• Induced fit– Enzyme forces substrate into unstable
transition state– Catalytic antibodies
• Binding substrate brings reactive groups together
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Catalysis by an enzyme
• E + S > ES > EP > E + P
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Enzyme Example Serine Proteases
• http://www.bio.cmu.edu/courses/03231/Protease/SerPro.htm
• Active site and substrate– Note side chain
interactions– Substrate diffuses into
active site
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Serine proteases
• Substrate binds to active site
• Chemical groups interact
• Force substrate into unstable intermediate
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Serine Proteases
• Peptide bond is cleaved
• Serine in active site is bound to carboxyl side of peptide
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Serine proteases
• Half of protein diffuses out
• Enzyme used up– Has to be regenerated
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Serine proteases
• Water diffuses into active site
• Juxtaposes chemical groups – Similar reaction to first
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Serine proteases
• New unstable intermediate generated
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Serine Proteases
• Carboxyl end of peptide diffuses out
• Enzyme regenerated • Ready for another
round
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Serine protease summary
• Enzyme stabilized by various side chain interactions
• Substrate binds to enzyme.– Fits pocket– Forms unstable intermediate– Chemical groups on enzyme do reaction
• Chemistry happens and product diffuses out
• Enzyme regenerated
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Kinetics
• Study of reaction rates• Why?
– Used to determine mechanisms
• Michaelis Menton kineticsV = rate of reaction
• Vmax = maximum reaction rate
[S]• Substrate concentration• Km = substrate concentration where rate is half maximal
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Michaelis-Menton Plot
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Enzyme inhibition, Competitive
• Both substrate and inhibitor bind to active site– Compete
• Inhibitor blocks substrate from binding
E S
I
EI
S
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Michaelis- Menton Plot for competitive inhibition
uninhibVmax
Vmax/2
Km
inhibited
Kmi
V
[S]
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Michaelis- Menton Plot for competitive inhibition
• Vmax is not changed
• Km increased
– Since substrate has to outcompete the inhibitor for the active site, it takes more substrate to get to the same rate.
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Noncompetitive Inhibition
S
I
• Inhibitor binds to an allosteric site on the enzyme• Changes active site so substrate doesn’t bind
I
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Michaelis- Menton Plot for noncompetitive inhibition
uninhib
Vmax/2
Km
Vmax uninhib
Vmax inhib
V
[S]
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Michaelis- Menton Plot for noncompetitive inhibition
• Inhibition lowers Vmax
• Km unchanged
– Since inhibitor doesn’t bind to active site, changing amount of substrate will have no effect
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Michaelis Menton Kinetics Summary
• Competitive inhibition– Inhibitor competes with substrate for active
site• Vmax unchanged• Km increased
• Noncompetitive inhibition– Inhibitor binds to allosteric site changing
active site• Vmax lowered• Km unchanged
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Allosteric Activation
• Active site will not bind substrate• Allosteric activator binds and changes shape of
active site• Now substrate binds
S
A
A
S