Upload
raj-kumar
View
360
Download
3
Tags:
Embed Size (px)
Citation preview
Enzymes
(If you don’t have the energy, we can help!)
www.freelivedoctor.com
Enzymes
• Biological catalysts
• Made primarily from proteins
• Bind to substrate
• Not used up
• Speed up reaction
www.freelivedoctor.com
CatalysisUncatalyzed
G
time
R
P
Transition
EA
time
R
P
G
Catalyzed
www.freelivedoctor.com
Catalysts
• Increase rate of reaction
• Provide activation energy– Energy is conserved– Activation energy has to come from
somewhere
www.freelivedoctor.com
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
www.freelivedoctor.com
Catalysis by an enzyme
• E + S > ES > EP > E + P
www.freelivedoctor.com
Enzyme Example Serine Proteases
• Active site and substrate– Note side chain
interactions– Substrate diffuses into
active site
www.freelivedoctor.com
Serine proteases
• Substrate binds to active site
• Chemical groups interact
• Force substrate into unstable intermediate
www.freelivedoctor.com
Serine Proteases
• Peptide bond is cleaved
• Serine in active site is bound to carboxyl side of peptide
www.freelivedoctor.com
Serine proteases
• Half of protein diffuses out
• Enzyme used up– Has to be regenerated
www.freelivedoctor.com
Serine proteases
• Water diffuses into active site
• Juxtaposes chemical groups – Similar reaction to first
www.freelivedoctor.com
Serine proteases
• New unstable intermediate generated
www.freelivedoctor.com
Serine Proteases
• Carboxyl end of peptide diffuses out
• Enzyme regenerated • Ready for another
round
www.freelivedoctor.com
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 regeneratedwww.freelivedoctor.com
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
www.freelivedoctor.com
Michaelis-Menton Plot
www.freelivedoctor.com
Enzyme inhibition, Competitive
• Both substrate and inhibitor bind to active site– Compete
• Inhibitor blocks substrate from binding
E S
I
EI
S
www.freelivedoctor.com
Michaelis- Menton Plot for competitive inhibition
uninhibVmax
Vmax/2
Km
inhibited
Kmi
V
[S]
www.freelivedoctor.com
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.
www.freelivedoctor.com
Noncompetitive Inhibition
S
I
• Inhibitor binds to an allosteric site on the enzyme
• Changes active site so substrate doesn’t bind
I
www.freelivedoctor.com
Michaelis- Menton Plot for noncompetitive inhibition
uninhib
Vmax/2
Km
Vmax uninhib
Vmax inhib
V
[S]
www.freelivedoctor.com
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
www.freelivedoctor.com
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
www.freelivedoctor.com
Allosteric Activation
• Active site will not bind substrate• Allosteric activator binds and changes shape of
active site• Now substrate binds
S
A
A
S
www.freelivedoctor.com