Enzymes Chapter 5

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Enzymes Chapter 5

• Enzymes kinetics. What does it mean?
• Enzyme Inhibition
• A. Reversible
• 1) Competitive
• 2) Non-competitive
• B. Irreversible

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3. Enzyme regulation A. Types B. Feedback
inhibition C. Allosteric regulation D. Covalen
t modification
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vo Vmax S Km S
1. Enzyme kinetics
The Michaelis-Menton equation
When S gtgt Km, vo Vmax When S Km, vo
Vmax/2 When S ltlt Km, vo Vmax S
Km
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Catalytic constant kcat
Vmax kcat Et kcat maximum number of
substrate molecules converted to product by
each active site/sec turnover number.
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What is the meaning of the Km?
k1 k2 k-1
E S ?? ES ? E P
Km k-1 k2 k1
If k2 ltlt k-1, then Km k1/k-1 which measure the
affinity of the enzyme for the substrate.
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• Enzyme inhibition
• A. Reversible
• B. Irreversible

• Reversible
• 1) Competitive
• 2) Noncompetitive

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Enzyme inhibition

• Reversible inhibitor a substance that binds to
  an enzyme to inhibit it, but can be released
• competitive inhibitor binds to the active
  (catalytic) site and blocks access to it by
  substrate
• noncompetitive inhibitor binds to a site other
  than the active site inhibits the enzyme by
  changing its conformation
• Irreversible inhibitor a substance that causes
  inhibition that cannot be reversed
• usually involves formation or breaking of
  covalent bonds to or on the enzyme

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Competitive Inhibitor
Non- Competitive inhibitor
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1) Competitive inhibitor
Binds at the acitve site.
Prevents binding of substrate
Can be reversed by adding more substrate.
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Competitive inhibitors
Substrate only
Active site
Substrate
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Add inhibitor
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Add more substrate
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Vmax
I
-I
v
0.5 Vmax
KM I
S
KM
Competitive Inhibition
-I
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Competitive inhibtion Increases Km Vmax stays
the same.
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Benzamidine is a competitive inhibitor of
trypsin.
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Drug design and treatment.
Purine nucleoside phosphorylase
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2) Noncompetitive Inhibition
Binds at a different site (than the active site)
Decreases Vmax
Km is unaffected
Can not overcome inhibition by adding more
substrate.
It changes the shape of the active site so
that the substrate can not bind.
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Non-competitive inhibiton
Substrate only
Active site
Site for Non-competitive inhibitor
Substrate
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Add inhibior
Active site
Inhibitor binds to Inhibitor site
Changes shape of active site and substrate can
not bind
Substrate
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Add inhibior
Active site
Inhibitor binds to Inhibitor site
Changes shape of active site and substrate can
not bind
Substrate
Adding more substrate does no good.
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Vmax - I
-I
Vmax I
v
0.5 Vmax -I
0.5 Vmax I
S
KM
Nocompetitive inhibiton
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Non-competitive inhibition. Vmax decreases Km
stays the same.
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• Irreversible inhibition
• Covalent
• Alkylation
• Acylation

which groups are at the active site.
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Formation of a Schiffs base reduction.
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Diisopropyl- Fluorophosphate Reacts with a
serine residue at the active site of
chymotrypsin
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Nerve gas inhibits acetylcholinesterase which
hydrolyses acetyl choline neurotransmitter.
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Triose phosphate isomerase
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Irreversible inhibitors allow one to determine
the residues at the active site of an enzyme.