Kinetics of Enzyme Reactions

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Kinetics of Enzyme Reactions

• Srbová Martina

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• E S ES E P

Rate of the conversion of substrate to products
(S ? P) v kcat ES
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Michaelis-Menten Equation
1. The ES complex is in a steady state. 2. All of
the enzyme is converted to the ES complex. 3.
Rate of formation of the products will be the
maximum rate possible. Vmax kcatEtotal Turnove
r number number of molecules of substrate that
one molecule of the enzyme can convert to product
per unit time
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Lineweaver-Burk Plot

• 1 Km 1 1
• v Vmax S Vmax

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Multisubstrate reactions

• 1. Ternary-complex mechanism

Random mechanism Two substrates A and B can bind
in any order P,Q - products
Ordered mechanism Binding of A is required before
B can bound
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• 2. Ping-pong mechanism

Substrate A reacts with E to produce product P
which is released before the second substrate B
will bind to modified enzyme E. The substrate B
is then converted to product Q and the enzyme is
regenerated.
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Enzyme activity

• Standard unit of enzyme activity (U) ?mol / min
  
• amount of enzyme that convert 1 ?mol substrate
  per 1min
• SI unit Katal (kat) mol /s
• - amount of enzyme that convert 1 mol substrate
  per 1s

Factors which effect enzyme activity
temperature optimum for human enzymes is between
35 45 C
pH
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Reversible Inhibition
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Competitive Inhibition
Competitive inhibitors bind at substrate binding
site and compete with the substrate for the enzyme
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Noncompetitive Inhibition
Noncompetive inhibitors bind at a site other than
the substrate binding site
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Uncompetitive Inhibition
Uncompetitive inhibitors bind only with the ES
form of the enzyme
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Irreversible Inhibition

• Irreversible inhibitors cause covalent
  modification of the enzyme
• Toxins e.g. Amanitin (Amanita phaloides)
• Diisopropylfluorophosfate (DFP)
• - binds to the serine in the active site ?
  deactivation of ezyme
• eg .inhibition of acetylcholine esterase
• Penicillin inhibits bacterial transpeptidase

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Control of enzyme activity

1. Allosteric enzymes

Negative feedback /feedback inhibition
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• 2. Covalently modulated enzymes
• zymogens

Glycogen phosphorylase
undergo cleavage to produce an active enzyme
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• Isozymes
• - catalyze the same reaction
• - differ in AA sequences, catalytic acitivity
  (substrates/coenzymes affinity..)
• Lactate dehydrogenase
• tetrameric, 2 types of subunits M, H
• M4, M3H, M2H2, MH3, H4
• Glucokinase x Hexokinase
• ?Km ?Km
• liver mostly in the other tissues
• not inhibited by Glc- 6P inhibited by Glc- 6P

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