CHAPTER 4 :
ENZYMES
4.2 MECHANISM OF ENZYME ACTION AND
ENZYME KINETICS
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Contents :
Mechanism of
Enzyme Action
 The lock-and-key
hypothesis
 The induced-fit
hypothesis
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Enzyme Kinetics
 Michaelis-Menten equation
 Effects of enzyme
concentration, substrate
concentration, temperature
and pH on the Rate of an
enzyme catalysed reaction
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Michaelis-Menten
Equation
A reaction model was proposed by
Leonor Michaelis and Maud Menten
in 1913 to account for enzymecatalysed reaction
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The Relationship between The Rate of an EnzymeCatalysed Reaction and The Substrate Concentration
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Lineweaver-Burk Plot
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The Significant of Vmax
- Vmax : Maximal Velocity
The velocity of an enzyme-catalysed reaction when there is
a saturating level of substrate
-Vmax show the turnover number ( catalytic constant ) of an
enzyme.
This is the number of substrate molecules that can be
converted into product molecules by the catalytic site of one
enzyme molecule per unit time (second)
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The Significant of Km
- Michaelis-Menten Constant (Km) : Substrate
concentration that produces one half Vmax
- Km provides a measure of the substrate
concentration required for significant enzyme
reaction to occur
- Km values also provide a measure of the
affinity of an enzyme for its substrate
- Different enzymes have different Km values
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Assumptions of Michaelis-Menten Kinetics
Substrate concentration
> Enzyme concentration
Vmax is obtained when
the catalytic sites of
enzymes are saturated
with substrate
The reverse reaction
of an enzyme and
product is negligible
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The total of enzyme
substrate remains
constant
The uncatalysed rate of
conversion of substrate
to product wuthout
enzyme is negligible
Enzyme is not
allosteric and not
cooperative
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Effects of Enzyme Concentration, Substrate Concentration,
Temperature and pH on the Rate of an Enzyme Catalysed
Reaction
The
Concentration
of the Enzyme
The Concentration
of the Substrate
pH
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Temperature
Presence pf
Competitive and
Non-Competitve
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Enzyme Concentration
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The rate of an enzymecatalysed reaction is directly
proportional to the
concentration of the enzyme
if substrates are present in
excess and no other factors
are limiting
When substrates are not in
excess, there is no increase
in reaction because the
substrates concentration
become the limiting factor
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Substrate
Concentration
At low substrate concentration, the
rate of an enzyme reaction
increases directly proportion to
increasing substrate concentration
As the substrate concentration
increases, more active sites are
involved in the binding of substrate
molecules
At a certain point, increasing the
substrate concentration does not
increase the net reaction rate
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Effects of Temperature
At low temperature, an
enzyme-catalysed reaction
occurs very slowly.
Increasing the temperature
increases the kinetic energy
of reactants
At suboptimal temperatues,
the Q10 reactions is
approximately 2
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Effects of pH
Most enzymes are
effective only within a
narrow pH range.
The optimum pH is the
pH at which the
maximum rate of reaction
occurs
Different enzymes have
different pH optima
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Example of Enzymes Optimum pH
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