Enzyme Action Model,and Factors Affecting Activity
Models Of Enzyme Substrate Complex Formation
1. Template or Lock-and-Key Mode
The
active site by itself provides a rigid, pre-shaped template fitting with the
size and shape of the substrate molecule.
2. Induced-Fit or Koshland Model
Koshland
in 1963 which is known as induced-fit model. The important feature of this
model is the flexibility of the region of active site.
The substrate during its binding induces conformational changes in
the active site to attain the final catalytic shape and form
•
Enzymes become inactive on denaturation
•
Saturation kinetics
•
Competitive inhibition
•
Allosteric modulation.
Kinetic
Properties Of Enzymes
Kinetic
analysis of enzymes was used for characterization of enzyme catalyzed reactions
even before enzymes had been isolated in pure form
Vo Definition
of initial velocity Vo in terms of [ES]: The initial velocity, according to
Michaelis and Menten theory is determined by the rate of dissociation of [ES]
in reaction (1) whose rate constant.
Vmax is the maximal reaction rate or velocity
of an enzymatically catalyzed reaction when the enzyme is saturated with its
substrate.
Km is
the substrate concentration at which half of the maximum velocity is achieved.
Factors Affecting Enzyme Action
Activity
of enzymes is markedly affected by several factors such as temperature, pH,
conc. of other substances, presence of activators or inhibitors, etc.
1.
Effect of Temperature
Each
enzyme is most active at a specific temperature which is called its optimum
temperature. Temperature increases the total energy of the chemical system
with the result the activation energy is increased.
The exact ratio by which the velocity changes of 10oC temperature rise is the Q10 or temperature coefficient. Reactions velocity almost doubles with 10oC rise (Q10 = 2) in many enzymes.
Activity of enzyme progressively decreases when the temperature of reaction is
below or above the optimum temperature. However, increase in temperature also
causes denaturation of enzyme.
The shape of the curve is bell-shape. Most of the enzymes of human system have an optimum temperature within the range of 35–40oC.
Thus,
the optimum temperature is that temperature at which the activity of the enzyme
is maximum.
2.
Effect of pH
The rate of enzymatic reaction also depends on pH of the medium.
The enzymatic
activity is maximum at a particular pH which is called its optimum pH and graph
is very similar to temperature graph.
3.
Effect of Enzyme
Concentration In the beginning velocity of the enzymatic reaction is directly proportional to the enzyme concentration.
When the substrate concentrations in large excess exceeding
that of Vmax, because enzyme is the limiting factor in the enzyme-substrate
reaction and providing more enzyme molecules enables the conversion of
progressively larger numbers of substrate molecules
4.
Effect of Product Concentration
Products
formed as a result of enzymatic reaction may accumulate and this excess of
product may lower the enzymatic reaction by occupying the active site of the
enzyme.
5.
Effect of Substrate Concentration
As
already described a known quantity of enzyme, the reaction is directly
proportional to the substrate concentration.
6.
Effect of Activators and Coenzymes
The
activity of certain enzymes is greatly dependent of metal ion activators and
coenzymes. The role of metal ions and coenzymes is already discussed.
7.
Effect of Modulators and Inhibitors
Whenever the active site is not available for the binding of the substrate the enzyme activity may be reduced. The substances which stop or modify the enzymatic reaction are called inhibitors or modulators.
Presence of these substances in
reaction medium can adversely affect the rate of enzymatic reaction.
8.
Effect of Time
The
time required for completion of an enzyme reaction increases with decreases in
temperature from its optimum. However, under the optimum conditions of pH and
temperature time required for enzymatic reaction is less.
Enzyme Inhibition And Effect On Reaction Kinetics
Enzymes are protein and they can be inactivated by the agents that denature them. The chemical substances which inactivate the enzymes are called as inhibitors and the process is called as enzyme inhibition.
Inhibitors are sometimes
referred to as negative modifier. They may be small inorganic ions, or organic
substances. Enzyme inhibition is classified under three major groups:
•
Competitive inhibition (Reversible).
•
Non-competitive inhibition (Irreversible or reversible).
•
Allosteric inhibition.
1. Competitive Inhibition
When the active site or catalytic site of an enzyme is occupied by a substance other than the substrate of that enzyme, its activity is inhibited.
The type of
inhibition of this kind is known as competitive inhibition. This is a type of reversible
inhibition.
Example
Lactate Dehydrogenase is competitive Inhibitor of Oxamate for Substrate Lactate.
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