Enzymes Note M.N. Chatterjea For Nurses

Afza.Malik GDA

Enzyme Action Model,and Factors Affecting Activity

Enzymes Note M.N. Chatterjea  For Nurses

Models Of Enzyme Substrate Complex Formation,Template or Lock-and-Key Mode,Induced-Fit or Koshland Model,Factors Affecting Enzyme Action,Enzyme Inhibition And Effect On Reaction Kinetics,Competitive Inhibition.

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.


Lactate Dehydrogenase is competitive Inhibitor of Oxamate for Substrate Lactate.

Post a Comment


Give your opinion if have any.

Post a Comment (0)

#buttons=(Ok, Go it!) #days=(20)

Our website uses cookies to enhance your experience. Check Now
Ok, Go it!