Mechanism and Specificity of Enzyme Action

  Enzymatic reactions include the formation or the destruction of chemical bonds. When two or more substrate molecules are Joined, chemical bonds are formed. When a complex molecule is split into simpler compounds chemical bonds are destroyed.

Both the formation and destruction of bonds generally requires the prior stretching or bending of existing bonds, creating a transition (Intermediate) state. The energy required to acquire transition state is called activation energy. Enzymes acts as catalyst and lowers the requirements for this activation energy. Therefore, the reaction can occur even at normal temperature and pressure.

Enzyme catalyzed reaction requires a lower activation energy as compared to uncatalyzed reaction.

Enzymes act by reacting with substrate through its active site. They are specific in action. Only a specific substrate can bind at the active site of the enzyme to form ES complex, which finally yields product. This can be expressed as under.
E+S ➞ ES ➞ ES* ➞ EP ➞ E + P.

  1. First the substrate binds to the active site of enzyme to form ES complex.
  2. Now the ES complex gets structurally Induced in such a manner that it is able to convert into product and forms EP complex. This involves a chemical change. The reaction energy required for this chemical conversion is lowered dowm by the enzyme.
  3. Now, the product dissociates from the complex and the enzyme molecule is made free. So that It is able to react with another substrate molecule again.

Specificity of enzyme action :

 Enzymes are highly specific in their action. They are specific for the substrate on which they attack and the reaction they catalyze. The basis of this specificity is their active site. The enzyme specificity, therefore, can be divided into two types as under:
1. Substrate specificity
2. Reaction specificity

Substrates specificity

Enzymes are specifie for the substrate on which they attack. This substrate specificity may further be categorized in to different types.
1. Absolute specificity
2. Group specificity
3. Stereo specificity

Absolute specificity

  When the enzyme possesses specificity for the entire substrate molecule, the specificity is called absolute specificity.
e.g. Urease acts on urea to degrade It into CO2 and NH3.

Group specificity

  Enzymes may be specifie for a particular group or chemical bond within the substrate and attack on them. This kind of specificity is called group specificity.
e.g. Transaminase acts only on (NH2) group of substrate and cause Its transfer.

Stereo specificity

Almost all enzymes are able to recognize orientation of groups within the structure of molecule. Therefore, they are able to distinguish structural as well as optical Isomers and attack them specifically. Such specificity is called stereo specificity.
e.g. Alanine recemase. It causes isomerization of alanine and is able to convert L-alanine to D-alanine.

Reaction specificity

  Enzymes are also specific for the reaction they catalyze. One particular enzyme will catalyze one particular type of blochemical reaction only. Such specificity is called reaction specificity. This kind of specificity is given prime importance for enzyme classification by IUB. Accordingly, they are divided Into six classes:

  1. Oxidoreductases
  2. Transferases
  3. Hydrolases
  4. Lyases
  5. Isomerases
  6. Ligases

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