All enzymes are macromolecules and have numerous specific areas called active sites on their surface.
During the enzymatic activity, substrate molecules attach at these active sites and an unstable enzyme-substrate complex is formed.
This enzyme-substrate complex dissociates to set free the product and the enzyme.
The mechanism of formation of the enzyme-substrate complex has been explained by two theories.
1. Lock and key hypothesis
2. Induced fit model theory
Formation of enzyme-substrate complex results in to lowering of the activation energy and hence the rate of reaction is accelerated.
Mode of Enzyme Action:
The mode of enzyme action depends upon the nature of the enzyme and the substrate molecule, and it can be understood by the following:
(1) Formation of Enzyme Substrate complex (ESC):
(2) Lowering of Activation energy.
(1) Formation of Enzyme Substrate Complex (ESC):
- In all enzyme governed reactions the enzyme and the substrate combine to form an unstable complex called enzyme-substrate complex. (ESC).
- This complex dissociates into the product and the enzyme.
- Enzyme + Substrate → Enzyme substrate complex.
- Enzyme substrate complex → Enzyme + Product(s).
- On the surface of each enzyme, several specific areas are found. These are called active sites.
- The substrate molecules combine with the enzyme at these sites.
- When the substrate molecules combine at the active sites, they can easily react with each other forming the product.
- The product formed by formation of new bonds dissociates from the enzyme.
- The enzyme becomes free to bind with other molecules of the substrate and new enzyme-substrate molecule complex is formed.
The mechanism of formation of the enzyme-substrate complex and the property of specificity of the enzyme has been explained by the following theories.
1. Lock and Key Theory:
- This theory was postulated by Emil Fisher (1898).
- According to this theory, like a lock can be operated by it’s key only, similarly, a specific substrate having a specific structure only can bind with the specific active site present on the surface of a specific enzyme.
- The enzyme remains unchanged after the product is released.
2. Induced Fit Model Theory:
- This theory has been proposed by Daniel Koshland (1966).
- According to this theory, the shape (structure) of active sites found on the surface of enzymes is not rigid but flexible.
- These active sites are initially not complementary to the substrate molecule, but as a specific substrate molecule comes in close proximity of the enzyme, it induces a change in the organisation of the active site and binds with it.
- This theory stipulates that there are two operative groups in the active sites one helps in binding of the substrate molecule and the other acts to convert the substrate into the product.
