When an enzymatic activity is reduced or stopped by the effect of some chemicals others than the substrate molecules, the act is called enzyme inhibition. The chemicals or substances which bring about inhibition are called enzyme inhibitors.
Enzyme inhibition is of two types:
1. Competitive inhibition
2. Non-competitive inhibition
Enzyme inhibitors: Some chemical substances inhibit enzyme activity. These substances are called enzyme inhibitors and are of two types.
1. Competitive inhibitors:
These substances have structure closely similar to the structure of the substrate molecules. Hence these substances compete with the substrate in binding with the active site of an enzyme. This results in a decrease in enzyme activity.
Example: Malonic acid is a competitive inhibitor of succinic acid.
The effect of this kind of inhibition can be overcome by increasing the concentration of substrate.
2. Non-competitive inhibitors:
Some substances do not show resemblance in structure with substrate molecule but attach with the active site of the enzyme permanently and bring about structural changes in their configuration.
These inhibitors permanently block or destroy the active site and thus act as a poison. These are called noncompetitive inhibitors or enzyme poisons.
Example: Pb++ , Hg++
Note: Cyanide causes denaturation of cytochrome oxidase enzyme which is used in respiration reactions and thus acts as a poison. The effect of non-competitive inhibitors can be overcome by increasing the concentration of the enzyme.