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Introduction

Class-XII/MEDICAL /Botany /Biomolecules /Enzyme /Introduction

There are certain substances present in this cell inhibit the activity of enzyme, such substances are called enzyme inhibitors and the phenomenon is enzyme inhibition. It is of following types–

1. Competitive Inhibition (Reversible Inhibition)

  • A competitive inhibitor resembles the substrate and bind to the active site of the enzyme.

  • The substrate is then prevented from binding to same active site.


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  • The activity of succinate dehydrogenase is inhibited by malonate.

  • Enzyme inhibition caused by a substances resembling substrate molecule is competitive inhibition.

  • Blocking enzyme action through blocking its active site is competitive inhibition.

  • Competitive inhibition is due to substrate analogue.

  • Sulpha drugs are competitive inhibitor of folic acid synthetase in bacteria.

2. Non competitive (Irreversible Inhibition)

  • There are certain inhibitors which are not similar in their molecular structure with substrate molecules. These substances do not compete with substrate molecules for attachment on active site but attaches some where as on the enzyme inhibiting the activity of enzyme.


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  • This is shown by Heavy Metals and poison.

  • Poison : Cyanide kills an animal by inhibiting cytochrome oxidase, cytochrome oxidase binds with and change in the active sites, and enzymes are deactivated.

  • Heavy Metals : and They attach with sulphohydril (–SH) of enzyme and inactive the enzyme.


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3. Feedback inhibition or Allosteric Inhibition

  • Allosteric enzymes discovered by Jacob and Monad.

  • This enzyme has three receptor sites-

    1. Active site : This site attach with substrate.

    2. Activator site : When enzyme attach with activator than many favourable changes occurs in active site of enzyme and active site attach to substrate and product is formed. This process is called allosteric activation.

    3. Inhibitor site : When inhibitor attach to enzyme then unfavourable changes occurs in active site of enzyme and enzyme not attach to substrate so product is not formed. This process is called allosteric inhibition.



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