• A chemical substance which can react in place of
substrate with the enzyme but is not transferred
into products and block the active site of the
enzyme temporarily or permanently is called
For example, poisons like
antibodies anti-metabolites and some drugs.
Types of inhibitor
• There are two type of inhibitor:
• Irreversible inhibitor
• Reversible inhibitor
• Irreversible inhibitors occupy or destroy the active
sites of the enzyme permanently and decrease the
They form covalent bond with the active sites and
sometime they physically block the active sites.
Example of irreversible
• Very small concentrations of chemical
reagents such as the heavy metal ions
arsenic (AS) or certain iodine-containing
compounds completely inhibit some
• They form weak linkages with the enzymes.
Their effect is completely or partly
neutralized by increasing the concentration
of the substrate. They are further divided
into two major types:
• and non-competitive
• 1)A competitive inhibitor is any compound which
closely resembles the chemical structure and
molecular geometry of the substrate.
2)The inhibitor competes for the same active site
as the substrate molecule.
• 3) a competitive inhibition is usually reversible if
sufficient substrate molecules are available to
ultimately displace the inhibitor.
• 4) Therefore, the amount of enzyme inhibition
depends upon the inhibitor concentration,
substrate concentration, and the relative affinities
of the inhibitor and substrate for the active site.
• 1) A non-competitive inhibitor is a
substance that interacts with the
enzyme, but usually not at the active site.
2) The non-competitive inhibitor reacts
either remote from or very close to the
3) The net effect of a non competitive
inhibitor is to change the shape of the
enzyme and thus the active site, so that
the substrate can no longer interact with
the enzyme to give a reaction.
• 4) Non competitive inhibitors are
usually reversible, but are not
influenced by concentrations of the
substrate as is the case for a
reversible competive inhibitor.
1) Cyanide combines with the Iron in the
enzymes cytochrome oxidase.
2) Heavy metals, Ag or Hg, combine with
• Heavy metals like lead, mercury (breaks
disulfide bonds), chromium will act as