2. Inhibitors
Chemicals that are able to modify the
activity of an enzyme by binding to it.
If the inhibitor attaches to the enzyme by
covalent bonds, inhibition is irreversible
and usually involves the destruction or
permanent modification of the enzyme
structure.
If it attaches by weak interactions, the
3. Competitive inhibitors
Chemicals that bind reversibly at the active
site of the enzyme and they compete with the
substrate for the binding position at the
active site.
Chemical structure similar to that of the
substrate. Block active site and make it
unavailable to the substrate.
Increasing the substrate concentration
decreases the effect of competitive inhibition
4. Effect on the rate of an enzyme-
catalyzed reaction
Vmax remains the same as
there is still a substrate
concentration where full
activity of the enzyme can
be achieved.
But as it takes a higher
substrate concentration to
reach this rate, Km is
increased.
5. Example :
The conversion that succinic
dehydrogenase carries out is:
Succinate Fumarate
The reaction is inhibited by malonate ions
which have a very similar shape to
succinate ions.
6. Non-competitive inhibition
Chemicals that bind reversibly away
from the active site of the enzyme
Binding causes conformational change
in the protein structure that alters the
active site, inhibiting the ability to bind to
the substrate.
Increasing the substrate concentration
has no effect as the enzyme’s shape still
7. Effect on the rate of an enzyme-
catalyzed reaction
Vmax is decreased and cannot be restored no
matter how high the substrate concentration.
The value of Km is unchanged because the
unhibited enzymes are perfectly functional.
8. Examples :
Silver ions (heavy metal) react with -SH groups in
the side groups of cysteine residues in the protein
chain:
If the cysteine residue is somewhere on the protein
chain which affects the way it folds into its tertiary
structure, then altering this group could have an
effect on the shape of the active site, and so stop the
enzyme from working.
9. B 7.7
State and explain the effects of heavy-
metal ions, temperature changes and
pH changes on enzyme activity
10. Effects of heavy-metal ions
Such as lead, copper, mercury and silver are
poisonous, due to their effects on enzymes.
When a heavy-metal ion is present at the
active site, substitution of a different metal
ion for the original ion can cause the enzyme
to malfunction and lose its activity.
This is particularly evident where heavy
metal ions can bind or chelate to the S-H
,sulfhydryl groups in proteins to form a
covalent bond with the sulfur atom and
11. Effect of temperature
Increasing the temperature will initially increase
the rate of enzyme-catalyzed reactions, as more
reactants will possess the minimum activation
energy.
The optimum temperature for most enzyme is
about 40°C. Above this temperature enzymes
rapidly become denatured as the weak bonds
holding the tertiary structure together break.
(Denaturation is the loss of the tertiary structure)
Lowering the temperature usually causes
deactivation of an enzyme. This prevents the
12. Effect of pH changes
At low or high pH values, the enzyme is
irreversibly denatured. Each enzyme works
optimally at a particular pH.
At different pH values the charges on the amino
acid residues change affecting the bonds
between them, and so altering the tertiary
structure and making the enzyme ineffective.
It is considered as a way of controlling their
activity as different enzymes has different
optimum pH.
Eg. pepsin is active in the stomach where pH is
