Factors Affecting Enzyme
Activity
Learning Outcomes
• apply knowledge of proteins to explain the effects on
enzyme activity of:
– pH
– temperature
– substrate concentration
– enzyme concentration
– competitive inhibitors, and non-competitive
inhibitors including heavy metals
• differentiate between the roles of enzymes and
coenzymes in biochemical reactions
• identify the role of vitamins as coenzymes
Factors affecting enzyme
activity
•
•
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•
Temperature
pH
Concentration
Activation
Inhibitors
• Co-enzymes
• What happens to the rate of most
chemical reactions as the temperature
is increased?
• Why?
Temperature
• All enzymes have an optimal temperature
• Up to this temperature, the reaction rate
increases as temperature increases
• Above this temperature, denaturation of
the enzyme occurs (irreversible)
• What would you expect to be the
optimal temperature of enzymes that
function in the human body?
Effect of temperature on
enzyme activity
Enzyme
activity
Optimal
temperature
Temperature
• How might a strong acid or base affect
a protein?
pH
• All enzymes have an optimal pH
• High or low pH causes reduced enzyme
activity and denaturation of the enzyme
Effect of pH on enzyme
activity
Enzyme
activity
Optimal pH
pH
Fig. 6.8
• What happens if we increase or
decrease the concentration of
substrate?
Substrate concentration
• Increasing the substrate concentration
increases the reaction rate
• only to the point where all enzymes are
being used
Effect of substrate
concentration
Enzyme
activity
Substrate concentration
• What if we add more enzyme?
• How would this happen in the body?
Enzyme concentration
• In cells, enzyme concentration is
genetically controlled (control of protein
synthesis)
• Increasing the amount of enzyme will
increase the reaction rate (as long as
substrate is present)
Effect of enzyme
concentration
Enzyme
activity
Enzyme concentration
Activation
• Enzymes may be “turned on” by the
presence of another molecule (ex. The
addition of a phosphate group, known
as phosphorylation)
Enzyme activation
• How could you prevent a substrate from
binding to an enzyme active site?
• Would this ever be a desirable
outcome?
Inhibition
• Inhibitors are molecules other than the
substrate that bind to the enzyme and
inhibit its activity
Competitive inhibition
• Competitive
inhibitors
compete with the
substrate by
binding to the
active site
Non-competitive inhibition
• Non-competitive
inhibitors bind to a
different site on the
enzyme (allosteric
site)
• Change the shape
of the enzyme’s
active site
• Toxic heavy metals such as lead,
cadmium and mercury can bind to
enzymes causing denaturation
• Act as non-competitive inhibitors
Feedback inhibition
• Sometimes the product of the pathway
acts as an inhibitor
• This stops the pathway when there is
enough of the product
Fig. 6.9a
Fig. 6.9b
Enzyme cofactors
• Cofactors or coenzymes are non-protein
molecules that assist the enzyme
• May be inorganic ions ex. Copper, zinc,
or iron
• Often are organic molecules derived
from vitamins
• Function is often to transfer electrons or
functional groups (ex. Phosphate) to the
substrate
• Vitamin deficiency
results in
interference with
enzyme activity in
various metabolic
pathways
(so eat your
veggies!)