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
• Temperature
• pH
• Concentration
• Inhibitors
• Activation
• Co-enzymes
Temperature
• 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

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
Substrate concentration
• Increasing the substrate concentration increases the reaction rate (Why?)
• only to the point where all enzymes are being used
Effect of substrate concentration
Enzyme activity
Substrate concentration
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
Enzyme activation
Activation
• Enzymes may be “turned on” by the presence of another molecule (ex. The addition of a phosphate group, known as phosphorylation)
Inhibition
• 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
Heavy metals
• 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 nonprotein molecules that assist the enzyme
• May be inorganic ions ex. Copper, zinc, or iron
• Often are organic molecules derived from vitamins

• Vitamin deficiency results in interference with enzyme activity in various metabolic pathways
(so eat your veggies!)