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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
• 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
Temperature
Enzyme activity
Optimal 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
pH
Optimal 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!)