What Factors Affect Enzyme Activity?
Learn how temperature, pH, enzyme concentration and substrate concentration affect enzyme activity for AQA A-level Biology, with key exam tips.
Temperature, pH, enzyme concentration and substrate concentration affect enzyme activity by changing collision frequency or the shape and availability of active sites. Enzyme activity is usually measured as the rate of an enzyme-controlled reaction.
Temperature
As temperature increases, enzyme and substrate molecules gain kinetic energy. They move faster, so successful collisions and enzyme–substrate complexes form more frequently. The reaction rate rises to an optimum temperature.
Above the optimum, increased vibration breaks bonds that maintain the enzyme’s tertiary structure. The active site changes shape, so fewer substrates bind. The enzyme is denatured, and the rate falls rapidly.
Low temperature does not normally denature an enzyme: it only reduces kinetic energy and collision frequency.
pH
Each enzyme has an optimum pH. Moving away from this optimum changes the concentration of hydrogen ions. This can disrupt ionic and hydrogen bonds maintaining the enzyme’s tertiary structure, changing the active site’s shape. Fewer enzyme–substrate complexes form. Extreme pH can denature the enzyme.
Enzyme and substrate concentration
Increasing enzyme concentration increases the number of available active sites. If substrate is in excess, the reaction rate increases because more enzyme–substrate complexes can form at the same time.
Increasing substrate concentration initially increases the rate because collisions with active sites occur more frequently. Eventually, all active sites are occupied. Enzyme concentration becomes the limiting factor, so the rate reaches a maximum and levels off.
Worked example
If substrate concentration is doubled while many active sites remain free, the rate will increase. If every active site is already occupied, doubling substrate concentration will have little or no effect. Adding more enzyme would increase the maximum rate, provided enough substrate is available.
Common exam mistake: Do not say that high temperature “kills” an enzyme. State that bonds maintaining its tertiary structure are disrupted, changing the active site so the substrate is no longer complementary.
