Factors Affecting Enzyme Activity (WJEC GCSE Biology: Combined Science)

• Temperature affects the movement of molecules, so influences the number of collisions that occur between enzymes and their substrate molecules in a solution

Increasing temperature

• As temperature increases more collisions occur between enzymes and substrates, increasing the rate of an enzyme-controlled reaction
  
  • This is because the molecules have more kinetic energy so they move faster and collide with more energy
• Enzyme-controlled reactions occur at the highest rate at their optimum temperature
  • In the human body, this optimum temperature for enzymes is around 37 ⁰C

High temperatures

• Heating solutions to high temperatures that are too far above the optimum will break the bonds that hold the enzyme together and the active site will lose its shape
  • This is known as denaturation and the enzyme is said to have denatured
  • Substrates cannot attach to denatured active sites as the complementary shape has been lost
  • Denaturation is irreversible; once enzymes are denatured they cannot regain their shape and activity will stop

Low temperatures

• Reactions occur slowly at low temperatures; this is because molecules move around more slowly and collisions occur less frequently and with less energy
  • Note that enzymes do not denature at low temperatures, so heating up a solution again will increase the rate of reaction

Enzyme denaturation diagram

• The effect of temperature on the rate of an enzyme-controlled reaction can be plotted on a graph as shown

Effect of temperature on enzyme activity graph

The effect of temperature on enzyme activity can be shown on a graph

• The optimum pH for enzyme activity varies between different types of enzymes
  • Enzymes that function in acidic conditions, such as the stomach, have a low optimum pH, e.g. pH2
  • Enzymes that function in alkaline conditions, such as the small intestine, have a high optimum pH, e.g. pH 8 or 9
• If the pH is too far above or too far below the optimum the bonds that hold the amino acid chain together can be disrupted or broken
• This changes the shape of the active site so the substrate can no longer fit into it, reducing the rate of activity
  • This is denaturation

Effect of pH on enzyme activity diagram

Extreme pH can also cause enzymes to denature

• The effect of pH on the rate of an enzyme-controlled reaction can be plotted on a graph as shown below

Effect of pH on enzyme activity graph

Enzymes in the small intestine have an optimum pH of around 8; pH levels that are too far below or above this can result in denaturation