Rate: pH (CIE AS Biology)

• All enzymes have an optimum pH which is a pH at which they operate best
• Enzymes are denatured at extremes of pH
  • Hydrogen and ionic bonds hold the tertiary structure of the protein (i.e. the enzyme) together
  • Below and above the optimum pH of an enzyme, solutions with an excess of H⁺ ions (acidic solutions) and OH^- ions (alkaline solutions) can cause these bonds to break
  • This alters the shape of the active site, which means enzyme-substrate complexes form less easily
  • Eventually, enzyme-substrate complexes can no longer form at all
  • At this point, complete denaturation of the enzyme has occurred
• Where an enzyme functions, can be an indicator of its optimal environment:
  • E.g. pepsin is found in the stomach, an acidic environment at pH 2 (due to the presence of hydrochloric acid in the stomach’s gastric juice)
  • Pepsin’s optimum pH, not surprisingly, is pH 2

Enzyme pH Diagram

The effect of pH on the rate of an enzyme-catalysed reaction for three different enzymes (each with a different optimum pH)

• When investigating the effect of pH on the rate of an enzyme-catalysed reaction, you can use buffer solutions to measure the rate of reaction at different pH values:
  • Buffer solutions each have a specific pH
  • Buffer solutions maintain this specific pH, even if the reaction taking place would otherwise cause the pH of the reaction mixture to change
  • A measured volume of the buffer solution is added to the reaction mixture
  • This same volume (of each buffer solution being used) should be added for each pH value that is being investigated