AQA A Level Biology复习笔记1.4.8 Limiting Factors Affecting Enzymes: pH

Rate: pH


  • All enzymes have an optimum pH or 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 (ie. 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:
    • Eg. 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



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