CIE A Level Biology复习笔记3.2.1 Rate: Temperature

• Enzymes have a specific optimum temperature – the temperature at which they catalyse a reaction at the maximum rate
• Lower temperatures either prevent reactions from proceeding or slow them down:
  • Molecules move relatively slow
  • Lower frequency of successful collisions between substrate molecules and active site of enzyme
  • Less frequent enzyme-substrate complex formation
  • Substrate and enzyme collide with less energy, making it less likely for bonds to be formed or broken (stopping the reaction from occurring)
• Higher temperatures speed up reactions:
  • Molecules move more quickly
  • Higher frequency successful collisions between substrate molecules and active site of enzyme
  • More frequent enzyme-substrate complex formation
  • Substrate and enzyme collide with more energy, making it more likely for bonds to be formed or broken (allowing the reaction to occur)
• However, as temperatures continue to increase, the rate at which an enzyme catalyses a reaction drops sharply, as the enzyme begins to denature:
  • Bonds (eg. hydrogen bonds) holding the enzyme molecule in its precise shape start to break
  • This causes the tertiary structure of the protein (ie. the enzyme) to change
  • This permanently damages the active site, preventing the substrate from binding
  • Denaturation has occurred if the substrate can no longer bind
  • Very few human enzymes can function at temperatures above 50°C
    • This is because humans maintain a body temperature of about 37°C, therefore even temperatures exceeding 40°C will cause the denaturation of enzymes
    • High temperatures causes the hydrogen bonds between amino acids to break, changing the conformation of the enzyme

The effect of temperature on the rate of an enzyme-catalysed reaction