Enzyme activity
Vary temperature and pH and watch the active site distort as the enzyme denatures — and why cooling it down again does not help.
- Lock-and-key model
- Optimum temperature and pH
- Denaturation
Relative rate
100%
Optimum pH
7
Enzyme state
active
Predict
An enzyme is heated to 70 °C and then cooled back to 37 °C. What happens to the rate?
Experiment
- 1Set pH to the optimum, then raise the temperature slowly from 0 °C and watch the rate rise.
- 2Note the temperature at which the rate peaks.
- 3Keep going past 50 °C and watch the active site distort as the rate falls away.
- 4Switch to pepsin and find its optimum pH — then explain why it differs from amylase.
Explain
Below the optimum, raising the temperature gives the molecules more kinetic energy, so enzyme and substrate collide more often and the rate rises. That is an ordinary rate effect and it is fully reversible.
Above the optimum, something different happens. The bonds holding the enzyme in its three-dimensional shape break, the active site changes shape, and the substrate no longer fits. The enzyme is denatured, and cooling it down does not restore it.
pH works the same way: far from the optimum the active site is distorted and no enzyme–substrate complexes form. Pepsin's optimum of pH 2 matches the stomach it works in — enzymes are adapted to where they are found.