Temperature
As temperature rises, molecules move faster and collide more often, so the rate of an enzyme reaction increases up to the optimum temperature (about 37 °C for human enzymes). Above the optimum, the enzyme is denatured: heat changes the shape of the active site permanently, so the substrate no longer fits and the reaction stops.
Key idea
Rate rises to the optimum, then falls sharply as the enzyme is denatured. A denatured active site can no longer bind its substrate. Low temperature only slows an enzyme — it is not denatured and can recover on warming.
pH
Each enzyme has an optimum pH. Very acidic or very alkaline conditions also denature the enzyme by changing the active site shape. For example, pepsin in the stomach works best in strongly acidic conditions (about pH 2), while salivary amylase works best near neutral (pH 7).
Substrate concentration
Increasing substrate concentration increases the rate — until all the active sites are occupied (the enzymes are saturated). After that, adding more substrate makes no difference unless more enzyme is added.
Worked example
A reaction is fastest at 40 °C. At 70 °C almost no product forms. Explanation: 40 °C is near the optimum, giving the greatest activity; at 70 °C the high temperature has denatured the enzyme, so the active site no longer fits the substrate.
Remember
- High temperature and extreme pH denature enzymes (permanent).
- Low temperature only slows enzymes (reversible).