Factors Affecting Enzyme Activity (SQA National 5 Biology): Revision Note

Enzymes work fastest at their optimum temperature

• The optimum temperature for enzymes that function inside the human body is around 37 °C

Heating enzymes to high temperatures will break the bonds that hold the enzyme together, and the shape of the active site changes

• The enzyme is said to have denatured

Substrates cannot fit into the active site of denatured enzymes, so the reaction will stop

Denaturation is irreversible; once enzymes are denatured, they cannot regain their shape

The rate of enzyme-controlled reactions increases with temperature until the optimum is reached

Above the optimum, high temperatures denature the enzyme, so the rate drops sharply to zero

Enzyme function is affected by the pH of the surrounding environment, e.g.:

• enzymes that function inside cells work best at or near neutral pH (around pH7), because the cytoplasm’s conditions are close to neutral

• enzymes that function in acidic conditions, such as the stomach, have a lower optimum pH

• enzymes that function in alkaline conditions, such as the small intestine, have a higher optimum pH

If the pH is too high or too low, the bonds that hold the enzyme together can be broken

This changes the shape of the active site, so the substrate can no longer bind to it, reducing the rate of activity

The rate of enzyme-controlled reactions increases as the pH approaches its optimum, and is at its highest at the optimum pH

Above the optimum, extreme pH denatures the enzyme, so the rate drops sharply to zero