2.3 Factors affecting enzyme activity

Key terms

Temperature

• As the temperature rises, enzyme and substrate molecules have more kinetic energy. They move faster and collide more often, so more enzyme–substrate complexes form and the rate increases.
• The rate is fastest at the optimum temperature. For most human enzymes this is about 37 °C, body temperature.
• Above the optimum, the bonds holding the enzyme's shape break. The active site changes shape and the enzyme is denatured, so the rate falls quickly to zero.

pH

Each enzyme has an optimum pH. A pH far from it changes the shape of the active site, so the substrate no longer fits, and the enzyme is denatured.

Substrate and enzyme concentration

• Substrate concentration: more substrate means more collisions with active sites, so the rate increases. Eventually every active site is occupied all the time. The rate levels off, because the enzyme is working as fast as it can. Adding more enzyme would now speed it up.
• Enzyme concentration: more enzyme means more active sites, so the rate increases, as long as there is plenty of substrate.

Calculating the rate

The rate is the amount of product made (or substrate used) divided by the time taken.

For example, if an enzyme makes 24 cm³ of oxygen in 2 minutes, the rate is 24 ÷ 2 = 12 cm³ per minute. When you only know the time taken for a reaction to finish, you can use 1000 ÷ time as a measure of rate (in s^-1). From a graph, the rate at any moment is the gradient of the curve: draw a tangent and divide the change in y by the change in x.