Enzyme action

Chemical reactions take place throughout living organisms. Many of these metabolic reactions would be too slow to sustain life without enzymes. Enzymes are proteins that increase the rates of these reactions.

An enzyme is a biological catalyst: it speeds up a reaction without being changed by it. This means the same enzyme can be used again.

How an enzyme acts on its substrate

The substance an enzyme acts on is called its substrate. The enzyme has a region called the active site, whose shape is complementary to the substrate. This allows the substrate to bind to the active site.

The enzyme catalyses the reaction, forming products from the substrate. The products are then released from the active site. The enzyme remains unchanged and can bind another substrate molecule and catalyse the reaction again.

Effect of temperature on enzyme activity

Temperature affects how quickly an enzyme-controlled reaction happens. As temperature rises, the enzyme and substrate particles gain kinetic energy and move faster. They collide more often, so more enzyme–substrate complexes form per second and the reaction rate generally increases.

The temperature at which the enzyme works fastest is its optimum temperature. Above this temperature, the reaction rate falls because heat changes the shape of the enzyme, including its active site. The substrate no longer fits the active site effectively, so fewer enzyme–substrate complexes form.

This loss of shape and function is called denaturation. At low temperatures, an enzyme works slowly but is not usually denatured; its activity can increase again when it is warmed.

Investigating temperature and enzyme activity

The effect of temperature can be investigated by measuring the rate of an enzyme-controlled reaction at a range of temperatures. Water baths can be used to keep the reaction mixtures at the chosen temperatures. The rate can be measured by recording how much product forms, or how much substrate disappears, in a fixed time.

Keep other conditions constant, including pH, the amounts and concentrations of enzyme and substrate, and the time allowed for the reaction. This makes temperature the factor being changed. Compare the rates to identify the temperature that gives the highest measured activity.

Effect of pH on enzyme activity

An enzyme works most effectively at a particular pH, called its optimum pH. At this pH, it catalyses its reaction at the highest rate. Moving to a higher or lower pH reduces the enzyme’s activity. Different enzymes can have different optimum pH values.

An enzyme’s active site is the region where its substrate binds. Changes in pH can alter the shape of the active site, making it harder for the substrate to bind. Large changes in pH can cause the enzyme to lose its functional shape; the enzyme is then denatured. If the substrate no longer binds effectively, the enzyme cannot catalyse the reaction effectively.

Investigating pH and enzyme activity

The effect of pH can be investigated by measuring the rate of an enzyme-controlled reaction at several pH values. For each test, use the same amounts and concentrations of enzyme and substrate, and keep the temperature constant. A buffer solution can be used to maintain the chosen pH.

Measure the rate in the same way each time, for example by recording how much product forms in a set time. Comparing the rates shows how activity changes with pH. The pH that gives the highest measured rate is the enzyme’s optimum pH under the conditions tested.

Effective enzyme–substrate collisions

A substrate molecule must meet an enzyme at its active site and bind to it for the enzyme-catalysed reaction to occur. A collision that leads to binding is an effective collision. Binding is possible because the substrate and active site have complementary shapes.

When the substrate binds, it forms an enzyme–substrate complex. The enzyme converts the bound substrate into product, which is then released from the enzyme.

Temperature and effective collisions

Increasing temperature gives enzyme and substrate molecules more kinetic energy. They move faster and collide more frequently. While the active site retains its functional shape, the increased collision frequency leads to more effective collisions per second, increasing the reaction rate.

At temperatures that denature the enzyme, the active site changes shape. The substrate is then less able to bind, so fewer collisions are effective. Although molecules are still moving faster, the increased collision frequency does not compensate for the reduced ability to bind, and the reaction rate falls.