3.5 Effect of temperature on enzyme activity

•Rate of reaction refers to how quickly a reaction occurs.

•Defined as the change in concentration of reactants or products per unit time.

Rate of reaction is determined by how quickly reactants are used up or how fast products are formed.

Case Study

Hydrogen peroxide decomposes to form water and oxygen. In this exercise, rate of reaction is monitored using the number of oxygen bubbles formed per unit time.

At low temperature

Above Figure 3.5.1 - At 0 deg celsius, number of oxygen bubbles produced per unit time is very low. If the rate of reaction is measured using number of bubbles formed per unit time, at 0 deg celsius, the rate of reaction is very low (Refer to Point A on Figure 3.5.2).

Figure 3.5.2 At Point A where temperature = 0 deg celsius, the rate of reaction is very low (ie very few bubbles are formed per unit time) because the enzymes are inactive. The enzymes and substrate have very low kinetic energy leading to few enzyme-substrate complexes formed per unit time. Thus, rate of reaction is very slow.

Figure 3.5.3 At 10 deg celsius, the number of oxygen bubbles formed per unit time increases. This is because the kinetic energy of enzymes and substrates increase leading to more collisions. Number of enzyme-substrate complexes formed per unit time increases leading to an increased rate of reaction. This is seen in the graph sloping upwards from Point A to Point B (Figure 3.5.4).

Figure 3.5.4 Increasing rate of reaction between Points A to B when temperature increases.

Figure 3.5.7 As the temperature increases beyond the optimum, the number of oxygen bubbles formed per unit time decreases as the enzymes start to denature. This can be shown through the decrease in the rate of reaction from Point C to Point D in Figure 3.5.8 below.

Figure 3.5.8 Decreasing rate of reaction from Points C to D as enzymes start to denature.

The reason for the decrease in the rate of reaction is due to the enzymes beginning to denature. ie active sites of enzymes start to be destroyed and there are fewer intact active sites of enzymes to bind to substrates. Refer to Figure 3.5.9 for information on denaturation of enzymes.

Figure 3.5.9 When the enzyme is denatured, the active site of the enzyme is distorted. The substrate cannot bind to the active site. Products cannot be formed.

At extremely high temperature, all the active sites of enzymes are completely denatured. Rate of reaction becomes zero as shown in the complete absence of oxygen bubbles in Figure 3.5.10. This relates to Point E on Figure 3.5.11.

Figure 3.5.10 No oxygen bubbles are released when liver is placed in hydrogen peroxide solution at a temperature of 100 deg celsius. Enzymes in the liver are completely denatured.

Figure 3.5.11 At Point E, rate of reaction at extremely high temperature becomes zero. All the enzymes are completely denatured.