Chapter 3: Enzymes
Temperature and enzyme activity
QUESTION 3.2 Why is it better to calculate the initial rate of reaction
from a curve such as the one in Figure 3.6, rather than simply measuring how much oxygen is given off in 30 seconds?
The effect of substrate concentration
Figure 3.8 shows the results of an investigation in which the amount of catalase was kept constant and the amount of hydrogen peroxide was varied. Once again, curves of oxygen released against time were plotted for each reaction, and the initial rate of reaction calculated for the first 30 seconds. These initial rates of reaction were then plotted against substrate concentration. As substrate concentration increases, the initial rate of reaction also increases. Again, this is only what we would expect: the more substrate molecules there are around, the more often an enzyme’s active site can bind with one. However, if we go on increasing substrate concentration, keeping the enzyme concentration constant, there comes a point where every enzyme active site is working continuously. If more substrate is added, the enzyme simply cannot work faster; substrate molecules are effectively ‘queuing up’ for an active site to become vacant. The enzyme is working at its maximum possible rate, known as Vmax. V stands for velocity.
Initial rate of reaction
Vmax
Figure 3.9 shows how the rate of a typical enzymecatalysed reaction varies with temperature. At low temperatures, the reaction takes place only very slowly. This is because molecules are moving relatively slowly. Substrate molecules will not often collide with the active site, and so binding between substrate and enzyme is a rare event. As temperature rises, the enzyme and substrate molecules move faster. Collisions happen more frequently, so that substrate molecules enter the active site more often. Moreover, when they do collide, they do so with more energy. This makes it easier for bonds to be formed or broken so that the reaction can occur. As temperature continues to increase, the speed of movement of the substrate and enzyme molecules also continues to increase. However, above a certain temperature, the structure of the enzyme molecule vibrates so energetically that some of the bonds holding the enzyme molecule in its precise shape begin to break. This is especially true of hydrogen bonds. The enzyme molecule begins to lose its shape and activity, and is said to be denatured. This is often irreversible. At first, the substrate molecule fits less well into the active site of the enzyme, so the rate of the reaction begins to slow down. Eventually the substrate no longer fits at all, or can no longer be held in the correct position for the reaction to occur. The temperature at which an enzyme catalyses a reaction at the maximum rate is called the optimum temperature. Most human enzymes have an optimum temperature of around 40 °C. By keeping our body temperatures at about 37 °C, we ensure that enzymecatalysed reactions occur at close to their maximum rate.
Substrate concentration
Figure 3.8 The effect of substrate concentration on the rate of an enzyme-catalysed reaction.
QUESTION 3.3 Sketch the shape that the graph in Figure 3.7b would
have if excess hydrogen peroxide were not available.
Rate of reaction
enzyme becoming denatured
optimum temperature
0
10
20
30 40 50 Temperature / °C
60
enzyme completely denatured
Figure 3.9 The effect of temperature on the rate of an enzyme-controlled reaction.
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