With enzyme-catalysed reactions, although the rate at which the reaction comes to equilibrium increases with temperature, there is a second effect of temperature — denaturation of the enzyme protein, leading to irreversible loss of activity (section 4.4.2. As the temperature increases, so the movement of parts of the protein molecules relative to each other increases, leading eventually to disruption of the hydrogen bonds that maintain the folded structure of the protein. When this happens, the protein chain unfolds and the active site is lost. As the temperature increases further, so the denatured protein becomes insoluble, and precipitates out of solution.
Figure 2.6 The effect of pH on enzyme activity. Enzyme A has apH optimum of 3.5, enzyme B apH optimum of 9.0.
As shown in Figure 2.7, temperature thus has two opposing effects on enzyme activity. At relatively low temperatures (up to about 50—55 °C), increasing temperature results in an increase in the rate of reaction. However, as the temperature increases further, so denaturation of the enzyme protein becomes increasingly important, resulting in a rapid fall in activity at higher temperatures. The rate of increase in the rate of reaction with increasing temperature depends on the activation energy of the reaction being catalysed; the rate of decrease in activity at higher temperatures is a characteristic of the enzyme itself.
The apparent temperature optimum of an enzyme-catalysed reaction depends on the time for which the enzyme is incubated. As shown in Figure 2.7, during a short incubation (e.g. 1 min) there is negligible denaturation, and so the apparent optimum temperature is relatively high, whereas during a longer incubation denaturation is important, and so the apparent optimum temperature is lower.
The effect of temperature is not normally important physiologically, as body temperature is normally maintained close to 37 °C. However, some of the effects of fever (when body temperature may rise to 40 °C) may be due to changes in the rates of enzyme-catalysed reactions. Because different enzymes respond differently to changes in temperature, there can be a considerable loss of the normal integration between different enzymic reactions and metabolic pathways.
1 minute incubation
0 20 40 60 80 100
Figure 2.7 The temperature dependence of enzyme activity. In a short (1 min) incubation the enzyme may have an optimum temperature as high as 90 °C, but in longer incubations this falls, so that in a 10-min incubation the optimum temperature is about 55 °C.
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