Cambridge International A Level Biology Revision Guide

Chapter 3: Enzymes
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 V max
. V stands for velocity.
V max
Initial rate of reaction
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.
Temperature and enzyme activity
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.
Rate of reaction
0
enzyme
completely
denatured
10 20 30 40 50 60
Temperature / °C
Figure 3.9 The effect of temperature on the rate of an
enzyme-controlled reaction.
enzyme
becoming
denatured
optimum
temperature
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