Cambridge International A Level Biology Revision Guide

Chapter 3: Enzymes
Mammals such as humans also use this method
of speeding up their metabolic reactions. Our body
temperature is maintained at 37 °C, which is usually
much warmer than the temperature of the air around
us. But even raising the temperature of cells to 37 °C is
not enough to give most substrates the activation energy
which they need to change into products. Enzymes avoid
this problem because they decrease the activation energy
of the reaction which they catalyse (Figure 3.5b). They
do this by holding the substrate or substrates in such a
way that their molecules can react more easily. Reactions
catalysed by enzymes will take place rapidly at a much
lower temperature than they otherwise would.
The course of a reaction
You may be able to carry out an investigation into the rate
at which substrate is converted into product during an
enzyme-controlled reaction. Figure 3.6 shows the results
of such an investigation using the enzyme catalase. This
enzyme is found in the tissues of most living things and
catalyses the breakdown of hydrogen peroxide into water
and oxygen. (Hydrogen peroxide is a toxic product of
several different metabolic reactions, and so it must be
got rid of quickly.) It is an easy reaction to follow, as the
oxygen that is released can be collected and measured.
The reaction begins very swiftly. As soon as the enzyme
and substrate are mixed, bubbles of oxygen are released
quickly. A large volume of oxygen is collected in the first
minute of the reaction. As the reaction continues, however,
the rate at which oxygen is released gradually slows down.
The reaction gets slower and slower, until it eventually
stops completely.
Total volume O 2 collected / cm 3
9
8
7
6
5
4
3
2
1
0
0 30 60 90 120 150 180 210 240 270 300 330 360 390
Time / s
Figure 3.6 The course of an enzyme-catalysed reaction.
Catalase was added to hydrogen peroxide at time 0. The gas
released was collected in a gas syringe, the volume being read at
30 s intervals.
The explanation for the course of the reaction is quite
straightforward. When the enzyme and substrate are first
mixed, there are a large number of substrate molecules.
At any moment, virtually every enzyme molecule has
a substrate molecule in its active site. The rate at which
the reaction occurs depends only on how many enzyme
molecules there are and the speed at which the enzyme
can convert the substrate into product, release it, and
then bind with another substrate molecule. However, as
more and more substrate is converted into product, there
are fewer and fewer substrate molecules to bind with
enzymes. Enzyme molecules may be ‘waiting’ for substrate
molecules to hit their active sites. As fewer substrate
molecules are left, the reaction gets slower and slower,
until it eventually stops.
The curve of a graph such as the one in Figure 3.6 is
therefore steepest at the beginning of the reaction: the
rate of an enzyme-controlled reaction is always fastest
at the beginning. This rate is called the initial rate of
reaction. You can measure the initial rate of the reaction
by calculating the slope of a tangent to the curve, as close
to time 0 as possible (see Figure P1.15, page 260, for advice
on how to do this). An easier way of doing this is simply to
read off the graph the amount of oxygen given off in the
first 30 seconds. In this case, the rate of oxygen production
in the first 30 seconds is 2.7 cm 3 of oxygen per 30 seconds,
or 5.4 cm 3 per minute.
Factors that affect enzyme
action
The effect of enzyme concentration
Figure 3.7a shows the results of an investigation in which
different concentrations of catalase solution (from celery
extract) were added to the same volumes of hydrogen
peroxide solution. Concentration was varied by varying
the initial volume of extract and then making up to a
standard volume. You can see that the shape of all five
curves is similar. In each case, the reaction begins very
quickly (steep curve) and then gradually slows down
(curve levels off). Because the quantity of hydrogen
peroxide is the same in all five reactions, the total amount
of oxygen eventually produced will be the same; so, if the
investigation goes on long enough, all the curves will meet.
To compare the rates of these five reactions, in order
to look at the effect of enzyme concentration on reaction
rate, it is fairest to look at the rate right at the beginning
of the reaction. This is because, once the reaction is under
way, the amount of substrate in each reaction begins
to vary, because substrate is converted to product at
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