Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
62
product binds to another part of the enzyme and prevents
more substrate binding. However, the end-product can
lose its attachment to the enzyme and go on to be used
elsewhere, allowing the enzyme to reform into its active
state. As product levels fall, the enzyme is able to top them
up again. This is termed end-product inhibition.
Comparing enzyme affinities
There is enormous variation in the speed at which different
enzymes work. A typical enzyme molecule can convert
around one thousand substrate molecules into product per
second. This is known as the turnover rate. The enzyme
carbonic anhydrase (Chapter 12) is one of the fastest
enzymes known. It can remove 600 000 molecules of
carbon dioxide from respiring tissue per second, roughly
10 7 times as fast as the reaction would occur in the absence
of the enzyme. It has presumably evolved such efficiency
because a build-up of carbon dioxide in tissues would
quickly become lethal. Speeds such as these are only
possible because molecules within cells move about very
quickly by diffusion over short distances, with tens or
hundreds of thousands of collisions per second occurring
between enzyme and substrate molecules.
Simple measurements of the rate of activity of enzymes
can be carried out. See for example, Box 3.1, Figures
3.6 and 3.7 and Questions 3.4 and 3.6. More precise
measurements of the rate at which enzymes work are
difficult and complex to make, but are important for
our understanding of how enzymes work together to
control cell metabolism. One of the key steps towards
understanding how well an enzyme performs is to
measure the theoretical maximum rate (velocity), V max
,
of the reaction it catalyses. At V max
all the enzyme molecules
are bound to substrate molecules – the enzyme is saturated
with substrate. The principle of how V max
is measured is
described on page 58. To summarise, the reaction rate
is measured at different substrate concentrations while
keeping the enzyme concentration constant. As substrate
concentration is increased, reaction rate rises until the
reaction reaches its maximum rate, V max
.
QUESTION
3.7 For each substrate concentration tested, the rate
should be measured as soon as possible. Explain why.
The initial rate for each substrate concentration is
plotted against substrate concentration, producing a
curve like that shown in Figure 3.8. This type of curve
is described as asymptotic and such curves have certain
mathematical properties. In particular, the curve never
completely flattens out in practice. In theory, it does so
at infinite substrate concentration, but this is obviously
impossible to measure. This makes it impossible to
accurately read off the value for V max
from the graph.
There is, however, a way round this problem. Instead of
plotting substrate concentration, [S], on the x-axis and
velocity (rate) on the y-axis, we can plot 1/[S] (the inverse
of substrate concentration) and 1/velocity (the inverse
of velocity) respectively. Such a plot is called a doublereciprocal
plot. (Remember, the word ‘reciprocal’ means
‘inverse’.) One advantage of doing this is that while it is
impossible to plot infinite substrate concentration,
1/infinity is zero, which can be plotted, so V max
can be
found accurately. Also, the resulting graph is a straight line.
It is easier to understand this if you use some specimen
QUESTION
3.8 a Copy the table and complete it by calculating the remaining values for
1/[S] and 1/v to one decimal place.
[S]
/ arbitrary units
1/[S]
/ arbitrary units
v
/ arbitrary units
0.02 50.0 0.025 40.0
0.04 0.041
0.06 0.052
0.08 0.061
0.10 0.067
0.20 0.085
1/v
/ arbitrary units
Table 3.1 Some results from an experiment to determine the effect of substrate
concentration on the velocity of an enzyme reaction. [S] = substrate concentration,
v = velocity (rate of reaction)
b
c
Draw two graphs, one with [S] on
the x-axis and v on the y-axis, and
a second with 1/[S] on the x-axis
and 1/v on the y-axis. (The second
graph is a double-reciprocal plot.)
Compare the two graphs. Note
that the double-reciprocal plot is
a straight line.
After reading the rest of this
section, calculate the values for
V max
and K m
using the data from
your double-reciprocal plot.