Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
Enzyme
Substrate
Maximum turnover K m
number / per second / mol dm 3
carbonic anhydrase carbon dioxide 600 000 8000
penicillinase penicillin 2000 50
chymotrypsin protein 100 5000
lysozyme acetylglucosamine 0.5 6
Table 3.2 Turnover numbers and K m
values for four enzymes. Note that the unit for
K m
is a concentration. (The turnover number per second is the number of molecules
of substrate that one molecule of an enzyme converts to product per second. This is
related to V max
.)
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The significance of V max
and K m
values
Knowing the values of V max
and K m
has a number of
applications.
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It enables scientists to make computerised models of
biochemical pathways or even the behaviour of whole
cells because it helps to predict how each reaction in a
proposed pathway will proceed and therefore how the
enzymes will interact. The consequences of changing
conditions such as temperature, pH or the presence of
inhibitors can be built into the models.
An enzyme’s preference for different substrates can be
compared quantitatively.
By understanding what affects enzyme efficiency,
scientists may in future be able to design better
catalysts, linking this to genetic engineering.
For a commercially important enzyme, the
performance of the same enzyme from different
organisms can be compared.
The calculations involved can be applied to other fields
of biochemistry, such as antibody–antigen binding.
Knowing K m
means the proportion of active sites
occupied by substrate molecules can be calculated for
any substrate concentration.
QUESTION
3.9 Which of the four enzymes in Table 3.2 has the
highest affinity for its substrate? Briefly explain
your answer.
Immobilising enzymes
Enzymes have an enormous range of commercial
applications – for example, in medicine, food technology
and industrial processing. Enzymes are expensive. No
company wants to have to keep buying them over and
over again if it can recycle them in some way. One of the
best ways of keeping costs down is to use immobilised
enzymes.
The enzyme lactase can be immobilised using alginate
beads (Box 3.2). Milk is then allowed to run through the
column of lactase-containing beads. The lactase hydrolyses
the lactose in the milk to glucose and galactose. The milk
is therefore lactose-free, and can be used to make lactosefree
dairy products for people who cannot digest lactose.
You can see that enzyme immobilisation has several
obvious advantages compared with just mixing up the
enzyme with its substrate. If you just mixed lactase with
milk, you would have a very difficult task to get the lactase
back again. Not only would you lose the lactase, but also
you would have milk contaminated with the enzyme.
Using immobilised enzymes means that you can keep and
re-use the enzymes, and that the product is enzyme-free.
Another advantage of this process is that the
immobilised enzymes are more tolerant of temperature
changes and pH changes than enzymes in solution. This
may be partly because their molecules are held firmly in
shape by the alginate in which they are embedded, and so
do not denature as easily. It may also be because the parts
of the molecules that are embedded in the beads are not
fully exposed to the temperature or pH changes.