Cambridge International A Level Biology Revision Guide

Chapter 3: Enzymes
substrate (key)
products
active site
enzyme–substrate complex
enzyme (lock)
a An enzyme has a cleft in its
surface, called the active site.
The substrate molecule has a
complementary shape.
b Random movement of enzyme and
substrate brings the substrate into the
active site. An enzyme–substrate
complex is temporarily formed. The
R groups of the amino acids in the
active site interact with the substrate.
Figure 3.2 How an enzyme catalyses the breakdown of a substrate molecule into two product molecules.
c
The interaction of the substrate with the
active site breaks the substrate apart.
An enzyme–product complex is briefly
formed, before the two product
molecules leave the active site, leaving
the enzyme molecule unchanged and
ready to bind with another
substrate molecule.
Each type of enzyme will usually act on only one type
of substrate molecule. This is because the shape of the
active site will only allow one shape of molecule to fit.
The enzyme is said to be specific for this substrate.
In 1959 the lock and key hypothesis was modified in
the light of evidence that enzyme molecules are more
flexible than is suggested by a rigid lock and key. The
modern hypothesis for enzyme action is known as the
induced fit hypothesis. It is basically the same as the lock
and key hypothesis, but adds the idea that the enzyme, and
sometimes the substrate, can change shape slightly as the
substrate molecule enters the enzyme, in order to ensure a
perfect fit. This makes the catalysis even more efficient.
An enzyme may catalyse a reaction in which the
substrate molecule is split into two or more molecules,
as shown in Figure 3.2. Alternatively, it may catalyse the
joining together of two molecules, as when making a
dipeptide. A simplified diagram is shown in Figure 3.3.
This diagram also shows the enzyme–product complex
which is briefly formed before release of the product.
Interaction between the R groups of the enzyme and the
atoms of the substrate can break, or encourage formation
of, bonds in the substrate molecule, forming one, two or
more products.
When the reaction is complete, the product or products
leave the active site. The enzyme is unchanged by this
process, so it is now available to receive another substrate
molecule. The rate at which substrate molecules can bind
to the enzyme’s active site, be formed into products and
leave can be very rapid. The enzyme catalase, for example,
enzyme
(lock)
enzyme–product
complex
substrates
(keys)
enzyme–substrate
complex
product
Figure 3.3 A simplified diagram of enzyme function. Note that in
this example the enzyme is catalysing the joining together of two
molecules.
55