BioFactsheet
April 1999Number 43
1
Factors Affecting Enzyme Activity
Enzymes are globular proteins which act as biological catalysts. This means
that they speed up the rate of reaction by lowering the activation energy,
that is the energy required to break bonds. Enzymes are a complex tertiary
and sometimes quaternary shape and catalyse reactions by forming a
complex (known as the enzyme substrate complex) at a specific region of
the enzyme called the active site.
Enzyme + substrate à enzyme substrate complex à product
Enzymes are specific; any individual enzyme can usually only catalyse
one particular reaction. The induced fit hypothesis has been put forward
to explain how enzymes work. The key points of the induced fit hypothesis
are as follows (Fig1):
The effect of temperature on the rate of a chemical reaction is described by
the term “temperature coefficient” (Q10).
Q10 = rate of reaction at T + 10oC
rate of reaction at ToC
Many enzymes have a Q10 of between 2 and 3. In other words, provided
that the temperature is not so high that it causes denaturation, an increase
in temperature of 10oC will speed up the reaction by a factor of 2-3, that is
it will double or treble it (Fig 2).
2. pH
The effect of a change in pH on enzyme activity is shown in Fig 3. As with
temperature, each enzyme has an optimum pH. If pH increases or decreases
much beyond this optimum, the ionisation of groups at the active site and
on the substrate may change, effectively slowing or preventing the formation
of the enzyme substrate complex. At extreme pH, the bonds which maintain
the tertiary structure – hence the active site – are disrupted and the enzyme
is irreversibly denatured.
Since most human enzymes are intracellular, most have a pH optimum of
7.3-7.4. However, pepsin, which works in the acidic environment of the
stomach, has an optimum of 2.4 (Fig 3).
Fig 1. Induced fit hypothesis
Enzyme Substrate ES complex
+
1. Substrate approaches the active site of the enzyme.
2. The shape of the active site then changes to fit precisely around the
substrate – in other words, the substrate induces the active site to
change shape.
3. The reaction is catalysed and products form.
4. The products are a different shape from the substrate and therefore
diffuse away from the active site. As they do, the active site reverts to
its original shape.
Factors affecting enzyme activity
1. Temperature
Enzymes have an optimum temperature – this is the temperature at which
they work most rapidly. Below the optimum temperature, increasing
temperature will increase the rate of the reaction. This is because
temperature increases the kinetic energy of the system, effectively increasing
the number of collisions between the substrate and the enzyme’s active
site.
Temperatures above the optimum will lead to denaturation. This occurs
because the hydrogen bonds and disulphide bridges which maintain the
shape of the active site are broken. Thus, enzyme substrate complexes can
no longer be formed.
Fig 2. Effect of temperature on enzyme activity
Temperature
Rate of reaction
Fig 3. Effect of pH on enzyme activity
pH
Rate of reaction
