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Biology for Advanced Level Secondary Schools

The rate of any enzyme catalysed reaction This change in shape means that the active
directly depends on the concentration of the site is less complementary to the shape of
enzymes. Provided that, the temperature and the substrate; therefore, it is less likely to
other conditions are suitable for the reaction catalyse the reaction. The change in shape
and there are excess substrate molecules, the prevents the enzyme and substrate from
rate of the reaction is directly proportional fitting together exactly. Finally, the enzyme
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to the concentration of enzymes. If the will become denatured, and it will no longer
amount of the substrate is restricted, it may be functional, hence decreasing the rate of
limit the rate of the reaction. Likewise, reaction.
addition of more enzymes cannot increase
the rate, thus, the graph tails off, as shown However, below normal temperature,
in Figure1.46. enzymes become less active due to reduction
in speed of molecular movement. When
the temperature is lowered below or near
Product concentration With 1x enzymes inactive. This condition will last untill when
freezing point, the enzymes are said to be
With 2x enzymes
higher temperature above the freezing point
is restored.

Time With no enzymes Therefore, as the temperature increases, the
Figure 1.46: The effect of enzyme concentration rate of reaction also increases due to increased
on the reaction rate of enzyme controlled reaction kinetic energy. There is a temperature
range at which the rate of enzyme’s action
is maximum. This is called an optimum
(c) Temperature temperature range. Below this temperature
Generally, increase in temperature increases range, enzymes are less active. Contrary,
the kinetic energy that molecules possess, above the optimum temperature, enzymes are
implying more random collisions between denatured. Within the optimum temperature
molecules per unit time. Since enzymes range there is optimum temperature at which
catalyse reactions by randomly colliding with the rate of the reaction is maximum. All
substrate molecules, increasing temperature enzymes work within a range of temperature
will increase the rate of reaction, leading specific to the organism. In human, the
to more products. However, increasing optimum temperature for many enzymes lies
temperature also increases the vibration around 40 °C and denaturation occurs at about
energy that molecules have; specifically 60 °C (Figure 1.47). However, enzymes of
in case of enzyme molecules which put some other organisms have different optimum
strain on the bonds that hold them together. temperatures. For example thermophilic
As temperature increases, more bonds, bacteria such as Thermus aquaticus have
especially the weaker hydrogen and ionic optimum enzymatic temperature ranging
bonds will break because of this strain. from from 51 °C to 80 °C and in Archaea
Breaking of bonds within the enzyme will the optimum enzymic temperature ranges
cause the active site to change in shape. from 41 °C to 122 °C.

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