The reason should be denaturing of the active site of the enzyme.
The rate of the enzymic
reaction depends on pH, concentration of substrate, enzyme molecule
concentration and the temperature. With the increase of temperature, the rate
of the enzymic reaction also increases. But this only happens up to a
certain point. After a certain temperature, the rate becomes to
decrease and eventually will stop. This is because denaturing of the enzyme.
Although other factors are perfect, the high temperature causes to break down
of bond interactions and hence the active site will be changed. If
the active site of an enzyme changed, then substrate cannot bind with that enzyme
anymore.
The temperature which the rate of the reaction is
maximum is called optimum temperature and it normally around 40°C. This can be
varied from enzyme to enzyme
