Astudent added an enzyme to a flask containing a substrate and placed the mixture over a flame. she observed the reaction for a few minutes when she noticed that the reaction appeared to have stopped. she was puzzled because there was still a large amount of substrate left in the reaction flask. a ph test confirmed that the ph of the solution was in an acceptable range. which is the most likely explanation for the decreased reaction rate?

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