Up to the optimum temperature (around 37 degrees Celsius) kinetic energy increases. So the likelihood of successful collisions between the active site of the enzyme and substrate increases. More enzyme substrate complexes are formed. Rate of reaction increases.Past the optimum temperature Kinetic Energy is too high. Enzyme denatures as (example)bond breaks affecting shape of tertiary structure and shape of Active Site. Less enzyme substrate complexes are formed. Rate of reaction decreases.