As the temperature is increased the kinetic energy of the molecules increases and the molecules move faster and collide more often and with more energy.There are more frequent and more effective collisions so more enzyme-substrate complexes are formed in a given time.At the optimum temperature, the rate of enzyme-substrate complex formation is a maximum.When the temperature is greater than the optimum temperature the hydrogen bonds and other bonds holding the enzyme molecule and active site in its specific 3D shape begin to break.The shape of the active site changes and is no longer complementary to the shape of the substrate.Around 60°C the enzyme stops working completely and is denatured.Denaturation is an irreversible change.