Why are enzymes affected by temperature?

At low temperature, there is low kinetic energy (=molecules move more slowly). So, an enzyme and a substrate are less likely to randomly collide with eachother (similarly, people are more likely to bump into eachother if they run around a room really quickly than if they move very slowly). Enzyme and substrate are also less likely to collide with enough energy for the reaction to occur. Therefore: rate of enzyme reaction is lower at lower temperatures.

Like other globular proteins, the tertiary structure of enzymes is maintained by many weak non covalent interactions such as ionic bonds and Van der Walls forces. At high temperature, there is high kinetic energy (the molecule 'vibrates' more), which can disrupt the weak non covalent interactions. If this happens, the shape of the active site changes, and it is no longer complementary to the substrate, so the reaction can't occur. Therefore: rate of enzyme reaction is lower at higher temperatures because some enzymes no longer function.