Enzymes are proteins that act like biological catalysts (speed up chemical reactions). They can act like scissors and break down substances more quickly, or act like builders and put together substances more quickly. It all depends on their structure.
All enzymes have an active site. This is where substances bind to the enzymes to speed up their chemical reaction. The active site has a very complex and specific shape, so therefore it is usually only complementary to one substance.
Enzymes have an optimal temperature, which is usually the average internal body temperature (37 degrees Celsius). At a lower temperature, the enzymes have less kinetic energy, so they move around less. Therefore they collide with their complementary substances, decreasing the rate of reaction. At a higher temperature, the enzymes have too much kinetic energy, breaking some of their bonds. This changes the shape of the enzyme’s specific active site. As a result, the enzyme cannot bind to its complementary substance and is now considered denatured.
Enzymes also have an optimal pH level. Enzymes located in the stomach have a low optimal pH, while enzymes located in the small intestine have a high optimal pH.