Enzymes are proteins that function as biological catalysts. They are molecules that speed up a reaction without getting used up
The lock and key hypothesis
This theory suggests that like a lock and key, enzymes active site (lock) and substrates (key) have to be complementary to each other to create a reaction. if they are not complementary no reaction will take place.
When enzymes are exposed to very high temperatures or pH levels they become denatured.
This is where the active site of the enzyme changes shape which means it is no longer complementary to the substrate. The key will no longer fit in the lock.