Denaturing an enzyme results in a permanent change to that enzyme. The permanent change comes from heat changing the shape of the enzyme, which stops it from working properly. Not only does heat change the shape of an enzyme, it changes its pH level as well, causing the enzyme to stop working.
Although enzymes are changed by heat, they cannot be killed because they are not living things. They work best at their optimal temperatures, and they are simply proteins that increase the speed of chemical reactions in the cells of living things. Generally, heat speeds up the rate of chemical reaction, which is not a bad thing unless the temperature becomes too hot. The optimal temperature for enzymes in the human body is around 98.6 degrees Fahrenheit.
Enzymes are like puzzle pieces. Only one molecule with the proper shape can fit into one enzyme, which makes the shape of these enzymes so important. Anything, like denaturing, that changes the shape of the active site of an enzyme renders it useless, and that is why proper temperature is so important.