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Increasing the temperature increases the kinetic energy of the enzyme and substrate molecules so that they move faster and are more likely to collide. So increasing the temperature increases the rate of the reaction up to a certain temperature. This temperature is known as the enzyme’s optimum temperature. Different enzymes have different optimum temperatures. The enzymes in animal bodies work best at 37˚C.
If the temperature is increased beyond the optimum the enzyme has so much kinetic energy that the bonds holding the enzyme molecule together start to vibrate and eventually break. The enzyme loses its specific shape so that the substrate no longer fits in to the active site. We say that the enzyme is denatured.
Enzymes also have an optimum pH. Different enzymes have different optimum pHs, e.g. pepsin found in the stomach works best at pH 2, which is why the stomach produces hydrochloric acid.
Although enzymes in our bodies work best at 37°C, some very special enzymes can even work well at 75°C and they form the basis of a technique used to make lots of DNA from tiny amounts found at crime scenes. This technique is called the Polymerase Chain Reaction (PCR) and uses an enzyme called Taq polymerase – an enzyme that works best at 75°C. Taq is a nickname for Thermus aquaticus, a bacterium that happily survives and reproduces in hot springs - an environment that is lethal to other organisms. You can find out more about PCR here.