What effect does temperature have on enzyme action?

Enzymes are biological catalysts and speed up reactions happening inside organisms.
An increase in temperature increases the rate of enzyme activity and therefore rate of reaction up to a certain point. In most organisms at around 40^oC protein structure of the enzyme is affected. The long amino acid chains that make up the enzyme begin to unravel. This changes the shape of the active site of the enzyme so that it is no longer complementary to the substrate. This is known as denaturing and the enzyme can no longer act as a catalyst as the substrate does not fit so the rate of reaction rapidly decreases beyond the optimum temperature.
The optimum of most human enzymes is around 37^oC so this is the temperature where they are most efficient. Other types of organisms such as extremophile bacteria live in hot springs so the optimum temperature for their enzymes is around 80^oC or more.