Since enzymes are made up of proteins, heating them at temperatures above 45 degrees Celsius changes its three dimensional structure. An enzyme denatures when the protein structure changes. It become less soluble and coagulates. In addition to heat, enzymes are known to denature by various chemical such as acid and alkalis.
The three dimensional shape of the enzyme is a very important aspect of its function. When this shape is lost, it can no longer act as a catalyst.
Extreme heat (e.g. boiling) brings about an irreversible damage to the enzyme. Enzymes usually have a low optimum temperature. In animals, this is usually the body temperature. Certain enzymes in plant have a high optimum temperature. For example, the enzyme papain found in the fruit pawpaw has a optimum temperature of 65 degrees Celsius. However, most enzymes are completely denatured at temperatures above 60 degrees Celsius.