The rate of enzyme reactions is affected by temperature. All enzymes have an optimum temperature range in which they work most efficiently. This is somewhere between 40-45 degrees Celsius. An enzyme is most active at its optimum temperature. A temperature rise beyond this point reduces enzyme activity till it completely stops. This happens because the enzyme has completely destroyed or denatured.
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.
Proteins are chains of peptides (primary structure) held together by covalent bonds that are hard to break. The secondary and tertiary (3D) structure is held together with non-covalent bonds that break and form at room temperature. The structure is held together in an equilibrium between folded and unfolded structures which favours the folded structure under normal conditions. In certain conditions (high salt urea/guanadinium) the 3D structure unfolds but the non-covalent bonds can sometimes form again and hold the structure together. Under really harsh conditions (boiling) the covalent bonds are broken so that the chain of peptides is broken.