Why do enzymes stop working at high temperatures?

Enzymes are biological catalysts. This means they spead up chemical reactions in the body. To understand why they might stop working, you must first look at the structure. Enzymes are proteins made up of primary, secondary and tertiary structures to give their complex final shape. The primary structure is made up of a sequence of amino acids. The secondary structure is formed by hydrogen bonds between the amino acids, and the tertiary structure is a 3D structure produced by interations of the amino acid side chains. 

The bonds and interations making up the teriary structure of the enzyme are sensitive to heat. In different temperatures, these bonds can change. If an enzyme is used in the human digestive system (e.g. amylase), it will work best at body temperature of 37 degrees. In high temperatures, the bonds of the enzyme will be altered and the structure of the enzyme will change. This means the active site (where the substrates interact), will be a different shape. The substrates will not fit this new shape and thus, the enzyme will no longer work. It is denatured.