Why do enzymes stop working at high temperatures?

With this question, you have to remember that enzymes are proteins. When the temperature gets too hot, the enzyme denatures. Denaturing is where the bonds that hold the enzyme together break, causing the active site of the enzyme to change shape. Because the active site has changed shape, the enzyme is unable to function properly and eventually the enzyme will stop working altogether. The effect of temperature of enzymes can be shown on a graph - you should be able to remember what the graph looks like.