Explain the lock and key model of enzyme action, including how they are denatured.

An enzyme is a protein that functions as a biological catalyst by speeding up chemical reactions and remaining unchanged by the reaction. The place on an enzyme where the molecule binds is called the active site. In the lock and key model, the shape of the active site exactly matches the shape of the substrate. This makes enzymes highly specific and means that each type of enzyme can usually only catalyse only one type of reaction - with some maybe able to catalyse a few types of reactions. 
In the example below, the enzyme splits one molecule into two smaller ones.Substrate collides with active site of the enzyme and becomes attached - the active site has a complimentary shape to the substrate, the fit between the enzyme and substrate is like a lock and keyEnzyme catalyses the breakdown of the substrate - the enzyme molecule is unchanged and can be reusedThe product is then released from the active site.
When enzymes are exposed to extreme levels of pH or high temperatures the shape of their active site may change. When this happens the enzyme will no longer to be able to bind the substrate and their shapes will not be compatible - the key will no longer fit the lock. In this case, we say that the enzyme has been denatured.