An enzyme is a substance that speeds up chemical reactions in your body. The part of a reaction an enzyme works on is called the substrate. An enzyme will work to break a substrate down into separate products. Enzymes, however, cannot work on just any substrate. Their active site has a specific shape that will only allow substrates that match their shape bind. This is referred to as the lock and key model. The enzyme is like the lock and the substrate is like the key. Only a certain key can fit into and unlock a lock. The same is true with enzymes. Enzymes can only work on certain specific substrates that will fit into it.