Figure 3 - A chemical reaction. Notice how the rate of reaction greatly decreases as the number of substrates decreases. Enzymes increase the rate of reaction by lowering the activation energy. They lower this activation energy by binding the substrates of the reaction. This causes a reaction to occur for two reasons.
Enzymes bind their substrates at what is called the active site.
When bound at the active site the substrates are brought into close proximity to one another and will react. The enzyme will bend the substrates in such a manner to make them more reactive. In biochemistry speak, the enzyme binds the substrates in a transition state complex. This makes it possible for substrates to react and form products. Figure 4 - A chemical reaction catalyzed by an enzyme.