To get the ride started you have to roll up an incline, this is the activation energy. Once beyond the top of the incline, the coaster is propelled forward by the force of gravity and no extra energy is needed. The activation energy represents the amount of work that goes into having the substrates come together in a manner that causes the reaction to occur. 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.