So enzymes increase the rate of the full reaction, but they also increase the rate of the reverse reaction. So this is a very important point that most people sometimes miss. So if we look at the following diagram, we have the full reaction going this way and we also have the reverse reaction going this way. Now for the full reaction, this is the activation energy. For the reverse reaction, this is our activation energy. It's the energy difference between the transition state and the energy of the product.