English: Diagram of a catalytic reaction, showing the energy needed at each stage of the reaction. The substrates (A and B) normally need a large amount of energy to reach the transition state, which then reacts to form the end product (AB). The enzyme creates a microenvironment in which A and B can reach the transition state more easily, reducing the amount of energy needed. Since the lower energy state is easier to reach and therefore occurs more frequently, as a result the reaction is more likely to take place, thus improving the reaction speed.