PP: Activation Energy

Exergonic reactions are spontaneous, while endergonic reactions are not; however, it is not the free energy change that determines the rate of a reaction. The rate of a reaction is determined by the activation energy, which is the energy needed to reach the transition state between reactants and products. Reactions with higher activation energies have slower rates because fewer molecules have enough energy to reach the transition state. When the activation energy is lower, more molecules can easily reach the transition state, accelerating the reaction. Enzymes speed up reactions by lowering activation energies. They do so by stabilizing the structure of the transition state, which then requires less energy to be reached. Enzymes do not affect the free energies of the substrates or products, and they do not alter the equilibrium of a reaction. They simply allow equilibrium to be reached faster. Enzymes can enhance the rate of a reaction in many ways: forming favorable interactions in its active site, orienting two substrates to react, directly participating in the reaction, or strain the substrate bonds. Enzymes usually use more than one of these strategies to stabilize the transition state, lower the activation energy, and speed up a mechanism.