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Enzymes' Effect on Activation Energy and Free Energy
Gibbs free energy is the free energy difference between the products and reactants of a chemical reaction. It can be used to determine what the equilibrium will look like and whether or not the given reaction is spontaneous. If a reaction has a zero Gibbs free energy, then the reaction is said to be at equilibrium and this implies that the rate of the forward reaction is equal to the rate of the reverse reaction. A negative Gibbs free energy implies that the reaction is exergonic and spontaneous. On the other hand, an endergonic reaction is said to be non-spontaneous and has a positive Gibbs free energy. An important biological process that is endergonic is the synthesis of ATP molecules. ATP molecules are high energy molecules that can be broken down to produce useful energy. The reverse of ATP synthesis - the break down of ATP molecules - is an exergonic reaction, which means its spontaneous and will produce energy that can be readily used by the cell. Enzymes do not affect the Gibbs free energy of a reaction. That means that they do not increase or decrease how much products are formed and how much reactants are used up nor do they increase or decrease the free energy values of the products and reactants. Gibbs free energy is independent of the pathway or mechanism of the reaction. That means that an uncatalyzed reaction will have the same Gibbs free energy as a catalyzed one. Another important quantity that can be used to understand enzyme activity is activation energy. Activation energy is the smallest amount of energy that must be inputed into the reaction to get it going. Whereas Gibbs free energy determines where the equilibrium will settle out and how much products are produced at the end of the reaction, the activation energy determines the rate at which the reaction ouccrs. Enzymes affect the activation energy by lowering it. Since the apex of the energy curve describes the energy of the transition state, we shall see shortly that enzymes actually stabilize the transition state and lower its energy.
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