They do not change the energy of the reactants, nor they actually change the energy of the products. And that's exactly why the difference, namely the delta g that gives free energy will remain exactly the same when an enzyme is used or when an enzyme is not used. Now, the final thing I'd like to mention about Gibbs free energy is what happens if Gibbs free energy is equal to zero? Well, if Gibbs free energy is zero, then no energy is being produced in that reaction that actually can be used in any useful way. In fact, when Gibbs free energy is zero, that reaction is said to have reached equilibrium. And at that moment in time, the rate of the four reaction is equal to the rate of the reverse reaction.