Human Biochemistry, Volume 1 - Murray R. 1993

Structure and Functions of Proteins and Enzymes
Enzymes: Kinetics
The Role of Catalysts in the Formation of Productive Transition States

Let us consider reaction profiles that differ only in The formation of two distinct transition states (Fig. 8.3).

In both cases, the standard Gibbs Free energy of activation characterizes the energy barrier of the overall reaction. However, the energy barrier of the reaction proceeding via the Transition State [Y^...R^...X]_b is lower than that of the reaction proceeding through the transition state [Y^...R^...X]_a. In the example above, [Y^...R^...X]_a represents the transition state of the uncatalyzed reaction, whereas [Y^...R^...Х]_b is the transition state of the catalyzed reaction. All catalysts, including Enzymes, lower the free energy of activation ∆G_F. Note further that catalysts have no effect on ∆G; the overall free energy change of the reaction is independent of the presence of catalysts. The Equilibrium Constant of a chemical reaction is a function of the Standard Free Energy change of that reaction:

Class="center">∆G⁰ = —RT In K_eq

Fig. 8.1. Energy profile of a substitution reaction characterized by a negative free energy change (∆G < 0).

Fig. 8.2. Energy profile of a substitution reaction characterized by a positive free energy change (∆G > 0).

Fig. 8.3. Reaction energy profiles involving the formation of two different transition states. [Y^...R^...Х]_а and [Y^...R^...X]_b, ∆G^a_F and ∆G_F^b are the free energies of activation for the formation of these complexes.

It follows that enzymes and other catalysts do not affect the reaction equilibrium constant.