Human Biochemistry, Volume 1 - Murray R. 1993

Structure and Functions of Proteins and Enzymes
Enzymes: Kinetics
Formation and Decay of Transition States

Let us consider a substitution reaction in which an incoming group Y replaces a leaving group X:

Class="center">Y + R — X ⇄ Y — R + X.

This reaction consists of two half-reactions: 1) The formation of a Transition State in which both Y and X are attached to the group R; 2) the decomposition of the transition state to yield the products:

Each of the half-reactions, like any other chemical reaction, is characterized by a specific change in Free energy. Let us denote by ∆G_F the free energy change associated with the Formation of the transition state, and by ∆G_D the free energy change associated with its decomposition and product formation:

The overall free energy change of the complete reaction, ∆G, is equal to the sum of the Free Energy Changes of each half-reaction:

∆G = ∆G_F + ∆G_D.

As in any equation containing two terms, it is impossible to determine the sign or magnitude of ∆G_F and ∆G_D solely from the algebraic sign and magnitude of ∆G. In other words, knowing the free energy change of the overall reaction ∆G tells us nothing about the free energy changes associated with the formation and decomposition of the transition states. Since catalysis is intimately related to The values of ∆G_F and ∆G_D, it is clear that the Thermodynamic parameters of the overall reaction (∆G) provide no insight into the reaction pathway (i.e., its mechanism). This very question is the subject of kinetics.