Relationship of pH to Standard Free Energy Change

Consider a hypothetical reaction: A ——> B.
If this is a thermodynamically unfavorable reaction the ΔGo\' value will be positive. Let\'s assume it is +4.0 kcal/mol. In order to drive this reaction in the direction written it can be coupled to the hydrolysis of ATP. The free energy of ATP hydrolysis to ADP is shown:

ATP + H2O ——> ADP + Pi: ΔGo\' = –7.3 kcal/mol

Coupling the two reactions together gives the equation:

A + ATP + H2O ——> B + ADP + Pi + H+

The ΔGo\' for this coupled reaction is the sum of the ΔGo\' values of the two separate reactions, i.e. (-7.3kcal/mol) + (+4.0kcal/mol) = -3.3kcal/mol. This indicates that coupling ATP hydrolysis provides the energy necessary to make the conversion of A to B thermodynamically favorable.

Another useful example is to examine one of the reactions of glycolysis. In this case we will look at the oxidation of phosphoenolpyruvate to pyruvate catalyzed by the enzyme pyruvate kinase (PK).

phosphoenolpyruvate ——> pyruvate: ΔGo\' = –14.7 kcal/mol
This reaction releases sufficient energy to drive the synthesis of ATP from ADP and Pi which would normally be thermodynamically unfavorable with a ΔGo\' of +7.3kcal/mol. Note that this value is the reciprocal of the hydrolysis of ATP. This points out another fact that the ΔGo\' for a reaction in one direction is equal but mathematically opposite for the reciprocal direction. Coupling the two reactions together yields:

phosphoenolpyruvate + ADP + H+ ——> pyruvate + ATP: ΔGo\' = –7.4 kcal/mol
For the simple enzyme-catalyzed reaction:
S <==> P
S is substrate; and P is product.
The enzyme forms a complex with the substrate, in much the same way as a protein-ligand complex, and performs some chemical reaction/transformation of the bound substrate. The resultant product is released.