Coupled Reactions

Two or more reactions in a cell sometimes can be coupled so that thermodynamically unfavorable reactions and favorable reactions are combined to drive the overall process in the favorable direction. In this circumstance the overall free energy is the sum of individual free energies of each reaction. This process of coupling reactions is carried out at all levels within cells. The predominant form of coupling is the use of compounds with high energy to drive unfavorable reactions.

The predominant form of high energy compounds in the cell are those which contain phosphate. Hydrolysis of the phosphate group can yield free energies in the range of –10 to –62 kJ/mol. These molecules contain energy in the phosphate bonds due to:

1. Resonance stabilization of the phosphate products

2. Increased hydration of the products

3. Electrostatic repulsion of the products

4. Resonance stabilization of products

5. Proton release in buffered solutions

The latter phenomenon indicates that the pH of the solution a reaction is performed in will influence the equilibrium of the reaction.