Lecture 5

Coupled reactions

Coupling between reactions

A change of state of a system which occurs independently of coupling to the surroundings cannot conserve the work potential associated with the change. For a chemical reaction in a test-tube, all the work potential (change in free-energy, or the -DG of the process) is squandered as heat. If the system is able to exchange heat with the surroundings, this heat appears in the surroundings as an increase in entropy. In order to conserve energy, a change in state of the system must be coupled to a change in the surroundings which increases the work potential of a part of the surroundings (work is performed on the surroundings, or a separate system undergoes a change in state with +DG). In the context of biological energy conservation, this means that the work potential (as represented by the -DG) for a biochemical reaction will be lost unless the reaction occurs in concert with (is coupled to) another reaction which has a +DG.

For example, the reaction of fermentation in an organism producing lactate as the sole product of glucose metabolism can be written as:

glucose <=> 2 lactate -47 kcal. mol^-1 (-197 kJ. mol^-1)

The reaction of glycolysis in the cytoplasm can be written as:

glucose + 2 Pi + 2ADP <=> 2 lactate + 2ATP + 2H₂O -32.4 kcal. mol^-1

The difference between these two reactions is:

2 x ( ADP + inorganic phosphate (Pi) <=> ATP + H₂O ) 2 x 7.3 kcal. mol^-1

If we sum the free energy changes for the fermentation reaction and ATP synthesis (-47 kcal.mol^-1 + 14.6 kcal. mol^-1) we get -32.4 kcal. mol^-1, the free energy change from the glycolysis reaction.

We can see that the change in the system represented by the fermentation reaction, with a -DG, is coupled to a change in the surroundings (the change in a separate system represented by the phosphorylation of ADP to ATP), with +DG.

When two systems are coupled in this way, it is often convenient to treat them as a single system. In this example, the new system is the reaction represented by the glycolysis equation, with a -DG equal to the sum of values for the two processes contributing.

From this example, it will be apparent that we can, from a thermodynamic perspective, treat metabolic processes in several ways. We can treat individual reactions as separate systems, or treat a set of coupled reactions (including the complete set representing the metabolism of the organism as a whole) as a single system. The choice is one of convenience, and the important points are that the system should be carefully defined, the reaction equation balanced in conformity with the Law of conservation of mass, and the energy equation balanced in accordance with the First Law of thermodynamics, and the properties of variables of state.

©Copyright 1996, Antony Crofts, University of Illinois at Urbana-Champaign, a-crofts@uiuc.edu