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.