Question

How do 'coupled reactions' allow life (as a biochemical process) to exist?

Answer 1

A reaction occurs that releases energy (like ATP losing a phosphate to become ADP + Pi). If this is uncoupled, the energy will merely turn into heat. If it is coupled, then it can be used to fuel some other process. For instance, if you couple the ATP -> ADP reaction to a certain protein, the energy can be used to modify the shape of that protein.

A reaction where the the free energy of a thermodynamically favorable transformation, such as the hydrolysis of ATP, and a thermodynamically unfavorable one, are mechanistically joined into a new reaction (or may be envisaged to be so joined) is known as a coupled reaction.

To put it another way, two or more reactions may be combined mechanistically such that a spontaneous reaction may be made 'drive' a non-spontaneous one, and we may speak of the combined reaction as being 'coupled'. The combined reaction may be catalyzed by an enzyme, in which case the 'thermodynamic push' is provided by the coupling agent (such as ATP) and the 'kinetic push' is provided by the enzyme.

We need to take into account a very important point. the coupled reaction is a different reaction to the reaction we are trying to 'drive', with different overall stoichiometry and hence a different overall equilibrium constant.

A coupled reaction does not "push a reaction past its equilibrium" (see Atkinson, 1977, p52). No enzyme, for example, can push any reaction past its position of equilibrium. This is forbidden by the second law. (or enzymes) can however, cause a reaction to proceed further than it normally would by catalyzing a different reaction