PP: Metabolic Coupling

The metabolism includes both anabolic and catabolic processes. Catabolic processes are exergonic and spontaneous while anabolic processes are endergonic and non-spontaneous; however, metabolic coupling between biochemical pathways allows endergonic reactions to still occur. In metabolic coupling, energy released from an exergonic reaction is used to provide energy needed for an endergonic reaction. This energy is often exchanged through phosphate groups. Molecules with phosphate groups tend to have high free energies, so the removal of a phosphate group by hydrolysis results in a largely negative change is free energy. Phosphorylation, on the other hand, requires energy input. The magnitude of free energy available in the bond is the phosphoryl group transfer potential, and molecules with larger phosphoryl group transfer potentials can phosphorylate molecules with lower potential. In other words, energy released through the hydrolysis of a phosphoryl group is used to power the phosphorylation of another mloecule. An example is the coupling that occurs to power muscle contractions. Creatine phosphate is hydrolyzed to form creatine, and this energy is used to phosphorylate ADP to ATP. ATP is the main energy currency molecule in nearly all cells.